Stéphane Hourdez
email: hourdez@sb-roscoff.fr

Cardiac response of the hydrothermal vent crab Segonzacia mesatlantica to variable temperature and oxygen levels

Keywords: Hypoxia, Oxyregulation, Critical temperature, Critical oxygen concentration

Segonzacia mesatlantica inhabits different hydrothermal vent sites of the Mid-Atlantic Ridge where it experiences chronic environmental hypoxia, and highly variable temperatures. Experimental animals in aquaria at in situ pressure were exposed to varying oxygen concentrations and temperature, and their cardiac response was studied. S. mesatlantica is well adapted to these challenging conditions and capable to regulate its oxygen uptake down to very low concentrations (7.3-14.2 µmol.l -1 ). In S. mesatlantica, this capacity most likely relies on an increased ventilation rate, while the heart rate remains stable down to this critical oxygen tension. When not exposed to temperature increase, hypoxia corresponds to metabolic hypoxia and the response likely only involves ventilation modulation, as in shallowwater relatives. For S. mesatlantica however, an environmental temperature increase is usually correlated with more pronounced hypoxia. Although the response to hypoxia is similar at 10 and 20˚C, temperature itself has a strong effect on the heart rate and EKG signal amplitude. As in shallow water species, the heart rate increases with temperature. Our study revealed that the range of thermal tolerance for S. mesatlantica ranges from 6 through 21˚C for specimens from the shallow site Menez Gwen (800 m), and from 3 through 19˚C for specimens from the deeper sites explored (2700 -3000 m).

Introduction

Environmental exposure to hypoxia in aquatic habitats can be common [START_REF] Hourdez | Hypoxic environments[END_REF]. Near hydrothermal vents, oxygen levels are often low and highly variable both in space and in time. These conditions result from the chaotic mixing of the hydrothermal vent fluid, which is hot, anoxic, and often rich in sulfide, with the deepsea water, cold and usually slightly hypoxic. Oxygen and sulfide spontaneously react, decreasing further the amount of available oxygen in the resulting sea water. The presence of reduced compounds in the hydrothermal fluid is paramount to the local primary production by autotrophic bacteria at the base of the food chain in these environments. To reap the benefits of this high local production in an otherwise seemingly barren deep-sea at similar depths, metazoans must possess specific adaptations to deal with the challenging conditions, among which chronic hypoxia is probably one of the most limiting. All metazoans that have been studied to date indeed exhibit oxygen requirements comparable to those of close relatives that live in well-oxygenated environments [START_REF] Childress | Metabolic rates of animals from hydrothermal vents and other deep-sea habitats[END_REF][START_REF] Hourdez | Adaptations to hypoxia in hydrothermal vent and cold-seep invertebrates[END_REF].

A study of morphological adaptations in decapodan crustaceans revealed that, contrary to annelid polychaetes [START_REF] Hourdez | Adaptations to hypoxia in hydrothermal vent and cold-seep invertebrates[END_REF], there is usually no increase in gill surface areas in vent decapods compared to their shallow-water relatives [START_REF] Decelle | Morphological adaptations to chronic hypoxia in deep-sea decapod crustaceans from hydrothermal vents and cold-seeps[END_REF]. In the vent decapods however, the scaphognathite is greatly enlarged, suggesting an increased ventilatory capacity. In situ observations of vent shrimp in settings typified by different oxygen concentrations also indicated that these animals increased their ventilation rates under lower oxygen conditions [START_REF] Decelle | Morphological adaptations to chronic hypoxia in deep-sea decapod crustaceans from hydrothermal vents and cold-seeps[END_REF]. This behavioral change is consistent with other decapods in which both the frequency and amplitude of scaphognathite beating are increased in response to hypoxia (see [START_REF] Mcmahon | Respiratory and circulatory compensation to hypoxia in crustaceans[END_REF][START_REF] Whiteley | Responses to environmental stresses: oxygen, temperature, and pH. Chapter 10. In: The Natural History of Crustaceans[END_REF][START_REF] Whiteley | Responses to environmental stresses: oxygen, temperature, and pH. Chapter 10. In: The Natural History of Crustaceans[END_REF] for reviews).

The vent crab Bythograea thermydron Williams 1980 is able to maintain its oxygen consumption relatively constant over a wide range of oxygen concentrations (i.e. oxyregulation capability), down to much lower concentrations than the shallow water species for which this ability was studied [START_REF] Gorodezky | Effects of sulfide exposure history and hemolymph thiosulfate on oxygen-consumption rates and regulation in the hydrothermal vent crab Bythograea thermydron[END_REF]. The capacity to oxyregulate can involve different levels of regulation. At the molecular level, the functional properties of the hemocyanins (in particular their oxygen affinity) play a central role. Hemocyanins from decapods that inhabit deep-sea hydrothermal vents exhibit very high oxygen affinities, allowing the extraction of oxygen from hypoxic conditions (see [START_REF] Hourdez | Adaptations to hypoxia in hydrothermal vent and cold-seep invertebrates[END_REF] for a review). The properties of these blood oxygen carriers can also be affected by allosteric effectors contained in the hemolymph. [START_REF] Gorodezky | Effects of sulfide exposure history and hemolymph thiosulfate on oxygen-consumption rates and regulation in the hydrothermal vent crab Bythograea thermydron[END_REF] showed that animals injected with thiosulfate, a byproduct of sulfide detoxification in the animals that increases hemocyanin affinity, allowed the crab to oxyregulate down to ever lower environmental oxygen concentrations. At the physiological level, adaptation to lower oxygen concentrations can involve ventilatory and cardio-circulatory responses (for a review, see [START_REF] Mcmahon | Respiratory and circulatory compensation to hypoxia in crustaceans[END_REF]. In the shallow water species studied to date, the circulatory response can be quite complex, involving modifications of the heart rate, stroke volume and peripheral resistance. Typically, decapods increase their ventilation (scaphognathite beating frequency and power), decrease their heart rate (bradycardia), and adjust the circulation of their hemolymph, decreasing its flow to digestive organs in favor of the ventral structures [START_REF] Mcmahon | Respiratory and circulatory compensation to hypoxia in crustaceans[END_REF].

The responses to variable oxygen levels involving modifications of heart parameters (contraction rate) and ventilation have however so far not experimentally been studied in hydrothermal vent species of crabs. We studied the cardiac response of the Mid-Atlantic Ridge (MAR) vent crab Segonzacia mesatlantica Williams 1988. This species has been collected at different sites from the MAR, with depths ranging from 850 m (Menez Gwen site) to 4080 m (Ashadze 1 site). To study the cardiac response to varying levels of environmental oxygen in S. mesatlantica, experimental animals were equipped with electrodes and their electrocardiograms (EKG) under different oxygen concentrations were recorded. As temperature affects oxygen demand as well, its effect on the EKG of the experimental crabs was also investigated.

