Littorina compressa, L. arcana and L. saxatilis s.s. are all common on this shore, but at this state of tide, about two hours before H.W. spring tide, when hundreds of L. arcana and L. saxatilis s.s. were exposed, nearly all L. compressa were submerged as they live lower in the tidal range.
L. compressa have been studied here by Hannaford Ellis (1979) and Naylor and Begon (1982). Unstable boulders in the foreground, that pose a danger of crushing, were found by Naylor & Begon to have L. compressa with thick shells resembling those associated by Reid (1996) with sheltered to moderately exposed shores where crabs pose a threat. In the stable crevices of the rock stack (left of image) they found specimens resembling Reid’s “wave-exposed ecotype” with thin shells (as crabs scarce on exposed shores), short spires, and wide aperture to accommodate a large foot for a firm grip.