Ecological relevance of a conspecific, waterborne settlement cue in Balanus amphitrite (Cirripedia)

Planktonic marine invertebrate larvae are now considered to exhibit varying degrees of control over their transition back to benthic habitats through behavioural and ontogenetic adaptations. Gregarious settlement in barnacles is attributed to the settlement-inducing protein complex (SIPC), the cypris larva temporary adhesive and a waterborne cue; the latter obtained by conditioning seawater with adult Balanus amphitrite (= Amphibalanus amphitrite). By responding to a waterborne settlement cue, a swimming larva may elect to settle without contacting a surface. The evidence for such a role is, however, limited. This theme is examined here by evaluating the behavioural response of cyprids to the cue through various laboratory techniques - settlement assays, motion tracking and enumeration of antennule movements - and linking the cue to recruitment of larvae in the field. The cue is detected in solution, remains active upon dilution, induces a similar response in young and aged larvae, and only a brief (3-15 min) exposure to conditioned seawater is required to stimulate settlement. Seawater collected in situ, close to piling fouled with B. amphitrite at Duke University Marine Laboratory, North Carolina, induced settlement over samples collected at a distance from the piling. The evidence derived from experiments on laboratory-conditioned and field-collected seawater is consistent with an important role for the waterborne cue in the settlement of barnacle cypris larvae.