Comparison of calcium storage between a Baikalian gastropod and holarctic relatives

In Lake Baikal, extremely thin shells are reported as a typical feature of endemic gastropods. This statement derived only from observations; no experimental data were available up to now. Therefore, we quantitatively investigated the calcium distribution in the endemic prosobranch gastropod Benedictia baicalensis and compared the results with those of Lithoglyphus naticoides, a near relative, non-endemic, palaearctic species. The shell of the endemic mollusc B. baicalensis consists of 94.9+/-26.0 microg Ca(2+)/microl animal volume (n=43), and in L. naticoides 865.0+/-271.5 microg Ca(2+)/microl (n=10). Calcium contents in the tissue of B. baicalensis vary between different sampling stations and different sampling dates (from 9.4+/-5.1 (n=33) to 20.5+/-8.4 microg Ca(2+)/mg dry weight DW (n=16)) and are only 1/5-1/10 compared to L. naticoides (88.5+/-39.1 microg Ca(2+)/mg DW (n=9)). But the values for hemolymph calcium concentration and osmolality in both species are identical (B. baicalensis: osmolality: 84.4+/-5.3 mosm/kg (n=40); hemolymph calcium concentration: 4.6+/-1.7 mmol/l (n=40). L. naticoides: osmolality: 85.0+/-2.0 mosm/kg (n=8); hemolymph calcium concentration: 5.2+/-5.0 mmol/l (n=40).). This is the first experimental study demonstrating, that - besides a similar hemolymph ionic composition - the Baikalian species is characterized by significantly lower calcium storage in shell and tissue than the nearly related non-endemic species.