Parasitism and the Movements of Intertidal Gastropod Individuals

Movements of marked individuals of Ilyanassa obsoleta (n = 500) were charted in an intertidal environment for about one week. At the end of observations, 260 marked individuals, which had been sighted 1017 times collectively, were recollected and examined for trematode infections. Six trematode species were found in 19 infection combinations including uninfected, singly, doubly, and triply infected snails. We know that most snails found high on beaches and on sandbars carry Gynaecotyla adunca infections. It has been hypothesized that this host behavior modification is a parasite adaptation to enhance cercarial transmission to a semi-terrestrial next host. Observations reported here support this hypothesis and reveal some of the complexity in the behavior imposed on I. obsoleta by G. adunca. Individuals that were uninfected or infected with other parasites demonstrated no unique movement patterns, but individuals infected with G. adunca made repeated excursions into the upper shore habitat. These excursions were timed so that host-parasites were left emerged at high elevations primarily during nighttime low tides. Because many snails were multiply infected, data presented support the idea that gastropod populations have the potential to be used as systems for the study of the nature of ecological and evolutionary interactions among parasite species.