Predator classification by the sea pen Ptilosarcus gurneyi (Cnidaria): role of waterborne chemical cues and physical contact with predatory sea stars

Using laboratory and field experiments we examined the defensive behaviour of the sea pen Ptilosarcus gurneyi (Gray) towards three species of sea stars representing three levels of predatory threat. In the laboratory we first quantified the behaviour of P. gurneyi following physical contact with the sea stars Dermasterias imbricata (specialist predator), Pycnopodia helianthoides (generalist predator), and Pisaster ochraceus (nonpredator). Whereas the majority (73%) of the sea pens rapidly burrowed into the sediment following contact with D. imbricata, their response to P. helianthoides was highly variable and only 23% exhibited burrowing. In contrast, the response of P. gurneyi to P. ochraceus was weak and similar to that elicited by contact with a glass rod (control). Also, whereas the majority of sea pens displayed colony-wide bioluminescent flashes towards D. imbricata and P. helianthoides, their responses to P. ochraceus and the control were weaker and more localized. We subsequently examined whether waterborne predator chemical cues alone could trigger the defensive responses of P. gurneyi to D. imbricata and P. helianthoides, using laboratory bioassays of varying stimulus intensity. Interestingly, although exposure to chemical cues from predatory sea stars did not elicit any defensive response in P. gurneyi, subsequent physical contact with these predators triggered complete burrowing. Field bioassays using SCUBA yielded similar results, as P. gurneyi did not respond to the proximity of predators but rather delayed its response until physical contact occurred. Our study thus provides the first experimental evidence of predator-classification abilities in cnidarians and suggests that physical contact with predatory sea stars is required to trigger defensive behaviours in P. gurneyi.