domain: sdv.aen sdv sde

text:
Lucien Besnard

Laurent Duchatelet

Christopher Bird

Gaël Le Croizier

Loïc Michel

Nicolas Pinte

Gilles Lepoint

Gauthier Schaal

Rui P Vieira

Jorge M S Gonçalves

Besnard Lucien

Duchatelet Laurent

Le Croizier Gaël

Michel Loic

Pinte Nicolas

Lepoint Gilles

Schaal Gauthier

Martin Ulrich

Mallefet Jérôme

Diet consistency but large-scale isotopic variations in a deep-sea shark: The case of the velvet belly lantern shark, Etmopterus spinax, in the northeastern Atlantic region and Mediterranean Sea

Keywords: Trophic ecology, Stable Isotopes, Stomach content, Mesopredator, Food webs, Benthopelagic predator

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Under the footprint of anthropogenic activities and climate change, many coastal and epipelagic fisheries have significantly declined [START_REF] Chavez | From Anchovies to Sardines and Back: Multidecadal Change in the Pacific Ocean[END_REF][START_REF] Pinsky | Unexpected patterns of fisheries collapse in the world's oceans[END_REF][START_REF] Tu | Fishing and temperature effects on the size structure of exploited fish stocks[END_REF], diverting fishing efforts toward deep-sea stocks [START_REF] Bailey | Long-term changes in deep-water fish populations in the northeast Atlantic: a deeper reaching effect of fisheries?[END_REF][START_REF] Devine | Deep-sea fishes qualify as endangered[END_REF][START_REF] Priede | A review of the spatial extent of fishery effects and species vulnerability of the deep-sea demersal fish assemblage of the Porcupine Seabight, Northeast Atlantic Ocean (ICES Subarea VII)[END_REF][START_REF] Vieira | Deep-water fisheries along the British Isles continental slopes: status, ecosystem effects and future perspectives[END_REF]. As a consequence, significant population decreases have been observed in these deep water ecosystems with unknown and potentially disruptive impacts on ecological processes and functions [START_REF] Benn | Human Activities on the Deep Seafloor in the North East Atlantic: An Assessment of Spatial Extent[END_REF][START_REF] Vieira | Deep-sea sponge aggregations (Pheronema carpenteri) in the Porcupine Seabight (NE Atlantic) potentially degraded by demersal fishing[END_REF].

However, despite their ecological importance, fragility and current state of exploitation, deepsea ecosystems are still relatively understudied compared to their shallower counterparts [START_REF] Benn | Human Activities on the Deep Seafloor in the North East Atlantic: An Assessment of Spatial Extent[END_REF][START_REF] Thurber | Ecosystem function and services provided by the deep sea[END_REF]. In order to predict how deep-sea ecosystems will respond to natural or human-induced changes, information on the functioning of deep-sea communities is urgently needed [START_REF] Howell | A decade to study deep-sea life[END_REF].

Previous descriptions of deep-sea habitats have established that food webs are complexly structured and include a range of trophic levels fuelled by a mixture of primary production, secondary production and benthic recycling [START_REF] Newman | High Mercury Concentrations Reflect Trophic Ecology of Three Deep-Water Chondrichthyans[END_REF]Shipley et al., 2017b;[START_REF] Trueman | Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes[END_REF]. Due to the absence of light for autochthonous primary production, deep-sea fauna relies on the downward vertical transport of nutrients [START_REF] Polunin | Feeding relationships in Mediterranean bathyal assemblages elucidated by nitrogen and carbon isotope data[END_REF][START_REF] Preciado | Food web functioning of the benthopelagic community in a deep-sea seamount based on diet and stable isotope analyses[END_REF], either actively by species diel vertical migrations or passively due to particulate organic matter sinking and re-suspension [START_REF] Trueman | Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes[END_REF]. Within these food webs, deep-sea elasmobranch species are meso-to top predators [START_REF] Churchill | Trophic interactions of common elasmobranchs in deep-sea communities of the Gulf of Mexico revealed through stable isotope and stomach content analysis[END_REF][START_REF] Simpfendorfer | Limited potential to recover from overfishing raises concerns for deep-sea sharks, rays and chimaeras[END_REF] which are of critical importance through their potential topdown or more complex regulation of communities [START_REF] Heithaus | Predicting ecological consequences of marine top predator declines[END_REF]Shipley et al., 2017a).

By connecting different depth layers, they also influence energy flux and carbon cycling, further enhancing their key ecological role [START_REF] Trueman | Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes[END_REF]. Unfortunately, information J o u r n a l P r e -p r o o f 1. Introduction on deep-sea sharks is still lacking due to the logistical challenges of accessing and studying them [START_REF] Hussey | Mark report satellite tags (mrPATs) to detail large-scale horizontal movements of deep water species: First results J o u r n a l P r e -p r o o f for the Greenland shark (Somniosus microcephalus)[END_REF][START_REF] Moura | Management of deep-water sharks' by-catch in the Portuguese anglerfish fishery: from EU regulations to practice[END_REF][START_REF] Simpfendorfer | Limited potential to recover from overfishing raises concerns for deep-sea sharks, rays and chimaeras[END_REF].

In deep-sea sharks, most trophic studies have relied upon stomach content analysis [START_REF] Barría | Feeding habits of four sympatric sharks in two deep-water fishery areas of the western Mediterranean Sea[END_REF][START_REF] J O U R N A L P R E -P R O O F Cortés | Standardized diet compositions and trophic levels of sharks[END_REF]. Even if this approach allows a complete qualitative description of the diet, it requires a large number of samples, represents only a snapshot of the last meal(s) and differences in digestion rates may also bias the importance of prey items [START_REF] Albo-Puigserver | Feeding ecology and trophic position of three sympatric demersal chondrichthyans in the northwestern Mediterranean[END_REF]. In contrast, biochemical tracers such as stable isotope ratios of carbon (δ 13 C) and nitrogen (δ 15 N) can help to describe the trophic structure, niche width and energy fluxes on a long-term basis with potential quantitative approaches [START_REF] Layman | Applying stable isotopes to examine food-web structure: an overview of analytical tools[END_REF]Shipley et al., 2017a). Carbon isotope signatures are considered as a good proxy to characterize the primary producers at the base of food webs [START_REF] Fry | δ13C Measurements as Indicators of Carbon Flow in Marine and Freshwater Ecosystems[END_REF][START_REF] Layman | Applying stable isotopes to examine food-web structure: an overview of analytical tools[END_REF]. Nitrogen isotope composition is mainly used as a proxy of trophic position due to a global increase in δ 15 N signal from prey to predator [START_REF] Cabana | Modelling food chain structure and contaminant bioaccumulation using stable nitrogen isotopes[END_REF][START_REF] Post | Using Stable Isotopes to Estimate Trophic Position: Models, Methods, and Assumptions[END_REF] and has been linked to foraging depth in benthopelagic communities [START_REF] Trueman | Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes[END_REF]. By extension, the combination of carbon and nitrogen isotopes constitutes a proxy of the trophic niche crucial for assessing the ecological role of a given species or population [START_REF] Newsome | A Niche for Isotopic Ecology[END_REF].

Deep-sea shark isotopic values depend on their diet but also on the isotopic compositions at the base of food webs, which exhibit spatial and temporal variations [START_REF] Magozzi | Using ocean models to predict spatial and temporal variation in marine carbon isotopes[END_REF][START_REF] Somes | Simulating the global distribution of nitrogen isotopes in the ocean[END_REF]. Environmental conditions and local biogeochemical processes are known to affect baseline isotopic profiles due to changes in nutrient dynamics. For example, latitudinal differences in baseline isotopic values can be influenced by temperature [START_REF] Magozzi | Using ocean models to predict spatial and temporal variation in marine carbon isotopes[END_REF][START_REF] Rau | CO2aq-dependent photosynthetic 13C fractionation in the ocean: A model versus measurements[END_REF], river discharges [START_REF] Chouvelon | Revisiting the use of δ15N in meso-scale studies of marine food webs by considering spatio-temporal variations in stable isotopic signatures -The case of an open ecosystem: The Bay of Biscay (North-East Atlantic)[END_REF] and upwelling events J o u r n a l P r e -p r o o f [START_REF] Lopez-Lopez | Does upwelling intensity influence feeding habits and trophic position of planktivorous fish? Deep[END_REF][START_REF] Puccinelli | Coastal upwelling affects filterfeeder stable isotope composition across three continents[END_REF]. The extent to which large-scale spatial variations in isotopic baselines are reflected in deep-sea ichtyofauna remains unclear.

Moreover, deep-sea sharks are mobile species that can shift their feeding ground while migrating. Isotopically, this shift will follow baseline changes and these variations need to be considered when analysing the trophic niche of a species over time or space [START_REF] Bird | A global perspective on the trophic geography of sharks[END_REF][START_REF] Lorrain | Nitrogen isotopic baselines and implications for estimating foraging habitat and trophic position of yellowfin tuna in the Indian and Pacific Oceans[END_REF].

Along the northeastern Atlantic and Mediterranean continental shelfs and slopes, the trophic ecology of a small deep-sea shark, the velvet belly lantern shark, Etmopterus spinax (Linnaeus, 1758), has been extensively studied using stomach content analysis [START_REF] Klimpel | Metazoan parasites and food composition of juvenile Etmopterus spinax (L., 1758) (Dalatiidae, Squaliformes) from the Norwegian Deep[END_REF][START_REF] Neiva | Feeding habits of the velvet belly lanternshark Etmopterus spinax (Chondrichthyes: Etmopteridae) off the Algarve, southern Portugal[END_REF][START_REF] Valls | Feeding strategies and resource partitioning among elasmobranchs and cephalopods in Mediterranean deep-sea ecosystems[END_REF]. Beyond those habitats, luminous velvet belly lantern sharks inhabit deep layers of fjords in Norway [START_REF] Claes | Phantom hunter of the fjords: Camouflage by counterillumination in a shark (Etmopterus spinax)[END_REF][START_REF] Duchatelet | Glow on Sharks: State of the Art on Bioluminescence Research[END_REF]. The first objective of this study is to present a description of the species diet inside these semi-enclosed habitats. More generally, throughout its distribution area, only a few studies took into account stable isotopes to specifically investigate E. spinax trophic habitat [START_REF] Albo-Puigserver | Feeding ecology and trophic position of three sympatric demersal chondrichthyans in the northwestern Mediterranean[END_REF][START_REF] Valls | Feeding strategies and resource partitioning among elasmobranchs and cephalopods in Mediterranean deep-sea ecosystems[END_REF]. Consequently, the second objective of this study is to investigate muscle carbon and nitrogen isotopic compositions of E. spinax at four different locations in the Mediterranean Sea and in the northeastern Atlantic to gain long-term dietary information on this species. Information on the trophic ecology of E. spinax gathered by stomach content analysis were ultimately compared to stable isotope values and discussed in terms of local habitat ecological characteristics and physical processes affecting nutrient dynamics.

