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

Abstract

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 retraction 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 aggregations 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.