Tendency of the amphipod Gammarus roeseli GERVAIS to colonize coarse sediment habitats under fish predation pressure

Many invertebrates react to the actual presence and/or to chemical cues fro m predators. However, there is lack of information on the interrelationship of different cues triggering the antipredator response. In a series of aqu arium experiments we investigated to which extent the antipredator response of a freshwater amphipod (Gammarus roeseli), i.e. seeking refuge in coarse substrate, is influenced by the actual presence of a natural fish predator (the ruffe, Gymnocephalus cernuus) including mechanical and visual cues, p urely chemical cues, as well as the thickness of the sediment layer. Presen ce of Gammarus roeseli on the substrate surface and activity was videotaped under different conditions and evaluated according to a standard protocol. Even without fish present and without fish chemicals Gammarus roeseli move d into the substrate because of thigmotactic behavior. The presence of ruff e accelerated this phenomenon and fish odor elicited weaker responses. Gamm arus roeseli was able to recognize the approach of fish predators both visu ally and/or mechanically.