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

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