MATERIALS AND METHODS

Stomach content

J o u r n a l P r e -p r o o f

In Norway, stomach content analysis of Etmopterus spinax came from eleven scientific expeditions from 2008 to 2012 inside the Raunefjord (Figure 1). Based on morphological analysis of less-digested or undigested component, preys were identified at the lowest taxonomic level possible. For Euphausiacea, eyes were digested at a lower rate than the rest of the body parts. Therefore, each pair of eyes was identified as one Euphausiacea individual unless morphological characteristics allowed identification to the species level. Cephalopods were identified through their remaining beaks. For large fishes, species identification was possible when the whole body was present or based on dental bone dimension. To assess and compare prey composition, four indices were calculated: the numeric percentage %N (a prey item abundance as a percent of the total prey abundance), the gravimetric percentage %W (a prey item remaining mass as a percent of the total prey mass), the occurrence percentage %O (number of stomachs containing a prey item in percent compare to all stomachs) and finally the index of relative importance IRI: IRI = (%N + %W) × %O [START_REF] Cortés | A critical review of methods of studying fish feeding based on analysis of stomach contents: application to elasmobranch fishes[END_REF]. Empty stomachs were not considered in indices calculation.

Stable isotope analysis

From 2014 to 2017, Etmopterus spinax specimens were sampled in four different areas of the northeast Atlantic and the Mediterranean Sea (Figure 1). Individuals from Rockall Trough and Portugal were sampled during annual-fisheries surveys run by Marine Science Scotland (MSS) and Instituto Português do Mar e da Atmosfera (IPMA) respectively. Additional samples were collected in Portugal from bycatches of the commercial black scabbardfish (Aphanopus carbo) longline and of the crustacean bottom trawl fisheries. In the Mediterranean Sea, E. spinax samples from the Balearic Islands were obtained from research cruises conducted by the Mediterranean International Trawl Survey (MEDITS). In the J o u r n a l P r e -p r o o f Raunefjord (Norway), sharks were caught using deep-water longlines during scientific surveys.

Each collected specimen was sexed and measured for total length (TL). Following dissection, white muscle was quickly frozen before analysis.

Samples were dried (48 hours at 60°C) and ground using pestle and mortar prior analysis. To avoid possible biases linked to polar compound contents (i.e. lipids, urea and trimethylamine N-oxide), all samples were washed in distilled water and their δ 13 C mathematically corrected [START_REF] Kiljunen | A revised model for lipidnormalizing δ13C values from aquatic organisms, with implications for isotope mixing models[END_REF][START_REF] Li | Urea and lipid extraction treatment effects on δ15N and δ13C values in pelagic sharks[END_REF]. Only samples from Norway obtained in 2017 underwent lipid chemical extraction using a modified Folch method based on repetitive wash in a 2:1 dichloromethane:methanol mix [START_REF] Folch | A simple method for the isolation and purification of total lipides from animal tissues[END_REF]. Carbon and nitrogen isotope compositions were subsequently compared between individuals sampled in Norway in 2014 (i.e. corrected δ 13 C values) and in 2017 (i.e. δ 13 C values measured after extraction) at the same locations. They were all kept in the dataset because no significant difference in mean isotopic values was observed. As pure protein samples exhibit a C:N ratio around 3.0 for shark muscle, a good lipid, urea and trimethylamine N-oxide extraction for all samples were considered as those with a C:N ratio lower than 4.0 [START_REF] Hussey | Stable isotopes and elasmobranchs: tissue types, methods, applications and assumptions[END_REF] leading to no discard or additional chemical extractions.

Isotopic ratios (δ) were expressed in per mille (‰) following: δX = [(Rsample/Rstandard) -1] × 1000 where X is 13 C or 15 N and R is 13 C/ 12 C or 15 N/ 14 N, respectively. Samples from Rockall were analysed in two separate laboratories at the Scottish University Environmental Research Council (SUERC) and at the OEA Labs (Exeter, United Kingdom). The remaining samples from 2014 (i.e. Norway, Portugal and Balearic Islands) were processed in Elemtex (Gunnislake, United Kingdom). SUERC samples were run on a continuous flow Elementar vario PYRO cube elemental analyser coupled with a Thermo Scientific Delta V plus isotope ratio mass spectrometer (SUERC, NERC LSMSF, East Kilbride facility). OEA Labs and Elemtex samples were run on a Thermo EA 110 elemental analyser linked to a Europa Scientific 2020 isotope ratio mass spectrometer running in continuous flow mode.

Data analysis

To assess isotopic baseline effects (i.e. change of primary producer isotopic composition) in each sampling site, isotope values were extracted from predictive models established for δ 13 C [START_REF] Magozzi | Using ocean models to predict spatial and temporal variation in marine carbon isotopes[END_REF] and δ 15 N [START_REF] Somes | Simulating the global distribution of nitrogen isotopes in the ocean[END_REF]. Baseline values were extracted from the shark specific sampling locations (details of the specific latitudes and longitudes can be found in supplementary information). To overcome the baseline effect when comparing sharks between locations, the modelled baseline (i.e. phytoplankton) value was subtracted from shark stable isotope composition for each individual geographic J o u r n a l P r e -p r o o f coordinate: ΔX = δXshark -δXphyto where X is 13 C or 15 N. Modelled δ 15 N baseline values inside the fjord were not available and values from the adjacent North Sea were subsequently used.

All statistical analyses were performed on the open source software R (R Core Team, 2020).

For each site, the SIBER package [START_REF] Jackson | Comparing isotopic niche widths among and within communities: SIBER -Stable Isotope Bayesian Ellipses in R[END_REF] was used to estimate the size of the isotopic niches (i.e. using Δ 13 C and Δ 15 N) and their associated Layman metrics [START_REF] Layman | Can Stable Isotope Ratios Provide for Community-Wide Measures of Trophic Structure?[END_REF]. Layman metrics were first used to characterize trophic niche space using four metrics.

Isotopic ranges (Δ 13 C rg and Δ 15 N rg) described the distance between the most 13 C-and 15 Nenriched and most depleted individuals respectively, the total area (TA), the size of the isotopic niche based on convex hull area and the mean distance to the centroid (CD), the mean distance of each individual to the Δ 13 C/Δ 15 N centroid. The other two metrics reflected trophic redundancy (i.e. the relative position of individuals from one another inside their isotopic niche). Mean nearest neighbor distance (NND and its standard deviation SDNND) measured the overall density of individuals clustering in a way that smaller NND and SDNND would describe small differences between individual isotopic values [START_REF] Layman | Can Stable Isotope Ratios Provide for Community-Wide Measures of Trophic Structure?[END_REF]. The SIBER package was finally used to calculate the standard ellipse area (SEA) encompassing 40% of the bulk δ 13 C/δ 15 N data at each site and the isotopic overlaps between them as a proportion of the non-overlapping area of two given ellipses.

For between-site comparisons, data was first checked for normality by Shapiro-Wilk tests and homoscedasticity by Bartlett's tests. As both conditions were not met, Kruskal-Wallis tests followed by Conover-Iman (C-I) post-hoc tests were applied. Comparisons between sexes were carried out using Student's t-tests or its non-parametric analogue, the Wilcoxon test.

Pearson correlation tests were used to assess linear correlations between Δ 13 C, Δ 15 N and TL.

Finally, an analysis of covariance (ANCOVA) was performed to test for slope differences J o u r n a l P r e -p r o o f between linear regressions. For all statistical analyses, the significance threshold was set at p<0.05.

RESULTS

Stomach content analysis

In the Raunefjord, a set of 255 stomachs was analysed (on different specimens than for stable isotope analysis), among which 80 contained remains of prey items. Individuals that had food remains in their stomach ranged from 14.3 to 52.0 cm (TL) and included 51 females and 29 males. Euphausiacea was the major prey items of E. spinax with occurrence of Meganyctiphanes norvegica. Other prey included other crustaceans (mostly decapods), teleost fishes and cephalopods (Table 1). Until they reached 36.0 cm, Euphausiacea was the only prey item identified in the guts of E. spinax. Beyond this size, a diet shift was observed with occurrence of teleost fishes (such as Scomber scombrus and Maurolicus muelleri) and, to a lesser extent, decapods and cephalopods, therefore decreasing the importance of Euphausiacea overall.

Stable isotopes

Muscle tissues from 147 E. spinax individuals were recovered from the four different stations (Table 2). As fishing methodologies differed between stations, capture depth significantly varied among sampling sites (²146,3 = 98.2, p<0.001). Samples from Norway were caught on average at a depth of 243 meters, which was significantly shallower than the other sites where catch depth was around 600 meters. E. spinax TL varied from 11.0 to 57.4 cm and was significantly different between sampling sites (²146,3 = 74.2, p<0.001). Individuals from

J o u r n a l P r e -p r o o f

Individuals from Portugal had a singular profile encompassing the highest Δ 13 C rg, TA and CD suggesting higher isotopic diversity (Table 3). In Norway and in the Balearic Islands, E. spinax presented the smallest TA, CD and the lowest Δ 13 C and Δ 15 N rg. While displaying intermediate TA and CD, E. spinax sampled in Rockall exhibited the highest Δ 15 N rg.

For all sampling sites, significant linear relations were established between Δ 15 N and TL (Figure 4a). Δ 15 N increased with TL in three sites: Portugal, Rockall and the Balearic Islands. E. spinax sampled in Portugal and Rockall had equivalent slopes while specimens from the Balearic Islands showed a flatter one (ANCOVA, F=5.83 p<0.05 for Rockall and F=21.32 p<0.001 for Portugal). Specimens in Norway were the only ones with a significant decrease in Δ 15 N values with TL. Δ 13 C increased with TL in the Balearic Islands and Rockall with no differences in slopes (Figure 4b). At each location, linear regressions did not significantly change between sexes or with depth.

DISCUSSION

Trophic ecology of E. spinax

Prey composition (Stomach content analysis)

In Norway, E. spinax stomach content composition inside the Raunefjord matched previous reports in the surrounding North and Norwegian Sea [START_REF] Bergstad | Predator-Prey Relationships and Food Sources of the Skagerrak Deep-water Fish Assemblage[END_REF][START_REF] Klimpel | Metazoan parasites and food composition of juvenile Etmopterus spinax (L., 1758) (Dalatiidae, Squaliformes) from the Norwegian Deep[END_REF]. The diet was dominated by M. norvegica, the most abundant prey in the ecosystem [START_REF] Bergstad | Predator-Prey Relationships and Food Sources of the Skagerrak Deep-water Fish Assemblage[END_REF][START_REF] Klimpel | Metazoan parasites and food composition of juvenile Etmopterus spinax (L., 1758) (Dalatiidae, Squaliformes) from the Norwegian Deep[END_REF], with an ontogenetic diet switch toward the consumption of teleost fishes. The only difference was a later switch observed in the fjord, probably due to a sampling difference with the two previous studies as sampled individuals inside the fjord were larger.

J o u r n a l P r e -p r o o f

At all locations sampled for stable isotope analysis, previously published studies have highlighted an ontogenetic shift in the diet of E. spinax, except for two studies on Portugal and the Balearic Islands (Table 4). Before the ontogenetic diet shift, Euphausiacea was generally the major prey item, mostly represented by the northern krill, Meganyctiphanes norvegica.

