Meher Naffouti

Marco Salvalaglio
email: marco.salvalaglio@tu-dresden.de

Thomas David

Jean-Benoît Claude

Monica Bollani

Axel Voigt

Abdelmalek Benkouider

Luc Favre

Antoine Ronda

Isabelle Berbezier

Anne Delobbe

Arnaud Houel

Marco Abbarchi
email: marco.abbarchi@im2np.fr

Deterministic 3D self-assembly of Si through a rim-less and topology-preserving dewetting regime

Capillary-driven mass transport in solids is typically understood in terms of surface-diffusion limited kinetics, leading to conventional solid-state dewetting of thin films. However, another mass transport mechanism, socalled surface-attachment/detachment limited kinetics, is possible. It can shrink a solid film preserving its original topology without breaking it in isolated islands and leads to faster dynamics for smaller film curvature in contrast with the opposite behavior observed for surface-diffusion limited kinetics. In this work, we present a novel rim-less dewetting regime for Si, which is ascribed to effective attachment-limited kinetics mediated by the coexistence of crystalline and amorphous Si phases. Phase-field numerical simulations quantitatively reproduce the experimental observations assessing the main mass transport mechanism at play. The process can be exploited to obtain in a deterministic fashion monocrystalline islands (with 95% probability) pinned at ∼500 nm from a hole milled within closed patches.

INTRODUCTION

Thin films of organic or inorganic compounds have the tendency to break as a consequence of the minimization of their surface energy density. When perturbed, liquid [1], polymer [2] and crystalline films [3] bead over time by dripping into tiny droplets featuring a particular size and shape determined by the interactions with the substrate, the surrounding atmosphere as well as by the initial film thickness.

Although dewetting of metals is a phenomenon conveniently exploited for important applications [4] (such as the formation of gold seeds for vapour-liquid-solid growth of nano-wires [5]), its use in silicon is largely unexplored in spite of the manifold advantages it offers with respect to common bottom-up and top-down nano-fabrication methods. Being a spontaneous phenomenon driving significant and controllable changes of morphologies, it offers interesting technological perspectives. Indeed, it has been exploited to i) implement 3D Si island in a strain-free system (in contrast with the conventional Stranski-Krastanov approach used in IV-IV and III-V semiconductor compounds) [6]; ii) form monocrystalline and faceted (atomically smooth) structures, free from defects and from the typical roughness produced by conventional etching methods [7][8][9][10]; iii) directly fabricate monocrystalline islands on electrically insulating substrates (SiO 2 ) [11]; iv) frame, when assisted by templates, complex, monocrystalline and ordered nano-architectures, with the additional advantage of reduced etching time with respect to conventional lithographic approaches [12,13]; v) form dielectric metasurfaces for application in photonics at visible and near-infrared frequen-cies [9,[14][15][16]; vi) implement devices over large scales in a time that is independent of their size; vii) tune the size of the particles by setting the initial film thickness and independently tune their density by adding germanium during growth [16]; viii) obtain core-shell structures [10].

So far, shape instabilities of thin crystalline films have been attributed mainly to capillary-driven mass transport at the crystal surface [17,18] and are well understood in terms of surface diffusion limited kinetics (SD) [19][20][21][22][23][24][25]. In this regime, common to metals and semiconductors, the film retracts forming a thick rim. In turn, the rim undergoes further instabilities (such as bulging and finger formation) and finally breaks into isolated islands. In templated crystalline films of semiconductors and metals, a remarkable example of dewetting via SD is the spontaneous pattern formation of complex nano-architectures [13,[26][27][28].

A different capillary-driven shape evolution ruled by surface attachment limited kinetics (SALK) was proposed in 1995 by Cahn and Taylor [29] and thoroughly discussed by Carter and coworkers [30] for faceted crystals [START_REF]phase and it could be named including both these terms[END_REF]. Such dynamics occurs when the phase surrounding the solid (e.g. a fluid, such as the atmosphere, or a thin surface layer having different properties) allows fast transport of atoms. In this case, the attachment or detachment of atoms to the surface of the solid is the rate-limiting step [30,[START_REF] Mullins | [END_REF]33]. If the surrounding fluid is the fast transport pathway, the mechanism is known as evaporation-condensation.

This mechanism has never been reported so far in the context of single-crystal thin-film dewetting. However, SALK at play in these systems would lead to a peculiar dynamics including volume conservation and shape-preserving evolution: in contrast with SD, no isolated islands are expected at the end of the process and one individual object preserves its topology while shrinking, as illustrated in Ref. [30]. Thus, given the broad interest in the stability of thin films, providing the conditions to realize and control this unexplored self-assembly method is of the utmost importance.

Here we report on solid-state dewetting of ultra-thin monocrystalline, solid films of silicon on SiO 2 (UT-SOI) which may be ascribed to SALK. A partially amorphous layer atop of a UT-SOI is obtained by patterning trenches and closed patches via focused ion beam. This amorphous layer provides the high-mobility phase necessary for fast mass transport during annealing while recrystallization takes place. Early stages of dewetting show the absence of a receding rim, a uniform thickening of the Si layer and a faster dewetting speed for larger patches with a corresponding lower height. This evidence is benchmarked against phase-field simulations of stripes and closed patches evolving under SALK. Finally, we show that this process can be controlled by milling pierced patches for the deterministic fabrication (with ∼95% probability over 180 trials) and positioning (within ∼500 nm from the milled hole) of monocrystalline silicon islands. The island size is, to a first approximation, independent on the initial patch surface.

RESULTS

Experimental methods

FIG. 1. 1) scheme of the UT-SOI (initial thickness h0 = 11 nm thick UT-SOI atop 145 nm thick buried oxide, BOX) and liquid-metal ionsource focused ion beam used to mill free patterns (e.g. trenches, squares, circles, pits, etc); 2) removal of native oxide via wet-etching; 3) annealing in the ultra-high vacuum (UHV) of a molecular beam epitaxy reactor. First the samples are annealed at 600 o C for 30 minutes followed by deposition of 2 monolayers of Ge and finally annealed at 780 o C for 120 minutes.

An 11 nm thick UT-SOI on a 145 nm thick buried oxide was milled with free patterns (e.g. trenches, circles squares, pits) with a liquid-metal ion-source focused ion beam (FIB, Ga + ions, milling current about 10 pA, beam energy 30 keV) and annealed at 780 o C in ultra-high vacuum (Fig. 1, further details of the experimental methods are reported in the supplementary information, S.I.) [9,12,15]. Two monolayers of Ge were supplied to enhance the surface diffusion and trigger the dewetting. From electron diffraction spectroscopy after dewetting, we estimate a Ge content of 2% circa, which is close to the sensitivity of the instrument (not shown).

Experimental results

FIG. 2. a) Square patch etched via FIB after annealing. The white dashed square highlights the original shape of the patch before annealing. b) Square patch etched via e-beam lithography and reactive ion etching (e-beam and RIE) after annealing (see reference [13] for details). The white dashed square highlights the original shape of the patch before annealing. c) Comparison between the patch edge profile after annealing for both FIB (top panel) and e-beam and RIE (bottom panel) etching extracted from a) and b) (highlighted by dashed lines).

We first compare the evolution of square patches etched via FIB against the case of electron beam lithography and reactive ion etching (e-beam and RIE, Fig. 2). Details for this second case are provided in Ref. [13,34]. AFM profiles at the edge of the patches (Fig. 2 c)) show a partial dewetting of the UT-SOI for both cases and a lacking rim for the FIB case in stark contrast with the e-beam and RIE case. Furthermore, in addition to the ∼30 nm thick rim all along the perimeter of the patch fond for the e-beam and RIE case, protrusions are formed at its corners as also predicted by sharp interface models [35][36][37][38]. These protrusions eventually lead to four islands depending on the initial aspect ratio of the square patch [13]. None of these features is observed when etching via FIB.

Further insight in the peculiarities of FIB etching with respect to e-beam and RIE can be obtained by modifying the initial patch design (Fig. 3) by milling a small hole at their ) crystallographic direction after annealing. The white dashed square highlights the original shape of the patch before annealing. See reference [13] for more details on this case. center. When using FIB milling we observe the formation of an island in contrast to the e-beam and RIE case where a much complex behavior takes place with formation of rims and protrusion also around the central hole [13]. Moreover, by comparing the evolution of patches oriented along the stable dewetting front ([110] in-plane crystallographic direction) with the unstable counterpart ([100]), we observe an identical outcome for the FIB milling (no rim and a single island at the center of the patch) against a completely different outcome in the e-beam and RIE case (bulging and fingers formation along the unstable dewetting front). Note that this latter feature is commonly observed for SD dewetting of thin silicon films [6,[39][START_REF] Danielson | Anisotropic dewetting in ultra-thin singlecrystal silicon-on-insulator films[END_REF][START_REF] Dufay | [END_REF][42][43][44].

A more systematic analysis of dewetting after FIB milling is provided for patches having different lateral width obtained from parallel trenches etched with different spacing (line-toline distance d LL , Fig. 4 a). An example of a patch obtained from parallel trenches is shown in the S.I., whereas here we focus only on their edges. Edge retraction (∆x) and patch height (h) are measured in 10 distinct points. The corresponding values and error bars are obtained as average and standard deviation for each d LL . This analysis highlights a faster dewetting speed for larger patches and a corresponding slightly lower film height (Fig. 4 b) andc)). This feature is in contrast with the conventional case of SD, where larger curvatures (smaller patch width) lead to faster kinetics. Height fluctuations, reflected by the error bars, are in the range of about 1 nm (one order of magnitude lower than the rim thickness found in SD dewetting).

