On wave-swept rocky shores, limpets are subjected to water velocities in excess of 20 m s(−1), which may impose large hydrodynamic forces. Despite the extreme severity of this flow environment, predictions from conical models suggest that limpets' shells are typically far from the optimal shape that would minimize the risk of dislodgment, a deviation that is allowed by the high tenacity of the limpets' adhesive system. In this study, we test this conclusion using an actual limpet. The shell of Lottia gigantea differs substantially from the hydrodynamic optimum in that its apex is displaced anteriorly to form a plough, which is used to defend the limpet's territory. The hydrodynamic effects of this shape are similar to those observed in conical models: the animal experiences an increased lift when facing into the flow and a decreased lift when the flow is at its back. However, neither effect has a substantial impact on the risk of dislodgment. When the animal is stationary, its adhesion to the substratum is very strong, and its risk of being dislodged is small regardless of its orientation to the flow and despite its sub-optimal shape. In contrast, when the animal is crawling rapidly, its adhesion is substantially decreased, and it would probably be dislodged by rapid flow even if the shell were shaped optimally. The risk of dislodgment by waves is therefore functionally independent of shell shape. In essence, despite the extremely high water velocities to which this species is subjected, its shell has had the ‘permission’ of the flow environment to respond to other selective factors, in particular those associated with its aggressive, territorial behavior. The result is a shell that is both a potent territorial weapon and a functional (albeit less than optimal) hydrodynamic shape.

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