Ontogenetic diet shift occurred at different lengths depending on the site, from 19.4 cm in Norway to 39.0 cm in Rockall Trough. Beyond these sizes, Euphausiacea decreased in importance due to the increasing occurrence of teleost fishes, cephalopods, or other crustaceans. This result was in line with previous analyses of E. spinax diet in the Atlantic Ocean and Mediterranean Sea [START_REF] Bengil | Feeding Ecology of Four Demersal Shark Species (<em>Etmopterus spinax, Galeus melastomus, Scyliorhinus canicula </em>and <em>Squalus blainville</em>) from the Eastern Aegean Sea[END_REF][START_REF] Isbert | Metazoan parasite communities and diet of the velvet belly lantern shark Etmopterus spinax (Squaliformes: Etmopteridae): a comparison of two deep-sea ecosystems[END_REF][START_REF] Preciado | Food web functioning of the benthopelagic community in a deep-sea seamount based on diet and stable isotope analyses[END_REF][START_REF] Preciado | Resource utilization by deep-sea sharks at the Le Danois Bank, Cantabrian Sea, north-east Atlantic Ocean[END_REF]. In Rockall, the later shift toward the consumption of the decapod crustacean, Pasiphaea tarda, has been explained by larger individuals foraging closer to the sea bottom where prey availability is different [START_REF] Mauchline | Diets of the sharks and chimaeroids of the Rockall Trough, northeastern Atlantic Ocean[END_REF]. Ontogenetic diet shifts are commonly observed in mesopredator fishes due to increasing body length, mouth gap and stomach size, hunting capacities and energetic demands, or to avoid intra-specific competition [START_REF] Klimpel | Metazoan parasites and food composition of juvenile Etmopterus spinax (L., 1758) (Dalatiidae, Squaliformes) from the Norwegian Deep[END_REF][START_REF] Neiva | Feeding habits of the velvet belly lanternshark Etmopterus spinax (Chondrichthyes: Etmopteridae) off the Algarve, southern Portugal[END_REF].

Stomach contents analysed in the Balearic Islands showed contrasting results between studies. E. spinax either foraged following the previously described ontogenetic shift from small Euphausiacea to teleost fishes [START_REF] Fanelli | Feeding habits of blackmouth catshark Galeus melastomus Rafinesque, 1810 and velvet belly lantern shark Etmopterus spinax (Linnaeus, 1758) in the western Mediterranean[END_REF][START_REF] Macpherson | Régime alimentaire de Galeus melastomus (Rafinesque, 1810)[END_REF] or fed on cephalopods throughout their entire size range in more recent studies [START_REF] Valls | Feeding strategies and resource partitioning among elasmobranchs and cephalopods in Mediterranean deep-sea ecosystems[END_REF][START_REF] Valls | Feeding ecology of demersal elasmobranchs from the shelf and slope off the Balearic Sea (western Mediterranean)[END_REF]. A higher proportion of cephalopod consumption by E. spinax was previously observed in the adjacent Catalan Sea and has been linked to the high exploitation rate of the area, depleting fish stocks in favour of cephalopods [START_REF] Barría | Feeding habits of four sympatric sharks in two deep-water fishery areas of the western Mediterranean Sea[END_REF][START_REF] Doubleday | Global proliferation of cephalopods[END_REF]. In deep-sea sharks, smaller adult size and earlier maturity due to oligotrophic conditions in the J o u r n a l P r e -p r o o f Mediterranean Sea could also explain cephalopod consumption by small specimens [START_REF] Catarino | The Pillars of Hercules as a bathymetric barrier to gene flow promoting isolation in a global deepsea shark (Centroscymnus coelolepis)[END_REF][START_REF] Massutí | Mediterranean and Atlantic deep-sea fish assemblages: Differences in biomass composition and size-related structure[END_REF]. However, this trend was not yet observed in E. spinax (Coelho et al., 2010).

Nitrogen and carbon isotope compositions (Δ 15 N and Δ 13 C)

Stable isotope values obtained in this study were measured in multiple laboratories which may result in a potential bias [START_REF] Mill | Mass-spectrometer bias in stable isotope ecology[END_REF]. The analysis of a reference sample (USGS 40 glutamic acid) to ensure comparability was performed on two of the four instruments used in this study (i.e. for Elemtex and OEA laboratories). δ 13 C values were on average 0.23 ± 0.40 ‰ higher from Elemtex than those obtained from OEA Labs. This difference, although significant, is quite small in regard of isotope patterns interpreted in this study, and comparable to error estimates considered acceptable in many ecological studies using stable isotope tracers. Nevertheless, this difference was subtracted from all shark δ 13 C values measured by Elemtex. There were no observed differences in δ 15 N values. Specimens sampled in Norway in 2017 and in Portugal in 2015 were analysed in facilities where glutamic acid samples could not be measured. However, all facilities follow recommended procedures for stable isotope analyses, and use certified materials that are all calibrated against the same international references (Vienna Pee Dee Belemnite for δ 13 C, Atmospheric air for δ 15 N).

Therefore, we argue that analytical biases are unlikely to have a major impact on stable isotope trends depicted here.

Ecological differences in E. spinax isotopic niches between sites were analysed by standardising values (i.e. Δ 15 N and Δ 13 C) to mitigate baseline effects [START_REF] Bird | A global perspective on the trophic geography of sharks[END_REF]. The absence of significant differences in isotopic ratios between females and males suggests a uniform diet across sexes, as globally observed in stomach content analyses of the species.

J o u r n a l P r e -p r o o f

Nitrogen isotope composition is commonly used as a proxy of trophic position in marine food web [START_REF] Cabana | Modelling food chain structure and contaminant bioaccumulation using stable nitrogen isotopes[END_REF]. M. norvegica, the main prey of E. spinax, is a low trophic level omnivorous species feeding on both phytoplankton and copepods [START_REF] Gomes | Spatial patterns of groundfish assemblages on the continental shelf of Portugal[END_REF][START_REF] Kaartvedt | Is the omnivorous krill Meganyctiphanes norvegica primarily a selectively feeding carnivore?[END_REF]. It is therefore unlikely that divergences in food web length might have contributed to the marked Δ 15 N differences. Moreover, the suspected Δ 15 N switch with TL was not observed at all sites. The relevance of Δ 15 N as a good proxy of the trophic level could be conditioned by the occurrence of a clear ontogenetic change as observed in Portugal and Rockall sharks. Along the Iberian slope, the rapid sedimentation of phytoplankton [START_REF] Lopez-Lopez | Does upwelling intensity influence feeding habits and trophic position of planktivorous fish? Deep[END_REF] results in a direct linear energy flow, with phytoplankton mainly consumed by Euphausiacea, themselves consumed by secondary predators such as juvenile E. spinax [START_REF] Gomes | Spatial patterns of groundfish assemblages on the continental shelf of Portugal[END_REF]. Older E. spinax fed on higher trophic level species such as Pasiphaea sivado or Micromesistius poutassou [START_REF] Neiva | Feeding habits of the velvet belly lanternshark Etmopterus spinax (Chondrichthyes: Etmopteridae) off the Algarve, southern Portugal[END_REF][START_REF] Santos | Trophic relationships in deep-water fish communities off Algarve, Portugal[END_REF] explaining the 15 N-enrichment (Figure 4a). In Rockall the same phenomenon is suspected to occur, with M. norvegica being replaced by possibly 15 N-enriched prey like the decapod crustacean P. tarda or the teleost M. muelleri [START_REF] Mauchline | Diets of the sharks and chimaeroids of the Rockall Trough, northeastern Atlantic Ocean[END_REF]. However, more recent information on E. spinax diet are needed to confirm this effect as the only stomach content description available came from samples obtained between 1973 and 1981 [START_REF] Mauchline | Diets of the sharks and chimaeroids of the Rockall Trough, northeastern Atlantic Ocean[END_REF] and major environmental and anthropogenic changes could have modified the shark diet over time.

In benthopelagic communities, the higher degradation of sinking particles is leading to higher δ 15 N values in deeper species [START_REF] Trueman | Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes[END_REF]. Bathymetric segregation at the interand intra-specific levels is commonly observed among deep-sea sharks [START_REF] Clarke | Comparisons of Trawl and Longline Catches of Deepwater Elasmobranchs West and North of Ireland[END_REF][START_REF] Neat | The diversity, distribution and status of deep-water elasmobranchs in the Rockall Trough, north-east Atlantic Ocean[END_REF] and has been reported for E. spinax (Coelho and Erzini, 2010). At each site, J o u r n a l P r e -p r o o f changes in capture depth were not associated with an increase in δ 15 N or Δ 15 N values. However, differences in E. spinax vertical segregation of maturity stages or bathymetric constraint among sites could explain Δ 15 N differences and would need to be specifically addressed in the future.

Except for the Balearic Islands, E. spinax likely shared a pelagic habitat owing to their lower Δ 13 C values (Figure 3b). This is in accordance with the high rate of phytoplanktonic production fuelling the deeper layers of Portugal and Rockall continental slope systems [START_REF] Gomes | Spatial patterns of groundfish assemblages on the continental shelf of Portugal[END_REF][START_REF] Mauchline | Diets of the sharks and chimaeroids of the Rockall Trough, northeastern Atlantic Ocean[END_REF]. In the Raunefjord, E. spinax Δ 13 C values agree with a diet dominated by pelagic preys [START_REF] Bergstad | Predator-Prey Relationships and Food Sources of the Skagerrak Deep-water Fish Assemblage[END_REF][START_REF] Klimpel | Metazoan parasites and food composition of juvenile Etmopterus spinax (L., 1758) (Dalatiidae, Squaliformes) from the Norwegian Deep[END_REF].

E. spinax sampled in the Balearic Islands exhibited higher Δ 13 C values (Figure 3b), suggesting reliance on different forms of primary production, such as 13 C-enriched benthic prey [START_REF] Madurell | Isotopic composition of carbon and nitrogen of suprabenthic fauna in the NW Balearic Islands (western Mediterranean)[END_REF]. It could correspond to the higher occurrence of cephalopods in E. spinax diet at that site (i.e. mainly Teuthoidea and Sepioidea). These cephalopods exhibit an ontogenetic diet shift from benthic to pelagic prey [START_REF] Valls | Feeding strategies and resource partitioning among elasmobranchs and cephalopods in Mediterranean deep-sea ecosystems[END_REF][START_REF] Valls | Feeding ecology of demersal elasmobranchs from the shelf and slope off the Balearic Sea (western Mediterranean)[END_REF]. As scavenging was not reported in the area and because mature cephalopods exceed E. spinax length, sharks are expected to forage on juvenile early benthic life stages possibly explaining their Δ 13 C values.

