Temperature Tolerances and Osmoregulation in some Estuarine Bivalves

Abstract

The salinity and temperature tolerances of some burrowing bivalves which occur in the middle reaches of the Swartkops estuary have been studied. Five species (Solen caperats, S. comeus, Dosinia hepatica, Macoma litoralis and PsammotelUna capensis) have upper lethal temperatures of 41-46°C when heated at a rate of 1 °/10 minutes. Two species have upper lethal temperatures of 37°C and 39°C when heated at a rate of 1 C°/day. It has been concluded that they can tolerate much higher temperatures than they normally encounter in the estuary.

The salinity tolerances of four species (S. capensis, S. comeus, Dosinia, Macoma) have been investigated. All four appear to be euryhaline osmoconformers and can tolerate a wider range of salinities than they normally encounter in nature. Most of them are, however, unable to survive very low salinities, such as occur during floods, for long periods. It has been noted that species with a large gape to their valves come rapidly into osmotic equilibrium with the external medium, while those with no gape can remain closed, and thus survive low salinities for long periods. A strong shell with little gape also has a noticeable insulating effect when the animals are rapidly heated.