Ecology, evolution, and the mechanics of the
wave-swept environment

Mark Denny
Hopkins Marine Station, Stanford University

Wednesday, May 26, 1999
3:00 p.m.—Pacific Forum

Ocean waves in the surf zone provide an unusual (if not unique) opportunity for the study of form and function. Although the height of individual waves is unpredictable, maximal wave heights can be accurately estimated and used to predict the maximal water velocity and acceleration to which organisms are subjected. This information regarding extreme flows can in turn be used to estimate the maximal hydrodynamic force that an organism experiences in its lifetime, providing an appropriate biomechanical context (the "wave exposure") in which to explore the evolution and ecology of wave-swept plants and animals. Water velocities of 25 m/s and accelerations in excess of 500 m/s² are predicted, and have been recorded. One might assume that these extreme flows would place severe constraints on the shape of intertidal plants and animals. Instead, the form of wave-swept organisms has been affected by episodes of preemptive evolution. For example, limpet shells are variable in shape and (on average) far from the "optimum" form that would minimize imposed hydrodynamic forces. It appears that evolution of a strong pedal adhesive (perhaps in response to predation by crabs) negated the ability of lift and drag to select among shell shapes. As a consequence, shells of limpets have the "permission" of the flow environment to evolve in response to ecological pressures related to territorial behavior, predation, and competition for space. Additional examples are found among kelps and sea urchins.

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