We performed laboratory experiments to investigate the perception and localization of prey by the subtidal asteroid Leptasterias polaris. Motivation to eat greatly influences its responsiveness to current and waterborne odors from prey. Fed asteroids behave erratically, whereas asteroids starved for 2 mo exhibit predictable behaviors revealing the species' foraging strategy. In the absence of prey, starved asteroids search by moving cross-current which increases the probability of encountering odor plumes. In conditions of varying current velocities (differences of 0.5 to 1 cm S-'), movement is cross-stream towards the stronger current and 2-to 3-fold faster in strong (ca l 7 cm S-') than in weak (ca 0.7 cm S-') current. We hypothesize that strong currents are preferred because they increase the spatial definition of odor plumes which facilitates localizing prey. When starved L. polaris perceive prey, they move upstream towards the odor plume. Because this should reduce the predator's conspicuousness, it may increase its chances of captunng the prey.

ABSTRACT. We examined size-related changes in vulnerability of juvenile Iceland scallops Chlamys islandica to crab and asteroid predators using field experiments in the Mingan Islands, northern Gulf of St. Lawrence, eastern Canada. Vulnerability to both crabs and asteroids markedly decreased with scallop size, likely because of increases in the effectiveness of the scallop's antipredator defenses. Multiple-choice experiments showed that the crabs Hyas araneus and Cancer irroratus preyed mainly upon small scallops (10 to 30 mm). At our study site H. araneus was the most abundant decapod and was likely the major predator of scallops. Small Iceland scallops were generally found in crevices, byssally attached to the undersides of dead shells and rocks. Use of these refuges did not show die1 periodicity but decreased markedly with scallop size, attaining 0% for >60 mm scallops. Tethering experiments indicated that refuge use decreased vulnerability to both crabs and asteroids. However, the advantage of refuge use, in terms of reduced predation risk, varied with scallop size, being maximal for 15 to 30 mm scallops and only marginal for larger, less vulnerable scallops.

The escape responses of many animals are finely adjusted to the risks of predation encountered in nature. Adult whelks Buccinum undatum from a population sympatric with the predatory asteroid Leptasterias polaris more readily exhibited violent leaping escape maneuvers (foot contortions), produced larger quantities of mucus, and more efficiently escaped predator attacks than adult whelks from a population allopatric with the asteroid. However, newly emerged whelks (recruits) from both populations did not resort to strong escape maneuvers to flee L. polaris, but they nevertheless increased their crawling activity. The behavioral response of recruits appears to be predator-specific in the sympatric population, but not in the allopatric population. In the sympatric population, the tendency of whelks to exhibit foot contortions under predation risk increased with size. Our results suggest that where whelks coexist with L. polaris the capacity to recognize this predator as a threat is innate, and elaborate antipredator behaviors develop during ontogeny. We hypothesize that the greater responsiveness of large individuals is adaptive as it enables them to take advantage of the feeding opportunities that arise from close associations with L. polaris.