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

Abstract

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.

Reply to the comment by Dalby and Elliott on "Predator classification by the sea pen Ptilosarcus gurneyi (Cnidaria): role of waterborne chemical cues and physical contact with predatory sea stars"

“…The following is a reply to the comment by Dalby and Elliott (2003) regarding a research note we recently published in the Canadian Journal of Zoology (Weightman and Arsenault 2002). Our study examined the behavioural responses (degree of contraction into the sediment and bioluminescent displays) of the sea pen Ptilosarcus gurneyi to sea stars representing different degrees of predation threat.…”

Comment on "Predator classification by the sea penPtilosarcus gurneyi(Cnidaria): role of waterborne chemical cues and physical contact with predatory sea stars"

“…In conclusion, Weightman and Arsenault (2002) neglected to cite much literature on cnidarians, including both primary and secondary and recent and historical literature. This oversight may have resulted from a reliance on one broad review article covering all major animal phyla.…”

“…In their note on behavioural interactions between a pennatulacean (sea pen) species and three asteroid (sea star) species, Weightman and Arsenault (2002) write:…”

“…Likewise, S. coccinea detaches and swims when touched by any of 3 predatory or 2 nonpredatory asteroid species, but does not show these responses to 12 nonpredatory asteroid species or any ophiuroid species (Yentsch and Pierce 1955;Sund 1958;Robson 1961;Ward 1965;Mauzey et al 1968). Weightman and Arsenault (2002) overlooked key information in at least two of the papers they cited (Yentsch and Pierce 1955;Mauzey et al 1968). …”

“…Our research, like that of Weightman and Arsenault (2002), was performed at Bamfield Marine Station and published in the Canadian Journal of Zoology. The actinian Urticina (formerly Tealia) piscivora releases its pedal disc from the substratum when brought into contact with its asteroid predator Dermasterias imbricata, but shows no such response to 19 other nonpredatory species (Elliott et al 1985).…”