Substratum heterogeneity, crypsis, and colour polymorphism in an intertidal snail (Littorina mariae)

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Abstract
On the British and Irish coasts, Littorina mariae showed extensive shell colour variation; the brown morph, dark reticulata, was common in exposed conditions, and the bright yellow morph, citrina, was common in shelter. Fucus serratus, the major substratum of L. mariae, contained a number of discreet colour backgrounds when viewed from the hunting position of Blennius pliolis, a predator on juvenile L. mariae. These backgrounds corresponded closely to the colours of the principal morphs. The opaque stem is dark brown, while the lamina, when viewed with reflected light, is olive–brown. With light transmitted through the lamina, that is. when viewed from beneath the fronds, it appears a brilliant yellow. In predation experiments with B. pholis, citrina was taken preferentially to dark reticulata on the stem and on the lamina perceived by reflected light; with transmitted light, dark reticulata was preferred.Examination of microdistributions showed juveniles prevalent on the lamina in sheltered shores but located on the stem and holdfast in exposed shores. The differences in adult morph frequencies between shores corresponded to the abundance of specific backgrounds on which juveniles were found. Stability in frequencies is discussed with reference to the heterogeneous backgrounds and density-dependent predation.
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