Anti-predatory chemical defenses of ascidians: secondary metabolites or inorganic acids?

Both secondary metabolites and inorganic acids have been hypothesized to protect adult ascidians from predation, raising the possibility of alternative defensive strategies in these sessile, soft-bodied, benthic invertebrates. The objective of this investigation was to determine if ascidian species from the Western Atlantic have these chemical defenses against fish predators, and if so, to determine their location within the body of the ascidian. The palatability of crude organic extracts of whole ascidians, as well as the dissected tunics, viscera, and gonads (when possible) were determined at natural volumetric concentrations using laboratory feeding assays with the bluehead wrasse, Thalassoma bifasciatum. Acidified food pellets were also assayed to determine the effect of lowered pH on predation. Sixteen of the 17 species tested had deterrent organic extracts from some region of the body (Aplidium constellatum, Aplidium stellatum, Ascidia interrupta, Ascidia nigra, Botrylloides sp., Clavellina picta, Didemnum candidum, Didemnum vanderhosti, Diplosoma listerianum, Ecteinasci-dia turbinata, Eudistoma capsulatum, Eudistoma hepaticum, Rhopalaea abdominalis, Styela plicata, Symplegma rubra, and Trididemnum solidum). The location of the deterrent secondary metabolites was isolated in the gonad in all three solitary species, raising the possibility that these defenses are passed on to eggs or larvae. Nine ascidian species sequestered acid in their tunics (A. interrupta, A. nigra, A. stellatum, D. candidum, D. vanderhosti, E. capsulatum, E. hepaticum, R. abdominalis, and T. solidum) at levels that were effective in deterring fish predation (pHV 3.0). Only one species (Botrylloides nigrum) had neither chemical defense. Results of this study indicate that there is not a clear trade-off between the presence of secondary metabolites and inorganic acid defenses in ascidians, suggesting that 0022-0981/02/$ see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S0022 -0981 (02 )00023 -0 * Corresponding author. E-mail address: [email protected] (J.R. Pawlik). www.elsevier.com/locate/jembe Journal of Experimental Marine Biology and Ecology 270 (2002) 203–214 these defenses are redundant, or that alternative chemical defenses may have evolved for different predators or for different stages in the life history of the ascidians producing them. D 2002 Elsevier Science B.V. All rights reserved.