The Biology and Evolution of Trematodes

In species assemblages of larval trematodes in individual snail hosts, fewer multispecies infections are observed than might be expected by chance. Both interspecific competition and the isolating effect of heterogeneity in recruitment may explain this pattern of community structure. Here, we analyzed the expected and observed frequency of double infections, using data culled from 62 studies. Our analysis included 296,180 host snails. Of these, 62,942 were infected with one or more species of trematode (23% pooled over all studies, 24% average across studies). By incorporating information from subsamples, we were able to estimate the proposed isolating effect of heterogeneity in recruitment. Surprisingly, spatial and temporal heterogeneity as well as differential prevalence among host size classes typically led to intensification of interactions (average increases in interactions by +19%, +19%, and +23%, respectively), while partitioning among host species usually led to isolation of potential competitors (a -1% average decrease in interactions). We calculated the expected number of interspecific double infections by applying rules of independent assortment to the frequency of trematode species. The majority of the 14,333 expected interactions did not persist; only 4,346 double infections were actually observed (a 69% decrease, 62% average). Competition, via a variety of interspecific competitive mechanisms by dominant species, is the structuring process most consistent with this paucity of observed multispecies interactions. How important is competition? Overall, we estimated that 13% (10% average) of the trematode infections were lost to interspecific interactions. Subordinate species in particular suffered very high losses. 189

https://www.annualreviews.org/doi/pdf/10.1146/annurev.es.25.110194.001201

Community Structure: Larval Trematodes in Snail Hosts

Life history traits expressed by organisms vary due to ecological and evolutionary constraints imposed by their current environmental conditions and genetic heritage. Trematodes often alter the life history of their host snails by inducing parasitic castration. Our understanding of the variables that influence the resulting changes in host growth, fecundity and survivorship is insufficient to confidently predict specific outcomes of novel snail-trematode combinations. In a literature review of the last 30 years, we found 41 publications examining various life history characteristics of trematode-infected snails. These publications reported 113 different field and laboratory experiments involving 30 snail species and 39 trematode species and provided a data set for assessing factors that potentially affect life history outcomes. Analysis of the diverse responses across various snail-trematode systems and experimental conditions teased out general patterns for the expression of host growth, fecundity and survival. These were used to address existing hypotheses and develop several new ones relating the response of snail-trematode interactions to environmental and genetic factors. Finally, we propose directions for future experiments that will better assess the ecological and evolutionary factors influencing snail life history responses to trematode parasitism.

Snail-trematode life history interactions: past trends and future directions.

A functional biology of parasitism

Spatial variation in parasitism is commonly observed in intermediate host populations. However, the factors that determine the causes of this variation remain unclear. Increasing evidence has suggested that spatial heterogeneity in parasitism among intermediate hosts may result from variation in recruitment processes initiated by definitive hosts. I studied the perching and habitat use patterns of wading birds, the definitive hosts in this system, and its consequences for the recruitment of parasites in snail intermediate hosts. Populations of the mangrove snail, Cerithidea scalariformis, collected from mangrove swamps on the east coast of central Florida are parasitized by a diverse community of trematode parasites. These parasites are transmitted from wading birds, which frequently perch on dead mangrove trees. I tested the hypothesis that mangrove perches act as transmission foci for trematode infections of C. scalariformis and that the spatial variation of parasitism frequently observed in this system is likely to emanate from the distribution of wading birds. On this fine spatial scale, definitive host behaviors, responding to a habitat variable, influenced the distribution, abundance and species composition of parasite recruitment to snails. This causal chain of events is supported by regressions between perch density, bird abundance, bird dropping density and ultimately parasite prevalence in snails. Variation between prevalence of parasites in free-ranging snails versus caged snails shows that while avian definitive hosts initiate spatial patterns of parasitism in snails through their perching behaviors, these patterns may be modified by the movement of snail hosts. Snail movement could disperse their associated parasite populations within the marsh, which may potentially homogenize or further increase parasite patchiness initiated by definitive hosts.

Spatial heterogeneity in recruitment of larval trematodes to snail intermediate hosts.

Etude de la dynamique des interactions entre H. anceps et H. occidualis pendant les mois d'hiver en utilisant des escargots marques individuellement. Les caracteristiques du parasitisme par le trematode dans la cohorte d'escargots sont etudiees et une methode de marquage-lâcher-recapture est utilisee pour suivre le cours des infections

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A Mark-Recapture Study of Trematode Parasitism in Overwintered Helisoma anceps (Pulmonata), with Special Reference to Halipegus occidualis (Hemiuridae).

Julie A. Williams

Papers

A Mark-Recapture Study of Trematode Parasitism in Overwintered Helisoma anceps (Pulmonata), with Special Reference to Halipegus occidualis (Hemiuridae).

Infra- and component community dynamics in the pulmonate snail Helisoma anceps, with special emphasis on the hemiurid trematode Halipegus occidualis