Varying salinities of coastal waters are likely to affect the physiology and ion transport capabilities of calcifying marine organisms such as bivalves. To investigate the physiological effect of decreased environmental salinity in bivalves, adult oysters (Crassostrea gigas) were exposed for 14 days to 50% seawater (14) and the effects on mantle ion transport, electrophysiology and the expression of Ca 2+ transporters and channels relative to animals maintained in full strength sea water (28) was evaluated. Exposure of oysters to a salinity of 14 decreased the active mantle transepithelial ion transport and specifically affected Ca 2+ transfer. Gene expression of the Na + /K +-ATPase and the sarco(endo)plasmic reticulum Ca 2+-ATPase was decreased whereas the expression of the T-type voltage-gated Ca channel and the Na + /Ca 2+-exchanger increased compared to animals maintained in full SW. The results indicate that decreased environmental salinities will most likely affect not only osmoregulation but also bivalve biomineralization and shell formation.

Calcium transport is essential for bivalves to be able to build and maintain their shells. Ionized calcium (Ca 2þ ) is taken up from the environment and eventually transported through the outer mantle epithelium (OME) to the shell growth area. However, the mechanisms behind this process are poorly understood. The objective of the present study was to characterize the Ca 2þ transfer performed by the OME of the Pacific oyster, Crassostrea gigas, as well as to develop an Ussing chamber technique for the functional assessment of transport activities in epithelia of marine bivalves. Kinetic studies revealed that the Ca 2þ transfer across the OME consists of one saturable and one linear component, of which the saturable component fits best to Michaelis-Menten kinetics and is characterized by a K m of 6.2 mM and a V max of 3.3 nM min 21 . The transcellular transfer of Ca 2þ accounts for approximately 60% of the total Ca 2þ transfer across the OME of C. gigas at environmental Ca 2þ concentrations. The use of the pharmacological inhibitors: verapamil, ouabain and caloxin 1a1 revealed that voltage-gated Ca 2þ -channels, plasma-membrane Ca 2þ -ATPase and Na þ /Ca 2þ -exchanger all participate in the transcellular Ca 2þ transfer across the OME and a model for this Ca 2þ transfer is presented and discussed.

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