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))

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Predation upon different size classes of juvenile sea scallops Placopecten magellanicus (Gmelin) (5–25 mm shell height) by different size classes of predatory sea stars Asterias vulgaris Verill (30–150 mm diameter) and crabs Cancer irroratus Say (45–120 mm carapace width) was studied 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 and active predator choice in determining observed prey selection. All sizes of sea stars consumed more small scallops than medium or large ones. Encounter rates between predator and prey were similar, irrespective of sea star and scallop size, and did not differ from encounter rates calculated on the basis of body sizes and movement velocities. Scallops 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 small scallops than for medium or large ones. Active selection was unimportant, since sea stars did not select a given scallop size more often in a choice situation than in a non-choice situation. Apparent preference for small scallops by sea stars was primarily due to size-related differences in prey vulnerability. In single prey size experiments with crabs, more medium scallops were consumed than small or large ones. However, in the multiple prey size experiment, large scallops were preferred over small and medium ones. The preference for large scallops over medium ones resulted from active selection, since crabs selected large scallops more often in a choice situation than in a non-choice situation. Conversely, low consumption rates of small scallops were due to low encounter rates with crabs, since encounter rates reflected body sizes and movement velocities. Scallops often closed their valves upon encounter with crabs, resulting in a higher probability of capture upon encounter (generally > 0.2) than that observed with sea stars. Therefore, prey size selection by crabs appears to be determined by both size-related differences in prey vulnerability, due to differential encounter rates, and active selection of larger prey.