Body size and competitor identity modulate prey consumption and feeding behaviour in a slow-moving benthic predator (Asterias rubens, Linneaus)

Abstract

Understanding interactive effects of body size and competition on predator-prey relationships is key to predicting 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.