Habitat characteristics influence on isotope compositions

E. spinax in the Iberian slope ecosystems and Rockall Trough, even if mainly relying on pelagic production, presented similarly high indices of isotopic diversity (Table 3). Both sites are large continuous continental slopes [START_REF] J O U R N A L P R E -P R O O F Mauchline | Oceanic prey of benthopelagic fish in the benthic boundary layer of a marginal oceanic region[END_REF][START_REF] Ribeiro | A study of the response of chlorophyll-a biomass to a winter upwelling event off Western Iberia using SeaWiFS and in situ data[END_REF] and topographic similarities might drive these similitudes. Indeed, ichthyofauna inhabiting continental slope ecosystems are known to integrate nutrient from a mixture of pelagic and J o u r n a l P r e -p r o o f benthic origins [START_REF] J O U R N A L P R E -P R O O F Mauchline | Oceanic prey of benthopelagic fish in the benthic boundary layer of a marginal oceanic region[END_REF][START_REF] Trueman | Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes[END_REF]. This trophic diversity is possibly further enhanced by the access to different topographic features (e.g. canyons, banks or steep slopes) [START_REF] Romero-Romero | Seasonal pathways of organic matter within the Avilés submarine canyon: Food web implications[END_REF][START_REF] Rowden | A test of the seamount oasis hypothesis: seamounts support higher epibenthic megafaunal biomass than adjacent slopes[END_REF].

In Portugal, from spring to late summer, changes in wind-driven mesoscale currents induce an upwelling of nutrient-rich cold water [START_REF] Loureiro | Microplankton composition, production and upwelling dynamics in Sagres (SW Portugal) during summer of 2001[END_REF] that influences shelf and slope ecosystems over great distances [START_REF] Pérez | Plankton response to weakening of the Iberian coastal upwelling[END_REF][START_REF] Ribeiro | A study of the response of chlorophyll-a biomass to a winter upwelling event off Western Iberia using SeaWiFS and in situ data[END_REF] and resulting in shifting isotopic signals. Due to upwelling seasonality and geographical influence, temporal and spatial variations in δ 13 C values are observed in species at the base of the food web [START_REF] Lopez-Lopez | Does upwelling intensity influence feeding habits and trophic position of planktivorous fish? Deep[END_REF]. The small-scale variability in the activity of the upwelling associated with the spatial scale at which individuals were fished might contribute to the overall isotopic variability, a dynamic also observed in Rockall where the pelagic production is seasonally stimulated over the shelf area [START_REF] White | Deep-water coral development as a function of hydrodynamics and surface productivity around the submarine banks of the Rockall Trough, NE Atlantic[END_REF].

The Balearic Islands deep ecosystem is characterized by a smaller continental slope, deep escarpments and canyons [START_REF] Acosta | The Balearic Promontory geomorphology (western Mediterranean): morphostructure and active processes[END_REF]. While topographic conditions might be similar to large continental slope systems, the main difference resides in the oligotrophic nature of the Mediterranean Sea [START_REF] Bosc | Seasonal and interannual variability in algal biomass and primary production in the Mediterranean Sea, as derived from 4 years of SeaWiFS observations[END_REF][START_REF] Estrada | Primary production in the northwestern Mediterranean[END_REF]. Such conditions might result in the observed small isotopic niche due to limited Δ 15 N variations and high redundancy index (Table 3). Still, their broad Δ 13 C rg, with some 13 C-depleted specimens, might reveal the existence of resource partitioning within the population, which could be fuelled by a diversity of prey of both benthic and pelagic origin [START_REF] Albo-Puigserver | Feeding ecology and trophic position of three sympatric demersal chondrichthyans in the northwestern Mediterranean[END_REF][START_REF] Madurell | Isotopic composition of carbon and nitrogen of suprabenthic fauna in the NW Balearic Islands (western Mediterranean)[END_REF][START_REF] Newman | High Mercury Concentrations Reflect Trophic Ecology of Three Deep-Water Chondrichthyans[END_REF]. Such coupling between reservoirs and reliance on other habitat is probably a response to oligotrophic conditions reducing pelagic production and prey availability [START_REF] Valls | Structure and dynamics of food webs in the water column on shelf and slope grounds of the western Mediterranean[END_REF]. Sharks could also forage inside canyons where higher benthic prey densities are J o u r n a l P r e -p r o o f present [START_REF] Massutí | Mediterranean and Atlantic deep-sea fish assemblages: Differences in biomass composition and size-related structure[END_REF]. Other 13 C-enriched sources have been identified in the Balearic Islands such as food falls [START_REF] Cartes | Contributions of allochthonous inputs of food to the diets of benthopelagic fish over the northwest Mediterranean slope (to 2300m)[END_REF] or seagrass exportation at deeper layers [START_REF] Boudouresque | The necromass of the Posidonia oceanica seagrass meadow: fate, role, ecosystem services and vulnerability[END_REF] but this remains speculative and will need further studies to investigate. Water temperature, higher in the Mediterranean Sea compared to the other locations, could also have led to differences in isotopic values compared to the other locations. Indeed, temperature can affect isotopic turnover rates and discrimination factors (Bloomfield et al., 2011), beyond latitudinal baseline δ 13 C and δ 15 N variations [START_REF] Magozzi | Using ocean models to predict spatial and temporal variation in marine carbon isotopes[END_REF].

Fjords are narrow, deep ecosystems delimited by steep flanking slopes [START_REF] Harris | Seafloor Geomorphology-Coast, Shelf, and Abyss[END_REF]. Therefore, possible bathymetric constraints (Coelho and Erzini, 2010) might reduce E. spinax foraging habitat diversity and explain isotopic niche limited space and high redundancy. Δ 15 N values (Figure 3a) may result from the important particle residency time [START_REF] Saino | 15 N natural abundance in oceanic suspended particulate matter[END_REF] in deep basins. Indeed, even if fjords are dynamic systems with quick surface water turnover [START_REF] Asplin | Nonlocal wind-driven fjord-coast advection and its potential effect on plankton and fish recruitment[END_REF], they are stratified with sometimes limited exchanges between reservoirs [START_REF] Aure | Observations of inshore water exchange forced by a fluctuating offshore density field[END_REF]. As sinking particles are trapped in deep layers, especially in fjords, they would exhibit higher baseline δ 15 N values than suggested by surface model values [START_REF] Saino | 15 N natural abundance in oceanic suspended particulate matter[END_REF][START_REF] Trueman | Trophic interactions of fish communities at midwater depths enhance long-term carbon storage and benthic production on continental slopes[END_REF]. Even if experiencing an ontogenetic shift toward higher trophic position preys, E. spinax Δ 15 N decreased with TL (Figure 4a). Sharks sampled in the Raunefjord were only mature individuals probably post-ontogenetic diet shift. This decrease could result from a shift in feeding habitat with early maturing sharks foraging strictly in fjords while larger sharks could forage outside in offshore areas. This hypothesis would ultimately lead to the observed decreasing Δ 15 N values with length, as modelled δ 15 N baseline was extracted from outside of the fjord. Another possibility is that large sharks J o u r n a l P r e -p r o o f change their foraging behaviour towards active predation of smaller organisms with lower trophic positions, although this is not supported by the stomach content analysis.

Finally, relatively small deep-sea sharks can exhibit important horizontal migrations [START_REF] Catarino | The Pillars of Hercules as a bathymetric barrier to gene flow promoting isolation in a global deepsea shark (Centroscymnus coelolepis)[END_REF][START_REF] Rodríguez-Cabello | Is Centrophorus squamosus a highly migratory deep-water shark? Deep Sea[END_REF]). E. spinax population structure suggested it might connect distant areas across the northeastern Atlantic with a potential isolation of Mediterranean individuals [START_REF] Gubili | Connectivity in the deep: Phylogeography of the velvet belly lanternshark[END_REF][START_REF] Mcmillan | Analysis of vertebral chemistry to assess stock structure in a deep-sea shark, Etmopterus spinax[END_REF]. Migrating E. spinax are likely to feed on different isotopic baselines and would integrate them throughout the course of their migration [START_REF] Carlisle | Using Stable Isotope Analysis to Understand the Migration and Trophic Ecology of Northeastern Pacific White Sharks (Carcharodon carcharias)[END_REF]. In this study, a significant overlap in bulk isotope niches occurred between Portugal and Rockall samples (Figure 2), reinforcing the idea of potential large-scale migration of E. spinax in the Atlantic Ocean. Conversely, the nonoverlapping and reduced isotopic niche spaces observed in the Balearic Islands and in the fjord suggest a certain level of residency, even more when diet is found homogeneous among distant sites. This strengthens the hypothesis of a separated population in the Mediterranean Sea, probably due to the bathymetric limitation at the Strait of Gibraltar [START_REF] Catarino | The Pillars of Hercules as a bathymetric barrier to gene flow promoting isolation in a global deepsea shark (Centroscymnus coelolepis)[END_REF][START_REF] Gubili | Connectivity in the deep: Phylogeography of the velvet belly lanternshark[END_REF]. In Norway, while changes in Δ 15 N values have been hypothetically linked to migrations outside the fjords, trophic redundancy and isotopic discrimination suggest E. spinax could be sedentary in the region. This hypothesis matches the separate stocks of E. spinax in Norwegian waters previously identified by vertebral chemistry variations [START_REF] Mcmillan | Analysis of vertebral chemistry to assess stock structure in a deep-sea shark, Etmopterus spinax[END_REF].

CONCLUSION

At each location, including inside the fjords, Etmopterus spinax appears as a benthopelagic mesopredator, mainly feeding on aggregations of Meganyctiphanes norvegica at a juvenile stage with increasing consumption of larger prey, such as teleost fishes, with J o u r n a l P r e -p r o o f increasing length. Nonetheless, isotopic niche spaces varied significantly among sampling sites. Observed differences in carbon and nitrogen isotope compositions are likely to be explained by differences in habitat features (e.g. productive continental slopes vs oligotrophic conditions in the Mediterranean Sea) and other oceanographic characteristics (e.g. upwelling regime). Resulting variations in pelagic primary production rates and sequestration across the water column are likely to affect the strength of mesopelagic linkage toward deep reservoirs and subsequently E. spinax trophodynamic and isotopic values. Our results suggest that considering the influence of nutrient cycle on isotopic baselines allows a better understanding of the trophic ecology of predators in deep-sea habitats.

Portuguese Science Foundation for the financial support to R.P.V. (SFRH/BD/84030/2012) and CCMAR (UIDB/04326/2020) through national funds. J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f J o u r n a l P r e -p r o o f  Stomach contents suggested a similar diet.

 E. spinax fed mainly on Euphausiacea with an ontogenetic diet shift.

 Muscle carbon and nitrogen isotope compositions differed between locations.

 Isotopic differences probably resulted from mechanisms affecting the baselines.

