Asterias rubens feed mainly upon molluscs, especially bivalves and snails. In addition, they act as scavengers on any dead animals they encounter. Like all sea stars, the stomach of the Asterias rubens turns inside out through the mouth and slips into the shell of the prey. Next, digestive enzymes enter the prey along with the everted stomach lining to further aid in digestion. The sea star also may use its tube feet to pry open a bivalve. (Pearse, et al 1987)
Studies have monitored the predation of the Asterias rubens upon sea scallops, Placopecten magellanicus. The experiments were designed to test the importance of prey size and vulnerability of the sea scallops to the sea stars. The Asterias rubens preferred small- to medium-sized scallops, based on the sea star's ability to slowly sneak upon the prey. When doing so, predatory sea stars emit saponins to which scallops and other prey species detect and use against the predator to monitor the distance between the two. This allows the scallop time to protect itself from the hungry predator by assuming the ready-to-swim position. Crabs and other fish have the advantage of agility compared to the slow creeping sea stars, allowing these quicker organisms better food selection, or in other words, larger scallops. (Barbeau and Scheibling 1994b, Barbeau, et al 1996).
Studies of the effect of temperature upon predation rates by sea stars concluded that the effectiveness of sea scallops' escape response decreases with increasing temperature, resulting in a higher success rate for the sea star. (Barbeau and Scheibling 1994a)