Effect of salinity on the swimming velocity of the water flea Daphnia magna.

The swimming velocity of the water flea Daphnia magna is dependent on its body size. Therefore, environmental factors that influence growth also influence swimming velocity. This study examined whether exposure to increased salinity reduces swimming velocity only through its effect on body size or whether it also reduces size-specific swimming velocity. Initially, size-specific swimming velocity decreased in a salinity-dependent way. Thereafter, swimming velocities gradually returned to their expected values in all treatments. This acclimation coincided with considerable mortality in the highest-salinity treatment, indicating that daphnids in this treatment either acclimated or died. The initial decrease in size-specific swimming velocity could not be explained by decreased uptake of food. Thus, the results indicate that salinity temporarily impaired physiology. The experiment illustrates how size effects can be accounted for in swimming-velocity analysis and how size-specific swimming-velocity analysis can be used as a non-invasive method to detect stress-induced deviations from normal physiology.