Comparisons of calcium sources between arboreal and ground-dwelling land snails: implication from strontium isotope analyses

Calcium (Ca) is an essential element for many animal species, especially for land snails with shell structures. The availability of Ca imposes constraints on their growth and survival as well as their density and distribution. However, little is known about how they obtain Ca in a natural environment. For example, many species of land snails feed on lichens, but lichens have Ca in the form of calcium oxalate, making it difficult for animals to utilize. Therefore, we predicted that lichens or a representative food source of land snails is not necessarily a major source of Ca, and there may be another source for shell development. The strontium isotope ratio (⁸⁷Sr/⁸⁶Sr) is a useful index for estimating the source of Ca. With this technique, we estimated Ca sources of an arboreal land snail, Euhadra brandtii sapporo, and tested the hypothesis described above. The data for the ⁸⁷Sr/⁸⁶Sr ratio show that E. brandtii sapporo did not obtain Ca from lichens and moss in tree canopies, although the data for the δ¹⁵N and δ¹³C show that the species’ main food source may be lichens and moss. We also conducted the isotope analysis of Ezohelix gainesi as a reference, which inhabits the forest floor. The data of δ¹⁵N and δ¹³C show E. gainesi may feed mainly on litter. The ⁸⁷Sr/⁸⁶Sr ratio was markedly different from that of arboreal E. brandtii sapporo, and this ground-dwelling snail likely obtains Ca from ground litter or soil. Our results indicate that Ca sources are different between arboreal and ground-dwelling land snails, and the former obtains Ca through something other than its major food sources, perhaps from stemflow and precipitation. Differences in the two species indicate that land snails utilize different Ca sources depending on their habitats.