Comparative biology of the predatory cladoceran Cercopagis pengoi from Lake Ontario, Baltic Sea and Caspian Sea

Cercopagis pengoi, an onychopod cladoceran native to the Ponto-Caspian-Aral region, was first found in the Baltic Sea in 1992 and in Lake Ontario in 1 998. Morphological studies of C. pengoi from Lake Ontario, Baltic Sea and C aspian Sea revealed a high degree of variability in body length, structure and length of the caudal process, and shape and size of the brood pouch. Th e female brood pouch changed dramatically in size and shape between instar stages, resulting in distinctive appearances. Males had significantly short er body length, caudal process length and distances between adjacent articu lar spines than did females. Instar III parthenogenetic females dominated p opulations of C. pengoi from all geographical localities. Body length of al l instar stages, genders, and reproductive modes from Lake Ontario was smal ler than that of comparable individuals from European localities. The ratio of caudal process length to body length also varied geographically, with h ighest values observed for individuals from Lake Ontario. Clutch size for i nstar LII parthenogenetic females from Lake Ontario was lower than values r eported from the Baltic Sea and Caspian Sea. Observed differences in morpho logical and reproductive characteristics of geographically remote populatio ns may reflect C. pengoi's vulnerability to zooplanktivory or responses to environmental conditions.