Shell size divergence in Littorina mariae and L. obtusata and predation by crabs

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Abstract
On the British and Irish coasts, Littorina mariae and L. obtusata show geographical variation in adult shell size, the former decreasing and the latter increasing in localities with increased protection from wave action. Within some shores, L. obtusata is relatively small in upper tidal positions but increases in size towards the lower shore where it overlaps with the much smaller shelled L. mariae.Characteristic signs of repair observed on the shells are considered to have resulted from unsuccessful attacks by crabs such as Carcinus maenas. High frequencies of these shell injuries were associated with the presence of large-shelled populations of L. obtusata both between localities and between different levels on the shore.Experimental work showed that large-shelled L. obtusata (16–17 mm) were exceptionally resistant to breakage by large C. maenas while smaller adult shells of L. obtusata (13–16 mm) and those of L. mariae (6–13 mm) were very susceptible. Small C. maenas were unable to successfully crack the thickened outer whorl of adult shells of either species and could only break the thin shells of juveniles. It is argued that small adult size, as seen in L. mariae, could represent an adaptation for a contracted juvenile stage and thus early maturity where small crabs are important predators.