The deep-sea hydrothermal ventmussel Bathymodiolus azoricus has been the subject of several studies aimed
at understanding the physiological adaptations that vent animals have developed in order to cope with the
particular physical and chemical conditions of hydrothermal environments. In spite of reports describing
successful procedures to maintain vent mussels under laboratory conditions at atmospheric pressure, few
studies have described the mussel's physiological state after a long period in aquaria. In the present study, we
investigate changes in mucocytes and hemocytes in B. azoricus over the course of several months after deepsea
retrieval. The visualization of granules of mucopolysaccharide or glycoproteinwas made possible through
their inherent auto-fluorescent property and the Alcian blue-Periodic Acid Schiff staining method. The density
and distribution of droplets of mucus-like granules was observed at the ventral end of lamellae during
acclimatization period. The mucus-like granules were greatly reduced after 3 months and nearly absent after
6months of aquarium conditions. Additionally,we examined the depletion of endosymbiont bacteria from gill
tissues, which typically occurs within a few weeks in seawater under laboratory conditions. The physiological
state of B. azoricus after 6 months of acclimatizationwas also examined by means of phagocytosis assays using
hemocytes. Hemocytes from mussels held in aquaria up to 6 monthswere still capable of phagocytosis but to a
lesser extent when compared to the number of ingested yeast particles per phagocytic hemocytes from freshly
collected vent mussels.We suggest that the changes in gill mucopolysaccharides and hemocyte glycoproteins,
the endosymbiont abundance in gill tissues and phagocytosis are useful health criteria to assess long term
maintenance of B. azoricus in aquaria. Furthermore, the laboratory set up to which vent mussels were
acclimatized is an applicable system to study physiological reactions such as hemocyte immunocompetence
even in the absence of the high hydrostatic pressure found at deep-sea vent sites.