BEHAVIORAL MECHANISMS OF PREY SIZE SELECTION BY SEA STARS (ASTERIAS-VULGARIS VERRILL) AND CRABS (CANCER-IRRORATUS SAY) PREYING ON JUVENILE SEA SCALLOPS (PLACOPECTEN-MAGELLANICUS (GMELIN))

Predation upon different size classes of juvenile sea scallops Placope cten magellanicus (Gmelin) (5-25 mm shell height) by different size cl asses of predatory sea stars Asterias vulgaris Verill (30-150 mm diame ter) and crabs Cancer irroratus Say (45-120 mm carapace width) was stu died in laboratory experiments using single prey size (no choice) and multiple prey size (choice) designs. Components of predation rate were quantified to assess the relative importance of prey vulnerability an d active predator choice in determining observed prey selection. All s izes of sea stars consumed more small scallops than medium or large on es. Encounter rates between predator and prey were similar, irrespecti ve of sea star and scallop size, and did not differ from encounter rat es calculated on the basis of body sizes and movement velocities. Scal lops assumed a ready-to-swim position when contacted by sea stars, and often actively escaped. The probability of capture by sea stars upon encounter was generally low (< 0.2), but significantly higher for smal l scallops than for medium or large ones. Active selection was unimpor tant, since sea stars did not select a given scallop size more often i n a choice situation than in a non-choice situation. Apparent preferen ce for small scallops by sea stars was primarily due to size-related d ifferences in prey vulnerability. In single prey size experiments with crabs, more medium scallops were consumed than small or large ones. H owever, in the multiple prey size experiment, large scallops were pref erred over small and medium ones. The preference for large scallops ov er medium ones resulted from active selection, since crabs selected la rge scallops more often in a choice situation than in a non-choice sit uation. Conversely, low consumption rates of small scallops were due t o low encounter rates with crabs, since encounter rates reflected body sizes and movement velocities. Scallops often closed their valves upo n encounter with crabs, resulting, in a higher probability of capture upon encounter (generally > 0.2) than that observed with sea stars. Th erefore, prey size selection by crabs appears to be determined by both size-related differences in prey vulnerability, due to differential e ncounter rates, and active selection of larger prey.