Materials and methods

Animal collection

Specimens of the crab Segonzacia mesatlantica were collected on the hydrothermal vent sites Menez Gwen (37˚50'N, 31˚31'W, 855 m water depth), Logatchev (14˚45.12'N, 44˚58.71'W, 3050 m water depth), and Irinovskoe (14˚20.01'N, 44˚55.36'W, 2700 m water depth), on the Mid-Atlantic Ridge. They were captured with the remotely operated vehicle (ROV) MARUM-Quest, deployed from the Research Vessel Meteor (Menez MAR M82/3 and M126 cruises). The animals were brought back to the surface in a thermally insulated box attached to the ROV and quickly transferred to a cold room (5-8˚C) before they were used for experiments. The few specimens from the shallow site (Menez Gwen) that were not used in the experimental system and were maintained at atmospheric pressure all survived for at least two weeks.

Experimental system

The experimental animals were fitted with three stainless-steel thin wire electrodes: two inserted on either side of the heart and the third in the general body cavity as a reference (Fig. 1A). The flexible leads where then glued directly onto the carapace to prevent the movements of the crabs from affecting the position of the implanted electrodes. Attempts to use the less-invasive infrared sensor simply attached to the shell [START_REF] Depledge | A computer-aided physiological monitoring system for continuous, long-term recording of cardiac activity in selected invertebrates[END_REF]Andersen, 1990, Robinson et al., 2009) proved unsuccessful on this species and on another vent crab, Bythograea thermydron (Hourdez, unpub. failure). The same sensor type also proved unsuccessful to measure scaphognathite beating frequency (possible penetration of the seawater into the sensor). The equipped animals were maintained in an anodized aluminum, custom-built, 500-ml pressure vessel (inside diameter 10 cm and 6.4 cm height) with a Plexiglas window that allowed regular visual inspection of the animals. The smaller specimens (LI-2 through LI-6 and LI-10, see Table 1) were maintained in a smaller pressure vessel (inside diameter 3.5 cm and 5 cm height). The crabs were free to move inside the pressure vessel. During the experiment, the crabs usually remained calm, with some short activity periods. The water flow was provided by a HPLC pump (Waters 515), set to 3-5 ml min -1 , depending on the size of the specimen to yield an oxygen concentration decrease (compared to the inlet concentration) that was measurable with confidence. The pressure was regulated by a pressurerelief valve (Swagelok SS-4R3A) (Fig. 1B). Oxygen concentration in the inlet water was modulated by bubbling nitrogen and/or air with various flows.

Oxygen concentrations are reported in µmol.l -1 rather than partial pressures as these latter depend on the total pressure and are therefore difficult to compare between experiments run at different pressures. Oxygen concentration was measured directly after the pressure relief valve with an oxygen optode (Neofox, Ocean Optics). Three-way valves allowed water to flow either through the vessel containing the animal or through a bypass without affecting the pressure in the system. Oxygen consumption rate (in µmol.h -1 ) was then simply calculated as the difference between these two values, taking the flow rate into consideration: Oxygen consumption rate= (O2 in-O2 out) * WFR where 'O2 in' is the inlet oxygen concentration (in µmol.l -1 ) measured with the bypass in place, 'O2 out' the oxygen concentration (in µmol.l -1 ) measured when the water was flowing through the pressure vessel, and WFR the water flow rate (in l.h -1 ) controlled by the HPLC pump.

Temperature in the pressure vessel was controlled by immersion in a temperature-controlled water bath (10 or 20 ± 0.2 ˚C). Temperature ramping to study the effect of temperature on the heart rate was obtained by progressively increasing temperature in the water bath. We were interested in the crabs' response to rapid temperature variation and therefore chose a rate of about 1˚C every 15 minutes. All experiments were run at a pressure equivalent to in situ pressure for the two sites (80 bars (8 MPa), equivalent to 800 m water depth for Menez Gwen and 270 bars (27 MPa) for the Logatchev and Irinovskoe sites).

Recording of electrocardiograms (EKG)

An electrical feed-through in the pressure vessel wall allowed the recording of the EKG of animals under pressure. We worked on a total of twelve specimens, including ten from Semyenov/Irinovskoe, and two from Menez Gwen (Table 1).

Out of these twelve specimens, six were equipped with electrodes to monitor the electrical activity of their heart, including both Menez Gwen specimens. The voltage variations were recorded with a LabPro (Vernier) interface equipped with an EKG sensor (Vernier) for 30 s for each of the conditions. Voltage values were recorded every 1/100 s. Recordings were made every 2-15 minutes, depending on the rate of change of the studied parameters. Specifically, temperature change was fast and recordings were made every 2-3 minutes, while changes in oxygen concentration were slower and recordings were made every 10-15 minutes. As the animals live in a highly variable environment, we were interested in the response to rapidly-changing conditions and did not give animals time to acclimate to various oxygen levels or temperature values.

The crabs sometimes went through transient cardiac arrests (some as long as 20 seconds), a phenomenon also reported in shallow-water crabs, in particular in response to tactile and visual stimuli (e.g. [START_REF] Stiffler | A comparison of in situ and in vitro responses of crustacean hearts to hypoxia[END_REF][START_REF] Florey | The effects of temperature, anoxia and sensory stimulation on the heart rate of unrestrained crabs[END_REF][START_REF] Defur | The effects of environmental variables on the heart rates of invertebrates[END_REF]. Recordings comprising such arrests were not used for the calculation of heart rate. There was no apparent correlation between the conditions and the occurrence of the arrests, although they seemed to occur less at lower oxygen tensions (pers. obs.).

Changes in the parameters of the EKG (amplitude, shape) could reflect important modifications of cardiac output. We studied the effect of both temperature and oxygen concentration on the shape and amplitude of the EKG.

Electrode implantation, recovery and effect of pressure

Shortly after electrode implantation, the EKG was directly recordable at atmospheric pressure, although a bit erratic while manipulating the animal. For one of the shallower site crabs, the EKG was recorded for 4 hours at atmospheric pressure and 10˚C in the closed experimental vessel. Once under stable conditions, the EKG quickly became regular, and its shape resembled that under pressure (data not shown). From an initial heart rate oscillating between 30 and Consequently, all experiments were run at pressures equivalent to that of the depth at which they were captured and, after re-pressurization, the animals were given 8-12 hours of recovery before experiments were initiated.

Determination of curve parameters

We used curve fitting to determine key values for the heart rate and oxygen consumption as a function of oxygen concentration. In particular, the critical oxygen concentration at which oxygen consumption or heart rate drops can be relatively subjective or its determination strongly dependent on the relatively small number of data points below that value. When the critical oxygen concentration is small (as it is the case for S. mesatlantica, see results), very few data points can be obtained below that value. Instead, we used the equation:

! = # * % -' 1 + * * % -'
where X is the oxygen concentration, Y is the physiological parameter (heart rate or oxygen consumption rate), b is a steepness coefficient, the ratio a/b is the value at plateau (X infinite), and c is the intercept of the curve with the x-axis.

