Post-capture hyperbaric simulations to study the mechanism of shell regeneration of the deep-sea hydrothermal vent mussel Bathymodiolus azoricus (Bivalvia: Mytilidae)

Abstract

This study describes the influence of hydrostatic pressure (HP) on regeneration of the artificially perforated shell in the vent mytilid Bathymodiolus azoricus. Pressure-dependent and size-dependent shell repair mechanism and haemocyte response was detected following post-capture hyperbaric simulations at environmentally relevant pressures at 1, 85, 175 and 230 bars as compared to control situation, i.e. prior to shell perforation. We provide evidence for the following: (1) shell wound repair is influenced by both hydrostatic pressure and depositional substrate: newly forming crystals emerge 10 days after induction, but only in animals maintained under HP similar to their site of provenience, in spite of the fact that B. azoricus is known to be euribarofilic as indicated by its biomass-dominance at various sites along a bathymetric gradient; shell repair also commenced at 1 bar, but only when the regeneration substrate was periostracum; (2) wound recovery, i.e. closure of the shell notch in B. azoricus is slower than in non-vent bivalves, i.e. it was not completed in 120 days at 1 bar; (3) shell repair involves the Ca bearing haemocytes, referred here as granulocytes, but this may not be automatically considered as the mechanism for (non-induced) shell calcification; blood cell counts showed that the raise in haemocyte number, where statistically significant, was reversible to pre-induction levels by day 10, which may indicate immune response rather than an active calcification process; (4) blood cells of the freshly collected animals contained bacteria resembling sulphur oxidiser endosymbionts from the gill, which may have profound immunological implications, and may be considered as an additional mechanism for environmental transmission of symbiosis.