An Adult Digenetic Trematode from an Invertebrate Host: Proctoeces Subtenuis (linton) from the Lamellibranch

Adult digenetic trematodes are typically parasites of vertebrates. Sexually mature trematodes occurring in invertebrate hosts are usually regarded as precocious last-larval stages, to which the term 'progenetic' is applied. These progenetic forms are often encysted, and can usually only contribute to the further life history of the species, and, in many cases, can only attain the definitive form of the adult trematode, after transference to a vertebrate host. Specimens of the mud-burrowing lamellibranch Scrobicularia plana, collected from the region of the Thames estuary, were found to be infected with unencysted, sexually mature trematodes. These parasites occurred in the kidneys of the molluscs, and, as will be discussed later, there seems no reason to regard them as other than adult digenetic trematodes. They were identified as specimens of Proctoeces subtenuis (Linton, 1907) Hanson, 1950. This species has been recorded, hitherto, only as a parasite of the hind-gut of marine fishes belonging to the families Labridae and Sparidae, from the Red Sea, New Zealand, and the eastern seaboard of America. In this paper an account is given of the occurrence, distribution, environment, morphology, and taxonomy of Proctoeces subtenuis from Scrobicularia plana, and the possible life history of this trematode in the Thames estuary is discussed with reference to the phenomenon of progenesis.