Crystalline calcium in littorinid mucus trails

Abstract

Previous work has shown that the feet of terrestrial and freshwater snails are important in calcium regulation, often secreting granules of CaCO3. This phenomenon has not, until now, been observed in marine snails. Here we report the presence of CaCO3 granules in the trail mucus of Littorina littorea (L.), L. saxatilis (Olivi) and L. obtusata (L.) Fixed mucus trails on plastic coverslips were examined by X-ray microanalysis under the SEM. Of the single-metal granules observed in the mucus trails the most abundant were of calcium (means: L. littorea, 440 mm−2; L. saxatilis, 401 mm−2; L. obtusata, 348 mm−2) followed for each species by silicon (maximum mean density: L. saxatilis, 120 mm−2) and iron (maximum mean density: L. saxatilis, 65 mm−2) granules. Single-metal granules of Al, Ti, Mg and P were also found but only in the mucus trails of L. obtusata, perhaps reflecting its different collection site from the other two species. The mean size of the calcium granules showed significant interspecific variation (L. littorea, 1.32 µm diameter±0 08 µm, n = 143; L. saxatilis, 1.80 µm±0.12, n = 113; L. obtusata, 2.14 µm±0.09, n = 167). Most calcium granules (L. littorea, 80%, n = 35; L. saxatilis, 57%, n = 113; L. obtusata, 69%, n = 167) were attached to, or embedded within, microthreads of mucus which tended to run parallel to the direction of locomotion. The significance of this is unknown although it may imply that the CaCO3 granules are secreted with the mucus. It is concluded that calcium losses via this route are too small for pedal mucus to function significantly in ionoregulation of calcium. The calcium in the trail may therefore perform other functions, for example indicating trail polarity.