Abstract
Morphological aberrations of the 5th pair of legs (P5) of females of the calanoid copepod genus Acartia from the Baltic Sea were observed. The aberrant shape of P5 ranged from thickening to doubling or tripling of the apical spines on one or both legs or even introduction of additional segments. From 7 to 16% of A. bifilosa females showed a deformation of P5 in the samples taken recently from German coastal waters, the Bornholm Sea, and the Lithuanian coast, but lower frequencies were observed in the specimens from the Polish coast (no abnormalities) as well as from the Gotland Sea (2%). In A. tonsa, A. longiremis, and A. discaudata, less than 4% of the females were abnormal. Coastal-open-sea or pollution gradients did not explain this. In A. bifilosa from the Kiel Bight, females from a sample collected in 1924 showed a similar frequency (14%) and the same structure of P5 aberrations in comparison with recent ones. It is suggested, that the described abnormalities are most likely a natural phenomenon for this genus.
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