Use of a wave tank to study the effects of water motion and algal movement on the displacement of the sea star Asterias vulgaris towards its prey

Abstract

We carried out experiments in a wave tank using a factorial design (in the presence and absence of waves and kelp blades) to evaluate the impact of water motion, and of wave-induced movement of kelp blades, on (1) movement of the predatory sea star Asterias vulgaris towards its prey, the blue mussel Mytilus edulis, and (2) on its success in capturing its prey. The wave tank mimicked the back-and-forth flow caused by waves. The displacement of the sea stars was 2 times greater in the absence than in the presence of waves. Movements of A. vulgaris were more directed towards the prey under back-and-forth water movement than under still conditions. The presence of kelp blades without waves also reduced the movement of sea stars towards prey as the sea stars largely stayed in the portion of the tank without kelp. Sea stars only became detached in treatments with waves, and a greater proportion detached when both waves and kelp were present. The success rate of sea stars in capturing mussels was null in the treatment with both waves and kelp. These observations support the hypothesis that the kelp canopy in shallow water, and movement of the kelp blades by waves, provide mussels with a spatial refuge from sea star predation. We show for the first time that a sea star can use distance chemodectection to localize prey under conditions of back-and-forth flow.