Behavioural and morphological changes in polymorphic Daphnia related to different predation regimes

Morphology and behaviour of Daphnia galeata in Lake Lille Lungegaardsvann, Bergen, Norway, were studied in two consecutive years. A portion the population showed helmet cyclomorphosis in 1992, but no helmets developed in 1993. The non-helmeted individuals in 1992 were found in the littoral habitat during summer, whereas the helmeted morph stayed mainly in the pelagial. These differences in morphology and behaviour within and between years are likely to be caused by changes in predation regimes. Larvae of the predacious phantom midge, Chaoborus flavicans, were observed only in 1992, whereas the three-spined stickleback, Gasterosteus aculeatus, was present both years.