J o u r n a l P r e -p r o o f

  δ 13 C values are expressed with reference to the Vienna Pee Dee Belemnite and δ 15 N values are expressed relative to Atmospheric air. Depending on sites and sampling year, samples were analysed in different facilities. Samples from Norway obtained in 2017 were processed in Liege (Oceanology, ULiege, Belgium) using a continuous flow isotope ratio mass spectrometer (Isoprime 100, Isoprime, United Kingdom) coupled to a C-N-S elemental analyser (MicroVario, Elementar, J o u r n a l P r e -p r o o f Germany). The certified substances, provided by the International Atomic Energy Agency IAEA (Vienna) were IAEA-CH-6 (sucrose) for δ 13 C and IAEA-N-1 (ammonium sulfate) for δ 15 N. Cod (Gadus morhua) muscle was used as a natural replicate showing precision of ± 0.05 ‰ for δ 13 C and ± 0.23 ‰ for δ 15 N (based on the standard deviation of the replicate measurements). Stable isotope ratios of samples obtained along the southern Iberian coast of Portugal in 2015 were measured using a Thermo Scientific Delta V Advantage IRMS via Conflo IV interface at Marinnova -Marine and Environmental Innovation, Technology and Services (Portugal).

  J o u r n a l P r e -p r o o f Norway and Rockall were significantly larger than in Portugal and in the Balearic Islands (C-I test, p<0.001 for all pairs of comparison). Females were larger than males considering the entire dataset (W = 1860, p<0.01), as observed in Portugal (W = 148, p<0.05) and in Norway (t = -6.91, p<0.001). No significant difference in TL was found between sexes in the Balearic Islands and Rockall Trough. Modelled phytoplanktonic baselines significantly differed among sites (²146,3 = 101.4, p<0.001 for δ 13 Cphyto; ²146,3 = 142.5, p<0.001 for δ 15 Nphyto). δ 13 Cphyto was significantly lower in the Balearic Islands than in Rockall and Portugal, while Norway displayed higher values than all other sites (C-I test, p<0.001). Concerning δ 15 Nphyto, all locations significantly differed (C-I test, p<0.001, for all pairs of comparison) with Portugal being the most 15 N-enriched, followed by the Balearic Islands, Rockall Trough and southwestern Norway. Shark bulk isotopic values (i.e. δ 13 C and δ 15 N) differed among sampling sites (Figure 2) with the only overlapping SEAs (Standard Ellipse Areas) reported between E. spinax sampled in the Portugal Iberian system and Rockall Trough (39.89%). Δ 15 N values differed between locations (²146,3 = 118.3, p<0.001; Figure 3a). Norway individuals exhibited the highest Δ 15 N and individuals sampled in Portugal the lowest (C-I test, p<0.001 for all pairs of comparison).Rockall Δ 15 N value was also significantly higher than in Portugal (C-I test, p<0.01). Δ 13 C also varied significantly among sampling locations (²146,3 = 89.4, p<0.001; Figure3b) with individuals from the Balearic Islands having the highest Δ 13 C and individuals from Norway the lowest (C-I test, p<0.001 for all pairs of comparison). E. spinax sampled in Rockall Trough and Portugal had similar Δ 13 C (C-I test, p>0.05). At each location, there was no depth-related or sex differences in Δ 13 C and Δ 15 N except in the Norwegian fjord where both values were significantly higher in males.

  r n a l P r e -p r o o f

Figure 1 :

 1 Figure 1: Sampling locations of velvet belly lantern sharks, Etmopterus spinax. Sampling area in Norway both corresponds to specimens analysed for stomach content and stable isotopes analysis.

Figure 2 :

 2 Figure 2: Muscle δ 13 C and δ 15 N values of E. spinax. Brown square points correspond to the Balearic Islands, blue circle points to Norway, green triangle points to Portugal and red cross points to Rockall samples. Solid lines delimit the standard ellipse areas (SEA) and dashed lines the convex hull areas for each sampling location. The only overlapping regions between SEA occurs between Portugal and Rockall individuals (39.89%).

Figure 3 :

 3 Figure 3: Boxplots of Δ 15 N (a) and Δ 13 C (b) values of E. spinax at each location. Significant differences are indicated by letters (p < 0.05).

Figure 4 :

 4 Figure 4: Relationships between individual Δ 15 N (a)/Δ 13 C (b) values and total length (TL) of E. spinax. Pearson linear regressions were applied for each location with their corresponding R² and p-value reported in the figure.

J

  o u r n a l P r e -p r o o f HIGHLIGHTS  E. spinax were sampled across the northeastern Atlantic and Mediterranean Sea.

Table 1 :

 1 Number (N), numeric index (%N), gravimetric index (%W), occurrence index (%O) and index of relative importance (%IRI) for each prey item found in stomachs of Etmopterus spinax sampled from 2008 to 2012 in the Raunefjord (Norway).

	Prey items	N	%N	%W	%O	%IRI
	Euphausiacea	39	41.94	7.04	55.10	45.55
	Meganyctiphanes norvegica	12	12.90	3.29	18.37	7.63
	Euphausiacea unidentified	27	29.03	3.75	44.90	37.75
	Decapoda	7	7.53	17.17	14.29	5.95
	Pasiphaea sivado	2	2.15	1.71	4.08	0.40
	Pasiphaea multidentata	1	1.08	1.28	2.04	0.12
	Pandalus montagui	1	1.08	11.78	2.04	0.67
	Decapoda unidentified	3	3.23	2.40	6.12	0.88
	Crustacea unidentified Teleostei Scomber scombrus Maurolicus muelleri Teleostei unidentified Cephalopoda Rossia macrosoma Cephalopoda unidentified Unidentified prey J o u r n a l P r e -p r o o f 10 10.75 5.92 20.41 7 7.53 52.76 14.29 1 1.08 50.00 2.04 2 2.15 2.32 4.08 4 4.30 0.45 8.16 10 10.75 4.53 18.37 5 5.38 1.18 10.20 5 5.38 3.35 10.20 20 21.51 12.57 40.82	5.74 14.54 2.67 0.47 0.99 4.74 1.72 2.28 23.48

Table 2 :

 2 Summary of stable isotope data from white muscle tissue of E. spinax, from different locations, showing sampling depth (in meters), number of samples analysed (including male:female sex ratio) and individual total length (TL). Sampled depth and TL are expressed in mean (minimum-maximum values). Carbon and nitrogen ratios are given as mean (± standard deviation). All isotopic values are expressed in ‰ with δ 13 C/δ 15 N representing E. spinax muscle isotopic composition, δ 13 Cphyto and δ 15 Nphyto the phytoplankton values extracted from predictive models and Δ 13 C and Δ 15 N the difference between shark muscle isotope values and phytoplankton modelled isotopic baselines.

	Location	Depth (m)	N (M:F)	TL (cm)	δ 13 C	δ 15 N	δ 13 Cphyto	δ 15 Nphyto Δ 13 C	Δ 15 N
	Balearic Islands 589 (230-754) 33 (17:16) 26.

Table 3 :

 3 Summary of Layman metrics calculated on E. spinax isotopic niches (Δ 13 C/Δ 15 N values) and based on convex hull areas. Δ 13 C rg and Δ 15 N rg represent both isotopic ranges, TA the total area, CD the mean distance to centroid, NND (as mean ± standard deviation) the nearest neighbor distance (details of Layman metrics calculation can be found in Materials and Methods). All values are presented in ‰ except for TA (in ‰²).

	Location	Δ 13 C rg	Δ 15 N rg	TA	CD	NND
	Balearic Islands 2.19	1.53	1.85	0.51	0.16 ± 0.16
	Norway	1.18	2.88	1.95	0.66	0.12 ± 0.07
	Portugal	3.45	2.88	5.88	0.95	0.21 ± 0.14
	Rockall	1.71	3.04	2.84	0.79	0.28 ± 0.21

Table 4 :

 4 Summary of E. spinax feeding habits. For each study, total length of sampled individuals (TL), length at which ontogenetic diet shift occurred (TL OS) and major prey before and after E. spinax ontogenetic diet shift (except for two studies describing constant diet) are reported. Prey groups accounting for more than 50% IRI (Index of Relative Importance) are given in bold character. Specimens < 15.0 cm presented a diet focused on Euphausiacea, fishes and to a less extend Decapoda, specimens between 15.0 and 25.0 cm had a diet mainly focused on Cephalopoda and to a less extend Decapoda and specimens > 25.0 cm focused mainly on fish. (**) OS corresponded to the absence of Euphausiacea in the diet after 22.0 cm. (***) Two OS with the first one observed around 17.0 cm when E. spinax individuals evolved from a diet focusing at 95.11 % (IRI) on Euphausiacea to a diet with 50.31 % (IRI) of Euphausiacea completed with Natantids (39.98 % IRI) and teleost fishes (9.03 % IRI). The second OS is described in the Table.

	Location	TL (cm)	TL OS (cm)	Major prey before OS	Major prey after OS	Publication
		10.0-49.0 ~20.0	Euphausiacea (M. norvegica), Cephalopoda	Osteichthyes, Cephalopoda	Macpherson, 1980
	Balearic Islands	15.0-45.0	~25.0 (*)	Natantian decapods, Euphausiacea, Cephalopoda	Osteichthyes (Stomiidae, Cephalopoda Myctophidae),	Fanelli et al., 2009
		11.0-47.0	-	Cephalopoda, Telesotei, Natantids	Valls et al., 2011
		10.2-48.3	~22.0 (**)	Cephalopoda, Telesotei, Decapods, Euphausiacea	Cephalopoda, Telesotei, Decapods	Valls et al., 2017
	Norway	31.0-52.0 ~36.0	Euphausiacea (M. norvegica)	Euphausiacea (M. Cephalopoda norvegica), Teleostei,	This study
		11.0-33.0	-	Euphausiacea (M. norvegica), Teleostei, Cephalopoda	Santos & Borges, 2001
	Portugal	9.1-40.1	~28.0 (***)	Euphausiacea (M. norvegica), Natantids, Teleostei	Natantids, Teleostei (Gadoids mainly), Cephalopoda Euphausiacea,	Neiva et al., 2006
				Euphausiacea (M.
	Rockall	12.6-53.0 ~39.0	norvegica), Teleostei (M. muelleri), Cephalopoda,	Decapoda (Pasiphaea tarda), Cephalopoda, Other Teleostei	Mauchline & Gordon, 1983
				Decapoda
	(*)

ACKNOWLEDGMENT

The authors would like to thank Christopher J. Somes, Sarah Magozzi and Katie St John Glew for providing the phytoplankton-modelled values, and to James Bell for his suggestions to early stages of the manuscript; as well as Tomas Sørlie, from the Espeland marine station, for the technical support during the shark sampling in Norway. This work was partially supported by FRIA grant (F.R.S-FNRS Belgium) to L.D. and N.P.. G.L and J.M are appointed by F.R.S-FNRS Belgium. The authors thank the Université de Bretagne Occidentale (UBO) and the École Doctorale des Sciences de la Mer et du Littoral (EDSML) for L.B. doctoral grant. This project is supported by ISblue project, interdisciplinary graduate school for the blue planet (ANR-17-EURE-0015) and co-funded by a grant from the French government under the program "Investissements d'Avenir". This study is the contribution BRC #374 of the Biodiversity Research Center (UCLouvain) from the Earth and Life Institute Biodiversity (ELIV) and the "Centre Interuniversitaire de Biologie Marine" (CIBIM). Thanks are also due to the J o u r n a l P r e -p r o o f

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:


Jennifer Morvan

Dylan Bouëtard

Lingyu Kong

Yajie Chou

Thomas Vives

Muriel Albalat

Thierry Roisnel

Christophe Crévisy

Paola Nava

Stéphane Humbel

Chiral Atropisomeric-NHC Catechodithiolate Ruthenium Complexes for Z-Selective Asymmetric Ring-Opening Cross Metathesis of Exo-Norbornenes

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J Di Martino

S Michon

P Berthod

C Rapin

R Podor

P Steinmetz

ELECTROCHEMICAL STUDY OF COBALT BASE SUPERALLOYS' CORROSION BY A MOLTEN GLASS ; INFLUENCE OF ALLOY'S MICROSTRUCTURE AND OF THE CHEMICAL COMPOSITION OF THE GLASS

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION CONCLUSIONS

Thus, high chromium superalloys are generally passivable in molten glass. They can be protected against glass corrosion by an external chromia scale, the corrosion rate strongly decreasing. In some cases no precaution is required for such alloys to reach the passive state, as it was observed for a glass containing strong oxidant species (Fe III ). In other glasses, the alloys need to be helped. In this field, a preliminary oxidation in air at high temperature gives them a first chromia scale and they get immediately the passive state. Minimal values of temperature and duration of this preoxidation have to be respected, depending of the alloy's characteristics. Among these latter, it appeared that existence and types of carbides can influence the reaching easiness of the passive state and its everlastingness on long times. Finally, certain glasses do not allow chromia-former alloys to become passive, maybe because a too high solubility of chromia into the glass melt. In some cases, a Ni base superalloy (Ni-30Cr-0.7C-7Fe-7W in %wt) and pure chromium were also tested for comparison.

