PREDATION RISK AND FEEDING IN AN INTERTIDAL PREDATORY SNAIL

Because avoidance of predators may affect a prey's foraging success and resultant fitness, we examined how a top predator, the stone crab Menippe adina (Williams & Felder), altered feeding and growth rates of rock snails [Stramonita (= Thais) haemastoma (Kool)]. In small-scale laboratory experiments, feeding rates of snails were reduced by chemical or acoustical cues from stone crabs. However, small snails were not differentially affected, even though they were more susceptible to predation. In large-scale laboratory experiments with free-ranging crabs, snail feeding and growth were again reduced because snails spent more time in refuges. Although small snails fed as often as large snails when exposed to crabs, they did spend more time in refuges. Small individuals may thus focus on feeding when out of refuge and devote less time to other activities that may extend their exposure to predators. Feeding by snails was reduced in field experiments only in enclosure cages in which crabs had direct access to snails. There was no measurable effect either in adjacent enclosure cages receiving chemical or auditory cues, or when crabs were tethered near the cage. Thus, reduced feeding in the field may occur only when predator abundances are high enough to cause frequent direct contact. Predator avoidance by intermediate predators may alter how intermediate predators affect their prey populations, but only if top predators are very abundant.