Coloration type of two allied cowries (Ovulidae:Gastropoda) tested through palatability evaluation in feeding experiments with fish

Abstract

Marine invertebrates have a great variety of defensive strategies, with taste deterrence and protective coloration among the most important. Species of the gastropod family Ovulidae are closely associated with chemically defended soft corals and can accumulate food-derived defensive substances. A painted mantle covers the shell, camouflaging some ovulid mollusks and making other conspicuous, which makes these mollusks a good model object for studying correlations between types of protective coloration and taste deterrence. We compared the palatability of two ovulids: Ovula ovum, whose coloration most researchers have interpreted as aposematic (although never tested directly), and Calpurnus verrucosus, whose coloration cannot be confidently assigned as either aposematic or cryptic. In feeding experiments, agar pellets flavored with water extracts of two mollusks and six soft coral species were offered to sympatric fishes, the Indo-Pacific sergeant (Abudefduf vaigiensis) and the juvenile barramundi (Lates calcarifer). Additionally, we studied fish feeding behavior. We found that both the foot and mantle of Ovula ovum have a strong aversive taste for fish, which proves aposematism in this species. On the contrary, Calpurnus verrucosus had an attractive taste for fish, which excludes aposematism in this mollusk. We contemplate possible crypticity over a long distance and in low visibility, as well as unlikeness of mimetic nature of such coloration. All soft corals tested, including host species for mollusks (Sarcophyton and Sinularia) were strongly aversive. Because both mollusks feed on soft corals but differ dramatically in palatability, it is probable that Ovula ovum and Calpurnus verrucosus transform soft coral antifeedants in different ways; however, we cannot rule out the possibility of de novo synthesis of taste deterrents in Ovula ovum. Both fish species orosensorially tested flavored pellets several times before swallowing or rejecting them. For pellets that produce aversive taste, the number of repeated grasps is smaller and retention time is longer than for pellets that produce attractive taste.