EXPERIMENTALS CHARACTERIZATION OF ALLOYS' CORROSION BY MOLTEN GLASS SYNTHESIS OF THE ALLOYS

Chemical compositions of the studied alloys (%wt)

The alloys' corrosion behavior was characterized using an electrochemical apparatus. It is adapted for molten glasses from the classical three-electrodes one usually used for aqueous corrosion studies.

Polarization curves, corrosion potential and polarization resistances were measured for several alloy-glass couples and different air oxidation conditions before immersion into glass. Polarization curves showing a passivation plateau after the anodic peak, all alloys, as pure Cr, are able to become passive.

If oxidised by air at high temperature before immersion ("preoxidation"), our high chromium superalloys can be stabilised in a passive state. It is characterised by the presence of an external chromia layer which protects the alloy from fast corrosion. E corr and Rp are higher (>-0.7V / EMZS and >200Ω.cm 2 , corresponding to corrosion rates of a few mm/year.

The anodic reaction is Cr -> Cr III + 3e and the cathodic one is reduction of other oxidant species than Si IV (dissolved O 2 , Fe III ). The E corr values give in each case some indications about the main acting oxidant. Glass chemical composition can also influence the behavior of the alloys. In some cases no preoxidation is necessary for alloys to be passive as early as immersion. For example, Co1 directly dived into SA glass at 1200°C is immediately in a passive state, thanks to the presence of Fe III oxidants. E corr and Rp evolutions show that Fe III acts first, then oxidation continues with O 2 ).

At the opposite, Co1 appears not to be able to become passive in the BS glass at 1250°C, even after a preoxidation.

There is no real passivation plateau and corrosion current is largely higher at all potentials. A possible reason is a too high solubility of Cr 2 O 3 in this glass. 1050°C (rot. 60rpm, preox 15' at 1150°C and 20' cooling to 1050°C ) Obtaining a passive state depends on both preoxidation parameters and microstructure of the alloys. The higher the oxidation temperature is and the longer its duration is, the higher Rp is and the better the resistance against corrosion is. Minimal values of T(°C) and d(h) exist for each alloy, when the glass allows passivation. For instance, Co2 is easier to passive than Co1 which needs a longer preoxidation for a same T, probably because it contains no tungsten and only one type of carbide.

Carbides seem to play an important role.

Indeed, CoO which represents only the matrix of Co2, shows a different behavior, with notably a worse stability of the passive state.

Chemical compositions of the glasses (%wt)

Passivation plateau for pure Cr Ecorr Rp

  Electrochemical study of cobalt base superalloys' corrosion by a molten glass ; influence of alloy's microstructure and of the chemical composition of the glass J Di Martino, S Michon, P Berthod, C Rapin, R Podor, P Steinmetz To cite this version: J Di Martino, S Michon, P Berthod, C Rapin, R Podor, et al.. Electrochemical study of cobalt base superalloys' corrosion by a molten glass ; influence of alloy's microstructure and of the chemical composition of the glass. 7th International Conference "Advances in Fusion and Processing of Glass", Jun 2003, Rochester, United States. hal-04062371

  of the Co2 alloy at two magnifications Below: microstructures of the Co1 and CoO (matrix of Co2) base superalloy being in a passive state (14h in SL glass at 1050°C, preoxidation before immersion, no rotation) Generally, Co base and Ni base superalloys are in an active state when directly dived into molten SL glass without any air oxidation before. Corrosion is then fast, corresponding to about a few cm/year of alloy dissolved in glass. Corrosion potential (E corr ) and polarization resistance (Rp) are about -1.2V / EMZS and 20Ω.cm 2 . The anodic reaction is Cr -> Cr II + 2e and the cathodic one is Si IV +4e -> Si o .

o

  curve of Co1 dived in SA glass without preoxidation ( in left upper corner: following of both Ecorr and Rp on 80 hours) ; rot. 60rpm Below: polarization curve of Co1 in SB glass at 1250°C compared to the one in SL glass at 1050°C (no rotation) Evolution of the Co2 polarisation resistance with temperature and duration of preoxidation Left: polarization curves of Co2 and CoO ( matrix of Co2) in SL glass at 1050°C ( rot. 60 rpm) Right: evolution with time of Ecorr and Rp of CoO in SL glass at


Ivan Miro Panades
email: ivan.miro-panades@st.com

Alain Greiner
email: alain.greiner@lip6.fr

Bi-Synchronous FIFO for Synchronous Circuit Communication Well Suited for Network-on-Chip in GALS Architectures

Keywords:

The distribution of a synchronous clock in Systemon-Chip (SoC) has become a problem, because of wire length and process variation. Novel approaches such as the Globally Asynchronous, Locally Synchronous try to solve this issue by partitioning the SoC into isolated synchronous islands. This paper describes the bisynchronous FIFO used on the DSPIN Network-on-Chip capable to interface systems working with different clock signals (frequency and/or phase). Its interfaces are synchronous and its architecture is scalable and synthesizable in synchronous standard cells. The metastability situations and its latency are analyzed. Its throughput, maximum frequency, and area are evaluated in function of the FIFO depth.

Introduction

In deep sub-micron processes, the largest parts of the delays are related to the wires. In multi-million gates System-on-chip (SoC), achieving timing closure is difficult, as place & route tools have difficulty coping with long wires and balancing the clock tree.

The Globally Asynchronous, Locally Synchronous (GALS) [START_REF] Chapiro | Globally-Asynchronous Locally-Synchronous systems[END_REF][START_REF] Muttersbach | Globally-Asynchronous Locally-Synchronous Architectures to Simplify the Design of On-CHIP Systems[END_REF] approach attempts to solve this problem by partitioning the SoC into isolated synchronous islands that have frequency and phase clock independency. With this approach, the timing constraints of the SoC can be bounded to the isochronous limit of each island. In this case, the communications between islands should be carried out by mixed-timing interfaces that adapt the clock frequency and phase discrepancy. Such interfaces are not trivial [START_REF] Ginosar | Fourteen ways to fool your synchronizer[END_REF] since the synchronization failure (metastability) of the registers can corrupt the transferred data. Many architectures have been proposed to solve this issue, some of them restricted to study only skew [START_REF] Chakraborty | A minimal source-synchronous interface[END_REF][START_REF] Elboim | A clock tuning circuit for System-on-Chip[END_REF][START_REF] Kol | Adaptive synchronization for multi-synchronous systems[END_REF], and jitter [START_REF] Kol | Adaptive synchronization for multi-synchronous systems[END_REF] correction. Others use pausible or stretchable clocks which pause [START_REF] Sjogren | Interfacing synchronous and asynchronous modules within a high-speed pipeline[END_REF][START_REF] Dobkin | Data synchronization issues in GALS SoCs[END_REF], or stretch [START_REF] Moore | Self calibrating clocks for globally asynchronous locally synchronous systems[END_REF][START_REF] Zipf | A switch architecture and signal synchronization for GALS System-on-Chip[END_REF] the receiver clock to synchronize the data in a safe rising edge of the receiver clock.

A number of approaches interface systems with a rational clock relation [START_REF] Chakraborty | Efficient selftimed interfaces for crossing clock domains[END_REF][START_REF] Sarmenta | Rational clocking[END_REF]. A recent published paper from The University of Pierre and Marie Curie proposes a novel synchronous↔asynchronous converter well suited for Network-on-Chip (NoC) in GALS architectures [START_REF] Sheibanyrad | Two efficient synchronous↔asynchronous converters well-suited for network on chip in GALS architectures[END_REF]. Also in [START_REF] Beigné | Design of On-chip and Offchip interfaces for a GALS NoC architecture[END_REF] asynchronous to synchronous and synchronous to asynchronous interfaces for GALS NoC are implemented using Gray FIFOs (first-in, first-out). Finally, the most generic architecture where frequency and phase are unknown is resolved by robust designs where metastability is well analyzed. The basic architecture is the "two-flop synchronizer" [START_REF] Ginosar | Fourteen ways to fool your synchronizer[END_REF] where only the request and acknowledge signals are synchronized. Fastest architectures use FIFOs [START_REF] Glasson | Device for transferring data between two asynchronous subsystems having a buffer memory[END_REF] or a RAM bank [START_REF] Pham | A high throughput, asynchronous, dual port FIFO memory implemented in ASIC technology[END_REF], where write and read pointers are transferred between clock domains using the Gray code. J. Jex et al. [START_REF] Jex | Fully asynchronous interface with programmable metastability settling time synchronizer[END_REF] and Chelcea-Nowick [START_REF] Chelcea | Robust interfaces for mixed-timing systems[END_REF] propose optimized architectures for mixed-timing systems. The J. Jex et al. solution was originally designed to be used on supercomputer interfaces. The solution can modify the number of registers in the synchronizer to increase the robustness against metastability, and is suited to high bandwidth communications since the control signals are optimized. The solution of Chelcea et al. [START_REF] Chelcea | Robust interfaces for mixed-timing systems[END_REF] proposes a mixedtiming interface for mixed synchronous to synchronous, asynchronous to synchronous, and synchronous to asynchronous interfaces. Its architecture is modular and requires low area overhead. However, it requires full custom cells to implement the Full and Empty detectors, which becomes less attractive for synchronous standard cell flows.

