Characterisation by X-ray microanalysis of metal granules in the mucus trails of Littorina littorea (Gastropoda) along a putative pollution gradient

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Metal-containing granules in the mucus trails of the marine gastropod Littorina littorea from nine sites in north-east England were analysed for elemental composition by X-ray microanalysis and characterised relative to a putative gradient of pollution. Overall granule density varied significantly between sites, means of 6.5-17.0 per field of view (2688 μm2). Most granules found (64%) were poly-metal of a wide variety of compositions, but could be classified as Si+X, Mg+X, S+X, Na+X, P+Ca, P+Al, where X indicates any other combination of elements. Si+Al+X accounted for 61% of the poly-metal granules found and was considered to be contamination from the beach substratum. In single-metal granule form only Ca, Si, Fe, Ti, Al and Na were found. The most common single-metal granule at each site was of Ca, except at two sites, where the most common single-metal granule was of Si. The densities of these granule types varied between sites but differences were found to be significant only in the case of Si granules. Across all sites, single-metal granules of Si (mean=2.49 μm ± 1.44 SD, n=141) and Ca (2.22 μm ± 1.08 SD, n=147) were significantly larger than granules of Fe (1.74 μm ± 0.95 SD, n=63) and Ti (1.24 μm ± 0.52 SD, n=18). The range of sizes was large: Ca (0.5-6 μm), Si (0.5-10 μm), Fe (0.3-4.1 μm), Ti (0.5-2.5 μm). Between the sites there were significant differences in the size of Fe and Si granules but not Ca or Ti granules. Despite these variations in granule type and size, there was no evidence of a relationship with pollution and consequently a detoxifying function of the mucus trail in metal polluted environments is not apparent.

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