Ecophysiological Responses of Cerastoderma Edule (L.) and C. Glaucum(bruguière) to Different Salinity Regimes and Exposure to AIr

The estuarine bivalve Cerastoderma edule and the lagoonal C. glaucum were exposed in the laboratory to different salinity regimes and the effects in the osmotic concentration of the mantle cavity liquid, extrapallial fluid and haemolymph determined. The behavioural responses of the two cockles to changing salinity and exposure to air were investigated by monitoring valve movements. Both C. edule and C. glaucum are euryhaline osmo-conformers when exposed to different environmental salinities, but differences are seen in their behavioural responses to sudden exposure to low salinity and cyclic changes in salinity. C. glaucum postponed the effect of sudden exposure to low salinity by complete valve closure whilst C. edule partially closed its valves and intermittently gaped during the same period. During exposure to a sinusoidal fluctuating salinity regime, C. edule responded first to the falling salinity by retracting siphons and partially closing valves. In contrast, C. glaucum responded more slowly to falling salinity than did C. edule. C. glaucum closed valves faster, accompanied by erratic adductions. During an acclimation period in a continuous automatic tidal system, C. edule and C. glaucum were entrained to the tidal rhythm of immersion and exposure to air and demonstrated different responses of valve gaping and adductions. This rhythm was lost upon continuous submergence indicating the rhythm is exogenously induced. The two cockles gaped during exposure to air at low tide, suggesting that both can respire in air, contrary to previous reports. The different physiological and behavioural responses of C. edule and C. glaucum to changing salinity and exposure to air were found to be a reflection of their respective environments. C. edule is adapted to daily changes and stresses in its estuarine habitat whilst C. glaucum is subjected to seasonal changes.