maenas (intertidal zone) and Segonzacia mesatlantica - Station ...

5.4. MANUSCRIT : RESPIRATORY ADAPTATIONS OF S. MESATLANTICA 211
of the lactate sensitivity (the lactate-bearing form being more compact for Homarus americanus)
compared to data collected at sea-surface pressure.
5.4.5 Conclusion
Segonzacia mesatlantica Hc has some common features present in hydrothermal crustacean Hc :
an intrinsic high affinity for O 2 , a strong Bohr effect and a high hemolymphatic concentration resulting
in a high capacitance. No phenotypic plasticity at the subunit level was involved in the respiratory
response to the conditions tested here but complex proportions varied according to temperature, with
an increased hexamer proportion at high temperature. The specific properties of each complex remain
to be determined to physiologically interpret this observation. Hemolymph composition responses
included a dramatic increase in urate levels in repressurized crabs, regardless of the experienced
conditions, a higher Hc level at low temperature and at high oxygenation, and higher magnesium
levels at high temperature and in moderate hypoxia. No lactate response was observed. Magnesium
can act at the metabolic level by decreasing the activity of the crab but also increases Hc affinity for
O 2 . Interestingly, S. mesatlantica Hc was sensitive to lactate but not to urate. This results in a physiological
uncoupling between the usual organic effectors of Hc and Hc properties since no effector
exhibits simultaneously a concentration response to stress and an effect on Hc affinity. This is also
the case in other hydrothermal crustaceans for lactate at least. The common adaptive strategies for
hydrothermal crustaceans can be summarized as 1) a constant high affinity of Hc for O 2 , but not or
weakly modulated by organic cofactors and temperature, for efficient loading at the gills in the whole
range of conditions they experience and 2) a strong Bohr effect to facilitate the unloading near the
metabolizing tissues. The absence of an excessively strong increase of affinity by L-lactate or urate in
hypoxia is adaptive since it permits to keep unloading O 2 at the tissues. Changes in complex proportions
and thiosulfate levels are also potential functional modulators for which further investigations
are needed. To reach an integrated understanding of the respiratory response in S. mesatlantica and
in other hydrothermal crustaceans, it would be of interest to measure physiological variables such as
ventilation, cardiac frequency and output, arterial and venous pH and oxygen partial pressure, and
oxygen consumption under various conditions including severe hypoxia and in the presence of sulphides.
These experiments should be made under pressure and the physiological rest status of the
individuals should be checked before applying the stress, since the urate data in our study suggest that
repressurization itself can act as a stressor.
Acknowledgments :
P. Legendre got involved in this research as recipient of a CNRS Associate Research Director-