SPECIFICITY OF THE HOST RECOGNITION BEHAVIOR OF LARVAL UNIONICOLA (ACARI, UNIONICOLIDAE) - THE EFFECTS OF LARVAL ONTOGENY AND EARLY LARVAL EXPERIENCE

The water mites Unionicola formosa and U. foili are obligate symbionts of the freshwater unionid mussels Pyganodon cataracta and Utterbackia imbecillis, respectively. Although behavioural specificity of adult U . formosa and U. foili is well documented, behaviour of the larvae of these species is poorly understood. The present study revealed that ho st-influenced behaviour of larval U. formosa and U. foili is highly ho st-specific, but changes during larval ontogeny. Larvae that had compl eted their parasitic phase with chironomid dipterans (=post-chironomid larvae) preferentially responded to a chemical signal from their resp ective host mussel, and exhibited the host-induced negative phototaxis that characterizes adult mites. Specificity of the host recognition b ehaviour of post-chironomid larvae was modified when larvae were expos ed to water modified by a non-host mussel species early in development . The chemically mediated behaviour patterns exhibited by post-chirono mid larvae were absent among mite larvae prior to parasitizing chirono mids (=pre-chironomid larvae). These data suggest that parasitism of a chironomid is adaptive for the ontogeny of behaviour of these mussel- mites at specific stages in the mite's life cycle. Differences in the behaviour of pre-chironomid and post-chironomid larvae may reflect the different demands of contacting an insect host prior to establishing the symbiotic association with the final molluscan host. The behaviour al preferences of larval U. formosa and U. foili complement those of a dult mites and suggest that host specificity is an important mechanism in maintaining reproductive isolation between these two species. (C) 1995 The Association for the Study of Animal Behaviour