Chemical defense of the Caribbean ascidian Didemnum conchyliatum

Field and laboratory experiments were performed to investigate the palatability to predatory fishes of organic extracts of the Caribbean ascidian Didemnum conchyliatum. This tan-colored compound ascidian grows as an epibiont on seagrass blades. A dichloromethane/methanol extract of the ascidian incorporated into carrageenan food strips at the same volumetric concentrat~on as the extract occurred in the ascidian tissues deterred feeding of a natural assemblage of consumers in the same seagrass beds from which the ascidians were collected. Bioassay-directed fractionation of this extract revealed that the deterrent property was restricted to fractions containing novel indolemaleimide-imidazole alkaloids, didemnimides A to D. Laboratory assays of purified metabolites revealed that didemnimides C and D deterred feeding by a generalist predatory reef fish, but that didemnimides A and B were not deterrent. Only didemnimide D deterred feeding in the field; neither didemnimide C nor the fraction contalning didemnimides A and B were deterrent when assayed in the natural environment of the ascidian. Antipredatory activity of the didemnimides was highly dependent on compound structure: only didemnimide D, which bears both a bromine on the indole ring and a methyl group on the imidazole ring, inhibited feeding in both laboratory and field assays. Didemnimide D deterred feeding at approximately 0.035 mg ml-l, and is among the most potent antipredatory chemical defenses described from tunicates to date.