Abstract
Daphnia are well known for their ability to protect themselves against various predators by developing inducible morphological, behavioural and life history defences. The influence of body size as a life history trait on behavioural defences has rarely been investigated, although it may be a decisive trait for the evolution of defensive strategies. Therefore, we investigated the behavioural reaction of the relatively small species D. barbata and the two relatively large species D. longicephala and D. magna in response to the heteropteran backswimmer Notonecta (ambush predator) and the tadpole shrimp Triops (cruising predator). We hypothesised that apart from prey body size, especially the predator hunting strategy (ambush vs. cruising) is relevant for the evolved defensive strategies. We found that the small D. barbata reacted with distinct behavioural alterations in depth selection, swimming velocity and swarming behaviour to Triops and Notonecta , while the large D. longicephala adjusted only swimming velocity and depth selection. Large D. magna did not respond behaviourally at all. Moreover, daphniids seemed to react more strongly to Notonecta than to Triops . Based on these observations, we describe a potential pattern explaining the interplay between body size, predator hunting type and resulting behavioural response, which may be validated in further species.