A Limpet‐Coralline Alga Association: Adaptations and Defenses Between a Selective Herbivore and its Prey

The population density of the limpet Acmaea testudinalis is highest on the crustose coralline alga Clathromorphum circumscriptum in both tide pool and subtidal environments in the Gulf of Maine. Juvenile limpets recruit to C. circumscriptum and both juveniles and adults preferentially feed on this species (not its epibionts) over a choice of other corallines, foliose algae, microalgae (diatoms), and detritus. Abundances of the predator (limpet) and the prey (coralline) are positively correlated, and the fitness of both organisms may be increased by the association. Acmaea testudinalis possesses a radula apparently adapted to eat efficiently C. circumscriptum, a food of low caloric value. The coralline is not only an abundant and dependable food throughout the geographic range of the limpet but also provides a superior holding surface for it. Clathromorphum circumscriptum requires grazing to remove potentially lethal epiphytes, but is not harmed by limpet grazing since it possesses a uniquely thick protective tissue over the region of growth. Reproductive structures of C. circumscriptum develop in winter and are buried below the surface so the effects of grazing on them are minimal. Over a wide range of depths, the rate of cell removal by limpets matches the rate of cell production by Clathromorphum. The number of apparently specific adaptations of both organisms, with mutual advantages, suggests evolution toward a coevolved interdependency. Trophic specialization of small organisms with low mobility (limpets) on larger, longer lived prey (coralline algae and kelps) is more likely than in other herbivore/prey interactions. By specializing on a long—lived and predictable food source, A. testudinalis may have evolved a strategy of economy by minimizing its cost of foraging (risk to predation and energy expenditure) relative to its absolute intake of calories. This foraging strategy may have been reinforced over time since it also improves the survivorship of the preferred prey.