The curve fitting parameters a, b, and c were obtained with the software JMP11, based on an exploration of possible values for the parameters a, b, and c, and the best values were determined by minimizing the difference between the observed (experimental) and expected values (based on the curve equation). Because of the very steep drop of both heart rate and oxygen consumption rate (see Fig. 3), the intercept c is hereafter referred to as the critical oxygen concentration.

Results

Oxygen consumption rates

The oxygen consumption rates (in µmole O2 per hour) were measured for all 12 specimens (Table 1, Fig. 2). With a size range of 0.4-41.5 g wet weight, the oxygen consumption rate increases with an allometry coefficient of 0.48 (p=2.8 10 -8 ). The sex of the animals does not have a significant effect on the regression (ANCOVA, p=0.1784). The oxygen consumption rates for the two specimens from Menez Gwen (800 m depth) do not differ markedly from the specimens from the other sites (2700-3050 m depth), and fall within the 95% confidence interval established for the ten specimens from the deeper sites.

Effect of oxygen concentration on heart rate and oxygen consumption

In all investigated specimens, both the oxygen consumption rates and heart rates follow the same pattern. For all specimens equipped with electrodes (n=6), oxygen concentration does not affect the heart rate over most of the range of concentrations, until a critical low concentration is reached (Fig. 3). Below that concentration, the heart rate and the oxygen consumption both drop sharply.

The oxygen consumption reaches zero at oxygen concentrations ranging from 7.3 to 9.9 µmole.l -1 for the deeper sites and 11.3-14.2 µmole.l -1 for the shallower site (Table 1). At 10˚C, the heart rate typically oscillates between 61 and 68 beats per minute (b.p.m.) while it usually varies between 90 and 108 b.p.m. at 20˚C for the specimens from the shallower site (Supplementary data Fig. S2). For the specimens from the deeper sites, the heart rate at 10˚C is higher (72.3-81.5 b.p.m.; Table 1) than that of the specimens from the shallower site (62.5 and 69.0; Table 1). Although the heart rate tends to decrease with increasing wet weight, the correlation is not significant (log/log transform linear correlation p=0.15).

Effect of temperature

Temperature affects the beating frequency of the heart for the three specimens tested (Fig. 4). The Arrhenius plot for the two LI individuals shows a biphasic curve between 3˚C and the Arrhenius break point at 19˚C. At a temperature higher than 19˚C or lower than 3˚C, the heart rate is more variable and drops sharply in warmer water, indicating that 19˚C is the upper temperature limit for this species. Below 3˚C (normal deep-sea temperature in the area), the heartbeat is also irregular, possibly indicating a lower temperature limit for this species. This phenomenon is also observed at 6˚C for the specimen from the shallower site (normal deep-sea water temperature 8˚C for this area). In addition to the upper and lower breakpoints, there is an inflection point for the two deeper specimens at 10.7˚C (Fig. 4). The colder part of the curve has a slope of ca. 4, while the upper part of the curve has a slope of 1.7-2. This inflection point could also be present for the shallower specimen but the temperature range below that value is too short to allow a proper estimate of the slope.

Modifications the EKG characteristics

In addition to the beating frequency, temperature also affects the overall shape of the EKG (Fig. 5). It is characterized by two large peaks at 12˚C, in addition to a smaller one preceding the large peaks. The second large peak increases in height in respect to the first one as temperature increases. At 16˚C, the two peaks have approximately the same amplitude, and fuse completely at higher temperatures.

The height of the second peak increases with temperature while that of the first peak remains unchanged up to 16˚C. Beyond that temperature, the height of the fused peaks keeps increasing at a rate similar to that of the second peak, suggesting it is the contribution of that second peak that is responsible for the changes in amplitude. Beyond 20˚C, the amplitude tends to level off or decrease slightly.

Over most of the range tested, oxygen concentration does not seem to affect the amplitude of the EKG (Fig. 6). Below 25 µmole.l -1 of oxygen, however, the amplitude of the EKG drops sharply. This phenomenon occurs at lower oxygen concentrations than the drop in heart rate (32 µmole.l -1 at this temperature for the same animal). At values below 25 µmole.l -1 of oxygen, the shape of the EKG is also significantly affected (Fig. 7), with a drastic decrease of the second peak height, to the point it may completely disappear (Fig. 7, 17 µmole.l -1 oxygen inset). Upon return to oxygen concentrations greater than 25 µmole.l -1 , the EKG returns to its pre-hypoxia characteristics (Fig. 7, 62 µmole.l -1 oxygen inset).

Discussion

Oxygen consumption rate

As expected, the oxygen consumption rates increase with increasing size, and this increase follows an allometry with coefficient 0.48. This coefficient is in the low end of the range reported for other marine crustaceans [START_REF] Vidal | Rates of metabolism of planktonic crustaceans as related to body weight and temperature of habitat[END_REF]. This value is lower to that reported for the shore crab Carcinus maenas (0.598; [START_REF] Wallace | Activity and metabolic rate in the shore crab Carcinus maenas (L.)[END_REF]. There is evidence that the allometry of metabolism is linked to activity, metabolic rate, and habitat [START_REF] Carey | Economies of scaling: More evidence that allometry of metabolism is linked to activity, metabolic rate and habitat[END_REF].

Compared to the other hydrothermal vent species studied to date, Bythograea thermydron, the rate is very similar for the large animals (Mickel and Childress, 1982b). Mickel and Childress (1982) however report an allometry coefficient not significantly different from 1.0, although the total size range in their study was reportedly not sufficient (20.0-111.4 g wet weight) to obtain reliable data. The wet weights in our study cover two orders of magnitude (0.4-41.5 g wet weight), yielding a more reliable allometry coefficient. The oxygen consumption rates are also similar to other deep-sea and shallow-water crustaceans [START_REF] Childress | Metabolic rates of animals from hydrothermal vents and other deep-sea habitats[END_REF]. Deciphering the meaning of the small allometry coefficient will require a comparative study of crabs closely related to Segonzacia and inhabiting different habitats.

Changes in the EKG shape characteristics

In decapod crustaceans, the heartbeat is initiated within the cardiac ganglion, where a small number of pacemaker neurons control this heartbeat (for a review, see [START_REF] Mcmahon | Intrinsic and extrinsic influences on cardiac rhythms in crustaceans[END_REF]. [START_REF] Wilkens | Re-evaluation of the stretch sensitivity hypothesis of crustacean hearts: hypoxia, not lack of stretch, causes reduction in the heart rate of isolated[END_REF] reports that it is maintained in isolated hearts, provided that the partial pressure of oxygen is sufficient. The overall shape of the EKG resembles that recorded for Bythograea thermydron, another hydrothermal vent crab studied by Mickel and Childress (1982) and other, shallow-water, crabs [START_REF] Burnovicz | The cardiac response of the crab Chasmagnathus granulatus as an index of sensory perception[END_REF]. For B. thermydron, the authors did not consider changes in amplitude as a function of pressure because the amplitude seemed to be affected by the time elapsed since electrode implantation and pressure affected the electrical connectors in the vessel. In the present study however, pressure remained unchanged and changes in amplitude of the EKG were accompanied by modifications of the shape of the EKG, suggesting the amplitude changes were not artifacts.

