THE EFFECT OF WAVE ACTION, PREY TYPE, AND FORAGING TIME ON GROWTH OF THE PREDATORY SNAIL NUCELLA-LAPILLUS (L)

Growth rates of intertidal snails vary among populations differentiall y exposed to wave action; individuals from sheltered habitats typicall y grow more quickly than do those from more exposed coasts. A series o f field and laboratory experiments were conducted to separate the gene tic and phenotypic components of this variation in Nucella lapillus (L .) and to investigate the extent to which prey type and foraging time, which also vary across the wave-exposure gradient, affect growth. Juv enile and adult whelks were reciprocally transplanted between an expos ed and a protected shore and subsequent growth followed. Independent o f origin, whelks grew more on the sheltered shore. By contrast, growth rates for snails from exposed and protected shores were similar when reared under uniform conditions in the laboratory. Together these find ings suggest that the variation observed in nature does not represent genetic differentiation, but reflects the influence of proximal factor s that depress growth on wave-swept shores, Growth rates of juveniles from exposed and protected shores maintained in the laboratory on a di et of an overabundance of (1) barnacles, (2) mussels, (3) both, and (4 ) both, but only 67% of the time, indicated that prey type and foragin g time affect growth. Whelks grew best on a diet of mussels, either si ngly or in combination with barnacles, grew less on barnacles alone, a nd least when foraging time was restricted. Because growth rates on sp ecific prey in the laboratory were opposite the observed trend in natu re, variation in prey across the exposure gradient cannot be invoked t o account for the difference in growth between N. lapillus from expose d and protected shores. The slower growth rates when foraging time was restricted are consistent with the notion that wave energies on expos ed coasts depress growth by limiting foraging time or by reducing fora ging efficiency.