No abstract

Extracts of the Atlantic sea pansy Renilla reniformis, which inhibit the settlement of barnacle larvae, have been found to contain three new diterpenes: renillafoulins A (1), B (2), and C (3). All three compounds inhibit barnacle settlement, with EC50 values ranging from 0.02 to 0.2 pg/mL. The structures were elucidated by using two-dimensional NMR data and spectroscopic comparisons to briarein-type diterpenes isolated from other Pennatulacea. Single-crystal X-ray diffraction data for 3 confirmed the structure.Barnacles are well-known to the maritime industry as fouling organisms. Toxic materials used to control fouling by barnacles have included metallic copper and lead and coatings containing mercury, copper, or arsenic.1 Organotin coatings, the latest biocides of choice, are currently under criticism. Environmental concerns over the use of these toxic metals have prompted investigations into alternative methods of inhibiting fouling by these organisms.2 Studies of the response of barnacle larvae to allelochemicals, using a settlement assay, have demonstrated the presence of both settlement inducers and inhibitors in homogenates of octocorals.3We report here the structures of three diterpenes (1-3) isolated from Renilla 2. R' = Ac, R2= C2H5CO-_3 R' = Ac, R2= n-CjHjCOreniformis (common sea pansy, a pennatulacean octocoral), which inhibit settlement of larvae of the barnacle Balanus amphitrite amphitrite Darwin 1854.4 These compounds are related to briarein-type diterpenes found in other Pennatulacea.5

Laboratory-reared barnacle larvae were used to study natural products from whip corals. Biological assays used barnacle responses in behavioral assays and in larval settlement assays. Whip corals contained substances that were active in both assays. Substances inhibiting swimming and reversible attachment of barnacle larvae were found in an aqueous extract of whip corals. Low-molecular-weight substances inhibiting barnacle settlement were found in a methylene chloride extract of material soluble in aqueous methanol. Antisettlement activity was associated with substances with specific mobilities in several chromatography systems. Partially purified material was effective in preventing settlement at concentrations of less than 0.2 μ/ml. Settlement inhibition involves adsorption of the natural product to surfaces.

Barnacle settlement inhibitors and inducers are present in the gorgonianLeptogorgia virgulata and the pennatulaceanRenilla reniformis. The inhibitors are low-molecular-weight compounds (<20,000 daltons) that were detected in soft tissue homogenates and dialysates of homogenate and in ambient "gorgonian water." Settlement was almost completely inhibited at a dialysate concentration of 1.0 g wet weight equivalents/liter. The inhibitors probably function in chemical defense against predation and fouling, and could prove useful in ship fouling control. The settlement inducers are high-molecular-weight substances (>20,000 daltons) that adsorb to surfaces.