Macrophyte‐gastropod associations: observations and experiments on macrophyte choice by gastropods

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Abstract
SUMMARY. The abundance and microdistribution of thirteen gastropod and ten macrophyte species were monitored over 2 years in a 1 ha shallow eutrophic pond near Oxford, England. While areas of allochthonous leaf litter supported a depauperate and nondistinctive gastropod fauna, three different macrophyte habitats supported more diverse and distinctive assemblages of gastropods. Most gastropod species were clearly more abundant (number m‐2pond bottom) on one macrophyte type than on other substrates. One exception, the limpetAcroloxus lacustris(Linn.) was abundantion both the water lilyNymphaea albaL. and on emergent macrophyles.Planorbis vortex(Linn.) was associated with graminoid emergent macropytes, especiallyGlyceria maxima(Hartm.) Holmberg, whileLymnaea peregra(Mull.) was associated with submersed macrophytes, perhaps more particularly withElodea canadensisMichx. The association of, 4.lacuxtriswithN. albaand emergent macrophytes is perhaps explained by constraints of the morphology of the limpet requiring a relatively broad smooth substrate for attachment and locomotion. The reason for the associations ofP. vortexwith G.maxima, and ofL. peregrawithE.canadensisare less obvious. Experiments giving each snail species a choice between the two macrophytes showed thatP. vortexregularly exhibited a preference for G.maximabut failed to reveal substrate selection byL. peregra.Neither snail species affected the macrophyte choice of the other. The results suggested thatL. peregradid not behave naturally in experiments. The preference for G,maximabyP. vortexwas not affected by prior conditioning of the plant by the snail, but was affected by the emergent nature of G.maximaand by the presence/absence of a natural periphyton assemblage and associated detritus. The distribution ofA. lacustrisis probably determined by the physical structure of the substrate, while that ofP. vortexis determined by the presence and quality of epiphytic periphyton‐detritus. The observations and experiments withL. peregrareported here leave the reasons for its distribution in doubt.