Life-History Variation, Phenotypic Plasticity, and Subpopulation Structure in a Freshwater Snail

Abstract

Earlier studies of habitat‐specific subpopulations of mixed clonal and sexual freshwater snails of the species Potamopyrgus antipodarum have revealed clinal variation by depth in several life‐history traits, risk of parasite infection, mixed population structure, and the genetic structure of the clonal population. Clinal variation is pronounced in life‐history traits: snails are larger and start reproduction later in the deeper habitats. The proportion of clonal individuals increases with depth, and many clones are habitat‐specific. While these patterns are well documented, it is not known which processes have led to the observed genetic divergence in the clonal population. In this study, we experimentally investigated the contribution of phenotypic plasticity to habitat‐specific life‐history trait variation using reciprocal transplant experiments with adult and juvenile snails. Assessment of phenotypic plasticity is important because canalized habitat‐specific life‐history trait variation is one of the alternative explanations for habitat‐specific genetic divergence seen in the clonal population. However, if life‐history trait variation is largely due to adaptive phenotypic plasticity, canalized optimization of life‐history traits is unlikely to explain the divergence observed in the clonal structure. We found significant habitat‐induced variation for growth rate, proportion of brooding females, brood size, number of surviving offspring, and juvenile survival, indicating that much of the life‐history variation must be considered the result of phenotypic plasticity. Based on these results, it seems that life‐history trait divergence is unlikely to explain habitat‐specific clonal structure. In contrast, we found genetically based differences in resistance to parasite infections; snails of the deeper Isoetes habitat were more susceptible to infection than snails of the shallow shorebank habitat. To our surprise, we found only a few habitat‐by‐origin interactions that could directly contribute to the maintenance of the observed habitat‐specific clonal structure. One potentially important interaction, however, was that in the deeper Isoetes habitat, reproductive output of snails transplanted from the shallow habitat was lower than that of resident snails. In addition, we also found that survival of clonal snails may be poorer in unfamiliar habitats than that of sexual P. antipodarum, potentially promoting the maintenance of habitat‐specific clonal assemblages. Thus, higher parasite resistance of shallow‐water snails in both habitats and lower reproductive output of shallow‐water snails in the deep habitat are likely to contribute to the maintenance of the habitat‐specific clonal structure in P. antipodarum populations, whereas most of the observed variation in life‐history traits seems to be due to phenotypic plasticity, which is likely to be adaptive.

A Periphyton-Based Diet Results in an Increased Growth Rate Compared to a Detritus-Based Diet in the Invasive New Zealand Mud Snail (Potamopyrgus antipodarum).

“…Individual clones in New Zealand differed significantly in size at maturity, brood size and susceptibility to parasites (Jokela et al 1997a, Dybdahl andLively 1998). These same traits may exhibit plastic variation under different environmental conditions, and these clone-specific traits remain distinct (Negovetic andJokela 2001, Dybdahl andKrist 2004). A thorough understanding of the complex interplay between physiological tolerances and resulting life history traits of individual mud snail clones in invading populations is important for managing and predicting the spread of this species in North America.…”

Explaining variation in life-history traits: growth rate, size, and fecundity in a marine snail across an environmental gradient lacking predators

“…food) should lead to different phenotypic responses (Stearns 1992, Chase 1999. Variability in life-history traits has been extensively studied in both land and aquatic snails (Baur & Raboud 1988, Johannesson & Johannesson 1996, Johnson & Black 1998, Trussell 2000, Negovetic & Jokela 2001. These animals are particularly well-suited to such studies since their gene flow can be limited -adults move little (Little 1989), and many species have direct development of juveniles (Johannesson 1988, Little 1989, Kyle & Boulding 2000.…”

Clinal variation in shell morphology of the freshwater gastropodPotamopyrgus antipodarumalong two hill‐country streams in New Zealand

“…This combination of attributes has rendered P. antipodarum an ideal model on which to test hypotheses of life history theory (e.g., Lively 1987;Jacobsen & Forbes 1997;Levri 1999;Negovetic & Jokela 2001). In this context, shell morphology has been investigated most rigorously.…”