In predator?/ prey relationships such as those between crabs and their bivalve prey, interference competition is a topic of intense investigation as it can have profound consequences on the dynamics of both predator and prey populations. However in laboratory experiments ?/ also thoseoncrab?/ bivalve systems ?/ workers never adequately disentangled interference competition from exploitative competition, as prey depletion was never compensated. Hitherto, experimental studies on crab?/ bivalve systems lack direct behavioural observations and have provided only indirect and thus inconclusive evidence of interference competition.We studied interference competition in adult male shore crabs Carcinus maenas that foraged on bluemussels Mytilus edulis.We developed a novel type of experimental tank to replenish eachconsumedmussel, andthus tokeep prey levels constant.We conducted two experiments inwhich we varied number of crabs (1, 2, 4)andnumberofmussels (first experiment: 4, 8, 16, 32; second experiment: 8, 32, 128)and directly observed the foraging behaviour of crabs (foraging area0/ 0.25 m2). In the first experiment, feeding rates decreasedwith increasing crab density only atmussel density 16 because both search time and time spent in agonistic interactions increased. At other mussel densities, variation in crab density did not affect feeding rates, possibly because of low statistical power and the narrow range of mussel densities offered. In the second experiment feeding rates decreasedwith increasing crab density because crabs spentmore time in agonistic interactions and handling their prey. Feeding rates increased with increasingmussel density.Overall, crabs spent on average 14?/ 18% of their foraging time in agonisticbehaviours,whileonthree out of 64 occasions feeding rates decreased because mussels were stolen (kleptoparasitism). Concluding, we have shown that interference competition occurs in absence of prey depletion, while conducting direct behavioural observations aid to identify the behavioural processes that underlie interference competition.
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