Predator-specific inducible morphological defenses of a water flea against two freshwater predators.

The expression of inducible morphological defenses in Daphnia in response to a single predator is a well-known phenomenon. However, predator-specific modifications of the same defensive traits as an adaption to different predator regimes is so far only described for Daphnia barbata. It is unknown if this accounts only for this species or if it is a more widespread, general adaptive response in the genus Daphnia. In the present study, we therefore investigated whether a clone of the pond-dwelling species Daphnia similis responds to different predatory invertebrates (Triops cancriformis; Notonecta maculata) with the expression of predator-specific modifications of the same defensive traits. We showed that Triops-exposed individuals express a significantly longer tail-spine, while body width decreased in comparison to control individuals. Additionally, they also expressed inconspicuous defenses, that is, significantly longer spinules on the dorsal ridge. The Notonecta-exposed D. similis showed a significantly longer tail-spine, longer spinules and a larger spinules bearing area on the dorsal ridge than control individuals as well. However, a geometric morphometric analysis of the head shape revealed significant, predator-specific changes. Triops-exposed individuals expressed a flattened head shape with a pronounced dorsal edge, while Notonecta-exposed individuals developed a high and strongly rounded head. Our study describes so far unrecognized inducible defenses of D. similis against two predators in temporary waters. Furthermore, the predator-dependent change in head shape is in concordance with the 'concept of modality', which highlights the qualitative aspect of natural selection caused by predators.