In our recordings, the EKG pattern clearly comprises two major peaks that fuse at temperatures greater than 16˚C. The relationship between each on the peaks and its potential physiological role (pacemaker, cardiac output control) would be an interesting avenue to explore but the need to work under pressure for S. mesatlantica renders this line of study difficult in this species.

Effect of temperature

For the Menez Gwen animals, the Arrhenius plot of the heart rate revealed that the normal range of functioning for this species lies between 6˚C and 21˚C at 80 bars (8 Mpa). The temperature of the deep-sea water at these depths is close to 8˚C, and the animals are then unlikely to encounter limiting conditions in the colder end of the range. Hydrothermal fluid, mixing with the deep-sea water, can however yield temperatures far greater than the upper end of the range, likely limiting the distribution of the crabs in their natural environment, in combination with other limiting factors (e.g. oxygen, sulfide). The upper temperature is however lower than that reported for B. thermydron, the eastern Pacific relative of S. mesatlantica (Mickel and Childress, 1982), although pressure is very likely to affect the physiology of the animals. These authors report that at 238 atm (23.8 MPa, corresponding to their typical environmental pressure) B. thermydron is capable of surviving 1 h at 35˚C but died when exposed for the same duration at 37.5 or 40˚C. Animals maintained at 238 atm and 30˚C however exhibited a very disrupted EKG, and three of the five experimental animals died within 2 hours. Contrary to the East Pacific Rise species B. thermydron, the Mid-Altantic Ridge species S. mesatlantica is found living as shallow as 800 m.

Animals collected from this shallow site can survive at least 2 weeks at atmospheric pressure (provided they are kept in a cold room at 5-8˚C), suggesting that they do not experience disruptions of the heart function as severe as those observed in B. thermydron at 1 atm (Mickel and Childress, 1982). This hypothesis is supported by the observations performed at 1 atm on freshly collected animals that showed a normal aspect of the EKG (see 'Materials and Methods' section). S. mesatlantica is also found at greater depths on other sites of the Mid-Atlantic Ridge (down to at least 3000 m depth). Our work on specimens from these deeper sites did not reveal an extended upper temperature tolerance, on the contrary the Arrhenius breakpoint is 2˚C lower for these animals (19˚c for the two specimens tested instead of 21˚C for the shallower site specimen). The same shift is observed for the low temperatures: the lower thermal tolerance is about 6˚C for the shallow water specimen and about 3˚C for the deeper ones.

Overall, it seems the total thermal range is about 16˚C, with a shift towards colder temperature in deeper specimens. The absolute heart rates recorded for our species do not differ greatly from other crabs of similar sizes for comparable temperatures. They are very similar to those obtained for the shore crab Carcinus maenas [START_REF] Ahsanullah | Factors affecting the heart rate of the shore crab Carcinus maenas (L.)[END_REF][START_REF] Giomi | A role for haemolymph oxygen capacity in heat tolerance of eurythermal crabs[END_REF], the mud crab Panopeus herbsti, and the blue crab Callinectes sapidus [START_REF] Defur | The effects of environmental variables on the heart rates of invertebrates[END_REF]. Some other species have much greater heart rates (235 b.p.m. for Hemigrapsus nudus at 18˚C) or much lower values (82 b.p.m. for Libinia emarginata at 25˚C; [START_REF] Defur | The effects of environmental variables on the heart rates of invertebrates[END_REF].

The characteristics of the EKG pattern also changed with temperature. Although the amplitude of the signal for the first large peak remained unchanged, that of the second large peak increased linearly with temperature up to the maximal temperature. This could reflect modifications of the cardiac output in S. mesatlantica. In Cancer magister and C. productus, the cardiac output declines but an increased oxygen delivery to the organs is possible through a concomitant decrease of hemocyanin oxygen affinity [START_REF] Florey | The effects of temperature, anoxia and sensory stimulation on the heart rate of unrestrained crabs[END_REF]. There is however no study linking EKG parameters to cardiac output in crustaceans.

Oxygen and capacity limited thermal tolerance (OCLTT)

The concept of oxygen and capacity limited thermal tolerance (OCLTT) was developed to explain the observations on temperature tolerance [START_REF] Frederich | Oxygen limitation of thermal tolerance defined by cardiac and ventilatory performance in spider crab, Maja squinado[END_REF][START_REF] Pörtner | Climate change and temperature-dependent biogeography: oxygen limitation of thermal tolerance in animals[END_REF]. The authors hypothesized that a mismatch between oxygen demand and oxygen supply results from limited capacity of the ventilatory and the circulatory systems at temperature extremes. They argue that limitations in aerobic performance are the first parameters that will affect thermal tolerance. In Segonzacia mesatlantica, the Arrhenius plot of the heart rate exhibits a biphasic profile for these animals from the deeper sites, with an inflection point at 10.7˚C. In the temperate eurythermal crab Carcinus maenas, [START_REF] Giomi | A role for haemolymph oxygen capacity in heat tolerance of eurythermal crabs[END_REF] report a similar observation. The authors interpret this inflection as the pejus temperature, beyond which hypoxemia sets in until the critical temperature (onset of anaerobic metabolism). This would then indicate an optimal temperature range of 3-10.7˚C from S. mesatlantica, beyond which the exploitation of hemocyanin-bound oxygen reserve delays the onset of hypoxemia. Contrary to the C. maenas hemocyanin, however, the hemocyanin from S. mesatlantica does not release oxygen in response to increased temperature, a lack of temperature sensitivity that is found in other hydrothermal vent crustacea hemocyanins (Chausson et al., 2004, Hourdez and[START_REF] Hourdez | Adaptations to hypoxia in hydrothermal vent and cold-seep invertebrates[END_REF].

Effect of oxygen concentration

At hydrothermal vents, temperature and oxygen concentration are negatively correlated (Johnson et al., 1986). The animals therefore need to extract even more oxygen to meet their metabolic demand when it is less abundant in their environment. The conditions also fluctuate rapidly and animals need to respond quickly to the chronic hypoxia they experience.

In most crustaceans, exposure to hypoxia below the critical oxygen tension induces a bradycardia, coupled with a redirection of the hemolymph from the digestive organs towards ventral structures [START_REF] Mcmahon | Respiratory and circulatory compensation to hypoxia in crustaceans[END_REF]. In our species, the heart rate remained relatively stable over a very wide range of oxygen concentrations and only dropped below a critical oxygen concentration similar to that of the oxygen consumption. This critical oxygen concentration ranges from 7.3 to 14.2 µmol.l -1 . These values are greater than the half-saturation oxygen tension of the hemocyanin (P50=3.7 µmol.l -1 at 15˚C, [START_REF] Chausson | Respiratory adaptations to the deep-sea hydrothermal vent environment: the case of Segonzacia mesatlantica, a crab from the Mid-Atlantic Ridge[END_REF], suggesting that hemocyanin is not the limiting factor in the failure of oxyregulation at lesser environmental oxygen concentrations. Diffusive and convective (ventilation) processes are likely to limit oxygen uptake. However, the ability to maintain a stable heart rate down to low environmental tensions, along with the high affinity hemocyanin, likely accounts for the very low critical oxygen concentration observed for the vent crabs (this study; Mickel and Childress, 1982) compared to their shallow water relatives (e.g. 100-130 µmol.l -1 in Carcinus maenas; [START_REF] Taylor | The respiratory responses of Carcinus maenas to declining oxygen tension[END_REF].

