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Dense swarms of Daphnia longispina (up to 4000 animals 1 -I) were recorded along the littoral zone in a lake where Chaoborus flavicans is considered the main predator. D. longispina coexisted with D. pulex, but there were no D . pulex in the littoral swarms . Swarms were less dense at night (about 1/10 the density), and D. longispina exhibited diel horizontal migrations by aggregating in the littoral during the day and spreading out at night . Laboratory experiments showed that Chaoborus capture efficiency on juvenile daphnids was higher in the light compared to darkness, and that Daphnia exhibited a behavioural response to water that had previously contained Chaoborus . We conclude that predation from Chaoborus can be an important factor affecting the distribution patterns of Daphnia observed in this lake. The behavioural experiments indicated that this influence might be partly mediated by chemical agents .

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