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

5.4. MANUSCRIT : RESPIRATORY ADAPTATIONS OF S. MESATLANTICA 183
5.4.1 Introduction
Deep-sea hydrothermal vents are hypervariable environments in which the physical and chemical
properties of the water exhibit rapid and large changes. These vents are located on geologically active
zones such as oceanic spreading centers and subduction zones. Dense faunal assemblages live around
hydrothermal vents, in contrast to the usually low biomass of the deep-sea environment. Such ecosystems
rely on chemosynthetic bacteria for their primary production of organic carbon. Hydrothermal
organisms live in the mixing zone between the hot, anoxic, acidic and toxic hydrothermal fluid and
the cold oxygenated surrounding deep-sea water. The chaotic emission of hydrothermal fluid causes
fluctuations of the environmental variables both spatially and temporally, thereby modulating oxygen
availability. Since metabolic levels seem to be comparable between species from this environment and
shallow water species (Childress, 1995), hydrothermal vent animals must cope with these conditions
to maintain an adequate oxygenation of their tissues. Original respiratory adaptations are thus likely
to exist in such environments (Hourdez et Lallier, 2007).
Crustaceans are useful models for comparative physiology since they have colonized a wide range
of environments (marine, freshwater, terrestrial), including deep-sea hydrothermal vents. Segonzacia
mesatlantica is the only known endemic crab living on hydrothermal vents on the Mid-Atlantic Ridge.
Like all other endemic deep-sea hydrothermal brachyuran species described up to now, it belongs to
the Bythograeidae family. In decapod crustaceans, the respiratory pigment responsible for oxygen
transport from the gills to the tissues is the blue copper-protein hemocyanin (Hc). Hc is made of
non-covalent hexamers of 75 kDa subunits and can bind dioxygen reversibly on the two copper
atoms of the active site of each subunit (Markl et Decker, 1992). Different subunit types exist for
each species thus permitting potential combinatory diversity of the complexes. Hc is freely dissolved
in the hemolymph and its functional properties can be modulated by a wide range of effectors (pH,
temperature, divalent cations, L-lactate, urate, thiosulfate) (Bridges, 2001). The phenotypic plasticity
due to the different subunit types can also result in changes in the intrinsic affinity of the pigment for
O 2 (Giomi et Beltramini, 2007). All these regulatory mechanisms of Hc affinity enable crustaceans
to adjust the functional properties of their pigment to a variety of stressful conditions (hypoxia, high
temperature, exercise, terrestrial way of life).
Crustacean Hc affinity usually decreases with decreasing pH (normal Bohr effect) and with in-