M.Mascaro á R. Seed Foraging behavior of juvenile Carcinus maenas L.) and Cancer pagurus L. Received: 21 July 2000 /Accepted: 18 July 2001 / Published online: 1 November 2001 Ó Springer-Verlag 2001 Abstract Information concerning the way juvenile crabs choose their diet from a variety of prey types can be useful for a better understanding of community dynamics, as well as for the adequate management of natural resources. Prey size and species selection by juvenile Carcinus maenas 15±35 mm carapace width, CW)and Cancerpagurus 20±40 mmCW)feedingonfour bivalves of contrasting shell morphology were investi- gated. When oered a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas,and Cerastoderma edule presented individually, crabs generally showed evidence of size-selective predation. Cancer pagurus selected larger mussels relative to the size of their chelae relative prey size, RPS) than did Carcinus maenas of similar and even larger carapace width. However, the RPS of se- lected O. edulis and Cerastoderma edule were similar for all crabs, suggesting that certain prey features constitute eective barriers even to the powerful chelae of Cancer pagurus. When oered a wide size range of mussels and oysters simultaneously, all crabs consistently selected mussels. When oered O. edulis and Crassostrea gigas, crabs consumed both these oyster species in similar numbers. Carcinus maenas consumedsimilarnumbersof mussels and cockles; Cancer pagurus, however, showed no preference for either prey in the smaller size classes but selected more mussels than cockles as prey increased in size. Although previous studies report that adult Carcinus maenas select prey species according to their pro®tability amount of food ingested per unit of han- dling time, milligrams per second), consumption rates of the size classes of prey selected by juvenile shore crabs did not always parallel prey value. Although variations in crab strength can account for many of the dierences between the foraging strategy of juvenile and adult C. maenas,ourresultssuggestthatjuvenilecrabsareless speciesselectivethanadultsasaresultoftherestrictions imposed on small individuals that have limited access to larger prey. Introduction There have been many previous studies concerning the feeding ecology of brachyuran crabs, for example, Por- tunidae Haddon and Wear 1987; Seed and Hughes 1997), Cancridae Boulding and Hay 1984; Juanes and Hartwick 1990), Xanthidae Hughes 1989; Lin 1990), and Calappidae Hughes and Elner 1989), and various aspects of foraging have been extensively documented. Most of the research has been dedicated to the study of the feeding behavior of adult crabs, and very little in- formation has been published regarding juveniles. Not only is the spatial distribution of juveniles on the shore dierent from that of adults Naylor 1958; Eriksson and Edlund 1977), but their feeding habits and food prefer- ences also contrast with those of larger conspeci®cs Ropes 1968; Scherer and Reise 1981; Rangeley and Thomas 1987). The variability in the feeding patterns throughout a wide size range of crabs can be particularly relevant when considering the impact of crab predation on prey populations. The size-frequency distribution and abun- dance of crabs vary seasonally, thus aecting the overall average food intake of a crab population over a period of time Walne and Dean 1972). On the shore, juvenile crabs are typically more abundant than adults Crothers 1970), and the relative abundance of certain age groups, in turn, varies throughout the intertidal and shallow subtidal regions Dare and Edwards 1981; Hunter and Marine Biology 2001) 139: 1135±1145 DOI 10.1007/s002270100677 Communicated by P.W. Sammarco, Chauvin M.Mascaro &) á R. Seed School of Ocean Sciences, University of Wales, Bangor, Menai Bridge, Anglesey LL59 5EY, UK Present address: M. Mascaro Laboratorio de BiologõÂa Marina Experimental, Depto. de BiologõÂa, Fac. de Ciencias, UNAM, Apdo. Post. 69, Ciudad del Carmen, Campeche, Mexico e-mail: mmm@hp.fciencias.unam.mx Tel.: +52-938-28730 Fax: +52-938-28730