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We ran field experiments under wave conditions (in the subtidal zone in Chile) to examine responses of the sea urchin Tetrapygus niger to predatory seastars.Trials involving simulated attacks showed that the urchin differentiated between predatory and non-predatory seastars.We compared the urchins responses to different threat levels presented by the two predatory seastars,Heliaster helianthus and Meyenaster gelatinosus,first simulated attack,then mere contact,and subsequently seastars placed at different distances from the urchin.In trials with both seastars,100％ responded to stimulated attacks and contact.The proportion decreased with distance and more rapidly in the trials with H.helianthus (0 ％ at 30 cm) than with M.gelatinosus (33 ％ at 50 cm).Urchins generally responded more rapidly to M.gelatinosus.When a seastar was added to circular areas where undisturbed urchins were present,urchins fled more rapidly in trials with M.gelatinosus than with H.helianthus.Our observations suggest that M.gelatinosus represents a stronger predatory threat than H.helianthus and this corresponds to field observations showing that urchins are more frequently consumed by M.gelatinosus.These are the first experiments demonstrating distance chemodetection by marine invertebrates under wave conditions.