The quick and the dead: might differences in escape rates explain the changes in the zooplankton community composition of Lake Michigan after invasion by Bythotrephes?

We demonstrate that zooplankton escape abilities are consistent with the composition of the zooplankton community in the Great Lakes following the invasion of the visually preying invertebrate predator Bythotrephes longimanus. Escape abilities were analyzed by videotaping responses of free-swimming zooplankton to encounters with tethered Bythotrephes. Both maximum speed and maximum acceleration of the escape response were appreciably greater in Daphnia mendotae and diaptomids, whose populations remained relatively unchanged, than those of Daphnia retrocurva and Daphnia pulicaria, whose populations greatly decreased after the Bythotrephes invasion. Maximum speed of all species was higher in the light than in complete darkness, likely due to a different level of activity of Bythotrephes. Contrary to treatments with Bythotrephes, mean and maximum swimming speeds of all species were similar to each other and the same in light and dark in treatments without Bythotrephes. This implies that the prey were responding to infochemicals produced by Bythotrephes.