In order to understand the origin of these differences between the two dewetting dynamics, we perform microscopic analysis of the initial state of the Si crystal after FIB etching. Apart from sputtering, ion milling has several consequences on the adjacent areas (Fig. 5). A monocrystalline (001) Si sample etched via FIB in parallel trenches shows implantation of the Ga + ions used for milling and consequent amorphization of the superficial layers (below the etched trenches up to ∼70 nm depth, dark area am-Si in Fig. 5 a) and b)) as well as in the nearby areas. More precisely, at ∼800 nm from the milled trenches the silicon is crystalline (cr-Si), at ∼700 nm the crystal shows some disorder (pol-Si) whereas at ∼600 nm the material is amorphous (am-Si). A similar analysis on thin films on insulator shows that the FIB-induced amorphization of the top layers extends over micrometric distances (Further details are shown in S.I.) confirming previous reports [45] and pointing out that patches having an extension of a few µm have their superficial layers partially amorphized.

Differently from FIB milling this amorphous layer is not present in crystalline materials etched by RIE. In fact, in this latter case, the surface of the patches is protected by a resist or a metallic mask and only the exposed parts are affect by the etching, eventually leading to some roughness on the sidewalls in the range of ∼10 nm [46]. Thus the dewetting dynamics of UT-SOI patches etched by e-beam and RIE can be simply ascribed to SD [13,17,18,[35][36][37][38].

The evolution observed in patches etched by FIB shows compelling similarities with the features typical of SALK which is enabled by the presence of a high mobility phase at the surface [30]. Owing to the much larger atom mobility of amorphous silicon with respect to the crystalline counterpart (with a difference of two orders of magnitude in the diffusion coefficient [47]) a reservoir of mobile material is present at the patch edges (as well as at its surface). This leads to an effective fast material redistribution over long distances (not limited by the film curvature as in SD) from the edges to the center.

In the following section, the comparison of the experimental data with simulations reproducing the SALK dynamics is shown, assessing the main features of the mechanism at play during the evolution.

PHASE FIELD SIMULATIONS AND COMPARISON WITH EXPERIMENTS

The outward-normal velocity of a surface evolving by SALK is v SALK = M (K -µ), with M a mobility coefficient depending on the material properties (such as the density of attachment sites and the attachment rate [30]), µ the local chemical potential on the surface and K the average of the chemical potential along the surface to impose volume conservation. For isotropic surface energies, µ is proportional to the local surface curvature κ. This dynamics ruled by SALK and described by v SALK differs from that by SD, as

v SD ∝ ∇ 2 Γ µ with ∇ 2
Γ the Laplacian evaluated along the surface (Γ). Material transport under SALK prevents the local accumulation of mass typical of SD leading to non-conventional dewetting features, such as a bulk thickening of the film, shape preservation, and lack of a receding rim [30].

To assess the analogies of the morphological evolution reported above with SALK, phase-field numerical simulations were performed [48,49]. This approach can deal with complex evolution possibly including topological changes regardless of the dimensionality of the system: an auxiliary order parameter, ϕ (set to 1 in the solid and 0 on the outside with a continuous variation in between) is considered defining implicitly the surface of the solid phase as the isosurface ϕ = 0.5. Morphological evolution of the solid is obtained by setting the evolution law for ϕ [49,50], here meant to reproduce v SALK . Within the considered phase field approach, SALK is accounted for by

∂ϕ ∂t = ∆ϕ + 1 (B (ϕ) + α), (1)
with B(ϕ) = 18ϕ 2 (1 -ϕ) 2 and a parameter controlling the extension of the interface between phases. α is a term enforcing mass conservation at each time [48,51,52]:

α = 2B(ϕ) Ω 2B(ϕ)dΩ Ω B (ϕ)dΩ, (2)
with Ω the simulation domain. No-flux boundary conditions are considered, enforcing a contact angle of 90 • with respect to the substrate. We consider isotropic surface-energy focusing on the features of the process neglecting the additional contributions of surface faceting (although feasible within the phase-field framework [53][START_REF] Torabi | Proc. R. Soc. London[END_REF][START_REF] Salvalaglio | [END_REF][56]). Moreover, the same framework can be adapted to account in detail for other mechanisms occurring at the surface and driving forces (see e.g, [57,58]). The simulations were performed exploiting the finite element toolbox AMDiS [59,60] and established numerical methods for phase-field approaches [61]. Additional details about the model are reported in the S.I. We first compare full-3D simulations of square patches having an aspect ratio of 1/40 evolving under SALK and SD FIG. 6. a) 3D phase-field simulations reproducing the dewetting by SALK of a patch having aspect ratio 1/40. b) Same as a) for SD dynamics (for more details on this case see Ref. [13]).

(Fig. 6). In the first case we observe a rim-less, conformal dewetting with a homogeneous thickening of the patch, whereas in the second case rim and protrusion at the corners are found, as expected for pure SD dewetting dynamics [26,27,[35][36][37][38]. For more details on simulations of SD see reference [13]). This 3D case is of particular interest as differences with respect to standard SD dewetting leading to rims and protrusion at the patch corners are very evident and are similar to those previously discussed for the experimental cases (Fig. 2).

Owing to the high-temperature annealing used to induce the dewetting, the amorphous phase is not only partially displaced towards the center of the patches but it is also re-crystallized. This second process is confirmed by TEM imaging after annealing showing a slight crystal disorder in the area affected by the FIB amorphization (see the S.I.). This implies that the SALK regime can be only observed in a relatively short time window, rendering a comparison of the 3D temporal dynamics of square patches with simulations (e.g. as those shown in Ref. [13]) not feasible.

A quantitative comparison between data and theory is provided for the evolution of long patches having different lateral with d LL . 2D phase-field simulations of dewetting via SALK were performed to compare with film aspect ratio r 1 = 1/200, r 2 = 1/400, mimicking d LL = 2µm and d LL = 4µm, (Fig. 4). A rim-less thickening of the film is observed, together with a faster dewetting for larger stripes (Fig. 7 a),b) and supplementary video SV1). In fact, these trenches are further away from the equilibrium with respect to more closely spaced ones: the lower values of K for large stripes leads to larger driving forces at the edges of the film where µ > K, against a smaller tendency to thickening at its center as µ ∼ 0 and v SALK ∼ K.

Deeper insight can be obtained from Fig. 7 b) and c), where a faster dewetting speed for r 2 against a faster thickening for r 1 is shown during the evolution towards equilibrium. This is in qualitative agreement with the experiments (Fig. 4 b) and c)). Finally, a direct comparison between theory and experiments for all the four investigated patch width shows a good agreement for time step t = 4, supporting the SALK-like dewetting mechanism at play (Fig. 7 d)).

We conclude this section observing that dewetting via SALK is mass-preserving. This is accounted for by a precise evaluation of the mass displacement from the sides of the patches towards their center (and eventually feeding the central island in pierced patches) as shown in the S.I..

SELF-ASSEMBLY OF ISLANDS

Islands formation is often observed in closed patches especially when a central pit is milled at their center (Fig. 3 a)). Thus, we considered different patch sizes, shape (triangle, circle, square), presence of lack of a central pit and orientation with respect to the crystallographic axes, in order to study and drive this phenomenon.

The general picture is described as follows: -for small patches (aspect ratio >1/230), the film is shrunk and a pyramidal island is found at the edge (Fig. 8 a) left panel); -for larger patches the film is shrunk but no island is observed (Fig. 8 -for pierced patches, the film is shrunk and the formation of the island is triggered next to the hole (Fig. 8 a) right panel).

These features are common to all the investigated patch shapes (e.g. square, circle or triangle) sizes and orientations with respect to the crystallographic axes.

The presence of an island depends on the patch size and the presence of holes in a highly reproducible fashion, as shown by the probability of forming a single island within a patch (p(1)) as a function of the initial patch surface S 0 = L 2 (Fig. 8 b)). Without a hole and S 0 >10 µm 2 , p(1) is below 0.5 and it shows a decreasing trend when increasing S 0 ; for smaller S 0 , p(1) approaches 1 and the islands are found at the patch edges. For pierced patches, p(1) is always larger than 0.8 and it approaches 1 for S 0 <10 µm 2 .

A remarkable peculiarity of the islands formed via SALK with respect to those obtained via conventional SD, is that in the former case all the islands have a similar size irrespective of the starting patch extension (Fig. 8 c) and d)), whereas in the latter case the initial patch surface and UT-SOI thickness set the final island dimension [12,15]. This suggests that size tuning by simply controlling the dewetting time could be achieved.

DISCUSSION

Although rim-less dewetting has been reported earlier in thin films of metals (e.g. Ag, [62] Fe [63] and Al [64]) in all these cases the material was polycrystalline. As such, these examples should not be confused with our report, where the underlying material is monocrystalline UT-SOI surrounded by a high-mobility, amorphous phase. A rim-less morphology, per se, does not account for SALK-like dewetting and it is not inconsistent with the diffusion-controlled process. In fact, it was attributed to SD combined with diffusion along the grain boundaries [62,63], and along the film-substrate/over-layer interfaces [63,64]. The features of SALK-based dewetting go beyond this specific aspect common to other systems as it also involves 1) a faster retraction speed for lower overall curvature and 2) a bulk thickening of the solid that was not reported in the aforementioned cases. Similar considerations hold for the case of UT-SOI dewetting [65,66] for patterned patches and spontaneous dewetting [67]: these rim-less processes did not account for SALK but were interpreted as conventional SD dewetting. Besides, the partial characterization of the system does not allow for a more direct comparison.

