Signal transduction in barnacle settlement: Calcium re-visited

The search for marine natural product antifoulants is being hampered by problems associated with conventional settlement assays. Yet it has been recognised that the study of how chemical cues are perceived by fouling organisms may offer clues to settlement inhibitors and may identify novel biochemical assays for antifourants based on signal transduction pathways. Here the role of calcium in barnacle settlement is re-examined. A requirement for calcium in settlement of the cypris larva of Balanus amphitrite has been confirmed; settlement was inibited in low calcium, and calcium-free, seawater. Although 10 mM (19.27 mM) excess calcium had no effect and higher concentrations were inhibitory, a 5 mM excess stimulated settlement. Stimulation is proposed to be effected by an increase in intracellular calcium. The release of calcium from intracellular pools with thapsigargin (but not cyclopiazonic acid) induced settlement and an antagonist of intracellular calcium, TMB-8, generally inhibited settlement. Nevertheless, the calcium ionophore A23187 did not induce settlement at the concentrations tested. Consequently, the relative importance of external calcium and intracellular pools to increased intracellular calcium has yet to be determined. Pharmacological manipulations of calcium channels with organic and inorganic channel blockers strongly indicate calcium channel involvement in barnacle settlement. The data are summarised in an hypothetical scheme for signal transduction at settlement and are compared to those obtained for other marine invertebrate larvae.