Involvement of the barnacle settlement-inducing protein complex (SIPC) in species recognition at settlement

A cuticular glycoprotein known as the settlement-inducing protein complex (SIPC) induces gregarious settlement in the barnacle, Balanus amphitrite. Here the ability of B. amphitrite cyprids to discriminate among the SIPCs of three related Balanidae, B. improvisus, Megabalanus rosa and Elminius modestus is examined. Using a laboratory settlement assay, the effective surface-adsorbed concentration of B. amphitrite SIPC that achieved 50% settlement of B. amphitrite cypris larvae (EC50) was ∼ 100 ng 0.8 cm− 2. When the B. amphitrite cyprids were exposed to conspecific and allospecific SIPC extracts at their EC50, they settled preferentially (P < 0.05) on the conspecific extracts (24.3 ± 1.8%) rather than on the SIPCs of B. improvisus (9.2 ± 1.6%), M. rosa (9.7 ± 2.9%) and E. modestus (3.1 ± 2.1%). Moreover, the cyprids did not discriminate significantly (P > 0.05) between the SIPCs of B. improvisus and M. rosa. These results are consistent with the phylogenetic relationships of the species and suggest that variation in the SIPCs may play an important role in species recognition and barnacle evolution.