THE METACERCARIAE OF SIBLING SPECIES OF MEIOGYMNOPHALLUS, INCLUDING M-REBECQUI COMB-NOV (DIGENEA, GYMNOPHALLIDAE), AND THEIR EFFECTS ON CLOSELY-RELATED CERASTODERMA HOST SPECIES (MOLLUSCA, BIVALVIA)

The morphology and ecology are described for four sibling species of d igenean metacercariae: Meiogymnophallus minutus (Cobbold 1859), M. str igatus (Lebour 1908) from the estuarine cockle Cerastoderma edule L. a nd M. fossarum (Bartoli, 1965) and M. rebecqui (Bartoli, 1983) comb. n ov. from the brackish water lagoon cockle C. glaucum (Poiret). Althoug h occupying different sites within the two species of cockle, both M. minutus and M. fossarum infections result in pathological effects that may cause heavy host mortality. Remarkably, they also result in an in version of the cockles in situ in the sand such that predation by the final host species is enhanced.