Long-term analysis and effectiveness of inducible behavioural and morphological traits of Daphnia longicephala in response to Notonecta

The freshwater crustacean Daphnia is well-known to adapt to changing predator environments by forming inducible defences. While defensive traits in many species have already been described, the actual mode of action reducing the predation rate is mostly unknown. Furthermore, the ontogenetic time line of defence development is often not described. We investigated inducible behavioural and morphological defences of Daphnia longicephala against the heteropteran backswimmer Notonecta sp. We documented defensive traits throughout seven successive adult stages. While juvenile Notonecta-exposed D. longicephala swim slower, Notonecta-exposed adults swim faster. We found morphological defences maximally expressed already at maturity. In this stage, we observe a significantly increased body length, an extended crest and an elongated tail spine. During the following adult stages body lengths of both morphs become equal, relative tail spine length is relatively reduced and crest height is kept constant, respectively. Morphological and behavioural parameters were backuped with a second clone. In line with this, we investigated the defensive effect in predation experiments with Notonecta. We found that defended D. longicephala are more frequently attacked, but also evade attacks more often and thus escape from the predators' grasp more successfully. This significantly decreases prey consumption. Moreover we tested effectiveness and mode of action of the prey defences. In summary, we provide a comprehensive view on the long-term expression of behavioural and morphological defences in D. longicephala in response to continuous exposure to Notonecta specific chemical cues.