Abstract
Prevailing theory suggests that many parasite species have evolved in tight congruence with their hosts, with the parasite phylogeny mirroring that of the host. This theory is based largely on studies of host-parasite interactions between species where strong links would be expected. For example, the highly congruent evolutionary trees of procellariform seabirds and their feather lice (Paterson et al., 1993) are expected as oceanic seabirds breed in large, often monospecific, colonies and lice do not survive long away from their host. Consequently, in this case, there are few opportunities for host switching. Our research employs molecular techniques to test the theory of tight host-parasite congruence in a host-parasite system where high host specificity is not necessarily expected; Trochoidea (topshell snails) and digenean trematodes (flatworms). Due to the fine-scale sympatry of the topshell hosts and the complex life cycle of digeneans, the parasites are likely to encounter a range of potential hosts, allowing ample opportunity for host-switching. Topshells belonging to the genera Melagraphia and Diloma are ubiquitous in the New Zealand intertidal zone, with seven species currently recognised. Six of these species are endemic, the exception being D. nigerrima, which also occurs in Chile. Related topshells are found in other parts of the Pacific, most notably along the southern coast of Australia, where seven species are classified as belonging to the genus Austrocochlea. Despite different topshell species having slightly different ecological requirements, species may exist in sympatry e.g. during a preliminary study at Purakaunui Inlet, near Dunedin, six species were found within a 20 m radius. Melagraphia, Diloma and Austrocochlea are all potential first-intermediate hosts of digenean trematodes, which infest the snail’s digestive gland. During their life cycle digeneans usually parasitise three hosts. The second-intermediate and definitive hosts of these digeneans are unknown, but are likely to be a crustacean and a fish, respectively. Digenean eggs are shed into the water column in the faeces of the definitive host and so, due to the sympatric nature of topshell distribution, developing larvae are likely to encounter a number of potential host species and opportunities for host-switching are great. Topshell phylogeny is currently unresolved, with poorly defined generic boundaries and digenean phylogeny is even less well understood; digeneans which parasitise Melagraphia, Diloma and Austrocochlea consist of a single morphotype which has tentatively been placed in Opecoelidae (Clark, 1958; Miller and Poulin, 2001). Species assignments and phylogenies of both topshells and their digeneans trematodes are being resolved using DNA sequence data. Questions concerning the dispersal, biogeography and evolution of both host and parasite are being addressed by analysing this molecular data in conjunction with data on their geographic distributions. Ultimately the goal of this research is to answer questions about the co-evolution of topshells and their parasites, such as: are the two trees congruent? is there evidence of host-switching and host-addition?