DESCRIPTION OF A NEW ZYGOCERCOUS CERCARIA (OPISTHORCHIOIDEA, HETEROPHYIDAE) FROM PROSOBRANCH GASTROPODS COLLECTED AT HERON-ISLAND (GREAT-BARRIER-REEF, AUSTRALIA) AND A REVIEW OF ZYGOCERCARIAE

The new zygocercous cercaria is described from an intertidal prosobranch gastropod, Clypeomorus batillariaeformis Habe and Kosuge, collected at Heron Island (Great Barrier Reef, Australia), It is of the opisthorchioid type with its oral sucker modified into a penetration organ, eye-spots, seven pairs of prevesicular penetration glands, an oval or rounded epithelial excretory bladder, and a large pigmented tail with dorsal proximal finfold and slender distal portion. Cercariae form large aggregates of up to 700 individuals that swim as one. These assemble in the mantle cavity and escape through the exhalant siphon of the snail. Large aggregates are elongate and hollow, and the cercariae are arranged in a tight spiral at the periphery. The swimming of clusters is performed by simultaneous rotation of the free proximal part of the tails of the cercariae. Aggregates are photopositive. Attempts to infect several species of fish proved unsuccessful. The present account is the eleventh description of a new zygocercous cercaria and the fourth among opisthorchioid trematodes.