Escape and aggregation responses of three echinoderms to conspecific stimuli

In marine invertebrates, waterborne chemical stimuli mediate responses including prey detection and predator avoidance. Alarm and flight, in response to damaged conspecifics, have been reported in echinoderms, but the nature of the stimuli involved is not known. The responses of Asterias rubens Linnaeus, Psammechinus miliaris (Gmelin), and Echinus esculentus Linnaeus to conspecifics were tested in a choice chamber against a control of clean seawater (no stimulus). All three species showed statistically significant movement toward water conditioned by whole animals or homogenate of test epithelium. P. miliaris and E. esculentus displayed a statistically significant avoidance reaction, moving away from conspecific coelomic fluid, gut homogenate, and gonad homogenate. A. rubens was indifferent to conspecific coelomic fluid, pyloric cecum homogenate, and gonad homogenate but moved away from cardiac gut homogenate. P. miliaris was indifferent to gametes, but the other two species were significantly attracted to them. No species showed preference for one particular side of the chamber during trials to balance water flow. These echinoderms can distinguish between homogenates of conspecific tissues that might be exposed when a predator damages the test, and those that may emanate from the exterior surface during normal activities.