Hydrodynamics, shell shape, behavior and survivorship in the owl limpet Lottia gigantea

On wave-swept rocky shores, limpets are subjected to water velocities in ex cess of 20 ms(-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 sub stantially from the hydrodynamic optimum in that its apex is displaced ante riorly to form a plough, which is used to defend the limpet's territory, Th e hydrodynamic effects of this shape are similar to those observed in conic al models: the animal experiences an increased lift when facing into the ho w and a decreased lift when the how is at its back. However, neither effect has a substantial impact on the risk of dislodgment, When the animal is st ationary, its adhesion to the substratum is very strong, and its risk of be ing dislodged is small regardless of its orientation to the how and despite its sub-optimal shape. In contrast, when the animal is crawling rapidly, i ts adhesion is substantially decreased, and it would probably be dislodged by rapid flow even if the shell were shaped optimally, The risk of dislodgm ent by waves is therefore functionally independent of shell shape. In essen ce, despite the extremely high water velocities to which this species is su bjected, its shell has had the 'permission' of the flow environment to resp ond to other selective factors, in particular those associated with its agg ressive, territorial behavior, The result is a shell that is both a potent territorial weapon and a functional (albeit less than optimal) hydrodynamic shape.