PARASITISM AND THE MOVEMENTS OF INTERTIDAL GASTROPOD INDIVIDUALS

Movements of marked Ilyanassa obsoleta were charted. Six trematode species were found in 19 infection combinations. 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 support this hypothesis. Individuals 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. -from Author