Velacumantus australis : (gastropoda: potamididae) and its trematode parasites: some aspects of the host parasite relationship.

Abstract

The mud whelk, Velacumantus australis, common in estuaries and coastal lakes in southern regions of Australia, is parasitised by several trematode species. These include an echinostome, Acanthoparyphium spinulosum; a heterophyid, Stictodora lari; two philophthalmids, Philophthalmus burrili and Cloacitrema narrabeenensis; a schistosome, Austrobilharzia terrigalensis and three other species, as yet undescribed, but herein called Cercaria I, Cercaria II and gercaria III. Previous studies on the relationship of V. australis to these trematodes have concentrated on describing the life cycles of the parasites and on determining aspects of distribution and prevalence.

In this study, concerned to examine this host-parasite relationship at the level of the individual host, three areas in Sydney were sampled, Tambourine Bay on the Lane Cove River, and Drummoyne and Rodd Point in Iron Cove. Snails collected were dissected and thin sections prepared from paraffin embeded specimens so as to determine the sites of trematode development, the pathology caused by the trematodes and any reaction by the host. Except for C. III, all the trematodes develop in the haemocoelic spaces surrounding the gonad tubules, two species, A. terrigalensis and C. II also inhabiting the spaces between the digestive gland tubules. The rediae of C. III are found in blood sinuses in the kidney-rectum area. Those species developing in the gonad region in mature snails cause the destruction of the host's gonad, presumably by indirect effects such as enzymatic action or metabolic inhibition, as there was little evidence for direct ingestion of gonad tissues. In sexually immature snails the germinal sacs were still found in the same sites, and prevented any gonad from developing. The fact that such immature snails were found infected resolved the question of the susceptibility of immature V. australis to trematode infection and demonstrated clearly that they are not immune until they become sexually mature. Trematodes developing in sites other than the gonad region caused little pathology and normally developing germinal sacs provoked no response by the host. Under abnormal circumstances, however, the host did react. Some of the trematodes were infected with a haplosporidian parasite which greatly reduced their viability and allowed the host to react against them. Reactions were also found to sporooysts and rediae developing in abnormal sites, especially the head-foot area. Trematode species which were subordinate in mixed infections were attacked by the other parasite species and then, following damage, were destroyed by a host reaction. In mixed infections the schistosome A. terrigalensis was always dominant and retarded the development of the other species by a form of indirect inhibition. In fact, this species was never found in single infections and apparently needs the presence of other trematodes in V. australis in order to survive and develop. It is able to make maximum use of the host's tissues by preventing the other species from growing normally, both in size and in numbers.

The relationship of V. australis to its trematode parasites has been shown to be typical of such relationships from the points of view of sites of development, pathology and host reactions to the parasites. It is unusual, however, in the apparently obligatory association of A. terrigalensis with other trematodes.