Local and Latitudinal Variation in Predation on an Herbivorous Marine Snail

Abstract

Intertidal grazing molluscs show striking local and geographic variation in distribution and body size. Their density is generally greatest in the upper and mid—intertidal zones on rocky shores of the Pacific coastal of North America, and least in the low intertidal zone. However, the elevation of the lower limits of their shore—level distribution varies considerably among sites. In addition, the lower limits are generally higher south of central California than in the north, and the average body size is usually smaller in the south than farther north. This is exemplified by the commonest molluscan grazer in the mid—intertidal region of many rocky shores on this coast, the trochid snail Tegula funebralis. A hypothesis to explain these patterns, that they are due to variation in predation, was tested. At sites where predator density was high, regardless of latitude, the lower limit of Tegula's intertidal range was higher on the shore, compared with areas containing few predators. Shell width was usually smaller, and increased with elevation wherever predators were abundant. Predators were more abundant at southern than at northern sites. The primary predators of Tegula are octopuses (Octopus bimaculoides and O. bimaculatus), the starfish Pisaster ochraceus, and crabs of the genus Cancer. The octopuses occur only at the southern sites, whereas Pisaster and Cancer are found throughout the geographic range of the study. Reciprocal transplants of marked Tegula, done between several southern and northern sites, showed that the lower limit of distribution of adult Tegula on the shore at a given site is influenced by the snails' defensive behavior. Where predators were abundant, both southern and northern Tegula placed low on the shore rapidly climbed to higher levels than in locations where predators were scarce. The probable cause of this upward migration is an avoidance response to chemical exudates from predators. Southern Tegula also have an inherently different response than northern Tegula; southern snails always climbed upwards sooner and reached higher levels than northern snails. This resulted in predators killing more of the northern snails in these experiments. The behavioral difference between southern and northern Tegula may be the result of long—term differences in the intensity of predation by octopuses, which are much rarer in the intertidal north of central California than south of there. Young—of—the—year Tegula were found mainly in the mid—intertidal region. The shift in the direction of shore—level size gradients of Tegula populations between high and low predation sites may be caused by movements of adult snails to levels above or below the less mobile juveniles. The relative scarcity of Tegula and other grazing molluscs in the low intertidal zone is probably due both to upward migration and high mortality rates, with predation being a major cause of both.