Journal of the Royal Society of New Zealand abstracts

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Clinal variation in shell morphology of the freshwater gastropod Potamopyrgus antipodarum along two hill-country streams in New Zealand

Martin Haase

National Institute of Water and Atmospheric Research Limited,
P.O. Box 11 115, Hamilton, New Zealand.
Email: m.haase@niwa.co.nz

Abstract  The extremely polymorphic gastropod Potamopyrgus antipodarum is widespread in freshwater habitats throughout New Zealand. This study describes morphological variation of P. antipodarum along two streams in Waikato (North Island, New Zealand), and explores relationships between morphology and selected environmental variables (temperature, conductivity, pH, flow, shade, vegetation). Along each stream, fully grown snails were collected at 11 sites extending from the source of flow to 1 or 1.5 km downstream. In both streams, shells increased in size and in one stream they also became more obese with distance from the source. Of the environmental variables assessed only flow, which increased downstream, was found to have potential influence on shell morphology. Larger snails have a larger foot and thus a larger attachment area and therefore may be able to withstand higher currents, despite having a higher Reynolds number resulting in increased drag on them relative to smaller individuals. Other studies have found that clines of morphological variation in lakes are apparently driven by the differential risk of infection by parasites. This can be excluded for the stream populations in this study, because parasitised individuals were absent at upstream sites, suggesting that similar morphological patterns may have entirely different causes.

Keywords  flow; Gastropoda; morphology; polymorphism; Potamopyrgus antipodarum; Waikato

R02027 Received 22 July 2002; accepted 14 February 2003; published 18 June 2003
© Journal of the Royal Society of New Zealand, Volume 33, Number 2, June 2003, pp 549-560

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