Maintenance of a Trematode, Aspidogaster conchicola, outside the Body of Its Natural Host

Abstract

Aspidogaster conchicola is a trematode which usually spends its entire existence within the viscera of fresh-water mussels (Williams, 1942). Faust (1922) has recorded the same parasite from snails and from the digestive tract of a turtle in China. This species, which undergoes direct development with neither alternation of generations nor alternation of hosts, has been considered along with other species of related genera as constituting a distinct subclass of trematodes, intermediate between the MONOGENEA and the DIGENEA. Faust and Tang (1936) have proposed the name ASPIDOGASTREA for this subclass. A fairly full account of the life history stages of Aspidogaster conchicola adds support to the argument for recognizing this as a distinct subclass (Williams, 1942). The occasional occurrence of A. conchicola in other than the normal host offers evidence that this species is not rigidly fixed in its host relationships. However, the regularity with which it completes an entire cycle in the fresh-water mussels makes it seem plausible that presence in the turtle and in molluscs other than UNIONIDAE is accidental. Appearance in the body of a vertebrate might be explainable through the agency of food chains, wherein infected mussels, eaten by a turtle, permit the worms to become temporarily established in the body of an unusual host. Such secondary attainment of an additional host will be recalled as the basis commonly advanced to explain the origin of alternation of hosts among the parasitic worms. Besides the forms occurring characteristically in molluscan hosts, there are other species and genera of ASPIDOGASTREA showing normal transitions toward the regular inclusion of a vertebrate host into which the worms must pass before they can reach sexual maturity. Thus in the genus Cotylaspis, one North American species, C. insignis, lives externally on the body of UNIONIDAE, while another species, C. cokeri, is found in the intestine of fresh-water turtles and has been recorded by Simer (1929) from the intestine of a paddle-fish (Polyodon). In yet another genus, Cotylogaster, an American species attains maturity in the intestine of the fresh-water drum (Aplodinotus grunniens). Of the food habits of this fish, Forbes and Richardson (1908: 324) state that it feeds especially on mollusks, the shells first being crushed by the powerful, paved, millstone-like pharyngeal jaws. This memorandum suggests that in this species the vertebrate host might be superimposed upon a life cycle primitively complete in the molluscan body. This explanation seems particularly plausible since free swimming larval stages are wanting in the ASPIDOGASTREA as a possible means of affecting active transfer from molluscan to vertebrate host. The versatility expressed by members of the subclass ASPIDOGASTREA in their host relationships suggests the possibility that physiological adjustment between the parasite and its host is not so rigorously fixed as it is among many trematodes. It