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The polyps of Discophyton rudyi (Verseveldt and van Ofwegen, 1992), a small, aggregating, alcyonacean soft coral found on rocky shores in the northeast Pacific Ocean, are selectively preyed on by the nudibranch Tritonia festiva (Stearns, 1873). In the laboratory, D. rudyi retracted their polyps when exposed to water-borne cues from a conspecific colony that was successfully attacked by T. festiva. This same inter-colony response was elicited by attacks simulated with fine scissors, but not by (i) the presence of T. festiva attempting to feed but prevented from damaging its prey, (ii) the simple withdrawal of the soft coral polyps, or (iii) seawater controls. The cue(s) eliciting polyp retrac- tion therefore emanate from the soft coral and not its nudibranch predator. Tritonia festiva often attacks neighboring colonies, which are usually separated by only a few millimetres, in rapid succession but will not attack colonies with retracted polyps. It also cannot move rapidly to reach more distant colonies. Therefore, polyp retraction by one colony in response to predation on a neighboring colony effectively serves as an anti-predatory alarm response. Although ag- gregations of D. rudyi are largely clonal, the response to water-borne cues from injured conspecifics does not appear to be clone-specific. Few examples of intra-specific alarm responses are known from sessile marine invertebrates, but the similarities between them suggest that other examples may be found in suspension or deposit-feeding taxa that form dense aggregations and are preyed on by stealthy partial-predators likely to attack adjacent individuals or colonies in rapid succession. Resume : Les polypes de Discophyton rudyi (Verseveldt et van Ofwegen, 1992), un petit corail mou alcyonace qui forme des agregats sur les plages rocheuses du nord-est du Pacifique, subissent la predation selective du nudibranche Tritonia festiva (Stearns, 1873). En laboratoire, D. rudyi retracte ses polypes lorsqu'on l'expose a des signaux transmis dans l'eau provenant d'une autre colonie effectivement attaquee par T. festiva. Cette meme reaction d'une colonie a l'autre peut etre provoquee par une attaque simulee avec des ciseaux fins, mais non par (i) la presence de T. festiva qui cherche a se nourrir, mais qui est empeche d'endommager sa proie, ni par (ii) le simple retrait des polypes du corail mou, ni enfin (iii) dans un temoin d'eau de mer. Le ou les signaux qui declenchent la retraction des polypes emanent donc du corail mou et non de son predateur nudibranche. Tritonia festiva attaque souvent des colonies adjacentes (qui sont ordinairement a quelques mm l'une de l'autre) en succession rapide, mais il n'attaque pas les colonies a polypes retractes. Il ne peut non plus se deplacer rapidement pour atteindre les colonies plus eloignees. C'est pourquoi, la re- traction des polypes dans une colonie en reaction a la predation dans une colonie adjacente sert effectivement de reac- tion d'alerte contre le predateur. Bien que les agregats de D. rudyi soient des clones, la reaction aux signaux transmis dans l'eau par les coraux blesses de la meme espece ne semble pas etre specifique au clone. On connait peu d'exemples de reactions d'alerte intra-specifiques chez les invertebres marins sessiles, mais les similarites qui existent entre elles laissent croire que d'autres exemples pourraient exister chez les taxons qui se nourrissent de particules en suspension ou de depots de surface, qui forment des agregats importants et qui sont exploites par des predateurs par- tiels furtifs susceptibles d'attaquer des colonies ou des individus adjacents en succession rapide. (Traduit par la Redaction) Goddard 71

Stealthy slugs and communicating corals: polyp withdrawal by an aggregating soft coral in response to injured neighbors