Motility patterns and egg production were investigated in two populations of Acartia tonsa, field animals from the Öresund and laboratory animals from a 12-year-old (≈ 120 generations) culture. When observed in aquaria with a layer of Thalassiosira weissffogii in the middle, laboratory animals displayed weak aggregation behaviour, while field animals did not aggregate at all. Both populations made longer and more frequent feeding bouts inside the patch. Egg production measurements were in accordance with the behaviour of the laboratory population if no diel feeding rhythm was assumed. The field population produced fewer eggs than predicted from activity measurements, probably due to a diel feeding rhythm. It is concluded that laboratory reared A. tonsa can be used for experiments involving behaviour, but that the possible loss of diel rhythms should be a concern. Both populations differed considerably from field-caught A. tonsa from the eastern United States, where both behaviour and egg production changed consistently and in accordance with a strong aggregation in food patches.
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