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 differentially exposed to wave action; individuals from sheltered habitats typically grow more quickly than do those from more exposed coasts. A series of field and laboratory experiments were conducted to separate the genetic 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. Juvenile and adult whelks were reciprocally transplanted between an exposed and a protected shore and subsequent growth followed. Independent of 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 findings suggest that the variation observed in nature does not represent genetic differentiation, but reflects the influence of proximal factors that depress growth on wave-swept shores, Growth rates of juveniles from exposed and protected shores maintained in the laboratory on a diet of an overabundance of (1) barnacles, (2) mussels, (3) both, and (4) both, but only 67% of the time, indicated that prey type and foraging time affect growth. Whelks grew best on a diet of mussels, either singly or in combination with barnacles, grew less on barnacles alone, and least when foraging time was restricted. Because growth rates on specific prey in the laboratory were opposite the observed trend in nature, variation in prey across the exposure gradient cannot be invoked to account for the difference in growth between N. lapillus from exposed and protected shores. The slower growth rates when foraging time was restricted are consistent with the notion that wave energies on exposed coasts depress growth by limiting foraging time or by reducing foraging efficiency.