The FIFO presented in this document is a bisynchronous FIFO [START_REF] Panades | Buffer memory control device[END_REF] able to interface two synchronous systems with independent clock frequencies and phases. A latency optimization of the architecture in which both interfaces have the same clock frequency but different clock phase (mesochronous) is also proposed. The main features of the bi-synchronous FIFO are low latency, robustness to metastability, small area, scalability, maximized throughput, and synthesizability on synchronous standard cell flow. This bi-synchronous FIFO is been used on the DSPIN [START_REF] Panades | A low cost Network-on-Chip with guaranteed service well suited to the GALS approach[END_REF][START_REF] Sheibanyrad | Systematic comparison between the asynchronous and the multi-synchronous implementations of a Network on Chip architecture[END_REF] Network-on-Chip (NoC), where ten bi-synchronous FIFOs of 34 bits are used per router: Five have 8 words depth, and five have 4 words depth. Since a SoC can contain more than 30 routers, the area of this component have to be careful designed to minimize the total SoC area. Furthermore, the depth of the bi-synchronous FIFOs in this kind of NoC is no deeper than 10 words, therefore a register-base FIFO is more suited than a RAM-base FIFO due to its lower area and its simplicity of test. Thus, its test method can be the scan-chain method. Moreover, the bisynchronous FIFO makes the DSPIN NoC well suited to the GALS approach as the DSPIN router communicates asynchronously with the local subsystem, and mesochronously with the neighbor routers.

The paper is organized as follows. Section 2 presents a novel encoding algorithm well suited to interface two clock systems. In Section 3, all the bisynchronous FIFO modules are detailed. In Section 4, the analysis of latency, throughput, area, and maximum frequency are analyzed and compared with existing solutions.

Bubble encoding

In this section, a novel-encoding algorithm based on a token ring is demonstrated to be useful on the synchronization of pointers between two independent clock domains.

Token ring

A token ring is a succession of nodes interconnected in a circular manner that contain tokens. It can be described with N registers (with enable signal) interconnected in serial, like a cyclic shift-register. Figure 1 shows an example of a token ring with 5 registers.

Figure 1. Token ring

If the enable signal is true, the content of the register is shifted (register i is shifted to register i+1, an register N-1 to register 0) at the rising edge of the clock, otherwise the register maintains the data. A token is represented by the logic state 1 of the register. The number of tokens in a token ring can be from one to N, where N is the number of registers.

A token ring with one token can be used as a statemachine. The position of the token defines the state of the state-machine. The enable signal of the token ring is equivalent to the next-state of the state-machine. If this condition is true, the token ring shifts and the statemachine changes. It is also possible to define a statemachine when the token ring contains two consecutives tokens, the state of the state-machine can be defined, for example, as the position of the first one.

Synchronizing the Token

Since the position of the tokens defines the state of the state-machine, the synchronization of the position can be exploited to interface the two clock domains. To synchronize the state of the state-machine, a parallel synchronizer (two registers per bit) can be used, as shown in Figure 2. However, as described by R. Ginosar, the parallel synchronizer [START_REF] Ginosar | Fourteen ways to fool your synchronizer[END_REF] does not guarantee the correctness of the result. Figure 3 shows an example of synchronization. Solution A, B, C and D are all the possible solutions when the metastability of the register changes its content. Solution A and B are correct since the token is well determined. Solution C is exploitable using some logic but Solution D is useless due to absence of information. Because the token was moving, the metastability of the register can be resolved to a useless result. In that case, one clock cycle should be waited to attempt to obtain a useful data.

To solve this issue, we propose to use two consecutive tokens (bubble encoding) in the token ring. As the metastability affects the changing registers, the use of two consecutive tokens prevents some registers from changing. Assuming that registers i and i+1 have the tokens, if the token ring shifts, register i+2 gets a token, register i loses its token, and register i+1 does not change (it shifts its token and gets a token). In terms of logic value, register i and i+2 change state but register i+1 remains unchanged. Because there always exists a register that does not change state, it is always possible to detect a token. Figure 4 shows an example of synchronization. For example, we can define the position of the detected token by the position of the first logic 1 after a logic 0 (starting from the left). In this case, all solutions A, B, C, and D are correct because the token can be well defined; it is always possible to detect a transition between 0 and 1. This encoding algorithm does not avoid the metastability on the synchronizer. It just guarantees that the position of the token will be detected and a useless situation will never occur.

The token ring and the bubble encoding presented in this section are used on the definition of the statemachines of the bi-synchronous FIFO and will be detailed in next section.

Bi-synchronous FIFO

This section presents the architecture of the bisynchronous FIFO and its application in multi-clock systems. The goal of this FIFO is to interface two synchronous systems having different clock signals.

Each system is synchronous with its clock signal but can be asynchronous (frequency and/or phase) to the others. The challenge of this architecture is to hide all synchronization issues while respecting the FIFO protocol on each interface. Furthermore, this architecture is scalable and synthesizable in a synchronous standard-flow without using custom cells.

As shown in Figure 5, five modules compose the bisynchronous FIFO architecture: Write pointer, Read pointer, Data buffer, Full detector, and Empty detector. The Write and Read pointers indicate the position to be written and to be read in the Data buffer, the Data buffer contains the buffered data of the FIFO, and the Full and Empty detectors signal the fullness and the emptiness of the FIFO.

To better understand the bi-synchronous FIFO, its interfaces and protocol are detailed.

Bi-synchronous FIFO interface and protocol

The bi-synchronous FIFO has a sender and a receiver interface. As shown in Table 1, each interface has its own clock signal, Clk_write for the sender and Clk_read for the receiver. The FIFO protocol is synchronous; all input and output signals in the sender and receiver interfaces are synchronous to their clock signal Clk_write and Clk_read, respectively. The queuing and dequeuing of data elements in the FIFO follows the next synchronous protocol. The Data_write is queued into the FIFO, if and only if, the Write signal is true and the Full signal is false at the rising edge of Clk_write. Symmetrically, data is dequeued to Data_read, if and only if, the Read signal is true and the Empty signal is false at the rising edge of Clk_read.

The clear partitioning of the sender and receiver interfaces into synchronous and independent interfaces simplifies the timing constrains analysis for all the modules connected to the FIFO ports.

Write and read pointers

The Write and Read pointers are implemented using the described token rings with the bubble-encoding algorithm. The position of the tokens determines the position of the pointer. The position of the Write_pointer is defined by the position of the register containing the first token (starting from the left) as shown in Figure 6a. Likewise, the position of the Read_pointer is defined by the position of the register after the second token (starting from the left). The Full and Empty detectors exploit this particular definition of the pointers and will be explained hereinafter.

The Write_pointer shifts right when the FIFO is not full and the Write signal is true. Likewise, the Read_pointer shifts right when the FIFO is not empty and the Read signal is true.

As the write and read interfaces belong to different clock domains, the token rings are clocked by their clock signal, Clk_write and Clk_read respectively.

Data buffer

The Data buffer module is the storage unit of the FIFO. Its interfaces are: Data_write, Data_read, Write_pointer, Read_pointer, and Clk_write. It is composed by a collection of data-registers, AND gates, and tri-state buffers as shown in Figure 7.

The input data, Data_write, is stored into the dataregister pointed by the Write_pointer at the rising edge of Clk_write. AND gates recode the Write_pointer into a one-hot encoding which controls the enable signals of the data-registers. Likewise, the Read_pointer is recoded into one-hot encoding which controls the tristate buffer on each data-register. Finally, the Data_read signal collects the outputs of the tri-state buffers. It is also possible to replace the tri-state buffers with multiplexers to simplify the Design for Test (DfT) of the FIFO.

The width and number of data-registers determine the width and the depth of the FIFO. The depth also determines the range of the Write and Read pointers.

Full detector

The Full detector computes the Full signal using the Write_pointer and Read_pointer contents. No status register is used as in the J. Jex et al. [START_REF] Jex | Fully asynchronous interface with programmable metastability settling time synchronizer[END_REF] or Chelcea-Nowick [START_REF] Chelcea | Robust interfaces for mixed-timing systems[END_REF] solutions. The Full detector requires N twoinput AND gates, one N-input OR gate, and one synchronizer, where N is the FIFO depth (Figure 8). The detector computes the logic AND operation between the Write and Read pointer and then collects it with an OR gate, obtaining logic value 1 if the FIFO is Full (Figure 6e) or quasi-Full (Figure 6c and6d) otherwise is 0. This value is finally synchronized to the Clk_write clock domain into Full_s. Since the synchronization has a latency of one clock cycle and the situation on Figure 6c can potentially be metastable, the detector has to anticipate the detection of the Full condition. Is for this reason that the output of the OR gate detects the Full and the quasi-Full conditions. The Full detector in Figure 8 can be optimized since the synchronization latency inhibits, in some cases, the FIFO from being completely filled. For example, if the FIFO is in the situation of Figure 6c, and the sender does not write any other data, the Full_s will be asserted even if the FIFO is not filled completely. As the fullness has to be anticipated, the detector is dimensioned to stop the sender before overflowing the FIFO, if the sender was writing continuously.

An improved Full detector implementation would be more complex (as the Empty detector), and would therefore require greater die area. However, a nonoptimal Full detector does not penalize the throughput of the FIFO as much as a non-optimal Empty detector. For example, assuming an optimal Empty detector and a non-optimal Full detector, the Full condition occurs when the receiver is not able to consume all the data. In this case, even with a non-optimal Full detector, the receiver limits the throughput of the FIFO. Therefore, design effort and chip area should be devoted to improving the performance of the Empty detector. Even when using a non-optimized Full detector, a low cost optimization can improve its performance. Figure 9 shows an additional module connected to the Full_s signal, which improves the Full detector. The module's operation is as follows: if the writer was not writing before asserting the Full_s signal, the Full signal is delayed one clock cycle, giving a second chance to the writer to fill completely the FIFO.

Empty detector

The implementation of the Empty detector is similar to the Full detector because both employ the Write and Read pointer contents. As seen in the previous paragraph, the Full detector has to anticipate the detection of the Full condition to avoid FIFO overflow. As the Empty detector is correlated to the FIFO throughput, its detection has to be optimized, and no anticipation detector should be used.

Figure 10 shows the Empty detector for a five word FIFO. First, the Write_pointer is synchronized with the read clock into the Synchronized_Write_pointer (SW) using a parallel synchronizer. Next, the Read_pointer is recoded into the AND_Read_pointer (AR) using twoinput AND gates, operation also done in the Data_buffer module. The output of AR is a one-hot encoded version of the Read_pointer. Finally, the Empty condition is detected comparing the SW and AR values using three-input AND gates. As the metastability can perturb some bits of the SW (as seen on Figure 4), each pair of consecutive bits is compared to find a transition between 0 and 1. Their analysis is as follows, if the values of SW i = 0 and SW i+1 = 1 that means that the SW pointer is on position i+1. Furthermore, when AR i = 1 that means that the AR pointer is on position i+1 (Figure 10). The FIFO is considered empty (see Figure 6a) when the Write_pointer points the same position of the Read_pointer. This can be detected when SW i =0, SW i+1 =1 and AR i =1 for any i. These comparisons are computed by means of the three-input AND gates. Finally, a N-input OR gate collects all the values of the three-input AND gates to generate the Empty signal. This N-input OR gate and the one on the Full detector can be decomposed with log 2 N levels of two-input OR gates.

