Analysis of the swimming velocity of cadmium-stressed Daphnia magna

The swimming velocity of the waterflea Daphnia magna is dependent on its body size. Therefore, environmental Factors like toxic stress that influence growth also influence swimming velocity. An experiment was set up to lest whether exposure to cadmium would reduce only growth, with a concomitant decrease in velocity, or whether it would reduce velocity below the swimming velocity of similarly-sized control animals. Daphnids were exposed for 10 days to free cadmium ion concentrations ranging from 1 x 10(-8) to 1 x 10(-7) M Cd2+, and body size and swimming velocity were measured every 2 days. The results showed that cadmium decreased both growth and velocity, i.e. exposed daphnids swam slower than similarly-sized control daphnids. Swimming velocity provided no indication of successful acclimation in any cadmium treatment. Food consumption and assimilation were reduced by exposure to cadmium. This reduced food intake may have, at least partially, caused the decreased growth rates. However, since reduced food intake does not affect swimming velocity, the reduced swimming velocity must be attributed to toxic effects of cadmium, other than those on food intake. (C) 1999 Elsevier Science B.V. All rights reserved.