Age, Sex, and Parasites: Spatial Heterogeneity in a Sandflat Population of Ilyanassa Obsoleta

Abstract

An intensive sampling study, undertaken in a natural sandflat population of an estuarine neogastropod, Ilyanassa obsoleta, revealed a marked degree of spatial heterogeneity for a number of important characteristics. Growth rate could be inferred by observing changes in size—classes over time, but the inequitable spatial distribution of size—classes renders size/frequency analysis impractical and of dubious predictive value for this population. Overall parasitism rate in this population is high, generally increases exponentially with increasing age, and creates a relatively narrow reproductive fenestra temporalis by apparently permanent sexual impairment of infected snails. In fact, the strong influence of parasites on behavior and physiology of snails may necessitate distinguishing between apparent and functional individuals, with regard to their role in estuarine benthic communities. However, incidence of both parasitism and parasite species is highly variable in space. Nearly half of all samples taken departed significantly from the overall sex ratio, indicating that distribution of sexes is also heterogeneous. Since there is little predictive relationship between specific samples and overall population trends, it is likely that differentiating between trends that are truly temporal and those that are in reality spatial is difficult at best. Moreover, high spatial heterogeneity in the field has implications for the interpretation of laboratory studies as well as those done in the field and indicates that generalizations about the biology and ecology of this species gleaned from small, albeit random, samples should be viewed with caution.

Trematode Infections inLittorina littoreaon the New Hampshire Coast

“…A bias for females to be infected by trematodes in populations of intertidal snails was also shown for Ilyanassa obsoleta (Say) (Eastern Mudsnail;Curtis and Hurd 1983), L. littorea (Hughes andAnswer 1982, Pohley 1976), L. saxatilis (Olivi) (Rough Periwinkle), and L. obtusata (L.) (Yellow Periwinkle) (Pohley 1976). However, Pohley (1976) also found more male L. saxatilis and L. obtusata than female snails infected in populations in Eastport, ME.…”

Can the studies at a spatial scale of 100s meters detect the spatiotemporal fluctuations of a parasite assemblage?

“…Positive relationship between bird abundance and the presence of larval digenean parasitizing snail hosts on small to medium spatial scales were found by several studies (e.g. Curtis and Hurd 1983;Hechinger and Lafferty 2005;Koprivnikar et al 2007;Merlo and Etchegoin 2011). In addition, Prinz et al (2010) observed that the temperature increase coincides with a large number of definitive hosts and with a consequent increase in parasite prevalence in the mollusc hosts.…”

Impact of trematodes on host survival and population density in the intertidal gastropod Zeacumantus subcarinatus

“…In snail-trematode associations it is commonly observed that trematode prevalence increases with increasing snail age (Whitlatch 1974, Hughes & Roberts 1981, Hughes & Answer 1982, Curtis & Hurd 1983, Sokolova 1995, Kube et al 2002. The simplest explanation is that older snails have been exposed to trematode infections for a longer period of time and therefore have a greater probability of being infected.…”