Variation in shell morphology of Calliostoma zizyphinum (Gastropoda: Trochidae)

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Differences in shell morphology in the intertidal prosobranch mollusc Calliostoma zizyphinum were studied from a number of sites within four geographical regions of the British Isles with varying exposures to wave action and crab predation. Mean values of damage scarring were highest in shells sampled from Strangford Lough, Northern Ireland, and lowest in individuals from the Atlantic Coast. Shells collected from the Isle of Man were smaller than those from either the Atlantic coast of Ireland or Strangford Lough. Shells from Strangford Lough had taller shells (higher aspect ratio) than shells from the County Down and Atlantic Coast and shells from the County Down coast were more squat (lower aspect ratio) than those from all other areas. Shells from Strangford Lough were significantly thinner than shells from all other geographical areas. The relationships between shell damage scarring and shell size and tallness were not consistent among areas. Shells from Strangford Lough are notable in showing a steep, positive relationship between shell size and scarring and a steep, negative relationship between scarring and tallness. The unusual shell phenotypes observed in shells from Strangford Lough may be explained by rapid shell growth, which would not only allow Calliostoma to attain a size refuge from crab predators but also to recover successfully from repeated crab attacks on the shell lip. Such a strategy would result in larger, thinner shells with a high number of damage scars.

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