The latency introduced by the synchronization of the Write_pointer cannot corrupt the FIFO, because a change in this pointer cannot underflow/overflow the FIFO, it just introduces latency into the detector.

The advantage of the bubble-encoding algorithm in this detector relies on the continuous detection of the Write_pointer position. Otherwise, as seen on Figure 3 Solution D, its position cannot be detected and the Empty condition should be asserted to avoid a possible underflow of the FIFO, thus, introducing one additional clock cycle latency to the FIFO.

Mesochronous adaptation

The FIFO architecture was originally designed to interface two independent clock domains, but can be adapted to interface mesochronous clock domains where the sender and the receiver have the same clock frequency but different phase. The difference of phase can be constant or slowly varying. We find examples interfacing mesochronous clock domains that employ the predictability of the rising edges to avoid the metastability situations [START_REF] Mu | Self-tested selfsynchronization circuit for mesochronous clocking[END_REF][START_REF] Mesgarzadeh | A new mesochronous clocking scheme for synchronization in SoC[END_REF]. The proposed adaptation lowers the FIFO latency by reducing the number of registers on the synchronizer module. Since metastability can be avoided when the rising edges of the clock signals are predictable, the two rows of registers on the synchronizer can be reduced to a single row of registers as shown in Figure 11b. The remaining row of registers is clocked using a delayed version of the read clock. This delay must be chosen to exchange the data without metastable situations (Figure 11a). The delay can be a programmable delay, or any other metastability-free solution, as for example the Chakraborty-Greenstreet [START_REF] Chakraborty | Efficient selftimed interfaces for crossing clock domains[END_REF] architecture allowing the FIFO to work also on plesiochronous (small difference of frequency) clocks. Likewise, if the write and read clock are out of phase by 180º (clock-inverter), no programmable delay is needed because, byconstruction, the communication is free of metastability. The interface is free of metastability also if the difference of phase varies between 90° and 270°. This type of implementation is done on the DSPIN NoC routers. The clock-tree of each DSPIN router is balanced with a 5% skew. Then, in the routers placed on the Y row and X colon position, where (Y+X) is an odd number, the first clock-buffer of the clock-tree have been replaced by a clock-inverter. Finally, the top pins of the clock-trees routers are balanced with a 50% skew, which is easier to design and less power consuming than a fully synchronous clock tree balance. With this procedure, neighbor routers are 180° out of phase with a tolerance of 50% skew.

This mesochronous adaptation of the bisynchronous FIFO is simple and allows switching between mesochronous and asynchronous modes. This adaptation is interesting in the design of a multi-million gate SoC in deep sub-micron technology, where the delay of long wires can drastically vary with temperature, voltage, and process. In such a system, the mesochronous clock distribution could fluctuate to an undesirable metastable situation, making the FIFO data useless. By switching the bi-synchronous FIFO into the asynchronous mode, robustness against metastability is improved, preventing the SoC from requiring redesign.

Simulation and analysis

Both synthesizable VHDL models and cycle accurate SystemC models of the bi-synchronous have been designed. We have simulated the bi-synchronous FIFO to characterize its latency, throughput, frequency, and area.

Latency analysis

As the sender and the receiver have different clock signals, the latency of the FIFO depend on the relation between these two signals.

The latency of the FIFO can be decomposed in two parts: the state machine latency and the synchronization latency. As the state-machines are designed using Moore automates, its latency is one clock cycle. Two registers compose the synchronizers and its latency is ∆T plus one clock cycle. Where ∆T is the difference, in time, between the rising edges of sender and receiver clock. As this difference is between zero and one Clk_read clock cycle, the latency of the bi-synchronous FIFO is between two and three Clk_read clock cycles. Figure 12 shows the detail of the latency. Sync_1 and Sync_2 are the synchronization registers. The latency of the bi-synchronous FIFO is equivalent to the latency of the J. Jex et al. [START_REF] Jex | Fully asynchronous interface with programmable metastability settling time synchronizer[END_REF] solution. This latency can be lower, but the robustness to the metastability would be penalized [START_REF] Dike | Miller and noise effect in a synchronizing flip-flop[END_REF][START_REF] Ginosar | Fourteen ways to fool your synchronizer[END_REF].

When the bi-synchronous FIFO is adapted to a mesochronous clock distribution, the latency of the FIFO is reduced, because a single register replaces the two-register synchronizer. In addition, the ∆T is constant as the difference of phase is constant. In that case, the latency of the FIFO is one clock cycle plus ∆T, as shown in Figure 13.

Throughput Analysis

The throughput of the bi-synchronous FIFO was analyzed in function of the FIFO depth. As the synchronizers add latency, the flow control of the FIFO is penalized and its performances are influenced. In case of deep FIFO, those latencies do not decrease the FIFO throughput since the buffered data compensate the latency of the flow control. Table 2 shows the minimum FIFO depth for 50% and 100% throughput in function of the clock relation. For FIFO depth of 6 or above, the synchronization latency has no influence on the flow control and the FIFO is able to deliver one word per cycle (100% throughput) even on asynchronous clock relation. For the asynchronous analysis, the write and read clock signals frequencies are similar, otherwise it is not possible to obtain 100% throughput.

Area and frequency estimation

The area and frequency estimation of the FIFO was computed once synthesized on CMOS 90nm GPLVT STMicroelectronics standard cells. Different FIFO depths are used to illustrate the scalability of the architecture and its performances in terms of maximum frequency. To minimize the power consumption, a clock gating technique is used. Two architectures were synthesized, one with the tri-state buffers and another with multiplexers.

Table 3 shows the area and frequency estimation of a 32-bit bi-synchronous FIFO in function of the FIFO depth and type of output port. Note that the maximum frequency of the write clock is greater than the one of the read clock. The limitation of the read clock is due to the Empty detector.

The architecture with tri-state buffers has greater area than the one with multiplexers. This phenomenon is due to the large area of the tri-state buffers. Moreover, the maximum clock frequency of the read part with tri-states is greater than the one with multiplexers, since the multiplexers are decoded in a log 2 N manner rather than in parallel.

Comparison with other existing designs

This architecture has been compared with similar architectures to analyze its area and latency. As its architecture is synthesizable with standard cells, the comparison with the others is done after Synopsys synthesis with the same timing constraints.

The selected architectures are a register-base Gray FIFO and the J. Jex et al. [START_REF] Jex | Fully asynchronous interface with programmable metastability settling time synchronizer[END_REF]. Their VHDL description has been written for different FIFO depths: 4, 8 and 16 words of 32bits. A clock-gating technique is applied but no tri-state buffer is used. Table 4 shows the estimated area and the area overhead percentage of these architectures compared to the presented solution. The register-base Gray FIFO has a 50% bigger area than the presented architecture. Even if the number of registers and synchronizers is lower than our architecture, the Gray code algorithm adds complexity to the read and write state machines. Nevertheless, the Full and Empty detectors are fully optimized, as the write and read sides know the exact position of the read and write pointer (after synchronization).

The J. Jex et al. [START_REF] Jex | Fully asynchronous interface with programmable metastability settling time synchronizer[END_REF] architecture has similar complexity as ours, but its area increases more than ours when the FIFO depth increases. Moreover, its Full detector is not optimized and suffers the same problem of the non-optimized Full detector presented in Figure 9. To correct this issue, the optimization of Section 3.4 could be used, regardless the increase of the total area.

In terms of FIFO latency, all three have the same latency, 2-3 clock cycles, since all of them use Moore state-machines and two flip-flops synchronizers.

Conclusions

A new bi-synchronous FIFO has been presented and analyzed. It is well suited to interface different systems working with independent frequency and/or phase clock signals.

It uses a novel encoding algorithm combined with an astute definition of the FIFO pointers that avoids the utilization of status registers. Its write and read pointers are directly combined to obtain the Full and Empty detectors.

Both of its interfaces are synchronous to its relative clock signals. Moreover, its architecture is designed to be synthesized using a synchronous standard cell design flow. None of its modules requires custom cells.

A simple mesochronous adaptation is proposed which reduces the latency of the FIFO. Its latency is 2-3 clock cycles in asynchronous mode, and 1-2 clock cycles in mesochronous mode.

Both SystemC cycle accurate and VHDL models of the bi-synchronous FIFO has been designed. The FIFO throughput depends on the FIFO depth. Throughput is 100% when the FIFO depth is six or above.

Using CMOS 90nm GPLVT STMicroelectronics standard cells, we have synthesized and analyzed the FIFO area and maximum frequency for different FIFO depths. Two architectures are analyzed, one with tristate buffers and another with multiplexers. A 32-bit bisynchronous FIFO with eight words depth requires 6581µm 2 and its maximum clock frequency is 1GHz.

The comparison with existent synthesizable asynchronous FIFOs shows a better integration density at the same data latency.

Figure 2 .

 2 Figure 2. Synchronization of a token ring

Figure 5 .

 5 Figure 5. Bi-Synchronous FIFO architecture

Figure 3 .Figure 4 .

 34 Figure 3. Possible solution in the synchronization of a single token ring containing one token

Figure 6 .

 6 Figure 6. Write and Read pointer position definition and Full and Empty conditions in terms of tokens position

Figure 8 .Figure 7 .

 87 Figure 8. Full detector detail

Figure 10 .Figure 9 .

 109 Figure 10. Empty detector detail

Figure 11 .

 11 Figure 11. Mesochronous adaptation

Figure 13 .

 13 Figure 12. Latency analysis

Table 1 .

 1 Sender and receiver interface signals

			Signal	Description
			Data_write	Data to be written into the FIFO
			Write	Input signal requesting a write
	Sender	interface	Full Clk_write	into the FIFO Output signal indicating the fullness of the FIFO Sender clock signal
			Data_read	Output data from the FIFO
			Read	Input signal requesting a read in
	Receiver	interface	Empty Clk_read	the FIFO Output signal indicating the emptiness of the FIFO Receiver clock signal

Table 2 .

 2 Minimum FIFO depth in function of the clock relation and required throughput

		Minimum depth	Minimum depth
		for 50 %	for 100 %
		throughput	throughput
	Asynchronous	5	6
	Mesochronous	4	5

Table 3 .

 3 Area and frequency in function of FIFO depth and type of output port

	Type	FIFO	Area	Max. Write	Max. Read
			Depth	(µm²)	Freq. (MHz)	Freq. (MHz)
	Mux		4 8 16	3304 6581 13384	2000 2000 2000	1110 1000 769
	Tri-	state	4 8 16	4082 8032 16101	2000 2000 2000	1428 1250 1110

Table 4 .

 4 Area and overhead comparison between other existing designs

	FIFO	This	Register-base	J. Jex et al. [9]
	Depth	Design	Gray FIFO
		µm 2	µm 2 (%)	µm 2 (%)
	4	3304	5113 (+54%)	3364 (+1.8%)
	8	6581	9702 (+47%)	6858 (+4.2%)
	16	13384	20364 (+52%)	14362 (+7.3%)