Field expenments support previous laboratory assays Indicating that crude lipid-soluble extracts of the Canbbean encrusting gorgonian octocoral Erythropod~um canbaeorum (Duchassalng & Mlchelotti) detel predation by reef fishes An extract of E canbaeorum incorporated into carrageenan food strips at the same volumetric concentration as i t occurred in gorgonian tlssues deterred feeding of a natural assemblage of fishes on the same reef from which the gorgonian had been collected Fleld assays using carrageenan stnps conta~ning naturally occurnng concentratlons of several pulitled secondary metabolites isolated from the crude extract revealed that the f e e d~n g deterrent effects were present In a fraction containing chlonnated dlterpenoid (Cz0) compounds (erythrolldes), but not in a fraction conta~ning sesquiterpene (Cl5) hydrocarbons (of which erythrodiene, a previously unreported manne natural product was a major component) Three compounds were purlfled from the erythrolide fraction erythrolide A, erythrolide B and erythrolide D Only the last 2 of these compounds, when assayed independently, Inhibited feeding of reef flshes at the natural concentratlons found in E canbaeorun~ Whlle the erythrolides appear to defend E canbaeorum from reef predators, the funct~on of sesqulterpene hydrocarbons, which occur in high concentratlons in the llpid-soluble extracts of many octocorals, remains unclear

Field and laboratory experiments confirmed the feeding-deterrent properties of lipidsoluble extracts of the Caribbean gorgonian octocoral Pterogorgia anceps (Pallas) with subsequent determination of the secondary metabolite responsible for this effect. A dichloron~ethane/methanol extract of P. anceps incorporated into carrageenan food strips at the same volumetric concentration as the extract occurred in gorgonian tissues deterred feeding of a natural assemblage of fishes on the same reef from which the gorgonian was collected. Bioassay-directed fractionation of this extract revealed that the deterrent property was restricted to an ancepsenolide-containing fraction; there was no effect of the less polar hydrocarbon fraction or the more polar pigmented fraction. Field assays of the ancepsenolide-containing fraction at the same volumetric concentration as it occurred in gorgonian tissues confirmed that it deterred predation by reef fishes. Laboratory assays of purified metabolites isolated from the ancepsenolide-containing fraction revealed that the feeding-inhibitory effect was restricted to a diacetoxy derivative of ancepsenolide; neither ancepsenolide nor its monoacetoxy derivative were deterrent when assayed alone or in combination. This represents the first evidence of feeding-deterrence by gorgonian metabolites of the unusual acetogenin class.

Previous investigations have indicated that the Caribbean gorgonian coral Plexaura homomalla derives a chemical defense from prostaglandins present at high concentrations w~t h i n the coral's soft tissues. These conclusions were based on laboratory and field experiments with the hydroxy acids of prostaglandin A2 (PGA2), and not the acetoxy methyl esters of PGA2 that are present in the living gorgonian. In the present study, the consumption of food strips containing the acetoxy methyl ester of 15(R) PGA, from P. homomalla by a natural assemblage of Caribbean coral reef fishes did not differ from consumption of control stnps. In support of previous findings, treatment with the acetoxy acid, hydroxy methyl ester and hydroxy acid of 15(R) PGA2 inhibited consumption of food strips by reef fishes. Although the acetoxy methyl esters of PGA2 undergo hydrolysis when the soft tissues of P. homomalla are damaged, the process appears to be too slow to provide the gorgonian with an effective inducible defense mechanism.

Larvae of the sea hare Aplysia californica (Molluscs: Opisthobranchia) spend several weeks feeding in the plankton pnor to settlement and metamorphosis Previous work indicated that metamorphosis was tnggered by only one (or a t most a few) algal species. However, in the present laboratory study, a mean of 30 % or more of the larvae of this sea hare metamorphosed in response to 10 of 18 species of intertidal macroalgae (9 red. 7 brown, 2 green). Metamorphosis was greatest in response to the red algae Rhodymenja californica, Corallina officinalis, Plocamjurn cartilagineurn and Laurencia pacifjca. Juveniles of A. californica that had metamorphosed on the last 2 species grazed on them and began to grow, whereas juveniles on the other species tended to crawl off the alga and around the assay dish. Of the 8 algae least preferred, only 1 was red, the remainder brown or green. For larvae of A. californica, metamorphosis on a relatively wide spectrum of algal species may be more efficacious than metamorphosis on any one alga, because juvenile sea hares can readily crawl to nearby algal species that they prefer to eat after they have metamorphosed on an alga that is not t h e~r preferred food.

Collisella limatula, a limpet found abundantly in the rocky intertidal from Oregon to Baja California, has recently been found to contain a novel triterpene, llmatulone The present paper describes laboratory experiments and field observations that strongly suggest that limatulone acts as a potent chemical defense against some intertidal predators. Intertidal fish and crabs rejected pieces of the foot of C. limatula, but readily ate the foot tissue of CO-occurring gastropods. In contrast, individuals of C. limatula were eaten by seastars, an octopus and sea gulls. Fish antl-feedant activity was restricted to a slngle compound, limatulone, which was present In the foot of the limpet at a mean concentration of 3 5 ppt dry weight. Food pellets containing limatulone at concentrations of 2 0.5 ppt dry weight induced regurgitation in the intertidal fish Gibbonsia elegans, a known llmpet predator. Concentrations of limatulone in C. limatula varied from one geographic location to the next and from one mo to the next. Reasons for these variations are not understood Surveys of the 5 most common limpets in an undisturbed rocky intertidal site revealed that C, llmatula was the most abundant llmpet inside tidepools and on intertidal boulders. Limatulone may act to defend foraging limpets from predatory fishes In tidepools In the boulder field habitat, shell damage lncurred by limpets from wave-borne rocks and debris may expose foot tissue prior to shell regeneration, during which time the chemical defense may release C. limatula from predatory constraints imposed on other Intertidal limpets.