The ordering of Si islands is a necessary step for any attempt to integrate these structures into existing Si-based electronic devices (owing to the need for mandatory spatial addressability). In our case, the underlying SALK-like process at play allows forming Si islands directly on pristine UT-SOI, that is a possibility forbidden in homo-epitaxy as well as in conventional solid-state dewetting evolving under SD, where the buried oxide is completely denuded. Conventional hybrid top-down/bottom-up methods for deterministic 3D islands formation rely on complex fabrication steps (e.g. e-beam lithography and reactive ion etching) and epitaxial growth employing strain [68] (Stransky Krastanov). In contrast, our method is a direct FIB milling process followed by annealing.

The size-homogeneity of the dewetted islands, irrespective of the initial patch extension, is also a fingerprint of the main role played by amorphization of the UT-SOI skin. This is in stark contrast with conventional dewetting via SD and in general with most self-assembly processes (e.g. nucleation via Stranski Krastanov). This feature is important in view of the formation of 3D nano-objects on thicker SOI and could poten-tially permit the fabrication of AFM tips and cantilevers [69]. The range of applicability of our method for Si islands goes beyond that: dielectric Mie resonators [9,14,15], solid-state memories, strain arrays for 2D materials [70], are only a few examples of possible uses. Furthermore, given the similarities commonly found between dewetting of thin films of metals and semiconductors, we expect that, in analogy with the SD regime [13,27,28], such SALK-like process can be extended to metals, which will further widen the range of applicability of our method. By comparing our results with similar samples etched by FIB and annealed at a higher temperature (quickly recrystallized, S.I.), we confirm that the SALK-like features are specific of the amorphous phase surrounding the crystalline patch. In pierced and quickly re-crystallized solids [12], we observe the features of SD: formation of islands occurs at the patch corners or edges whereas no island is observed nearby the central hole.

Differently from what is expected for an anisotropic mass transport on a crystal evolving under SD (different dewetting speed along stable and unstable dewetting fronts (Fig. 3 b)) [71,72]), we observe the same dewetting outcomes for patches oriented along [110] and [100] axes (Fig. 3), further supporting the hypothesis of mass transport through the amorphous phase.

Despite the high reproducibility of the experimental results (tested also in other samples), justifying the island formation from analytical or numerical modeling is not straightforward: nucleation events for the island (that cannot be traced back to energy considerations leading to SALK or SD), should be taken into account. Moreover, a full model including the recrystallization dynamics in 3D (and not only an effective model focusing on the material transport at the surface) should be considered to capture the different morphologies and it is far beyond the scope of the present work. Nonetheless, we observe that all the rectangular islands have sides oriented along the [100] and [010] in-plane crystallographic directions. This is a feature often observed in (Si)Ge-based structures on Sibased (001)-oriented substrates, nucleated via Stranski Krastanov mechanism [73][74][75]. This analogy suggests that local strain accumulation nearby the central pit during dewetting may be the origin of island formation in our system.

Based on the results obtained for trenches, the central hole in pierced patches can be regarded as a dewetting front expelling mass but, at the same time, competing with the fast fluxes coming from the edges. Owing to the reduction of the dewetting speed at the center of the patch and to the local thickening of the layer fed by the inward flux from the edges, mass can be accumulated. Other local conditions (e.g strain accumulation, facet formation during re-crystallization), may act as seeds for island formation.

CONCLUSIONS

In conclusion, we realized the experimental conditions achieving a peculiar dewetting regime whose main feature corresponds to surface attachment limited kinetics. In contrast to conventional surface diffusion, the dewetting kinetics is here faster for lower average curvatures, it occurs without rim formation and brings a solid film to thicken while shrinking, as accounted for by experiments, numerical simulations and their detailed comparison. As a result, the topology of the system and the break up of the thin film, typical of standard dewetting mechanisms, is prevented. Beyond fundamental interest into this mass transport process for Si thin-films, we also demonstrated that it can be efficiently exploited to deterministically form monocrystalline sub-micrometric islands sitting on large patches of pristine UT-SOI. Our approach provides an alternative way to form 3D islands and combines the benefits of top-down fabrication and of bottom-up self-assembly (e.g. atomically smooth islands). Beyond the first experimental evidence of this mass transport regime in a dewetting process for Si, this method is a distinct approach for specific applications such as the implementation of position-controlled, nanocrystal-based memory devices, dielectric Mie resonators and cantilever tips for atomic force microscopes.

FIG. 3

 3 FIG. 3. a) Left (Right) panel: square patch etched via FIB after annealing oriented along the [110] ([100]) crystallographic direction. The white dashed square highlights the original shape of the patch before annealing. In both cases, sub-micrometric, rectangular island is formed next to the central hole after annealing. b) Left (Right) panel: square patch etched via e-beam lithography and reactive ion etching oriented along the [110] ([100]) crystallographic direction after annealing. The white dashed square highlights the original shape of the patch before annealing. See reference[13] for more details on this case.

FIG. 4

 4 FIG. 4. a) Height profile obtained from AFM images of parallel trenches etched by FIB after dewetting. An example of the AFM image showing the full patch width is provided in the S.I.. From the left to the right panel dLL = 2, 2.5, 3, and 4 µm. Shaded areas highlight the original shape of the UT-SOI before annealing. h0 highlights the original film height and h its final height. ∆x is a measure of the edge retraction distance from its original position before dewetting. b) Retraction distance ∆x (in unity of h0) as a function of dLL. c) Film height h (in unity of h0) as a function of dLL . h, x and the experimental errors are determined as average values and standard deviations over ten measurements. See also the S.I..

FIG. 5

 5 FIG. 5. a) Dark-field TEM image of Si-bulk (001) milled with a Ga + FIB. The platinum (Pt) protecting the TEM lamella, the amorphous (am-Si) and crystalline (cr-Si) silicon are highlighted. b) A blow-up of the area highlighted in a). c) d) and e) Blow up of the rectangles shown in b). For each case the 2D Fourier transform is shown.

FIG. 7

 7 FIG. 7. a) Representative stages of the evolution by SALK of a rectangle (the cross section of a stripe) with height-to-base aspectratio r1 = 1/200. Time is expressed in arbitrary units. Simulation are performed with h0 = 1. b) Edge displacement over time for patches with r1 = 1/200 and r2 = 1/400. c) Thickening over time for profiles with r1 and r2 (see also video S.V.1). d) Comparison between experimental and theoretical profiles. The red curve represents the initial condition of the UT-SOI before annealing, the black line the film at t = 4 and the symbols are the experimental data (see also the S.I.).

  FIG. 8. a) Respectively from the left to the right panel: four replicas of L = 2 µm squares, L = 4 µm square and a pierced, L = 4 µm square. The white lines highlight the FIB milling. The white dashed lines highlight the etched trenches and pit. b) Island formation probability p(1). Full symbols: probability p(1) to form one-and-only-one island pinned by the central hole in pierced patches, as a function of S0 = L 2 . Empty symbols: p(1) (regardless of its position within the patch) as a function of S0 for simple (non-pierced) shapes. The symbol shape corresponds to the patch shape. Each point represents at least 20 repetitions of the same patch size and shape. The statistics merge patches oriented along the [110] and [100] directions. c) From the left to the right panel: pierced, squared patches oriented along the [100] crystallographic direction. The patch side L ranges between 2.5 up to 4 µm. The white dashed lines highlight the etched trenches and pit. d) Island volume (right axis) and height (left axis) extracted from a) as a function of the initial patch surface. Lines are linear fit to the data. The first points (S0 = 4 µm 2 ) are relative to a non-pierced squared patch (as those shown in a), left panel).

ACKNOWLEDGEMENTS

We acknowledge the projects PHC MAGHREB (no. 32595SL), EMMAG-Erasmus Mundus Maghreb & Egypt, the EU H2020 R&I programme (654148 LASER-LAB Europe-ARES), the PRCI-ULYSSES (ANR-15-CE24-0027-01) and the FET-OPEN project NARCISO (No. 828890). M.S. acknowledges the support of the Postdoctoral Research Fellowship awarded by the Alexander von Humboldt Foundation. We also gratefully acknowledge the computing time granted by the John von Neumann Institute for Computing (NIC) and provided on the supercomputer JURECA at Jülich Supercomputing Centre (JSC), within the Project no. HDR06, and by the Information Services and High Performance Computing (ZIH) at TU Dresden. We thank the Nanotecmat Platform of the IM2NP Institute of Marseille. The authors thank Dominique Chatain and Rachel Zucker for the useful discussions.


Baptiste Le Bourg
email: baptiste.lebourg@doct.ulg.ac.be

Jérémy Kiszka

Paco Bustamante

Michael R Heithaus

Sébastien Jaquemet

Frances Humber

Jeremy J Kiszka

Effect of body length, trophic position and habitat use on mercury concentrations of sharks from contrasted ecosystems in the southwestern Indian Ocean

Keywords: Hg, elasmobranchs, stable isotopes, trophic ecology, foraging habitat, body size

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

Introduction

Elasmobranchs are present in many habitats and at various depths. They exhibit a range of foraging strategies and tactics, including planktivorous filter-feeding and predatory species (e.g. [START_REF] Bird | A global perspective on the trophic geography of sharks[END_REF]Kiszka and Heithaus 2014). They also have long lifespans [START_REF] Cortés | Standardized diet compositions and trophic levels of sharks[END_REF]) and some species grow to large body sizes. Sharks are heavily fished for human consumption [START_REF] Clarke | Global estimates of shark catches using trade records from commercial markets[END_REF]).

