Discrimination between several diterpenoid compounds in feeding by Gambusia affinis

The alcyonacean soft coral Sinularia flexibilis Quoy and Gaimard produces a number of bioactive complementary (secondary) metabolites. The ability of three of these diterpenes — flexibilide, sinulariolide and dihydroflexibilide — to elicit differential discriminatory feeding behavior in Gambusia affinis was assessed in a feeding deterrence study. Terpene-impregnated fish flakes were offered to fish trained to feed on such food, and their responses (acceptance, rejection, avoidance, or no response) were assessed as indicative of feeding deterrence. Food treated with sinulariolide (a compound previously determined to be non-bioactive) was generally accepted at a 1% concentration. It was avoided and rejected, however, at a concentration of 10%, a concentration level generally restricted to polyp-rich branchlets. This indicated negative olfactory and palatability cues, respectively. Flexibilide-treated flakes were accepted to some extent by the fish at the 1% concentration, but strongly avoided at the 10% concentration, indicating effective feeding deterrence via olfaction. Dihydroflexibilide-impregnated flakes were strongly rejected even at low concentrations (1%) after tasting, indicating a negative palatibility cue. They were strongly avoided or rejected at higher concentrations (10%), indicating a negative olfactory cue as well. This response at higher concentrations indicates that sinulariolide and flexibilide become effective at concentrations between 1% and 10%. Such concentrations may be found in the polypary (polyp-bearing portion) of the soft coral colonies. Dihydroflexibilide elicited the strongest negative palatability response from these test fish. The feeding deterrence characteristics of the compounds determined here represent the potentials of individual compounds to elicit differential feeding responses in organisms like Gambusia which are capable of discriminating between different but closely related complementary (secondary) metabolites.