The EKG amplitude and shape does not change in response to oxygen concentration variations over most of the tested range. Below the critical concentration however, both the amplitude and the presence of the second large peak are affected. This suggests that, although the pacemaker activity remains, the heart either contracts less strongly or does not contract at all. As a result, the hemolymph does not circulate and the animal is unable to regulate its oxygen uptake. As for shallow-water species, the animals are able to survive anoxia. In the crabs Cancer magister and C. productus, this can be tolerated for up to 1 hr [START_REF] Florey | The effects of temperature, anoxia and sensory stimulation on the heart rate of unrestrained crabs[END_REF]. Our experimental crabs were also maintained for the same duration below their ability to oxyregulate, a time during which, once oxygen reserves were depleted, they had to rely on anaerobiosis. During that time, the heart rate varied greatly (Fig. 7), possibly indicating attempts to reestablish oxygen uptake.

Measuring the modifications of blood flow to different parts of the body was unfortunately not feasible inside the pressure vessels. Similarly, we were not able to measure ventilation rate or ventilation flow in our set-up. However, the ability to oxyregulate while maintaining a stable heart rate (neither tachycardia nor bradycardia) strongly suggests that ventilation increases under hypoxic conditions as it does in for example in C. maenas [START_REF] Taylor | The respiratory responses of Carcinus maenas to declining oxygen tension[END_REF][START_REF] Giomi | A role for haemolymph oxygen capacity in heat tolerance of eurythermal crabs[END_REF]. In the hydrothermal vent shrimp Alvinocaris komaii, animals observed in situ in environments characterized by lower oxygen tensions exhibited a higher ventilation rate [START_REF] Decelle | Morphological adaptations to chronic hypoxia in deep-sea decapod crustaceans from hydrothermal vents and cold-seeps[END_REF], as typical of other decapods (see [START_REF] Mcmahon | Respiratory and circulatory compensation to hypoxia in crustaceans[END_REF]. Although not directly observed in S. mesatlantica, a similar response is very likely.

Conclusion

As all marine invertebrates, Segonzacia mesatlantica can experience hypoxia through both environmental exposure and as a consequence of increased metabolic consumption during exercise. Unlike most marine invertebrates however, environmental hypoxia is chronic -and possibly continuous-for this species. S. mesatlantica, like its East Pacific Rise congener Bythograea thermydron, is well adapted to these challenging conditions and capable regulate of regulating its oxygen uptake down to very low environmental tensions. In S. mesatlantica, this capacity most likely relies on an increased ventilation rate, while the heart rate remains stable. This is probably helped by the increased ventilatory capacity found in the vent species compared to their shallow water relatives [START_REF] Decelle | Morphological adaptations to chronic hypoxia in deep-sea decapod crustaceans from hydrothermal vents and cold-seeps[END_REF]. When not exposed to temperature increase, hypoxia corresponds to metabolic hypoxia and the response only involves ventilation modulation (and possibly circulatory adjustments). For this species, however, a temperature increase is usually correlated with more pronounced hypoxia. Although the response to hypoxia is similar at 10 and 20˚C, temperature itself has a strong effect on the heart rate and the characteristics of the EKG. It would be interesting to investigate whether the lack of temperature sensitivity [START_REF] Chausson | Respiratory adaptations to the deep-sea hydrothermal vent environment: the case of Segonzacia mesatlantica, a crab from the Mid-Atlantic Ridge[END_REF] impacts the cardiac output response to temperature in comparison to nonhydrothermal vent endemic species. ; in µmol.l -1 .h -1 ) as a 14 function of wet weight in grams (WW) for all specimens (n=12, see table 1 for 15 specimens characteristics). The linear regression has the equation log(Oxygen 16 cons.) = 0.48 * log(WW) + 0.87, and a correlation coefficient r 2 =0.959 (p<0.001). 17

  45 beats per minute (b.p.m.), the heart rate increased to 55 b.p.m. between 3.5 and 4 hours after implantation. After pressure was applied a bradycardia appeared (heart rate down to 35 b.p.m.), which lasted for about 1.5 hours before the heart rate returned to typical values for 10˚C at 80 bars (8 MPa; 60-70 b.p.m., see below). Similarly, animals from the Irinovskoe or Logatchev sites (2700-3050 m depth) and acclimated to their in situ pressure, exhibit transient bradycardia when exposed to lowered pressure. Within a few minutes, the animals stabilized their heart rate to values greater than simulated in situ pressure values. At pressure values lower than 150 bars (15 mPa), the heart rate remained relatively stable (see supplementary material S1). Upon return to the in situ pressure value, the heart rate rapidly returned to the initial value.

Figure 1 :Figure 2 :

 12 Figure 1: Experimental set-up. (A) Electrode implantation on an experimental 8 animal (cephalothorax width about 50 mm). (B) Flow-through pressure vessel, 9 control of oxygen concentration, and position of oxygen optode. The bypass line 10 allows the isolation of the vessel and the measurement of oxygen concentration 11 in the inlet water. 12 13 Figure 2: Oxygen consumption rates (oxygen cons.; in µmol.l -1 .h -1 ) as a 14

  18

Figure 3 :Figure 4 :Figure 5 :

 345 Figure 3: Oxygen consumption rate (open diamonds) and heart rate (black 19 diamonds) in response to oxygen levels in the pressure vessel. Conditions: 20 270 bars (27 MPa) of pressure at 10˚C for specimen LI-1 (see Table 1 for 21 specimen characteristics). The curves were fit to the datapoints as described in 22 the Materials and methods section. 23 24 Figure 4: Arrhenius plot of temperature-induced changes of the heart rate 25 (HR) of S. mesatlantica for three specimens under in situ pressure. 1000/ K: 26 reciprocal temperature in Kelvin (multiplied by 1000 for ease of reading). See 27 Table 1 for specimens characteristics. The Arrhenius breakpoint, inflection point 28 in the relationship, and slope values on either side of this point are also indicated. 29 30 Figure 5: Modifications of EKG characteristics in response to temperature 31 under 80 bars (8 MPa) of pressure and at non-limiting oxygen 32 concentrations (50-100 µmol.l -1 ). Open diamonds: amplitude of the first peak 33 in the EKG; black squares: amplitude of the second peak. Note that at 34 temperature values greater than 16˚C, the two peaks fuse, and only the black 35 symbols are used. Each datapoint corresponds to a mean of 30-40 measurements 36 (depending on the temperature) and its standard deviation. 37

