Differential susceptibility of horizontal and vertical swimming activity to cadmium exposure in a gammaridean amphipod (Gammarus lawrencianus).

In this study two indices of swimming behavior (horizontal and vertical swimming activity) in a gammaridean amphipod (Gammarus lawrencianus) were examined for their sensitivity to Cd exposure. G. lawrencianus were exposed for 72 h to a variety of Cd concentrations [background (approximately 12), 62, 125, 250 and 500 microg l(-1)] at 20 ppt. Subsequent to exposure, video surveillance of survivors held within grooved rings or clear boxes was used to assess horizontal swimming activity (percentage of time mobile) and vertical swimming activity (number of surfacings), respectively. Results show that control amphipods were quite active, being mobile approximately 61% of the time, with horizontal swimming activity decreasing (P < 0.01) to approximately 0.3% between a Cd exposure concentration of 125 and 500 microg l(-1). Vertical swimming activity in amphipods was also impacted by Cd exposure (P < 0.001), with the greatest decrease occurring between background (approximately 12 microg l(-1)) and 62 microg l(-1) (60 versus approximately 26 surfacings, respectively), which is approximately four-fold lower than the estimated 72 h LC50 (250 microg l(-1)) for G. lawrencianus. Based on fluid dynamic considerations, it is speculated that of the two behaviors, vertical swimming activity is more sensitive to Cd exposure because of the presumed greater energetic costs associated with producing enough thrust to attain the lift required to make a vertical ascent into the water.