Interference and aggregation: Close encounters of the starfish kind

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Abstract
The effects of inter and intraspecific encounters among the predatory asteroids Crossaster papposus (L.) and Asterias rubens L. were monitored in the laboratory by time lapse photography in the presence or absence of a common food item. A. rubens readily aggregated on food whereas C. papposus did not because of strong contact-mediated instraspecific avoidance. The lack of intraspecific avoidance among A. rubens aided its opportunistic exploitation of abundant food. Intraspecific avoidance is suggested as a possible dispersal mechanism of C. papposus. C. papposus interfered with A. rubens in the presence of a common food source and prevented the latter from aggregating due to interspecific avoidance resulting from encounters. This interference mechanism may enhance access to food for C. papposus in the presence of A. rubens.