  Figure 5 70 71

Table 1 :

 1 Collection, morphological and physiological characteristics of the experimental animals at 10˚C and under in situ pressure. 1 Animals LI-2 through LI-6 and LI-10 were too small for adequate electrode implantation. 2 Individuals from the Menez Gwen site; LI: Individuals from the Logatchev or Irinovskoe sites; a: Cephalothorax width; b: average 3 value for the plateau area of the graph; nd: not determined. 4

		Depth of	Sex	Size a	Wet	Heart rate b	Resp. rate	Critical O 2
	Specimen	capture (m)		(mm)	weight (g)	(b.p.m.)	(mmol.h -1 )	conc.
	ID							(µmol.l -1 )
	MG-1	800	M	51	41.5	69.0	41.1	14.2
	MG-2	800	F	34	24.2	62.5	35.4	11.3
	LI-1	3050	M	27	7.7	81.1	16.6	9.8
	LI-2	3050	M	13	0.9	No data	5.3	8.8
	LI-3	3050	M	21	3.0	No data	11.8	9.9
	LI-4	3050	M	17	1.7	No data	8.8	8.9
	LI-5	3050	M	7.5	0.2	No data	3.3	8.9
	LI-6	3050	M	12	0.6	No data	5.5	9.2
	LI-7	3050	F	23	3.6	81.5	12.3	8.9
	LI-8	2700	F	53	39.1	74.8	47.2	nd
	LI-9	2700	F	31	10.1	72.3	31.4	7.9
	LI-10	3050	M	12	0.4	No data	6.3	7.3
	MG:

Acknowledgements

All the work described here would not have been possible without the skills and help of the ROV MARUM-Quest crew, not only for animal collections but also for fixing a broken fiber optics cable used in my system: many thanks to a great crew. The crew of the RV Meteor has also been very helpful on board. I would also like to thank Nicole Dubilier, chief scientist, for inviting me on this cruise and for exciting scientific discussions. The modular pressure vessels used in this study were based with permission on a design by Raymond Lee. I would like to thank Jim Childress for very insightful discussions. This research was supported in part by the European Union FP7 Hermione programme (Hotspot Ecosystem Research and Man's Impact on European Seas; grant agreement no. 226354), and by the Region Bretagne HYPOXEVO grant. The German Research Foundation (DFG) and the DFG Cluster of Excellence "The Ocean in the Earth System" at MARUM, Bremen (Germany) are acknowledged for funding and support of the research cruise with the RV Meteor (MenezMar M82/3 and M126) and ROV MARUM-Quest.

The author declares that he has no competing interests.


Dan Radu
email: dan.radu@aut.utcluj.ro

Adrian Cretu
email: adriancr.ro@gmail.com

Benoît Parrein

Jiazi Yi
email: yi@lix.polytechnique.fr

Camelia Avram
email: camelia.avram@aut.utcluj.ro

Adina As

Benoit Parrein
email: benoit.parrein@polytech.univ-nantes.fr

Adina Astilean
email: adina.astilean@aut.utcluj.ro

Flying Ad Hoc Network for Emergency Applications connected to a Fog System

The main objective of this paper is to improve the efficiency of vegetation fire emergency interventions by using MP-OLSR routing protocol for data transmission in Flying Ad Hoc NETwork (FANET) applications. The presented conceptual system design could potentially increase the rescuing chances of people caught up in natural disaster environments, the final goal being to provide public safety services to interested parties. The proposed system architecture model relies on emerging technologies (Internet of Things & Fog, Smart Cities, Mobile Ad Hoc Networks) and actual concepts available in the scientific literature. The two main components of the system consist in a FANET, capable of collecting fire detection data from GPS and video enabled drones, and a Fog/Edge node that allows data collection and analysis, but also provides public safety services for interested parties. The sensing nodes forward data packets through multiple mobile hops until they reach the central management system. A proof of concept based on MP-OLSR routing protocol for efficient data transmission in FANET scenarios and possible public safety rescuing services is given.

Introduction

The main objective of this paper is to introduce MP-OLSR routing protocol, that already proved to be efficient in MANET and VANET scenarios Yi et al (2011a), [START_REF] Radu | Acoustic noise pollution monitoring in an urban environment using a vanet network[END_REF], into FANET applications. Furthermore, as a proof of concept, this work presents a promising smart system architecture that can improve the saving chances of people caught in wildfires by providing real-time rescuing services and a temporary communication infrastructure. The proposed system could locate the wildfire and track the dynamics of its boundaries by deploying a FANET composed of GPS and video enabled drones to monitor the target areas. The video data collected from the FANET is sent to a central management system that processes the information, localizes the wildfire and provides rescuing services to the people (fire fighters) trapped inside wildfires. The data transmission QoS (Quality of Service) for data transmission in the proposed FANET network scenario is provided to prove the efficiency of MP-OLSR, a multipath routing protocol based on OLSR, in this types of applications.

Wildfires are unplanned events that usually occur in natural areas (forests, prairies) but they could also reach urban areas (buildings, homes). Many such events occured the last years (e.g. Portugal andSpain 2017, Australia 2011). The forrest fire in the north of Portugal and Spain killed more than 60 people. During the Kimberley Ultra marathon held in Australia in 2011 multiple persons were trapped in a bush fire that started during a sports competition.

The rest of this paper is structured as folows. Section 2 presents the related works in the research field. Section 3 introduces the proposed system design. Section 4 shows and discusses the QoS performance evaluation results. Finally, Section 5 concludes the paper.

Related works

Currently there is a well-known and increasing interest for providing Public Safety services in case of emergency/disaster situations. The US Geospatial Multi Agency Coordination1 provides a web service that displays fire dynamics on a map by using data gathered from different sources (GPS, infrared imagery from satellites). A new method for detecting forest fires based on the color index was proposed in [START_REF] Cruz | Efficient forest fire detection index for application in unmanned aerial systems (uass)[END_REF]. Authors suggest the benefits of a video surveillance system installed on drones. Another system, composed of unmanned aerial vehicles, used for dynamic wildfire tracking is discussed in [START_REF] Pham | A distributed control framework for a team of unmanned aerial vehicles for dynamic wildfire tracking[END_REF].

This section presents the state of the art of the concepts and technologies used for the proposed system design, current trends, applications and open issues.

Internet of Things and Fog Computing

Internet of Things (IoT), Fog Computing, Smart Cities, Unmanned Aerial Vehicle Networks, Mobile Ad Hoc Networks, Image Processing Techniques and Algorithms, and Web Services are only some of the most promising actual, emerging technologies. These all share a great potential to be used together in a large variety of practical applications that could improve, sustain and support peoples life.

There are many comprehensive surveys in the literature that analyse the challenges of IoT and provide insights over the enabling technologies, protocols and possible applications [START_REF] Al-Fuqaha | Internet of things: A survey on enabling technologies, protocols, and applications[END_REF]. In the near future, traditional cloud computing based architectures will not be able to sustain the IoT exponential growth leading to latency, bandwidth and inconsistent network challenges. Fog computing could unlock the potential of such IoT systems.

