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

1. Dry weight, body length and spine length were measured for the exot ic cladoceran Byrhotrephes cederstroemii collected from offshore and i nshore stations in southeastern Lake Michigan. Average dry weight of e ach developmental stage exhibited seasonal variation by a factor of mo re than 5. 2. Mean dry weight of Bythotrephes was closely correlated w ith water temperature. Contrary to the inverse relationship between wa ter temperature and body size frequently observed for other invertebra tes, the dry weight of Bythotrephes increased at higher ambient temper atures. 3. No significant correlation was observed between abundances of major zooplankton taxa and the dry weight of Bythotrephes. An indir ect effect of temperature on prey consumption may cause seasonal varia tion in dry weight of Bythotrephes in Lake Michigan. 4. Distances betw een 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.