Journal of Experimental Marine Biology and Ecology Ž . 262 2001 13–30 www.elsevier.nlrlocaterjembe Use of prey resources by the seastars Leptasterias polaris and Asterias Õulgaris: a comparison between field observations and laboratory experiments Carlos F. Gaymer ) , John H. Himmelman, Ladd E. Johnson ( Departement de Biologie and GIROQ Groupe InteruniÕersitaire de Recherches Oceanographiques du ´ ´ ) Quebec , UniÕersite LaÕal, Quebec, Canada G1K 7P4 ´ ´ Received 29 December 2000; received in revised form 21 March 2001; accepted 8 April 2001 Abstract The subtidal predators Leptasterias polaris and Asterias Õulgaris strongly overlap in their size, distribution and use of prey resources. We evaluate the factors determining their use of prey Ž . feeding rate and prey selection by comparing prey use in the laboratory and in the field. Both seastars have the same preferred prey, the mussel Mytilus edulis, select medium-sized mussels Ž . Ž . 1.5–3.0 cm , and show similar responses to increasing mussel density functional response . Differences between the two seastars in their feeding abilities appear to affect both their size selection of mussels and use of alternative prey when mussels become rare or absent. A. Õulgaris selects larger mussels than L. polaris when mussel abundance is low, and this may optimize its energetic intake. L. polaris is be better able to prey on the crevice-dwelling clam Hiatella arctica and A. Õulgaris on the ophiuroid Ophiopholis aculeata. These differences in prey selection may reduce potential competitive interactions between the two seastars, as they cause prey partitioning when the preferred prey is rare or absent. The greater feeding rate of A. Õulgaris on larger mussels likely provides it with a competitive advantage when the abundance of preferred prey is limited. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Feeding behavior; Prey preference; Feeding experiments; Seastars; Leptasterias polaris; Asterias Õulgaris; Subtidal; Northern Gulf of St. Lawrence ) Corresponding author. Tel.: q 1-418-656-2131 ext. 5230; fax: q 1-418-656-2339. Ž . E-mail addresses: Carlos.Gaymer@giroq.ulaval.ca C.F. Gaymer , John.Himmelman@bio.ulaval.ca Ž . J.H. Himmelman . 0022-0981r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. Ž . PII: S0022-0981 01 00264-7