Effects of light and presence of fish on lure display and larval release behaviours in two species of freshwater mussels.

We investigated how two sympatric species of freshwater mussels transmit their parasitic larvae to fish hosts. We found that Villosa nebulosa and V. vibex both display large mantle lures to attract potential host fish, but V. nebulosa displayed only at night and V. vibex displayed mostly by day. Display periods were similar in the laboratory and in the field. In two laboratory experiments, we found that the frequency of lure display in both mussel species was unrelated to the presence of fish or to the species of fish present. However, both species released more larvae in the presence of a suitable host fish (Micropterus spp.) and a nonhost species (Cyprinella camura) than in the absence of fish. In all treatments, females released low numbers of larvae on a daily basis throughout the experiment. We also observed several, irregularly occurring major release events in which numbers of larvae released were from one to three orders of magnitude larger than minor, daily releases. In V. nebulosa, major releases occurred with suitable and nonsuitable host species; in V. vibex major releases occurred mostly with suitable host species. In an additional laboratory experiment, we found that V. vibex released large numbers of larvae only when the host fish was able to make physical contact with the mussel. Few larvae were released when no fish were present or when host fish were present but physical access to the mussel was restricted. These results show that, in mussel species that display lures, physical interaction with a fish is necessary to stimulate large releases of larvae and suggest that interactions with a suitable host species stimulate larger and more frequent releases than with nonhosts. Copyright 2000 The Association for the Study of Animal Behaviour.