Both spatial and temporal variation in environmental conditions can favour intraspecific plasticity in animal form. But how precise is such environmental modulation? Individual Balanus glandula Darwin, a common northeastern Pacific barnacle, produce longer feeding legs in still water than in moving water. We report here that, on the west coast of Vancouver Island, Canada, the magnitude and the precision of this phenotypic variation is impressive. First, the feeding legs of barnacles from protected bays were nearly twice as long (for the same body mass) as those from open ocean shores. Second, leg length varied surprisingly precisely with wave exposure: the average maximum velocities of breaking waves recorded in situ explained 95.6-99.5% of the variation in average leg length observed over a threefold range of wave exposure. The decline in leg length with increasing wave action was less than predicted due to simple scaling, perhaps due to changes in leg shape or material properties. Nonetheless, the precision of this relationship reveals a remarkably close coupling between growth environment and adult form, and suggests that between-population differences in barnacle leg length may be used for estimating differences in average wave exposure easily and accurately in studies of coastal ecology.

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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.