Major rocky intertidal predators in the northeast Pacific such as sea stars, whelks, and birds can consume foundation species such as mussels and thereby affect zonation patterns and diversity in these habitats. Predation specifically on juvenile intertidal invertebrates can also substantially impact population dynamics and influence community structure. The lined shore crab Pachygrapsus crassipes (Randall, 1840) is an abundant denizen of sheltered and exposed intertidal habitats in the northeast Pacific from Canada to Mexico. This study examined potential P. crassipes predation on juvenile mussels (Mytilus galloprovincialis), whelks [Nucella ostrina (Gould, 1852)], and abalone [Haliotis rufescens (Swainson, 1822)] due to conflicting reports on the diet of this species. Crabs consumed more juvenile mussels and abalone than seaweed (Ulva lactuca) and fed preferentially on the smallest mussels (6- to 10-mm size class). Further experiments showed that predation on mussels by P. crassipes was highly size dependent, with the largest crabs consuming over twenty-five 15-mm mussels per day. Field outplant experiments revealed that P. crassipes consumed high numbers of juvenile mussels in a natural setting, meaning that it could substantially affect mussel recruitment. This crab species appears to be an opportunistic predator that could have significant impacts on the recruitment and early life history of several invertebrate prey species.
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Vol. 36 • No. 1