A transmissible trematode affects the direction and rhythm of movement in a marine gastropod

Microphallus piriformes (Trematoda) is unusual In having only two hosts and no motile free-living stages. The intermediate host, the rough periwinkle, Littorina saxatilis, is present Lear-round on rocky shores and has a high parasite prevalence near breeding colonies of the definitive host, the herr ing gull, Larus argentatus, which is present in numbers at these sites for only 4 months per year. Given the seasonal availability of gulls for infect ion and a low incidence of periwinkles in the normal diet of herring gulls, specialized transmission between stages appears necessary for maintenance of the parasite's life cycle. We investigated the hypothesis that M. pirifo rmes alters its intermediate host's behaviour during the gull's breeding se ason in a manner that may facilitate predation of the infected periwinkle b y breeding gulls, We studied the movements of periwinkles during simulated tidal cycles in the laboratory; parasite status was established subsequentl y. Periwinkles with mature infections moved further upwards but showed less downwards and horizontal movement than uninfected periwinkles. The movemen t of periwinkles with immature (nontransmissible) infections was less affec ted by the parasite. During the tidal cycle, infected and uninfected periwi nkles differed in both timing and extent of movement. A field experiment co nfirmed the greater upwards movement of infected periwinkles. The parasite- induced changes in periwinkle behaviour may increase the chances of predati on by the final host and could represent an important survival strategy for M. piriformes. (C) 2000 The Association for the Study of Animal Behaviour.