Competition and its effects are difficult to demonstrate, as they vary depending on the species present and the environmental conditions in each community. We conducted laboratory and field experiments to evaluate the effects of intra-and interspecific interactions on the use of food resources and behavior of the sea stars Leptasterias polaris and Asterias vulgaris, 2 major predators in subtidal communities of the northern Gulf of St. Lawrence. We demonstrate interference (encounter)

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... between the two. For both species, the presence of the other (as detected by both contact and chemical signals) decreased feeding activity and provoked behavioral changes (avoidance, decrease in percentage feeding). These effects were reversible and short-lived, and were more pronounced when there was a shortage of food and when sea stars were exposed to simulated attacks by the other species (involving pedicellarial pinching). L. polaris seemed to be the inferior competitor as (1) it switched to less profitable prey size (smaller mussels, Mytilus edulis) when A. vulgaris was present, and (2) it avoided A. vulgaris and reduced its feeding activity when food was scarce. The dominance of A. vulgaris is possibly attenuated at our study site (near its northern distributional limit) by the reduction of its fitness when water temperatures are low (most of the year) and during the winter when it migrates to deeper water. Competition for food resources seems to occur periodically (mainly summer), in localized areas (patches and beds of mussels) or when sea stars form dense aggregations in which they are likely to interact frequently. Interactions, and thus the chance of competition, are less likely at greater depths. Coexistence seems to be permitted by both the attenuation of the dominance of A. vulgaris and by the great variations in how L. polaris and A. vulgaris interact (strongly affected by variations in prey availability, sea star abundance, and in the patterns of prey use by both sea stars).