Fog computing refers to a computing infrastructure that allows data, computational and business logic resources and storage to be distributed between the data source and the cloud services in the most efficient way. The architecture could have a great impact in the emerging IoT context, in which billions of devices will transmit data to remote servers, because its main purpose is to extend cloud infrastructure by bringing the advantages of the cloud closer to the edge of the network where the data is collected and pre-processed. In other words, fog computing is a paradigm that aims to efficiently distribute computational and networking resources between the IoT devices and the cloud by:

• allowing resources and services to be located closer or anywhere in between the cloud and the IoT devices; • supporting and delivering services to users, possibly in an offline mode when the network is partitioned by example; • extending the connectivity between devices and the cloud across multiple protocol layers.

In the near future, traditional cloud computing based architectures will not be able to sustain the IoT exponential growth leading to latency, bandwidth and inconsistent network challenges. Fog computing could unlock the potential of such IoT systems.

Currently the use cases and the challenges of the edge computing paradigm are discussed in various scientific works [START_REF] Lin | A survey on internet of things: Architecture, enabling technologies, security and privacy, and applications[END_REF][START_REF] Al-Fuqaha | Internet of things: A survey on enabling technologies, protocols, and applications[END_REF], [START_REF] Ang | Big sensor data systems for smart cities[END_REF]. Some of the well known application domains are: energy, logistics, transportation, healthcare, industrial automation, education and emergency services in case of natural or man made disasters. Some of the challenging Fog computing research topics are: crowd based network measurement and interference, client side network control and configuration, over the top content management, distributed data centers and local storage/computing, physical layer resource pooling among clients, Fog architecture for IoT, edge analytics sensing, stream mining and augmented reality, security and privacy.

There are numerous studies that connect video cameras to Fog & IoT applications. The authors of Shi et al (2016) discuss a couple of practical usages for Fog computing: cloud offloading, video analytics, smart home and city, and collaborative edge. Also, some of the research concepts and opportunities are introduced: computing stream, naming schemes, data abstraction, service management, privacy and security, and optimization metrics. Authors of [START_REF] Shi | The promise of edge computing[END_REF] present a practical use case in which video cameras are deployed in public areas or on vehicles and they could be used to identify a missing persons image. In this case, the data processing and identification could be done at the edge without the need of uploading all the video sources to the cloud. A method that distributes the computing workload between the edge nodes and the cloud was introduced Zhang et al (2016). Authors try to optimize data transmission and ultimately increase the life of edge devices such as video cameras.

Fog computing could be the solution to some of the most challenging problems that arise in the Public Safety domain. Based on the most recent research studies and previous works concerning public safety Radu et al (2012), [START_REF] Yi | Multipath routing protocol for manet: Application to h.264/svc video content delivery[END_REF] it can be stated that real time image & video analysis at the edge of a FANET network could be successfully implemented in the public safety domain, more specifically for fire detection and for rescuing emergency services provisioning.

One of the most important advantages of Fog computing is the distributed architecture that promises better Quality of Experience and Quality of Service in terms of response, network delays and fault tolerance. This aspect is crucial in many Public Safety applications where data processing should be done at the edge of the system and the response times have hard real-time constraints.

Flying Ad Hoc Networks

Unmanned Aerial Vehicles (UAV's, commonly known as drones) become more and more present in our daily lifes through their ease of deployment in areas of interest. The high mobility of the drones, with their enhanced hardware and software capabilities, makes them suitable for a large variety of applications including transportation, farming and disaster management services. FANET's are considered as a sub type of Mobile Ad Hoc Networks networks that have a greater degree of mobility and usually the distance between nodes is greater as stated in [START_REF] Bekmezci | Flying ad-hoc networks (fanets): A survey[END_REF].

A practical FANET testbed, build on top of Raspberry Pi c , that uses two WiFi connections on each drone (one for ad hoc network forwarding and the other for broadcasted control instructions is described in [START_REF] Bekmezci | Flying ad hoc networks (fanet) test bed implementation[END_REF]. Another FANET implementation that consists of quadcopters for disaster assistance, search and rescue and aerial monitoring as well as the design challenges are presented in [START_REF] Yanmaz | Drone networks: Communications, coordination, and sensing[END_REF] 2.3 Routing protocols OLSR (Optimized Link State Routing) protocol proposed in Jacquet et al ( 2001) is an optimization of link state protocol. This single path routing approach presents the advantage of having shortest path routes immediately available when needed (proactive routing). OLSR protocol has low latency and performs best in large and dense networks.

In [START_REF] Haerri | Performance comparison of aodv and olsr in vanets urban environments under realistic mobility patterns[END_REF] OLSR and AODV are tested against node density and data traffic rate. Results show that OLSR outperforms AODV in VANETs, providing smaller overhead, end-to-end delay and route lengths. Furthermore there are extensive studies in the literature regarding packets routing in FANET's. Authors of [START_REF] Oubbati | A survey on position-based routing protocols for flying ad hoc networks (fanets)[END_REF] give a classification and taxonomy of existing protocols as well as a complete description of the routing mechanisms for each considered protocol. An example of a FANET specific routing protocol is an adaptation of OLSR protocol that uses GPS information and computes routes based on the direction and relative speed between the UAV's is proposed in [START_REF] Rosati | Dynamic routing for flying ad hoc networks[END_REF].

In this paper authors use MP-OLSR (Multiple Paths OLSR) routing protocol based on OLSR proposed in Yi et al (2011a), that allows packet forwarding in FANET and MANET networks through spatially separated multiple paths. MP-OLSR exploits simultaneously all the available and valuable multiple paths between a source and a destination to balance traffic load and to reduce congestion and packet loss. Also it provides a flexible degree of spatial separation between the multiple paths by penalizing edges of the previous paths in an original Dijkstra algorithm execution.

Based on the above considerations, a system architecture that can improve the saving chances of people caught in wildfires by providing real-time rescuing services and a temporary communication infrastructure is proposed.

System Design

One of the main objectives of this work is to design and develop a smart system architecture, based on FANET networks, which integrates with the numerous emergent applications offered by the Internet of Things, that is:

• extensible: the system architecture should allow any new modules to be easily plugged in; • reliable: the system should support different levels of priority and quality of service for the modules that will be plugged in. For example, the public safety and emergency services that usually have real-time hard constraints should have a higher priority than other services that are not critical; • scalable: the architecture should support the connection of additional new Fog components, features and high node density scenarios;

• resilient: the system will be able to provide and maintain an acceptable level of service whenever there are any faults to the normal operation.

