Effects of ambient UV-radiation on the behaviour of mayfly larvae of the genus Deleatidium from trout bearing and fishless streams in New Zealand

Abstract

New Zealand ground levels of UV-radiation have always been comparatively high, however, little attention has been paid to the effects of UV-radiation on biotic interactions in New Zealand freshwater systems. In two outdoor short term microcosm experiments we tested the effects of UV-radiation on the behaviour of grazing mayfly Deleatidium (Ephemeroptera) larvae. The relationship between pigmentation of the Deleatidium larvae and their UV-related behavioural response was also examined. The first experiment showed that Deleatidium larvae were able to detect and actively avoid ambient levels of UV-radiation. The results of the second experiment suggested that Deleatidium larvae spent more time grazing on exposed surfaces if the UV-radiation was attenuated. There was also an effect of fish, larvae from trout streams were more reluctant to feed on periphytic algae than larvae from fishless streams. Larval pigmentation did not affect the UV-radiation related behavioural response. Although the presence of trout has been considered the main a major force for low daytime grazing in by Deleatidium, the present results suggest that UV-radiation can also be a factor responsible for the avoidanceance of exposed surfaces of stones during daytime.