The dynamics of two aquatic plant–snail associations

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Abstract
Submerged macrophyte communities in the Delta Marsh on the southern shore of Lake Manitoba were sampled quantitatively using a modified macrophyte sampler. Two major plant–snail associations, expressed in terms of numbers of snails per unit dry plant weight and per unit plant surface area, and in terms of snail biomass per unit dry plant weight, were found: Physa gyrina and Potamogeton pectinatus, and Lymnaea stagnalis and P. richardsonii. The associations were not constant but showed well-defined peaks correlated with growth of the respective host plants. The peaks were composed of newly hatched snails and some adults. Peaks for Physa gyrina coincided with carbohydrate maxima, especially of fructose and glucose, in the tissues of Potamogeton pectinatus. Peaks for Lymnaea stagnalis coincided with sucrose maxima in P. richardsonii. The hypothesis is presented that plant exudates may attract specific snail species. Such mechanisms reduce interspecific competition for food.