The overview of the proposed model, in the context of Internet of Things & Fog Computing, is given in Figure 1. The system could locate the wildfire and track the dynamics of its boundaries by deploying a flying ad hoc network composed of GPS and video enabled drones to monitor the target areas. The fire identification data collected from the FANET is sent to a central management system that processes the data, localizes the wildfire and provides rescuing services to the people (fire fighters) trapped inside the wildfires. The proposed system intends to support and improve emergency intervention services by integrating, based on the real-time data collected from the Fog network, multiple practical services and modules such as:

• affected area surveillance;

• establishing the communication network between the disaster survivors and rescue teams; • person in danger identification and broadcasting of urgent notifications;

• supporting the mobility of the first responders through escape directions;

• rescuing vehicle navigation.

Our FEA (FANET Emergency Application) network topology is presented in Figure 2 and it is composed of three main components:

• A MANET of mobile users phones;

• FANET -video and GPS equipped drones that also provide sufficient computational power capabilities for fire pattern recognition; • Fog infrastructure that supports FANET data collection at the sink node located at the edge of the network. This provides data storage, computational power and supports different communication technologies for the interconnection with other edge systems.

This last component can be done through an object store as proposed in Confais et al (2017b) where a traditional Bittorrent P2P network can be used for storage purpose. Combined with a Scale-out NAS as in Confais et al (2017a), the Fog system avoids costly metadata management (even in local accesses) and computing capacity thanks to an intensive I/O distributed file system. Moreover, the global Fog system allows to work on a disconnected mode in case of network partitionning with the backbone.

FEA uses a FANET network, to collect fire identification data from drones (GPS and video enabled), and a MANET network composed of users smartphones. Sensing nodes periodically transmit data to the central management system where the fire dynamics is determined for monitoring purposes. If a fire has been detected by a sensing drone, based on the dynamics of the fire, rescuing information will be computed and broadcasted back into the FANET and MANET so that the people trapped in the fire to be able to receive the safety information on their smartphones in real-time. We make the folowing assumptions, that will be taken into account for the simulation scenario modelling, regarding the FEA message forwarding:

• when fire is detected by sensing drones they start to periodically forward data packets with the information regarding fire dynamics over multiple hops in the mesh network towards the sink node; • the central management system processes the fire detection data received from FANET nodes and computes the fire dynamics using the GPS coordinates that are included in the received data messages; • the central management system sends back into the mobile network (FANET and MANET) rescuing information that will be received by people in danger on their smartphones.

In FEA system FANET nodes are responsible for: fire identification based on video recording, forwarding the processed information (alongside with GPS coordinates) towards the collector node and rescuing information forwarding to the MANET nodes. The proposed network architecture could also serve as a temporary communication infrastructure between rescuing teams and people in danger.

One of the many advantages of FEA is the ease of deployment, all the technologies and components of the system are widely available, inexpensive and easy to provide. Also the Quality of Service in the FANET network, which is essential in emergency services where delays and packet delivery rations are very important, is enhanced by using MP-OLSR routing protocol that chooses the best multiple paths available between source and destination.

System Evaluation

The simulations are performed to evaluate MP-OLSR in the proposed FANET scenario. This section is organized as follows. The simulation environment configuration and scenario assumptions are given in Section 4.1 and then the Quality of Service performance are compared between OLSR and MP-OLSR in Section 4.2

Simulation Scenario

For the simulations we designed a 81 nodes FANET & MANET hybrid topology placed in a 1480 square meters grid topology. The Random Waypoint Model mobility pattern was used for different maximal speeds suitable for the high mobility of drones: 1-15 m/s (3.6-54 km/h). We make the assumption that only a subset of nodes (possibly the ones that detect fire or the smart phones of people in danger) need to communicate with the Fog edge node through the mesh network so the data traffic is provided by 4 Constant Bit Rate (CBR) sources. Qualnet 5 was used as a discrete event network simulator. The detailed parameters for the Qualnet network scenario and routing protocols configuration parameters are listed in Table 1. The terrain altitude profile is shown in Figure 3.

Simulation Results

For each routing protocol a number of 80 simulations were executed (10 different seeds/speed ranges). To compare the performances of the protocols, the following metrics are used:

• Packet delivery ratio (PDR): the ratio of the data packets successfully delivered at all destinations. • Average end-to-end delay: averaged over all received data packets from sources to destinations as depicted in [START_REF] Schulzrinne | Rfc 1889: Rtp: A transport protocol for real-time applications[END_REF]. • Jitter: average jitter is computed as the variation in the time between packets received at the destination caused by network congestions and topology changes.

Figures 4, 5 and 6 show the QoS performance of MP-OLSR and OLSR in terms of PDR, end-to-end delay and Jitter results with standard deviation for each point.

From the obtained results it can be seen that PDR decreases slightly with the mobility as expected. For the proposed FANET scenario MP-OLSR delivers an average of 10% higher PDR than OLSR protocol.As expected, when the speed increases to values closer to the high mobility of FANET scenarios the links become more unstable so OLSR performance decreases while MP-OLSR provides a much better overall delivery ratio than OLSR (around 9% in average at higher speeds). MP-OLSR also performs much better than OLSR in terms of end-to-end delay and Jitter. The delay of OLSR is around 2 times higher at the highest speed while Jitter is 50% higher. This aspect is very important for the proposed emergency application where the re-sponse time must be provided as quickly as possible. Furthermore, the MP-OLSR standard deviation for all the results is smaller than for OLSR.

Conclusion and Future Work

We described FEA system as a possible emergency application for MP-OLSR routing protocol which uses a FANET network to collect fire dynamics data from drones and through a central management system it provides safety instructions back to the people in danger. The performance evaluation results show that MP-OLSR is suitable for FANET scenarios, most specifically emergency applications, where the mobility is high and response times have hard real-time constraints.

The folowing are some of our future works: system deployment on a real testbed, analysis of the cooperation between MANET & FANET, data analysis based on thermal cameras.

Fig. 1

 1 Fig. 1 System overview

Fig. 2

 2 Fig. 2 Emergency system architecture

Fig. 3

 3 Fig. 3 Qualnet altitude profile pattern for 100 m 2

Fig. 4

 4 Fig. 4 Delivery ratio

Fig. 5

 5 Fig. 5 End-to-end delay

Fig. 6

 6 Fig. 6 Jitter

Table 1

 1 Simulation parameters.

	Simulation Parameter	Value	Routing Parameter	Value
	Simulator	Qualnet 5	TC Interval	5 s
	Routing protocols	OLSRv2 and MP-OLSR	HELLO Interval	2 s
	Area	1480 x 1480 x 34.85 m 3	Refresh Timeout Interval	2 s
	Number of nodes	81	Neighbor hold time	6 s
	Initial nodes placement Grid	Topology hold time	15 s
	Mobility model	Random Waypoint	Duplicate hold time	30 s
	Speeds	1-15 m/s	Link Layer Notification	Yes
	Number of seeds	10	No. of path in MP-OLSR	3
	Transport protocol	UDP
	IP	IPv4
	IP fragmentation unit	2048 bytes
	Physical layer model	PHY 802.11b
	Link layer data rate	11 Mbits/s
	Number of CBR sources 4
	Sim duration	100 s
	CBR start-end	15-95 s
	Transmission interval	0.05 s
	Application packet size 512 bytes

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