New Zealand Journal of Marine and Freshwater Research abstracts

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Physical microhabitat effects on 3-dimensional spatial variability of the hydrobiid snail, Potamopyrgus antipodarum

Joseph R. Holomuzki

Department of Evolution, Ecology, and Organismal Biology
The Ohio State University
1760 University Drive
Mansfield, OH 44906, United States
email: holomuzki.3@osu.edu

Barry J. F. Biggs

National Institute of Water and Atmospheric Research Limited
P.O. Box 8602
Christchurch, New Zealand
email: b.biggs@niwa.co.nz

Abstract We studied how sediment size and current velocity affect epi- and endobenthic spatial variability of the hydrobiid snail, Potamopyrgus antipodarum, on New Zealand’s South Island. A multi-stream survey showed that P. antipodarum densities were lowest on gravel and highest on cobbles, and that densities on stones and sand were comparable. Sand patches were associated with the lowest current velocities, which likely enhanced the suitability of this microhabitat for snails. Snail densities in sample reaches (averages 19 to 442 individuals/m²) sharply declined at current velocities above c. 15 cm/s, suggesting a velocity-density threshold. A colonisation experiment conducted in a slow-velocity run revealed that more P. antipodarum use subsurface sediments than surface sediments, and that frequency of subsurface use may be habitat-mediated. Approximately 80% of snails on gravel, pebbles, and cobbles used subsurfaces, whereas only c. 50% of snails in sand occupied subsurfaces. A laboratory flume experiment showed that snails burrow into sand even at relatively slow current velocities (18 cm/s) apparently to avoid hydraulic forces or abrasion. Our results suggest that the large portion of snails occupying sediment subsurfaces must be accounted for in an accurate population census. Our findings also imply that Potamopyrgus surface densities could increase significantly, and possibly threaten local biodiversity, when the areal extent of slow velocity zones are increased by natural or human-caused flow reductions.

Keywords surface and subsurface sediment use; sediment size; current velocity-density threshold; hydraulic forces; burrowing; patch colonisation

New Zealand Journal of Marine and Freshwater Research, 2007, Vol. 41: 357–367
0028–8330/07/4104–0357     © The Royal Society of New Zealand 2007

M07033; Online publication date 9 November 2007
Received 3 July 2007; accepted 3 October 2007

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