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206 CHAPITRE 5. ADAPTATIONS RESPIRATOIRES DE S. MESATLANTICA were found previously, all of which are also observed in our study. Since the crabs came from different sites (Menez Gwen for the previous study, Lucky Strike and Rainbow here), these differences could be due to the existence of two separated population or to long term adaptations to different conditions of pressure or of hydrothermal fluid composition. ESI-MS is the method of choice for identifying such a high diversity among proteins so close in molecular mass. A high diversity was also observed for Cyanagraea praedator with at least 8 different subunits (Chausson et al., 2001). The observed hexamer proportion was always very high in S. mesatlantica, which is different from what is generally observed for shallow water brachyuran species. Dodecamer is usually the main complex in brachyurans, lobsters, crayfishes, hermit crabs species, and hexamer in isopods, krill, shrimps, prawns, spiny lobsters (Markl, 1986, Markl et Decker, 1992, Terwilliger, 1998). This hexamer abundance is also generally found in the other hydrothermal vent species, such as Bythograea thermydron (26.8 % dodecamer) and Cyanagraea praedator (46.5 % dodecamer) (Lallier et al., 1998, Chausson, 2001). It is also the case for the terrestrial crab Ocypode quadrata (44 % dodecamer) (Johnson, 1987) and two crabs from Amazon River (Dilocarcinus pagei cristatus, mainly hexamer, and Sylviocarcinus pardalinus, 30 % dodecamer) (Bonaventura et al., 1979). For Segonzacia mesatlantica Hc complexes are formed by non-covalent interactions since no covalently bound dimer of 75 kDa subunits was ever detected. Two subunits were found to be dodecamer-specific and were never found in hexamer. For some individuals (7/24), only one of the two subunit was detected from purified dodecamer, and for one individual amongst 24 none of them was detected, suggesting that these subunits promotes dodecamer assembly but the two of them are not compulsory. The canonical redundancy analysis showed a consistent strong correlation between the occurrence of these subunits and the type of complex analyzed, whereas the other subunits were non-specific of the complex type. Two subunits were also dodecamer-specific in Cyanagraea praedator (Chausson et al., 2001). In numerous crustacean species, dodecamer assembly is dependent on one or several specific subunits (Stöcker et al., 1988). Response to acclimation to different conditions In the hydrothermal zone, temperature and low oxygenation are correlated since a rise in temperature indicates a higher fraction of the anoxic hydrothermal fluid in the mix. Hence, it is of interest to know if crabs have a physiological response to high temperature, low oxygen, or both, if the same response is triggered for high temperature when associated with normoxia rather than hypoxia, and reciprocally, and what are the specific effects of these factors. The use of canonical redundancy analysis provided a global view of how the response variables changed over the 4 experimental conditions. The first observation is that both temperature and oxygenation had a significant effect, and that their effect was different depending on the state of the other factor (significant interaction). One could
5.4. MANUSCRIT : RESPIRATORY ADAPTATIONS OF S. MESATLANTICA 207 have hypothesized that in an environment in which two stresses (high temperature and low oxygen) are correlated, the physiological responses to them could have evolved to be triggered by only one of the factors. Here both temperature and oxygenation had a separate, uncorrelated effect. Mg 2+ levels increased at high temperature, particularly under low oxygenation, and decreased at low temperature (average values are 25.1 mM at high temp, 19.2 mM at low temp and 22.8 mM in control crabs). Mg 2+ is known to act as an anesthesiant that inhibits neuromuscular activity (Katz, 1936) and is often correlated with lower activity in Reptantia (Robertson, 1953). High Mg 2+ causes relaxation in crustacean (Frederich et al., 2000) and is correlated with a lower heart activity (Walters et Uglow, 1981). The same pattern was observed for the oxygenation effect at high temperature (average values 25.75 mM for low O 2 , 23.2 mM at high O 2 ). Thus Mg 2+ increase was general in respiratory stressful conditions. It has been shown that lower Mg 2+ level correlates with a higher heart rate and a lower O2 consumption in polar species (Frederich et al., 2001). In S. mesatlantica it may act at high temperature or low oxygenation by decreasing the metabolism of the crab and thus decreasing its O 2 consumption. It has been showed for B. thermydron that high temperature induces an increase of heart rate (Mickel et Childress, 1982a). Mg 2+ could limit the increase in heart rate due to the high temperature in S. mesatlantica. Both low temperature and high oxygenation conditions induced a strong production of hemocyanin. This results in an improved oxyphoric capacity that would rather be expected and adaptive at high temperature or at low oxygenation. A possible explanation for this is that whereas hemocyanin biosynthesis was reactivated under pressure at low temperature or high oxygenation, it was not at high temperature or low oxygenation because of the excessive metabolic cost. No clear lactate response to acclimation was observed. On the contrary, urate levels increased in all experimental crabs. This shows that a non-specific urate response to repressurization exists and thus repressurization itself could be considered as a stress. This has also been observed for Paralvinella grasslei for which higher levels of DNA damage were observed after 18 h in worms reacclimated under pressure compared to worms kept at sea surface pressure (Dixon et al., 2002) ; in this precise case oxidative stress can have been the cause since worms were acclimated with surface water with higher O 2 contents than their natural environment. Note that the experimental hypoxia was moderate and that crabs can undergo more severe hypoxia in their natural environment. The Pacific hydrothermal vent crab Bythograea thermydron was found to be able to survive up to 12 h without O 2 (Mickel et Childress, 1982b). Thus, it is possible that more severe hypoxia could have induced a lactate response, but this was not tested since specimens were rare and precious and those available were not exposed to conditions that could have been lethal to them and would have prevented using them for physiological studies. Na + levels were higher in acclimated crabs compared to the controls. The average values were
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THESE DE DOCTORAT DE L’UNIVERSITE
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i Résumé / Abstract Réponse adap
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iv SOMMAIRE 4.2 Objectifs de l’é
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FIG. 1.5 - Localisation des princip
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(a) FIG. 1.6 - Fonctionnement d’u
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134 CHAPITRE 4. PLASTICITÉ PHÉNOT
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Page 243 and 244: Bibliographie Adair, G. S. The hemo
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Page 247 and 248: BIBLIOGRAPHIE 241 Farmer, C. S., J.
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Page 253 and 254: BIBLIOGRAPHIE 247 Rainer, J., C. P.
Page 255 and 256: BIBLIOGRAPHIE 249 Svedberg, T. et I
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Page 261: LISTE DES FIGURES 255 4.17 Profils
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Table des matières Résumé Sommai
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TABLE DES MATIÈRES 261 4.3.5 Spect
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TABLE DES MATIÈRES 263 Table des m
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