However, mercury (Hg) concentrations reported in sharks are frequently above the maximum values recommended for fish consumption (1 µg.g -1 wet weight; [START_REF] Escobar-Sánchez | Biomagnification of mercury and selenium in blue shark Prionace glauca from the Pacific Ocean off Mexico[END_REF][START_REF] Hueter | Methylmercury concentration in shark muscle by species, size and distribution of sharks in Florida coastal waters[END_REF][START_REF] Kousteni | Total mercury concentrations in edible tissues of two elasmobranch species from Crete (eastern Mediterranean Sea)[END_REF][START_REF] Mckinney | Global versus local causes and health implications of high mercury concentrations in sharks from the east coast of South Africa[END_REF]. Hg is toxic to organisms at low concentrations with known sublethal effects including reduced growth, impacts on reproduction (e.g. reduced fertility and spawning success), abnormal behaviour (e.g. increased lethargy) and tissue damages [START_REF] Depew | Toxicity of dietary methylmercury to fish: derivation of ecologically meaningful threshold concentrations[END_REF][START_REF] Díez | Human Health Effects of Methylmercury Exposure[END_REF][START_REF] Sandheinrich | Methylmercury in freshwater fish-Recent advances in assessing toxicity of environmentally relevant exposures[END_REF]. Hg is released in the environment by both natural and anthropogenic sources [START_REF] Fitzgerald | Marine biogeochemical cycling of mercury[END_REF][START_REF] Selin | Global biogeochemical cycling of mercury: a review[END_REF]. It is known to be present in aquatic organisms mainly in the form of highly toxic methylmercury (CH3Hg) form [START_REF] Bloom | On the chemical form of mercury in edible fish and marine invertebrate tissue[END_REF][START_REF] Jones | Spatial variability in selenium and mercury interactions in a key recreational fish species: implications for human health and environmental monitoring[END_REF][START_REF] Pethybridge | Mercury in 16 demersal sharks from southeast Australia: biotic and abiotic sources of variation and consumer health implications[END_REF][START_REF] Storelli | Total and methylmercury residues in cartilaginous fish from Mediterranean Sea[END_REF][START_REF] Bush | Diet and diel feeding periodicity of juvenile scalloped hammerhead sharks, Sphyrna lewini, in Kāne'ohe Bay, Ō'ahu, Hawai'i[END_REF]. Due to its high assimilation rate and affinity for proteins, CH3Hg bioaccumulates in organisms through time and biomagnifies up food webs [START_REF] Atwell | Biomagnification and bioaccumulation of mercury in an arctic marine food web: insights from stable nitrogen isotope analysis[END_REF][START_REF] Mason | Bioaccumulation of Mercury and methylmercury[END_REF][START_REF] Power | Mercury accumulation in the fish community of a sub-Arctic lake in relation to trophic position and carbon sources[END_REF].

Various biological and ecological factors influence Hg concentrations in aquatic animals including elasmobranchs. Body length (a proxy for animal size) and age may be considered as the main factors determining Hg concentrations. Hg concentrations increase with size in various taxa such as cephalopods (e.g. [START_REF] Chouvelon | Interspecific and ontogenic differences in δ 13 C and δ 15 N values and Hg and Cd concentrations of cephalopods[END_REF], predatory teleosts [START_REF] Cresson | Mercury in organisms from the Northwestern Mediterranean slope: importance of food sources[END_REF][START_REF] Kojadinovic | Mercury content in commercial pelagic fish and its risk assessment in the Western Indian Ocean[END_REF][START_REF] Sackett | The influence of fish length on tissue mercury dynamics: implications for natural resource management and human health risk[END_REF]) and elasmobranchs [START_REF] Cresson | Mercury in organisms from the Northwestern Mediterranean slope: importance of food sources[END_REF][START_REF] Hueter | Methylmercury concentration in shark muscle by species, size and distribution of sharks in Florida coastal waters[END_REF][START_REF] Mckinney | Global versus local causes and health implications of high mercury concentrations in sharks from the east coast of South Africa[END_REF][START_REF] Pethybridge | Mercury in 16 demersal sharks from southeast Australia: biotic and abiotic sources of variation and consumer health implications[END_REF]2012;[START_REF] Rumbold | Mercury accumulation in sharks from the coastal waters of southwest Florida[END_REF]. Differences in life-history traits may also influence Hg dynamics in elasmobranchs (e.g. Le [START_REF] Bourg | Mother-embryo isotope (δ 15 N, δ 13 C) fractionation and mercury (Hg) transfer in aplacental deep-sea sharks[END_REF][START_REF] Lyons | Mechanisms of maternal transfer of organochlorine contaminants and mercury in the common thresher shark (Alopias vulpinus)[END_REF] the levels of species [START_REF] Cresson | Mercury in organisms from the Northwestern Mediterranean slope: importance of food sources[END_REF][START_REF] Pethybridge | Trophic structure and biomagnification of mercury in an assemblage of deepwater chondrichthyans from southeastern Australia[END_REF], taxonomic groups [START_REF] Mckinney | Global versus local causes and health implications of high mercury concentrations in sharks from the east coast of South Africa[END_REF][START_REF] Pethybridge | Trophic structure and biomagnification of mercury in an assemblage of deepwater chondrichthyans from southeastern Australia[END_REF]) and communities [START_REF] Atwell | Biomagnification and bioaccumulation of mercury in an arctic marine food web: insights from stable nitrogen isotope analysis[END_REF]Campbell et al. 2008;[START_REF] Lavoie | Trophic structure and mercury distribution in a Gulf of St. Lawrence (Canada) food web using stable isotope analysis[END_REF][START_REF] Power | Mercury accumulation in the fish community of a sub-Arctic lake in relation to trophic position and carbon sources[END_REF] show that feeding ecology of organisms dramatically influences Hg concentrations. Spatial variations of Hg concentrations in organisms [START_REF] Hisamichi | Levels of mercury and organochlorine compounds and stable isotope ratios in three tuna species taken from different regions of Japan[END_REF][START_REF] Jones | Spatial variability in selenium and mercury interactions in a key recreational fish species: implications for human health and environmental monitoring[END_REF][START_REF] Kojadinovic | Mercury content in commercial pelagic fish and its risk assessment in the Western Indian Ocean[END_REF][START_REF] Sackett | The influence of fish length on tissue mercury dynamics: implications for natural resource management and human health risk[END_REF] or correlations between depth of occurrence appears to be particularly important in driving Hg concentrations [START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF][START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (north-east Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF][START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF][START_REF] Monteiro | Mercury concentrations in prey fish indicate enhanced bioaccumulation in mesopelagic environments[END_REF], but the relative importance of environmental drivers on Hg concentrations in marine predators is still relatively poorly understood.

Furthermore, Hg concentrations are poorly documented for a number of ocean regions such as in the western Indian Ocean (but see [START_REF] Chouvelon | Chemical contaminants (trace metals, persistent organic pollutants) in albacore tuna from western Indian and south-eastern Atlantic Oceans: trophic influence and potential as tracers of populations[END_REF][START_REF] Mckinney | Global versus local causes and health implications of high mercury concentrations in sharks from the east coast of South Africa[END_REF][START_REF] Kojadinovic | Mercury content in commercial pelagic fish and its risk assessment in the Western Indian Ocean[END_REF].

The aim of this study was to investigate the effects of body size, trophic position, and broad habitat characteristics on Hg concentrations in sharks and other elasmobranchs of the south-western Indian Ocean (coastal, open ocean and bathyal). We used stable isotope ratios of carbon ( 13 C: 12 C, denoted δ 13 C) and nitrogen ( 15 N: 14 N, denoted δ 15 N) to respectively assess the effect of foraging habitat and trophic positions on observed Hg concentrations [START_REF] Hobson | Tracing origins and migration of wildlife using stable isotopes: a review[END_REF][START_REF] Michener | Stable isotopes ratios as tracers in marine food webs: an update[END_REF].

Increasing Hg concentrations with body length because of bioaccumulation and with δ 15 N because of bioamplification are expected. Furthermore, high Hg concentrations are expected in sharks from the deep sea because of the enhanced methylation processes in the deep-sea waters [START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF].

Material and methods

Sampling and samples preparation

White muscle samples were collected from the dorsal region of 14 shark and 2 batoid species in the south-western Indian Ocean from three sampling locations: southwestern Madagascar, La Réunion Island and the oceanic waters of the southwestern Indian Ocean, from southern Madagascar to the Mascarene archipelago (between 43° 30′ E and 70° 00′ E and 16°30′ S and 28° 30′S).

Elasmobranchs sampled in Madagascar were caught between April 2009 and May 2010 by traditional fishers (non-motorised boats) with gillnets and longlines and brought back to five landing sites where local data collectors took elasmobranch muscle samples for stable isotope and Hg analysis. Details on sampling in Madagascar are provided in Kiszka et al. (2014).

