Contents lists available at ScienceDirect Journal of Experimental Marine Biology and Ecology journal homepage: www.elsevier.com/locate/jembe Body size and competitor identity modulate prey consumption and feeding behaviour in a slow-moving benthic predator (Asterias rubens, Linneaus) Anne P. St-Pierre a, ⁎ , Hali R. Moreland b , Patrick Gagnon a a Department of Ocean Sciences, Ocean Sciences Centre, Memorial University of Newfoundland, 0 Marine Lab Road, St. John's, Newfoundland and Labrador A1C 5S7, Canada b Department of Biology, Memorial University of Newfoundland, 232 Elizabeth Avenue, St. John's, Newfoundland and Labrador A1B 3X9, Canada ARTICLE INFO Keywords: Common sea star (Asterias rubens) Rock crab (Cancer irroratus) Green crab (Carcinus maenas) Predation Competition Feeding behaviour ABSTRACT Understanding interactive effects of body size and competition on predator-prey relationships is key to pre- dicting shifts in marine communities, particularly under accelerating pace of introductions of non-native species on a global scale. The common sea star, Asterias rubens (Linneaus), is a dominant indigenous predator in rocky subtidal habitats in eastern Canada, affecting community structure through sustained consumption of bivalve prey, mainly blue mussel, Mytilus edulis (Linneaus). Yet, considerable spatial and dietary overlaps with the indigenous rock crab, Cancer irroratus (Say), and introduced green crab, Carcinus maeneas (Linneaus), together with comparatively lower mobility in A. rubens, suggest the latter is poorly fit to compete with both crabs for M. edulis. A laboratory mesocosm experiment was used to examine competitive interactions for variably-sized M. edulis between small and large, 3-week starved A. rubens, and large C. irroratus or C. maenas (in one-on-one sea star versus crab face-offs) from southeastern Newfoundland. Mussel consumption in starved A. rubens, as well as four components of its natural feeding behaviour, were quantified over 75-h trials in relatively cold (~4 to 5.5 °C) seawater representative of the current northern limit of C. maenas' distribution in the northwestern Atlantic. Results showed that starved A. rubens (1) prioritizes consumption, upon contacting mussel prey, over non-feeding activities such as responding to physical contacts initiated by crabs; (2) alters consumption and feeding behaviour under prolonged exposure to C. irroratus; and (3) increases movement in presence of green crab, more so in large than small sea stars. Overall, findings demonstrate that A. rubens' foraging decisions vary ontogenetically and with competitor's identity. Observed differences in consumption and feeding behaviour in A. rubens exposed to familiar (rock crab) or unfamiliar (green crab) competitors also suggest timing and frequency of encounters with competitors have the potential to alter feeding patterns throughout an organism's lifetime. 1. Introduction Competitive interactions can significantly alter prey consumption and feeding behaviour in marine benthic predators (McDonald et al., 2001; Menge, 1972; Salierno et al., 2003). Interference competition occurs when a predator's feeding is altered by direct physical contacts (e.g. aggressive interactions or kleptoparasitism) with a competitor (Boudreau et al., 2013; Gherardi and Cioni, 2004; Goss-Custard, 1980), and exploitative competition when feeding is indirectly affected by depletion of common, limited prey (Birch, 1957; Connell and Gillanders, 2007; Davey et al., 2006). In both types of competition, morphological (e.g. body size), physiological (e.g. starvation), or me- chanical (e.g. mobility) differences between predators may give one individual a net advantage, forcing the other to adjust its feeding re- sponse (Gaymer et al., 2001a; Menge, 1972; Ojeda and Dearborn, 1991; Smallegange et al., 2006). While competing, some predators may select suboptimal prey, retreat, or both to limit physical damage or energy expenditure associated with agonistic interactions, reducing the ability to search for, capture, manipulate, and consume prey (Gaymer and Himmelman, 2002; Salierno et al., 2003). Shifts in predators' prey consumption and feeding behaviour can significantly alter community structure (Menge and Lubchenco, 1981; Menge and Sutherland, 1987; Ripple and Beschta, 2004). Accordingly, understanding effects of competition on predator-prey relationships is key to predicting shifts in marine communities, particularly under accelerating pace of introduc- tions of non-native species on a global scale (Gilman et al., 2010; Harley, 2011; Kordas et al., 2011). The common sea star, Asterias rubens (Linnaeus), is a dominant in- digenous predator in rocky subtidal habitats in the northern Gulf of St. Lawrence and along the Atlantic coast of Newfoundland, Canada https://doi.org/10.1016/j.jembe.2018.07.002 Received 10 February 2018; Received in revised form 30 June 2018; Accepted 6 July 2018 ⁎ Corresponding author. E-mail address: apsp66@mun.ca (A.P. St-Pierre). Journal of Experimental Marine Biology and Ecology 507 (2018) 8–16 0022-0981/ © 2018 Elsevier B.V. All rights reserved. T