Surface active adrenoceptor compounds prevent the settlement of cyprid larvae of Balanus improvisus

The barnacle Balanus improvisus is the major fouling macroorganism in Swedish waters and it colonizes most man-made surfaces submerged in the sea. New or impending legislation restricts the use of traditional, hazardous antifouling coatings based on heavy metals, mainly copper and tin. This calls for the development of new non-toxic methods that prevent barnacle settlement. In this work several adrenoceptor compounds are shown to be very efficient in preventing the settlement of cyprid larvae of B. improvisus. The settlement rate of laboratory-reared cyprids was studied in hydrophilised polystyrene dishes containing adrenoceptor antagonists and agonists dissolved in seawater. Two of these drugs, medetomidine and clonidine, repeatedly inhibited settlement at concentrations between 1nM and 10nM. In the vertebrate adrenoceptor classification system, which separates pharmacological substances according to their receptor affinity, both of these substances are classified as alpha (2) adrenoceptor agonists. An inhibiting effect on presynaptic receptors is suggested, but the localization of the receptor effect requires futher studies. Experiments also revealed that the inhibiting effect of medetomidine was reversible. Cyprids incubated with medetomidine for 20 h attached and metamorphosed into juvenile barnacles after washing and transferrence to seawater. The antagonizing compound atipamezole reversed the effect of medetomidine. This observation supports the assumption that this substance acts at the receptor level. Studies of the surface affinity of medetomidine revealed a strong tendency to accumulate in solid/liquid phase boundaries. This ability makes it particularly attractive as a candidate for the development of a slow-release carrier in marine coatings. Panels coated with medetomidine in an acrylate polymer and exposed in the field reduced the recruitment of B. improvisus by 96% after 4 weeks and by 70% after 8 weeks.