Elasmobranchs of the oceanic waters of the region were sampled during scientific longline fishing cruises or by observers on commercial longline fishing vessels from January 2009 to November 2010. Details on sampling locations in the open ocean are available in [START_REF] Kiszka | Plasticity of trophic interactions among sharks from the oceanic south-western Indian Ocean revealed by stable isotope and mercury analyses[END_REF]. Bathyal sharks and bull sharks (Carcharhinus leucas) of La Réunion were caught in November 2011 and between June and July 2013, respectively, using longlines and drumlines. For all individuals, species, sex and basic morphometric measurements, including total and fork lengths were recorded and muscle samples were collected and frozen at -20°C until further processing. A total of 339 samples were collected. Each species was assigned to a preferred habitat (coastal, open ocean and bathyal) according to ecological characteristics indicated in the literature and the location where it was captured (Table A.1). Most scalloped hammerheads (Sphyrna lewini) were juveniles and were caught in the coastal waters of Madagascar [START_REF] Humber | Assessing the small-scale shark fishery of Madagascar through community-based monitoring and knowledge[END_REF]. Therefore, although adult scalloped hammerhead sharks tend to be oceanic, those in this study were considered coastal (Compagno et al. 1984b;[START_REF] Hussey | Stable isotope profiles of large marine predators: viable indicators of trophic position, diet, and movement in sharks? Can[END_REF]Kiszka et al. 2014). In the laboratory, samples were freeze-dried and then grounded into a homogenous powder using a porcelain mortar and a pestle previously decontaminated in an acid bath.

Stable isotope analyses

Subsamples of homogenised elasmobranch muscles were repeatedly (2 to 3 treatments) shaken for one hour in 4 ml of cyclohexane in order to remove lipids, which can affect δ 13 C values, and trimethyl-amine oxide (TMAO), which can affect δ 15 N values [START_REF] Hussey | Stable isotopes and elasmobranchs: tissue types, methods, applications and assumptions[END_REF]. They were then centrifuged for 5 min at 4000 g, and the supernatant containing lipids was discarded. Lipid-free subsamples (0.35-0.45 mg) were dried and then then weighted in tin cups and analysed with a continuous flow isotope-ratio mass spectrometer (Delta V Advantage, Thermo Scientific) coupled to an elemental analyser (Flash EA 1112, Thermo Scientific). Reference gas was calibrated against International Reference Materials (IAEA-N1, IAEA-N2 and IAEA-N3 for nitrogen; NBS-21, USGS-24 and IAEA-C6 for carbon). Results are expressed in ‰ in the δ notation relative to PeeDee Belemnite and atmospheric N2 for δ 13 C and δ 15 N, according to the equation: δX = [(Rsample / Rstandard) -1] × 10 3 , where X is 13 C or 15 N and R is the isotope ratio 13 C: 12 C or 15 N: 14 N. Percent C and N elemental composition of tissues were obtained using the elemental analyser and the calculated C:N ratios were always below 3.5, indicating good lipid removal efficiency.

Trophic position (TP) was calculated for elasmobranchs (except the spinetail devil ray Mobula japanica) according to Hussey et al. (2014a;2014b) and the baseline values used in [START_REF] Kiszka | Plasticity of trophic interactions among sharks from the oceanic south-western Indian Ocean revealed by stable isotope and mercury analyses[END_REF]: k is the averaged rate at which the ratio between 15 N incorporation and 15 N elimination changes relative to dietary δ 15 N averaged across the food web (0.14).

Hg analyses

Total Hg measurements were calculated using a solid sample atomic absorption spectrometer AMA-254 (Advanced Mercury Analyser-254; Altec®), except for the bull shark samples from La Réunion Island. At least two aliquots of 5-15 mg of homogenised dry muscle subsamples for each individual were analysed. The analytical quality (i.e. accuracy and reproducibility) of the Hg measurements by the AMA-254 was assessed by the analyses of blanks and certified reference material (CRM)

TORT-2 (Lobster Hepatopancreas from the National Research Council of Canada; www.nrccnrc.gc.ca; certified Hg concentration: 0.27 ± 0.06 μg.g -1 dw) at the beginning and at the end of the analytical cycle, and by running controls for every 10 samples [START_REF] Bustamante | Total and organic Hg concentrations in cephalopods from the North East Atlantic waters: influence of geographical origin and feeding ecology[END_REF]. Measured values were 0.26 ± 0.02 µg.g -1 dry weight (dw), n = 20. Mass of the CRM was adjusted to represent an amount of Hg similar to that in muscle samples. Blanks were analysed at the beginning of each set of samples and the limit of detection was 0.005 μg.g -1 dw. For bull sharks, Hg concentrations were measured from proton-induced X-ray emission (PIXE) analyses at GNS Sciences, New

Zealand. Hg concentrations in tissues reported are expressed in μg.g -1 dw but can be converted to wet weight by using a factor of 5 corresponding to an average 80% moisture [START_REF] Cresson | Mercury in organisms from the Northwestern Mediterranean slope: importance of food sources[END_REF] to compare our results with previously published ones.

Statistical analysis

Hg concentrations were log(x+1)-transformed (indicated as log-transformed in the following) to obtain a normal distribution of the data. As the maximum length differs between shark species and habitat, fork length values were standardised between 0 (juveniles) and 1 (adults) by dividing them by a theoretical maximum fork length for each species. The theoretical maximum fork length FLmax was estimated by using the following fork length-total length relationship:

FL max =a×TL max + b
with TLmax being the maximum total length reported for each species as the total length value and a and b being species specific constants [START_REF] Binohlan | The LENGTH-LENGTH table in FishBase[END_REF][START_REF] Kohler | Length-length and length-weight relationships for 13 shark species from the Western North Atlantic[END_REF]table A.1). In several species, the standardised length of the largest individuals may be higher than 1 if using the highest total length reported. Consequently, the standardisation was done by dividing fork length values by the maximum fork length value we measured for these species. Furthermore, as both species of spurdog (Squalus spp.) were not precisely identified, the standardisation of fork length for these sharks was done by dividing fork length values by a same maximum fork length value we got for both species combined (Table A .1).

Relationships between the continuous variables (log-transformed Hg concentrations, standardised fork length, δ 13 C and δ 15 N values) were preliminary investigated with a principal component analysis (PCA). Because the sampling was concentrated on restricted areas for coastal and bathyal elasmobranchs, latitude and longitude were not included in the PCA. However, because open ocean elasmobranchs have a broad distribution, the relationship between log-transformed Hg concentrations and longitude of sampling was investigated for this group. A general linear model (GLM) followed by type III analysis of covariance (ANCOVA, F) was performed to assess the influence of standardised fork length, δ 15 N values and the habitat (either coastal/inshore, open ocean and bathyal) on log-transformed Hg concentrations. Second order interactions and non-significant first order interactions were progressively removed from the model and a post-hoc Scheffe analysis was performed on the habitat factor. Species with low sample size (n < 5) were not included in the model. For each habitat, the influence of standardised fork length, δ 15 N values and species on logtransformed Hg concentrations were then assessed with GLM followed by type III ANCOVAs. Sex was not included in the models because no significant effects were observed in a first preliminary ANCOVA from which were removed five unsexed individuals and the pelagic thresher shark

Alopias pelagicus for which only males were sampled. Pearson correlation coefficients between log-transformed Hg concentrations and standardised fork length were computed for each species, excepting those with n < 10 individuals. The same analysis was done to study the relationship of log-transformed Hg concentrations with δ 15 N and δ 13 C values. All analyses were performed using R 3.3.3 (R development Core Team).

Results

Factors influencing Hg concentration differences between habitats

The PCA performed on Hg concentration, standardised size of the individuals, δ 15 N and δ 13 C values explained almost 70% of the variance of the data set with the two first axes. Results of the PCA showed that Hg concentrations were positively and strongly correlated to δ 15 N (proxy of trophic level) and negatively correlated to δ 13 C values (Fig. 1a). This suggested a potential effect of habitat on Hg concentrations, with habitats segregating along the first axis (Fig. 1b andA.1). Oceanic individuals had higher Hg concentrations and higher relative δ 15 N values. Coastal and bathyal elasmobranchs had lower Hg concentrations and lower δ 15 N values (Table 1).

No relationship between Hg concentrations and longitude was observed for oceanic sharks (r = - The GLM and subsequent ANCOVA results (Table 2) supported PCA results, with Hg concentrations increasing with δ 15 N values and varying with habitat. Open ocean elasmobranchs had the highest Hg concentrations, closely followed by bathyal species. In contrast, coastal elasmobranchs had the lowest Hg concentrations recorded (Fig. 2a; Table 1). Influence of standardised fork length, as well as its interaction with the habitat, on Hg concentrations were also observed. The interaction between habitat and fork length showed that an increase of Hg concentrations with standardised size occurred in open ocean and bathyal sharks but that Hg concentrations decreased with standardised size in coastal species (Fig. 3 andA

Factors influencing Hg concentrations within habitats

Within each habitat, Hg concentrations were influenced by standardised size thanks to the significant correlations between the standardised size and Hg concentrations within all species (tables 2 and 3). Variation in Hg concentrations amongst species occurred in the coastal habitat, with the sliteye shark (L. macrorhinus) having lower Hg concentrations (1.25 ± 0.96 μg.g -1 dw) than the other species, and in the open ocean habitat, with the crocodile shark (Pseudocarcharias kamoharai) having much higher Hg concentrations (17.25 ± 6.45 μg.g -1 dw) than the other species, but not in the bathyal habitat (Fig. 2b; Tables 2 and3). The interaction between species and standardised size also influenced Hg concentrations in the coastal habitat thanks to the negative correlation between the standardised size and Hg concentrations observed in the giant guitarfish djiddensis; Table 3) may explain this pattern.

Lastly, a significant positive correlation between Hg concentrations and δ 13 C values appeared only in the pelagic silky sharks (Carcharhinus falciformis) and blue sharks (Prionace glauca) and in the bathyal Centrophorus moluccensis and Squalus sp. 1 (Table 3), maybe as the result of ontogenetic change in foraging habitat in these species.

In a few cases, the correlations in Table 3 were not highly significant (or even not significant, if a

Bonferroni correction was applied), but this does not modify these conclusions.

