The hydrothermal vent bivalve Bathymodiolus azoricus

Hydrothermal environments are characterised by high metal concentrations related to interactions of the convective seawater circulation with basaltic rocks inside the ocean crust. Thus, hydrothermal fluids are enriched with, among other toxic metals, mercury (Hg) that becomes bioavailable to vent macro invertebrates and that may determine development of specialised defence mechanisms. Unusually high Hg concentrations in many deep sea hydrothermal vents have suggested that invertebrate communities inhabiting these extreme environments are naturally exposed to elevated levels of the metal (Prol-Ledesma et al., 2002; Ando et al., 2002; Martins et al., 2001; Stoffers et al., 1999; Costa et al., 1988; Falkner et al., 1997). Even though our recent investigations failed to detect markedly high Hg levels in the water column of mixing zones at vent sites of the Mid Atlantic Ridge (MAR), concentrations in the tissues of the vent bivalve Bathymodiolus azoricus exceed many fold those in mussels from industrially polluted areas (E. Kadar, J. J. Powell, V. Costa and R. S. Santos, unpublished). However, for a clear demonstration of organism–habitat interaction in hydrothermal vents, an indepth characterisation of this mixing zone is needed that is lacking to date, given the dynamic nature of the system with such large spatial and temporal variations of environmental conditions. Since vent chemistry is complex and, as yet, poorly understood even after two decades of intense research, postcapture experiments under controlled laboratory conditions designed to simulate the natural environment may be the way to find answers to the paradigm of densely populated hydrothermal vents in spite of the harsh conditions. Mercury is a ubiquitous element, which is highly toxic (Florentine, 1991), and is present in the environment under three main forms: elemental Hg, divalent ‘inorganic’ Hg and organo-Hg (methyl-, dimethyl-, aril-Hg, etc). Organic forms of Hg have been previously considered as having a greater lipid The Journal of Experimental Biology 208, 505-513 Published by The Company of Biologists 2005 doi:10.1242/jeb.01415