Comparison of antipredator responses of two related water striders to a common predator

This study compares the antipredator responses of two congeneric water stri ders that live in different habitats yet share a common predator. Aquarius nebularis and A. remigis are stream-dwelling species of water striders foun d in close association in central Kentucky. Aquarius nebularis typically in habit third- and fourth-order streams that support diverse and complex fish communities; A. remigis inhabit adjacent first- and second-order tributari es that tend to have either no fish or relatively few fish patchily distrib uted. The ecological conditions for these two species are distinctly differ ent, yet the green sunfish (Lepomis cyanellus) is a surface-feeding predato r and a potential risk to both species. In experimental pools with sunfish, an initial fish strike caused A. remigis to move to the edge of the pool a nd become inactive. For the rest of the observation period, A. remigis redu ced behaviour patterns that attract predators such as skating on open water , mating and male-male aggression. The proportion of A. remigis missing fro m sunfish pools was 5 times higher than in pools lacking fish, suggesting t hat 20% of water striders were killed by predation. In contrast, an initial fish strike caused A. nebular-is to exhibit a burst of activity as individ uals erratically skated over the open water. They then resumed the less act ive pre-attack behaviours a short time later. Aquarius nebularis showed non e of the antipredator behaviours typical for A. remigis. Furthermore, the p roportion of A. nebularis missing from sunfish pools did not differ from th at in pools without fish. Although antipredator behaviour has been examined for relatively few species of water striders, this study suggests that the behaviour of the most commonly studied species, A. remigis, may not be ind icative for water striders in general.