The use of subsurface sediments by benthic prey as spatial refugia from predators can potentially influence predator consumption, and hence density-dependent relationships. We studied how predatory fish affect subsurface sediment use by spiny- and smooth-shelled morphs of the mudsnail, Potamopyrgus antipodarum, on New Zealand's South Island. Spines reduce consumption risk by small benthic fishes (e.g., bullies [Gobiomorphus spp.]). Thus, we predicted that spiny morphs would be more likely to use sediment surfaces with relatively high algal food supplies than the more vulnerable smooth morphs when exposed to fish predators. Epi- and endobenthic samples from 5 streams with fish showed that ∼80% of snails typically were retrieved from subsurface sediments, whereas only 33% of snails from fishless Jackson's Creek occurred in subsurface sediments. A laboratory experiment revealed that significantly more snails used subsurface sediments when with common bullies (G. cotidianus) than when without. Subsurface sediment use in the presence of bullies was similar between shell morphs, a result suggesting that use of spatial refugia might be a more important predator defense than shell armature for Potamopyrgus. Regardless of fish presence, more spiny than smooth morphs burrowed into sand, but subsurface use did not differ between shell morphs in cobbles and gravel. Fewer snails were active on rock surfaces at night than during the day in a stream run, apparently to reduce encounters with nocturnally hunting bullies and shortfinned eels (Anguilla australis). Our results suggest that if fish predation pressure were reduced, then surface densities of Potamopyrgus would increase significantly. Given the snail's superior competitive abilities, this increase could alter top-down and bottom-up effects in food chains and threaten local biodiversity.
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Vol. 28 • No. 4