A commensal scale-worm of the genus Branchipolynoe is often found inside mussels clustered around deep-sea hydrothermal vents and cold seeps (Pettibone, 1984; Pettibone, 1986; Miura & Hashimoto, 1991). In addition to the constraints of the hydrothermal environment, such as rapid and large variations of temperature, pH, oxygen, sulphide, and carbon dioxide concentrations, living in the pallial cavity of a mussel may obviously affect respiration. The aim of this study was to explore anatomical and physiological respiratory adaptations in Branchipolynoe seepensis, an Atlantic species compared to B. symmytilida, an East-Pacific species. In contrast to littoral species belonging to the family Polynoidae, Branchipolynoe species possess two evident specificities: numerous gills, and abundant haemoglobin. Anatomical studies have shown that the gills have a high specific surface area, and present a reduced diffusion distance. The hollow gills are not vascularized but their coelom is connected to the large body cavity which contains a dissolved extracellular haemoglobin. The reduced vascular system also contains an extracellular haemoglobin. In B. symmytilida, gel chromatography revealed that, besides a small amount of a 3.5.10 super(6) Da Hb typical of annelid hexagonal bilayer Hb (Lamy et al., 1996) and most probably of a vascular origin, two different major Hbs, Cl and C2, coexist in the coelomic fluid. |
