Local adaptive pigmentation in Asellus aquaticus -effects of UV-radiation and predation regime

Sammanfattning: Pigmentation in the freshwater isopod Asellus aquaticus differed between two habitats. Asellus were lighter pigmented in stands of submerged stonewort vegetation, than in nearby reed stands. Moreover, Asellus pigmentation became lighter with time in recently established stonewort stands. Diet or pigment adjustment had minor influence on pigmentation and mating was random with respect to pigmentation. Crossings of light and dark Asellus showed that pigmentation had a genetic base. These results indicate that altered pigmentation instead resulted from responses to natural selection. Studies on selective forces indicated that predation was a possible explanation. Results from experiments with two common predators, a fish (perch) and a damselfly suggested that predation from visually oriented fish on the least cryptic Asellus could explain the differences in pigmentation. Asellus pigmentation did not affect predation risk from the damselfly. Further examined effects of perch and damselfly showed that perch fed on both large and small Asellus but preferred large. The damselfly fed on small Asellus but was not able to feed on the largest. This size refuge could explain why small Asellus reduced their activity and feeding when exposed to damselfly cues, whereas large Asellus did not. Moreover, both small and large Asellus reduced their activity when exposed to chemical cues from perch for less than one hour, whereas reduced activity was restricted to larger Asellus in a three weeks experiment. The difference in behaviour between small and large Asellus indicates that Asellus can assess the predation risk and adapt behaviourally. In mesocosms, with natural vegetation, perch had no effect on Asellus survival, but induced an aggregation of Asellus. Together, the results indicate that anti-predatory traits such as cryptic pigmentation, reduced activity and aggregation can lower predation risk from perch in structural complex habitats. Another laboratory experiment suggested that the damselfly could feed efficiently in structural complex environments. Since fish predation can be reduced in dense vegetation damselflies might thus be important predators. The stoneworts can reach the water surface during summer and Asellus in the vegetation might therefore be exposed to UV-radiation. I examined differences in tolerance and behaviour between dark and light pigmented Asellus when exposed to natural levels of UV-radiation. The results showed that dark Asellus survived exposure longer than light ones. Furthermore, Asellus avoided UV-radiation independent of pigmentation. The results suggest that also in lowland lakes UV-radiation may influence littoral food webs in via behavioural responses on invertebrates.