Traditionally, the field of parasitology has dealt with eukaryotic animals, to the exclusion of viruses, bacteria, fungi, etc., which is the way it will be approached here. The focus of the present paper will be on certain ecological aspects of the life cycles and life-history strategies employed by the Digenea, a diverse group of platyhelminths that includes some 25,000 species. More specifically, the review will consider the nature of host/parasite interactions within molluscan intermediate hosts and the manner in which these interactions, or lack thereof, function in structuring trematode infracommunities within these molluscan intermediate hosts. Literature in this area suggests that predation/competition may be a significant structuring force for infracommunities in certain marine prosobranchs, but not others, and that temporal/spatial factors may be involved as structuring mechanisms in at least some freshwater pulmonates.

This paper presents a perspective on the ecology of trematodes in snail hosts based on recent evidence. Because flukes use snails almost obligatorily as first intermediate hosts, we highlight the role of gastropods as keystone species for trematodes and their communities. After reviewing recent developments in the transmission of trematodes to and from snails, we discuss trematode communities within individual snails (infracommunities) and in snail populations (component communities). Results garnered using various protocols at the infracommunity level are reviewed. The few data available, all from marine systems, indicate that low colonization rates characterize infracommunities, suggesting that trematode infracommunities tend to be isolationist in character rather than interactive. The variety of trematode species present in a component community seems to be determined by spatial overlap of definitive hosts. Relative abundance of species in a component community shows little dependence on negative interspecific interactions at the level of the infracommunity. Temporal aspects of trematode communities are related to the life history of the host snail. The component communities of long-lived snails (mostly marine) integrate many infection episodes whereas shorter-lived snails (mostly freshwater) acquire new component communities each time host cohorts turnover.

A positive relationship between distribution and local abundance is often observed among species in a community. The resource-breadth hypothesis suggests that this pattern is the result of differential abilities among species to utilize available resources, such that generalists are widely distributed and locally abundant, and specialists are narrowly distributed and locally sparse. This hypothesis was tested in a community consisting of 22 species or morphospecies of parasites infecting members of 18 species of fish among 14 sites in 7 small streams in the Appalachian Mountains of North Carolina. A positive relationship between distribution (fraction of sites occupied) and abundance (average local abundance) was evident among parasite species. The number of host species infected by each parasite species was positively related to both distribution and average local abundance; both relationships held after statistical removal of the distribution and abundance of the hosts, respectively. These results support the resource-breadth hypothesis as an explanation for the distribution-abundance relationship in this system.

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The role of egg structure in transmission and habitat use of Leptorhynchoides thecatus and Pomphorhynchus bulbocolli (Acanthocephala) was investigated. During storage in tap water at 4 C, the outer membrane of L. thecatus eggs was lost, releasing ribbonlike filaments of the fibrillar coat. After similar storage, the outer membrane and fibrillar coat of P. bulbocolli eggs remained intact. Eggs of L. thecatus entangled in algae, whereas those of P. bulbocolli settled to the substratum. Leptorhynchoides thecatus infections in amphipod intermediate hosts were significantly more prevalent and dense when eggs were allowed to entangle than when they were not. Prevalence and relative density of P. bulbocolli infections in amphipods were not significantly different between trials in which entanglement was possible and those in which it was not. These results indicate that although the same species of amphipod, Hyalella azteca, is the intermediate host for both acanthocephalan species, mechanisms of transmission differ. Differences in fibrillar coats result in segregation of the environment in a manner that affects transmission and occurrence in intermediate hosts.