Water loss from aerially exposed mussels

The valve activity and water loss from aerially exposed Mytilus edulis L. and Modiolus modiolus (L.) have been investigated in a series of laboratory experiments. Mytilus is capable of maintaining long periods of complete, or almost complete, valve closure when exposed in air, and this allows the retention of water in the mantle cavity and protects the tissues from evaporative water loss. Over periods of three days or more the amount of water lost from emersed Mytilus was found to be less than that retained in the mantle cavity at the beginning of exposure, suggesting that during normal periods of exposure the tissues are never directly subject to water loss. In contrast, Modiolus shows periods of gaping during which water loss is rapid due to drainage from the mantle cavity and evaporation from the tissues. The exposure of the tissues to air that results from gaping, makes the water loss susceptible to environmental influences of which wind was found to be the factor which caused the greatest increase in water loss and, as a consequence, an important factor in causing death through dehydration. The different abilities of Mytilus edulis and Modiolus modiolus to control water loss may be related to their intertidal distributions.