Shell growth, reproduction and life cycles of Lymnaea peregra and L. palustris (Pulmonata: Basommatophora) in oligotrophic turloughs (temporary lakes) in Ireland

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Abstract
The life cycles and shell growth rates of populations of the freshwater basommatophoran snails, Lymnaea palustris (Müller) and L. peregra (Miiller), were studied over a year's period in two oligotrophic turloughs (temporary lakes) in Ireland. The vast majority of individuals in these populations were annuals, though in one L. peregra and the two L. palustris populations a few rare individuals survived to a second year of life and reproductive effort. Lymnaea palustris populations were completely univoltine, while there was evidence of very limited bivoltinism in one of the L. peregra populations, the other being purely univoltine. Mean generation shell growth rates were the lowest yet reported for either species (≅ 30–35% of maximal mean shell growth rate reported for other natural populations) and were apparently the result of the low productivity and temperature, and highly variable water levels. In these species, there is a direct correlation between shell growth rates and habitat productivity. The vast majority of interpopulation growth rate variation appears to result from non–genetic ecophenotypic environmental influences. Such plasticity allows both species to grow and reproduce over a wide range of trophic conditions of which the studied turloughs represent the lower extreme. In L. peregra, oviposition was limited to spring and early summer and followed closely by near complete adult mortality. In L. palustris, adults survived through, and oviposition periods extended into late summer or early autumn. This extended adult survival and oviposition is a life history trait adaptation that allows L. palustris to survive in marginal, unstable habitats subject to periodic drying and other catastrophic perturbations. In contrast, the contracted spring oviposition of L. peregra appears to be an adaptation for life in more stable and permanent aquatic habitats as it allows newly recruited generations maximal time for pre–reproductive growth and minimizes competition with adults.
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