Behavioural response to the bioavailability of inorganic mercury in the hydrothermal musselBathymodiolus azoricus

Abstract

The hydrothermal vent bivalve Bathymodiolus azoricus is naturally exposed to putatively elevated levels of mercury (Hg), exposure that dates back to the geological occurrence of vent ecosystems, and thus may have evolved evolutionary detoxification mechanisms. Therefore, it was used as a model organism in the present investigation to study the Hg-animal interaction. Mussels were exposed to inorganic Hg by daily administration of 20 microg l(-1) Hg for 21 days (cumulative added concentration was 420 microg l(-1), i.e. approximately 2 mmol l(-1)) under controlled laboratory conditions, and consequent bioaccumulation and detoxification patterns were investigated, while shell gaping behaviour indicative of filtering activity was monitored. As a result of Hg exposure, significant increase in duration, as well as decline in frequency of shell gaping occurred, which did not recover to pre-exposure levels following 21 days of Hg-free treatment. An increase in the duration of open-shelled status may indicate the absence of an avoidance reaction in the vent mussel coming in contact with Hg, unlike other bivalves that normally close their shells in response to stress compounds. Alternatively, it may suggest that Hg had an inhibitory effect on the adductor muscle function that is responsible for closing the shells. As a result, elevated Hg levels were measured in the soft tissues (270+/-71 microg g(-1) in gills, 245+/-52 microg g(-1) in digestive glands, 93+/-25 microg g(-1) in the mantle and 46+/-9 microg g(-1) in the foot), in byssus threads (peak levels of 442+/-89 microg g(-1)) and in pseudofaeces (reaching levels as high as 1000 microg g(-1)). Overall, gills contributed 75% to the total Hg body burden followed by mantle (13%), digestive gland (7%), byssus (3%) and foot (2%). Tissue Hg levels remained elevated in mussels transferred to Hg-free seawater even after 21 days, despite the high concentrations persistently eliminated with pseudofaeces both, during and after, exposure. This potential for bioaccumulation of inorganic Hg (concentration factors reached the order of magnitude of 10(4)) by the vent mussel, which does not seem to prevent uptake by shell closure, suggests that the main Hg-handling strategy is elimination via mucus.