We performed a factorial experiment to investigate the effects of current and prey odours (mussels) on the displacement behaviour of the sea star Asterias vulgaris (Verrill, 1866). This sea star is a common subtidal predator of sessile and slow-moving animals in the western North Atlantic Ocean. In the presence of current and prey odours in a flume, sea stars oriented themselves upstream and 70% succeeded in finding the prey. Also, the degree of orientation toward the prey increased as the sea star approached the prey. In contrast, only 5% of individuals tested in still water found the prey. Thus, for A. vulgaris the presence of macroscale flow is an essential condition for locating distant prey. Sea stars tested in current alone showed rheotactic behaviour, moving diagonally upstream. This behaviour should enhance the probability of encountering prey odour plumes in the field. Sea stars moved faster and straighter in flowing water than in still water. The slow movement of A. vulgaris in still water probably minimizes costs of foraging when there is a low chance of finding prey and the straight diagonal movement in current should ensure that the sea star continuously samples new areas, rather than resampling the same odour-free area.
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