Abstract

Two hypotheses have been advanced to explain the evolution of host responses to parasites: the arms race-evolutionary lag and equilibrium hypotheses. We investigated predictions from these hypotheses based on interspecies host preferences and adaptations in an obligate spawning relationship between a freshwater fish, the European bitterling (Cyprinidae) and four species of freshwater mussels (Unionidae), which the fish use as hosts for their eggs. We found a significant trend in preference by the fish for mussels in the following order: Unio pictorum, U. tumidus, Anodonta anatina and A. cygnea. Male and female bitterling both showed this ranking and the clutch sizes deposited into each species also followed this trend. These host preferences proved to be adaptive in terms of egg ejection, which was lowest in the most preferred species (U. pictorum). Furthermore, these hierarchical host preferences were flexible, as females switched species when individuals of the preferred species ejected a greater number of eggs. The similarity in mussel defences between the U.K. population and. a European population of ancient sympatry suggests that the absence of a defence in some mussel species may not be due to evolutionary lag. Mussel ejection behaviour may have reached an evolutionary equilibrium in each host species, or alternatively the fish may have evolved adaptive preferences that coincide with generalized mussel responses to foreign objects in their gills. (C) 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.