Abstract
The presented results provide the first comprehensive description of the C. pengoi population and document new records of its expansion to the west in the Polish coastal zone of the Baltic Sea. In June–August 2006, C. pengoi was observed for the first time continuously for nine successive weeks in a broad range of the water temperature and salinity. Its abundance fluctuated from some to several hundred specimens per cubic meter. In July the species was much more abundant in the water far from the coast, whereas in August its maximum density was observed at the most shallow station. The population of C. pengoi occupied the whole water column, from the surface water to 80 m depth, its vertical distribution was not uniform. C. pengoi was represented by specimens from all age-specific morphological stages (BS I–III). The largest part of the population (up to 81.53%) was composed of juvenile specimens (BS I and II). Parthenogenetic females in the second instar constituted the majority of BS II specimens.