Parasite transmission in the intertidal zone: Vertical migrations, infective stages, and snail trails

Ilyanassa obsoleta (Say) (Gastropoda), when infected by Gynaecotyla adunca (Linton) (Trematoda), repeatedly migrate to the higher intertidal zone. The parasite's next hosts are semiterrestrial crustaceans, which are infected by cercariae from snails. Previous papers suggested that host snail migrations are parasite-induced to favor cercarial transmission to semiterrestrial hosts. Two main questions were posed to address this possibiligy using G. adunca-infected snails on Cape Henlopen, Delaware, USA. When and where are cercariae released during snail migrations; and what schedule do these migrations follow? Few cercariae were released by infected snails when confined in jars, or caged, in the intertidal zone during high tide. However, abundant cercariae were found in the mucus trails made by migrating host snails when they are left aerially exposed by ebbing tides. Mid-day trails lacked cercariae, but they were often abundant in nighttime trails. Cercariae are thus deployed in the exposed, nighttime intertidal zone. There, contact with land-based, nocturnal crustacean hosts is possible. Migratory behavior occurs from late June through mid-September. Data show that most G. adunca-infected snails migrate upward when future transmission of cercariae can occur, i.e. on flood tides that precede nighttime low tides. The temporal occurrence of these flood tides during summer, and possible cues for responding to them, are considered.