Differential growth rates and calcium-allocation strategies in the garden snail Cantareus aspersus

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An optimal division of a key resource between growth and reproduction is expected to produce consistent life history schedules in habitats where its supply is highly predictable. However, differential growth rates are found between populations and within broods of Cantareus aspersus, a simultaneous hermaphrodite for which the reproductive benefits of a large body size may favour rapid growth. Although energy is usually assumed to be the limiting resource in allocation theory, calcium limits the distribution, growth and reproduction of snails. This is a very consistent resource and populations may have allocation strategies which reflect availability in their habitats. Three experiments compared Ca allocation in the progeny of six populations from Ca-rich and Ca-poor habitats. In the first, 100 d-old juveniles were compared between populations for their shell/soft-tissue dry weight ratio, their allocation of Ca to each compartment, and the variability within broods. The second measured growth, food consumption and shell ratios in growth trials of three populations on low Ca. Thirdly, five populations were compared on abundant or excess Ca. The relationship of shell Ca with soft-tissue levels differs between populations, but shell ratios changed with Ca availability in all populations. Most favoured soft-tissue growth when dietary Ca is low, but one population (LE) always had the highest shell ratios in these trials. Ca in the parental habitat was not a good predictor of juvenile-allocation strategies, but the consistency of LE shell ratios across several broods suggests theirs may be an inherited trait. LE has faster growth rates and a preference for shell building, which probably represents a strategy for early reproduction. The robustness of a snail's shell may thus be more indicative of its reproductive strategy rather than Ca availability in its habitat.

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