Influence of predation by two crabs,Pachygrapsus marmoratus(Fabricius, 1787) andLophozozymus incisus(H. Milne Edwards, 1834) (Decapoda: Brachyura: Grapsidae, Xanthidae), on intertidal mussels (Mytiloidea) in the Adriatic Sea

Abstract

We tested the hypothesis that predation on beds of different species of intertidal mussels (Mytiloidea, Mytilidae) by intertidal crabs could influence the abundance and population structure of the pray, changing dominance between the mytilid species over space and time. We tested the influence of selective-feeding behaviours by two Mediterranean intertidal crabs, Pachygrapsus marmoratus (Fabricius, 1787) (Grapsidae) and Lophozozymus incisus (H. Milne Edwards, 1834) (Xanthidae), on populations of the mussels Mytilus galloprovincialis Lamarck, 1819 and Mytilaster minimus (Poli, 1795). Two experiments were conducted, one involving the replacement of consumed prey individuals of different shell lengths by conspecifics of the same size, and a second where no such replacement was made. The predatory pressures were tracked through the preferences for the two species as prey, their selective behaviours in relation to prey sizes, and the type of attack behaviours used. Results demonstrated that the competition for space between the two mytilids is more influenced by the feeding behaviour of L. incisus, which showed a preference for Mytilaster minimus. Comparable data obtained for P. marmoratus suggests that the two resident crabs are not in competition for the same resources. The feeding strategy adopted by L. incisus on Mytilaster minimus showed predation on all of the size classes of Mytilaster minimus because their stronger chelae allow them to mechanically handle larger prey. Lophozozymous incisus uses a simple crushing attack, as opposed to overcoming larger prey using a larger variety of methods, which includes marginal breaking, mandibular biting around valve edges, or a combination of these.