SEASONAL SIZE VARIATION IN THE PREDATORY CLADOCERAN BYTHOTREPHES CEDERSTROEMII IN LAKE-MICHIGAN

1. Dry weight, body length and spine length were measured for the exotic cladoceran Byrhotrephes cederstroemii collected from offshore and inshore stations in southeastern Lake Michigan. Average dry weight of each developmental stage exhibited seasonal variation by a factor of more than 5. 2. Mean dry weight of Bythotrephes was closely correlated with water temperature. Contrary to the inverse relationship between water temperature and body size frequently observed for other invertebrates, the dry weight of Bythotrephes increased at higher ambient temperatures. 3. No significant correlation was observed between abundances of major zooplankton taxa and the dry weight of Bythotrephes. An indirect effect of temperature on prey consumption may cause seasonal variation in dry weight of Bythotrephes in Lake Michigan. 4. Distances between adjacent pairs of barbs, added to the caudal spine with each moult; are significantly shorter in Bythotrephes which produce resting eggs. Less material investment in the exoskeleton of sexually reproducing females was observed in favour of growth and reproduction.