Discussion

In this study, the influence of various factors on Hg concentrations in elasmobranchs from Indian Ocean was assessed. Foraging habitat, body length and trophic level appeared to contribute significantly to the bioaccumulation of Hg in elasmobranchs.

With the exception of the crocodile shark (P. [START_REF] Chiffoleau | Evaluation des valeurs de bruits de fond dans l'eau de mer des 4 métaux de l'état chimique DCE[END_REF]. The geographical origin of these animals might therefore explain this pattern and this is supported by Hg analyses in coastal waters of La Réunion that reveals unexplained high Hg concentrations [START_REF] Chiffoleau | Evaluation des valeurs de bruits de fond dans l'eau de mer des 4 métaux de l'état chimique DCE[END_REF]). Yet, coastal bull sharks (C.

leucas), which were sampled in La Réunion too, did not exhibit high Hg concentrations compared to both oceanic and bathyal species. Furthermore, the absence of relationship between Hg concentration and longitude in elasmobranchs from the open ocean also indicates that geographical origin is not a primary driver of mercury accumulation in elasmobranchs of the Indian Ocean.

Consumption of prey in a restricted latitude range and high mobility may explain the absence of link between Hg concentration and geographical origin for oceanic elasmobranchs [START_REF] Bird | A global perspective on the trophic geography of sharks[END_REF]. Hg concentrations in bathyal sharks could also be explained by the Hg distribution in the water column. Increasing concentrations of Hg with depth were observed in the water column of various regions [START_REF] Hammerschmidt | Vertical methylmercury distribution in the subtropical North Pacific Ocean[END_REF][START_REF] Horvat | Speciation of mercury in surface and deep-sea waters in the Mediterranean Sea[END_REF][START_REF] Kim | Gaseous mercury profiles in the tropical Pacific Ocean[END_REF], including in the southwestern Indian Ocean [START_REF] Sunderland | Distributions of mercury and methylmercury in the Indian Ocean[END_REF]. In particular, a maximum level of highly bioavailable CH3Hg may be present in the mesopelagic environment (ca 200-1000 m; [START_REF] Hammerschmidt | Vertical methylmercury distribution in the subtropical North Pacific Ocean[END_REF][START_REF] Horvat | Speciation of mercury in surface and deep-sea waters in the Mediterranean Sea[END_REF]. This process is due to the higher net CH3Hg production below the mixed layer by microorganisms' activity than at the surface where a large portion of methylmercury is photodegraded [START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF]. Consequently, considering that increasing levels of CH3Hg with depth were observed in southwestern Indian Ocean [START_REF] Sunderland | Distributions of mercury and methylmercury in the Indian Ocean[END_REF], that bathyal sharks rely on organic matter produced at the surface which is degraded and methylated while sinking [START_REF] Bird | A global perspective on the trophic geography of sharks[END_REF][START_REF] Blum | Methylmercury production below the mixed layer in the North Pacific Ocean[END_REF] and that most of the Hg present in muscles of sharks is CH3Hg [START_REF] Pethybridge | Mercury in 16 demersal sharks from southeast Australia: biotic and abiotic sources of variation and consumer health implications[END_REF][START_REF] Storelli | Polychlorinated biphenyl, heavy metal and methylmercury residues in hammerhead sharks: contaminant status and assessment[END_REF], similar Hg levels in open ocean and bathyal elasmobranchs despite differences of trophic positions are likely the result of higher bioavailability of CH3Hg in deep-sea waters as highlighted by higher proportions of CH3Hg relative to total Hg in deep-sea sharks [START_REF] Pethybridge | Mercury in 16 demersal sharks from southeast Australia: biotic and abiotic sources of variation and consumer health implications[END_REF]. Such a situation could also explain why bull sharks in coastal waters of La Réunion Island showed low Hg concentrations despite surface waters having high Hg concentrations [START_REF] Chiffoleau | Evaluation des valeurs de bruits de fond dans l'eau de mer des 4 métaux de l'état chimique DCE[END_REF], as the photodegradation of CH3Hg would make Hg less bioavailable.

Although sample size is limited, P. kamoharai appeared to have very high Hg concentrations and δ 15 N values despite the small body size of this species. The maximum concentration of CH3Hg in the mesopelagic environment may explain the high Hg levels observed in P. kamoharai, which have a mesopelagic distribution (Compagno 1984a;[START_REF] Last | Sharks and rays of Australia[END_REF], and high Hg concentrations previously reported in organisms foraging in the mesopelagic environment [START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF][START_REF] Kojadinovic | Bioaccumulation of trace elements in pelagic fish from the Western Indian Ocean[END_REF][START_REF] Monteiro | Mercury concentrations in prey fish indicate enhanced bioaccumulation in mesopelagic environments[END_REF]. Another hypothesis to explain this finding would be the existence of a longer food chain at smaller body sizes in the mesopelagic environment, as indicated by the high δ 15 N values and estimated trophic position for this species and the higher rates of carnivory for mesopelagic zooplankton [START_REF] Hannides | Midwater zooplankton and suspended particle dynamics in the North Pacific Subtropical Gyre: A stable isotope perspective[END_REF]. A third hypothesis would be that the high δ 15 N values in P. kamoharai would be the result of higher δ 15 N values in lower trophic positions and baseline sources of the mesopelagic food web where this species forages, as highlighted by increasing δ 15 N values in suspended particles and zooplankton along depth [START_REF] Hannides | Midwater zooplankton and suspended particle dynamics in the North Pacific Subtropical Gyre: A stable isotope perspective[END_REF].

Hg concentrations consistently increased with fork length at the scales of the whole sampling area, of the habitats and of the species. This pattern has previously been documented in several species of sharks in different marine environments [START_REF] Cresson | Mercury in organisms from the Northwestern Mediterranean slope: importance of food sources[END_REF][START_REF] Hueter | Methylmercury concentration in shark muscle by species, size and distribution of sharks in Florida coastal waters[END_REF][START_REF] Pethybridge | Mercury in 16 demersal sharks from southeast Australia: biotic and abiotic sources of variation and consumer health implications[END_REF]2012;[START_REF] Rumbold | Mercury accumulation in sharks from the coastal waters of southwest Florida[END_REF]. Body length may be used as a proxy of age in elasmobranchs (e.g. [START_REF] Cotton | Age, growth and reproduction of a common deep-water shark, shortspine spurdog (Squalus cf. mitsukurii), from Hawaiian waters[END_REF][START_REF] Natanson | Validated age and growth estimates for the shortfin mako, Isurus oxyrinchus, in the North Atlantic Ocean[END_REF][START_REF] Pajuelo | Population biology of the shark, Squalus megalops, harvested in the central-east Atlantic Ocean[END_REF][START_REF] Piercy | Age and growth of the scalloped hammerhead shark, Sphyrna lewini, in the north-west Atlantic Ocean and Gulf of Mexico[END_REF][START_REF] Skomal | Age and growth of the blue shark (Prionace glauca) in the North Atlantic Ocean[END_REF]. Consequently, we may consider that increasing Hg concentrations with length results from ageing: because Hg is more efficiently assimilated than eliminated, it accumulates in the body with age. However, age alone may not explain this phenomenon, because the trophic level of organisms tends to increase with body length [START_REF] Chouvelon | Speciesand size-related patterns in stable isotopes and mercury concentrations in fish help refine marine ecosystem indicators and provide evidence for distinct management units for hake in the Northeast Atlantic[END_REF][START_REF] Hussey | Stable isotope profiles of large marine predators: viable indicators of trophic position, diet, and movement in sharks? Can[END_REF]. Our results actually indicate that Hg concentrations and δ 15 N values, and thus trophic level, are generally linked at the scale of the whole sampling area and within species. However, the link between Hg concentrations and δ 15 N values appeared only in the bathyal habitat because there are still some species for which no relationship between Hg concentrations and δ 15 N values occurred in the two other habitats. Nevertheless, large-scale variations of trophic level may explain regional differences of Hg concentrations. Indeed, higher Hg concentrations in South Africans C. leucas and S. lewini may be linked to higher trophic levels for these species in this region than in our study area [START_REF] Mckinney | Global versus local causes and health implications of high mercury concentrations in sharks from the east coast of South Africa[END_REF]. In contrast, and in accordance with previous studies (e.g. [START_REF] Domi | Feeding ecology of five commercial shark species of the Celtic Sea through stable isotope and trace metal analysis[END_REF][START_REF] Pethybridge | Trophic structure and biomagnification of mercury in an assemblage of deepwater chondrichthyans from southeastern Australia[END_REF][START_REF] Rumbold | Mercury accumulation in sharks from the coastal waters of southwest Florida[END_REF], positive correlation between Hg concentrations and Many shark species are currently threatened by human activities [START_REF] Clarke | Global estimates of shark catches using trade records from commercial markets[END_REF][START_REF] Dulvy | Extinction risk and conservation of the world's sharks and rays[END_REF]) and the Hg accumulation in their bodies could have additional detrimental effects on the dynamics of their populations [START_REF] Depew | Toxicity of dietary methylmercury to fish: derivation of ecologically meaningful threshold concentrations[END_REF][START_REF] Sandheinrich | Methylmercury in freshwater fish-Recent advances in assessing toxicity of environmentally relevant exposures[END_REF]. The results of the present study suggest that bathyal and oceanic sharks should be considered more carefully for human consumption. It is now well-established that the toxicity of Hg in teleost fish decreased when selenium (Se) concentrations are elevated [START_REF] Kaneko | Selenium and mercury in pelagic fish in the Central North Pacific near Hawaii[END_REF][START_REF] Ralston | Dietary and tissue selenium in relation to methylmercury toxicity[END_REF][START_REF] Ralston | Dietary selenium's protective effects against methylmercury toxicity[END_REF]. However, the role of Se in commercial sharks could be minor, because its concentrations are usually not linked with Hg concentration, nor exceed it, particularly in oceanic blue and mako sharks [START_REF] Domi | Feeding ecology of five commercial shark species of the Celtic Sea through stable isotope and trace metal analysis[END_REF][START_REF] Escobar-Sánchez | Biomagnification of mercury and selenium in blue shark Prionace glauca from the Pacific Ocean off Mexico[END_REF][START_REF] Kaneko | Selenium and mercury in pelagic fish in the Central North Pacific near Hawaii[END_REF][START_REF] Florez-Martínez | Diet comparison between silky sharks (Carcharhinus falciformis) and scalloped hammerhead sharks (Sphyrna lewini) off the south-west coast of Mexico[END_REF]).

