Adaptive and non-adaptive variation in two species of rough periwinkle (Littorina) on British shores

Rough periwinkles are notoriously variable in shell characters. There are many reports of substantial local variation on single shores which not only make identification difficult but also may be difficult to understand in terms of likely selective pressures. We show that despite local variation in southern Britain there is evidence of a broader scale of change which is likely to be explicable in adaptive terms. At the same time, along an extreme environmental gradient on a single shore in south Wales we show that there are changes in morphology which are related to avoidance of water loss.