Evol Ecol Res 16: 77-89 (2014)     Full PDF if your library subscribes.

Distinct snail (Physa) morphotypes from different habitats converge in shell shape and size under common garden conditions

K.D. Gustafson, B.J. Kensinger, M.G. Bolek and B. Luttbeg

Department of Zoology, Oklahoma State University, Stillwater, Oklahoma, USA

Correspondence: K.D. Gustafson, Department of Zoology, Oklahoma State University, Life Sciences West, Stillwater, OK 74078, USA.
e-mail: kyle.gustafson@okstate.edu


Background: Aspects of snail shell morphology may be plastic or genetically fixed. Even within a single population, environmentally induced shell shape plasticity can lead to unclear species identifications as a result of extreme shape variation. Extrinsic factors, such as predation pressure and stream flow, tend to induce adaptive plastic changes in shell morphology, such as elongate shells with narrow apertures and short-spired shells with wide apertures, respectively. Snail populations from a local stream and adjacent wetland exhibit these distinct morphotypes.

Questions: Do the snail morphotypes represent a single cryptic species? Are the morphotypes environmentally induced and plastic, or epigenetic?

Organisms: We captured wild Physa snails from either a stream population (low predation, high flow site) or a nearby pond population (high predation, low flow site) in Stillwater, Oklahoma, USA.

Predictions: If distinct snail morphotypes represent a single cryptic species, their phenotypes may be plastic. In this case, raising snail offspring under similar conditions of predation and stream flow will result in one shell shape and size.

Methods: We reared and maintained snail offspring of both morphotypes in laboratory aquaria (low water flow, no predation). We measured the shell morphology of wild, of first-generation laboratory, and of second-generation laboratory snails using geometric morphometrics.

Results: Shell shape and size of wild snails from the two populations were significantly different. After a single generation, however, the shell shape of both populations resembled the wild snails from the pond site (elongate with narrow apertures). Shell size decreased in the first generation, but shell size in the two populations did not fully converge until the second generation.

Conclusions: The shape differences are plastic responses to environmental variation. Thus, the two morphotypes constitute a single snail species (Physa acuta). The single generation lag in size convergence suggests there is an epigenetic difference between generations within populations.

Keywords: geometric morphometrics, morphology, phenotypic plasticity, predation, water flow.

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