  TP= log(δ 15 N lim -δ 15 N base )-log(δ 15 N lim -δ 15 N TP ) k + TP base δ 15 Nbase and TPbase are the known δ 15 N value and TP of a consumer (9.7 and 3, respectively). Here, the values are based on four individuals of the species Mobula mobular (planktivorous) whose mean ±SD δ 15 N values in white muscle were 9.7 ±0.1 ‰. δ 15 NTP is the δ 15 N value of the consumer of interest. δ 15 Nlim is the δ 15 N value at which 15 N incorporation and 15 N elimination are equal (21.9).

Fig. 1 .

 1 Fig. 1. Results of PCA: a) correlations between log-transformed Hg concentrations (µg.g -1 dw), standardised fork length, δ 15 N and δ 13 C values (‰). b) plot of the individuals resulting from the PCA with symbols indicating the habitat: black squares: coastal, white points: open ocean, grey

(Fig. 2 .

 2 Fig. 2. Hg concentrations (µg.g -1 dw, mean + SD) in muscle of elasmobranchs by a) preferred habitat and b) by species (Cleu: Carcharhinus leucas, Lm: Loxodon macrorhinus, Rd: Rhynchobatus djiddensis, Sl: Sphyrna lewini, Ap: Alopias pelagicus, Cf: Carcharhinus falciformis, Clon: Carcharhinus longimanus, Io: Isurus oxyrinchus, Mj: Mobula japanica, Pg: Prionace glauca, Pk: Pseudocarcharias kamohrai, Cm: Centrophorus moluccensis, Hn: Hexanchus nakamurai, Ssp1: Squalus sp. 1, Ssp2: Squalus sp. 2, Zs: Zameus squamulosus). White: coastal, grey: open ocean, black: bathyal. P values are the results of the post-hoc Scheffe analysis on the habitat factor.

Fig. 3 .

 3 Fig. 3. Relationship between the standardised fork length and Hg concentrations (log(x+1)-scale, µg.g -1 dw) in muscle of elasmobranchs with symbols and lines indicating the habitat: black squares and black line: coastal; white points and dashed line: open ocean; grey triangles and grey line: bathyal. For a more detailed plot, see Fig. A.3.

(

  Rhynchobatus djiddensis). Hg concentrations increased with δ 15 N values in bathyal sharks only but the interaction between the species and δ 15 N values influenced Hg concentrations in coastal and open ocean elasmobranchs (Table 2). The significant and positive correlation between δ 15 N values and Hg concentrations within all bathyal species and the lack of correlation for several coastal and open ocean species (Carcharhinus leucas, Carcharhinus longimanus, Isurus oxyrinchus and R.

  δ 13 C values are rarely found. This lack of relationship may suggest that all the sampled individuals in each species forage inside a single preferred habitat with no ontogenetic changes of feeding habitat or individual foraging specialisation or segregation (e.g. size or sexual segregation). By comparison, a relationship between Hg concentrations and δ 13 C values were observed in the wandering albatross Diodema exulans. This relationship results from the foraging segregation between males preying mainly in Antarctic waters and females preying in subtropical areas (Carravieri et al. 2014). In conclusion, Hg concentrations in elasmobranchs living in three different types of habitat of the southwestern Indian Ocean appear to be linked to three factors: 1) trophic position, as shown by the relationship between Hg concentration and δ 15 N values at the whole sample level, and by top predator sharks from the open ocean usually having higher Hg levels than the mesopredatory coastal elasmobranchs, 2) habitat, as shown by mesopredatory bathyal species having almost similar Hg concentrations as the top predator species of the open ocean despite their lower trophic position, 3) body size, as Hg concentrations increased with body length.

Table 1 .

 1 Length ranges, mean ± SD stable isotope values, Hg concentrations and trophic positions of elasmobranchs sampled in south-western Indian Ocean.

		Abbreviation n	Fork length range (cm)	δ 15 N (‰)	δ 13 C (‰)	Hg (µg.g -1 dw)	TP
	Coastal		157	58.0-325.0	12.0 ± 0.9 -15.5 ± 1.0	1.95 ± 1.58	3.68 ± 0.31
	Carcharhinus leucas	Cleu	11	214.0-325.0	12.6 ± 0.6 -15.5 ± 0.7	2.18 ± 1.46	3.86 ± 0.21
	Loxodon macrorhinus	Lm	83	62.0-100.0	11.7 ± 0.6 -15.4 ± 1.0	1.25 ± 0.96	3.55 ± 0.18
	Rhynchobatus djiddensis	Rd	19	65.0-190.0	11.9 ± 0.4 -14.7 ± 0.4	3.07 ± 1.20	3.63 ± 0.13
	Sphyrna lewini	Sl	44	58.0-190.0	12.7 ± 1.2 -15.9 ± 1.1	2.74 ± 2.01	3.90 ± 0.43
	Open ocean		94	73.0-304.0	13.8 ± 1.2 -16.8 ± 1.0	6.49 ± 5.15	4.30 ± 0.44
	Alopias pelagicus	Ap	5	147.0-268.0	13.9 ± 0.7 -16.7 ± 0.4	4.97 ± 3.57	4.32 ± 0.26
	Carcharhinus falciformis	Cf	10	73.0-260.0	12.7 ± 1.2 -15.0 ± 1.4	7.13 ± 7.20	3.91 ± 0.42
	Carcharhinus longimanus	Clon	13	99.0-229.0	13.4 ± 1.1 -16.5 ± 0.5	7.41 ± 7.91	4.14 ± 0.38
	Isurus oxyrinchus	Io	29	122.0-304.0	14.6 ± 0.7 -16.7 ± 0.6	5.96 ± 2.78	4.62 ± 0.29
	Mobula sp.	Mj	1	221	9,7	-17,4	0,14	NA
	Prionace glauca	Pg	31	160.0-269.0	13.6 ± 1.1 -17.5 ± 0.5	5.11 ± 1.88	4.21 ± 0.42
	Pseudocarcharias kamoharai	Pk	5	82.0-95.0	14.3 ± 0.6 -16.6 ± 0.1	17.25 ± 6.45	4.49 ± 0.24
	Bathyal		88	26.0-120.0	12.3 ± 0.6 -16.9 ± 0.2	5.59 ± 3.76	3.74 ± 0.20
	Centrophorus moluccensis	Cm	16	29.0-77.5	12.0 ± 0.5	-16.7 ± 0.4	3.35 ± 2.18	3.65 ± 0.15
	Hexanchus nakamurai	Hn	1	120	12,8	-16,1	5,22	3,92
	Squalus sp. 1	Ssp 1	57	26.0-69.5	12.3 ± 0.6 -16.9 ± 0.1	6.25 ± 3.92	3.74 ± 0.17
	Squalus sp. 2	Ssp 2	12	31.0-64.0	12.2 ± 0.6 -16.9 ± 0.2	5.75 ± 4.02	3.71 ± 0.20
	Zameus squamulosus	Zs	2	25.7-29.8	14.3 ± 0.4 -16.8 ± 0.2	4.06 ± 3.53	4.46 ± 0.18

Table 2 .

 2 Effects of standardised fork length, δ 15 N values, habitat and species of elasmobranch on log-transformed Hg concentrations. Bold results are significant at the P = 0.05 level.

	df	F	P

Table 3 .

 3 Results of Pearson correlation tests between log-transformed Hg concentrations in

	elasmobranch muscles and standardised fork length, δ 15 N values and δ 13 C values. Bold results are
	significant at the P = 0.05 level. Italic results are no more significant if applying Bonferroni
	correction, when the 0.05 P-level is divided by the number of tests (33 tests and P = 0.0015, Rice
	1989).
			Standardised fork length		δ 15 N			δ 13 C
		df	r	P	df	r	P	df	r	P
	Coastal
	Carcharhinus leucas (n = 11)	9 0.612 0.045	9 0.010 0.976	9 -0.562 0.072
	Loxodon macrorhinus (n = 83)	81 0.379 < 0.001	81 0.517 < 0.001	81 0.059 0.598
	Rhynchobatus djiddensis (n = 19)	17 -0.758 < 0.001	17 0.313 0.192	17 -0.096 0.696
	Sphyrna lewini (n = 44)	42 0.625 < 0.001	42 0.495 < 0.001	42 -0.011 0.941
	Open ocean
	Carcharhinus falciformis (n = 10)	8 0.662 0.037	8 0.669 0.034	8 0.692 0.027
	Carcharhinus longimanus (n = 13)	11 0.756 0.003	11 0.453 0.120	11 -0.128 0.677
	Isurus oxyrinchus (n = 29)	27 0.630 < 0.001	27 0.064 0.742	27 0.072 0.709
	Prionace glauca (n = 31)	29 0.675 < 0.001	29 0.638 < 0.001	29 0.697 < 0.001
	Bathyal
	Centrophorus moluccensis (n = 16)	14 0.941 < 0.001	14 0.642 0.007	14 0.762 < 0.001
	Squalus sp. 1 (n = 57)	55 0.723 < 0.001	55 0.667 < 0.001	55 0.641 < 0.001
	Squalus sp. 2 (n = 12)	10 0.808 0.001	10 0.881 < 0.001	10 0.157 0.626

  kamoharai), mean Hg concentrations reported in elasmobranch muscles in this study were lower than those reported for sharks sampled off eastern South Africa[START_REF] Mckinney | Global versus local causes and health implications of high mercury concentrations in sharks from the east coast of South Africa[END_REF]. By contrast, lower Hg concentrations were reported in western South Africa than in the waters between La Réunion and Madagascar in albacore tuna Biton[START_REF] Biton Porsmoguer | Mercury in blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus) from north-eastern Atlantic: implication for fishery management[END_REF]. Similarly, higher Hg concentrations were reported in our study than those in Mexican Pacific waters[START_REF] Escobar-Sánchez | Biomagnification of mercury and selenium in blue shark Prionace glauca from the Pacific Ocean off Mexico[END_REF][START_REF] Maz-Courrau | Bioaccumulation and biomagnification of total mercury in four exploited shark species in the Baja California Peninsula, Mexico[END_REF] and in the Atlantic Ocean[START_REF] Biton Porsmoguer | Mercury in blue shark (Prionace glauca) and shortfin mako (Isurus oxyrinchus) from north-eastern Atlantic: implication for fishery management[END_REF] for blue sharks (P. glauca).

	Thunnus For bathyal
	sharks, Hg concentrations reported in spurdogs (Squalus spp.) were similar to those reported in
	Australia (Pethybridge et al. 2010), but lower than those reported in Brazilian waters (de Pinho et
	al. 2002). species were sampled near La Réunion, a remote island with active volcanism likely to enrich
	surrounding waters in Hg
	In our study, sharks from bathyal and open ocean habitats appear to have almost similar Hg
	concentrations while coastal elasmobranchs had the lowest Hg concentrations. Influence of habitat

alalunga

[START_REF] Chouvelon | Chemical contaminants (trace metals, persistent organic pollutants) in albacore tuna from western Indian and south-eastern Atlantic Oceans: trophic influence and potential as tracers of populations[END_REF]

. Thus, local factors may explain variation in Hg concentrations between regions. For coastal sharks, Hg concentrations reported in bull sharks (C. leucas) were two to four times lower than those reported near South Africa

[START_REF] Mckinney | Global versus local causes and health implications of high mercury concentrations in sharks from the east coast of South Africa[END_REF]

and off Florida

(Adams and McMichael 1999;[START_REF] Hueter | Methylmercury concentration in shark muscle by species, size and distribution of sharks in Florida coastal waters[END_REF][START_REF] Rumbold | Mercury accumulation in sharks from the coastal waters of southwest Florida[END_REF]

. For open ocean sharks, Hg concentrations reported in mako sharks (I. Oxyrinchus) were three times lower than those reported near South Africa

[START_REF] Mckinney | Global versus local causes and health implications of high mercury concentrations in sharks from the east coast of South Africa[END_REF]

) but higher than those reported in Mexican Pacific waters

[START_REF] Escobar-Sánchez | Biomagnification of mercury and selenium in blue shark Prionace glauca from the Pacific Ocean off Mexico[END_REF][START_REF] Maz-Courrau | Bioaccumulation and biomagnification of total mercury in four exploited shark species in the Baja California Peninsula, Mexico[END_REF]

) and in the Atlantic Ocean (

Acknowledgements

Our particular acknowledgements are due to sample collectors during longline fishing cruises and to IFREMER (IOSSS-Espadon project) and community data collectors and fishers (Blue Ventures) for providing shark samples. We are also very grateful to G. Guillou and P. Richard (University of La Rochelle, lab. LIENSs) for running the mass spectrometer analyses. We are grateful to the CPER (Contrat de Projet État-Région) and the FEDER (European Regional Development Fund). B. Le Bourg is a FNRS-FRIA fellow. The IUF (Institut Universitaire de France) is acknowledged for its support to P. Bustamante as a senior member. Sample collection and laboratory measurement of mercury and stable isotopes in some sharks from La Réunion Island were funded by the CHARC project (EU FEDER/French Government/ La Réunion Island regional council). C. Trystram participated in the production of the data. We thank the anonymous referees for their helpful comments.

Campbell, L., Verburg, P., Dixon, D.G., Hecky, R.E., 2008. Mercury biomagnification in the food web of Lake Tanganyika (Tanzania, East Africa). Sci. Total Env. 402,[184][185][186][187][188][189][190][191] https://doi.org/10. 1016/j.scitotenv.2008.04.017. Carravieri, A., Bustamante, P., Tartu, S., Meillère, A., Labadie, P., Budzinski, H., Peluhet, L., Barbraud, C., Weimerskirch, H., Chastel, O., Cherel, Y., 2014. Wandering albatrosses -24document latitudinal variations in the transfer of persistent organic pollutants and mercury to Southern Ocean predators. Environ. Sci. Technol. 48, 14746-14755.

The sample analysis of this work was supported by the French grant Contrat de Projet Etat-Région.

on Hg concentrations was reported in sharks [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (north-east Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF], teleosts [START_REF] Monteiro | Mercury concentrations in prey fish indicate enhanced bioaccumulation in mesopelagic environments[END_REF][START_REF] Chouvelon | Oligotrophy as a major driver of mercury bioaccumulation in medium-to high-trophic level consumers: a marine ecosystemcomparative study[END_REF][START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF], cephalopods [START_REF] Bustamante | Total and organic Hg concentrations in cephalopods from the North East Atlantic waters: influence of geographical origin and feeding ecology[END_REF][START_REF] Chouvelon | Interspecific and ontogenic differences in δ 13 C and δ 15 N values and Hg and Cd concentrations of cephalopods[END_REF], crustaceans [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (north-east Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF] and seabirds [START_REF] Monteiro | Mercury concentrations in prey fish indicate enhanced bioaccumulation in mesopelagic environments[END_REF], with higher Hg concentrations reported in organisms with deeper occurrence or foraging.

Differences in Hg concentrations between habitats may be explained by the species composition, size and trophic ecology in each habitat. Indeed, the low Hg levels of coastal elasmobranchs can be explained by the fact that most sampled coastal individuals are small sliteye sharks (L. macrorhinus), which are small mesopredatory sharks [START_REF] Cortés | Standardized diet compositions and trophic levels of sharks[END_REF][START_REF] Jabado | Diet of two commercially important shark species in the United Arab Emirates: milk shark, Rhizoprionodon acutus (Rüppell, 1837), and slit-eye shark, Loxodon macrorhinus (Müller & Henle, 1839)[END_REF], followed by mostly juvenile scalloped hammerhead (S. lewini), which are generalist feeders targeting smaller prey than adults [START_REF] Bush | Diet and diel feeding periodicity of juvenile scalloped hammerhead sharks, Sphyrna lewini, in Kāne'ohe Bay, Ō'ahu, Hawai'i[END_REF][START_REF] Florez-Martínez | Diet comparison between silky sharks (Carcharhinus falciformis) and scalloped hammerhead sharks (Sphyrna lewini) off the south-west coast of Mexico[END_REF]. Such a diet composition is supported by lower δ 15 N values in most coastal elasmobranchs. Conversely, the open ocean species we sampled included larger apex predators, which had bioaccumulated Hg to high levels and whose high δ 15 N values may reflect a diet composed of large and high trophic level prey [START_REF] Biton Porsmoguer | DNA evidence of the consumption of short-beaked common dolphin Delphinus delphis by the shortfin mako shark Isurus oxyrinchus[END_REF]2017;[START_REF] Camhi | Sharks of the open ocean: biology, fisheries and conservation[END_REF][START_REF] Kiszka | Plasticity of trophic interactions among sharks from the oceanic south-western Indian Ocean revealed by stable isotope and mercury analyses[END_REF]. However, Hg concentrations in bathyal sharks were almost similar to those of oceanic species despite the sampled bathyal shark species being smaller generalist feeders [START_REF] Braccini | Sources of variation in the feeding ecology of the piked spurdog (Squalus megalops): implications for inferring predatoreprey interactions from overall dietary composition[END_REF][START_REF] Ebert | A preliminary investigation of the feeding ecology of squaloid sharks off the west coast of southern Africa[END_REF]. Furthermore, the Hg concentrations in bathyal sharks are higher than in coastal species despite their similar trophic ecology and their smaller size. This is consistent with the observation of increasing Hg concentrations with depth in other taxa and in other locations [START_REF] Chouvelon | Enhanced bioaccumulation of mercury in deep-sea fauna from the Bay of Biscay (north-east Atlantic) in relation to trophic positions identified by analysis of carbon and nitrogen stable isotopes[END_REF][START_REF] Choy | The influence of depth on mercury levels in pelagic fishes and their prey[END_REF][START_REF] Monteiro | Mercury concentrations in prey fish indicate enhanced bioaccumulation in mesopelagic environments[END_REF] and this indicates that other factors than the trophic ecology of the sampled species explain the differences of Hg concentrations in sharks between habitats. All bathyal