Morphological and chemical camouflage of the Mediterranean nudibranch Discodoris indecora on the sponges Ircinia variabilis and Ircinia fasciculata

Abstract

The nudibranch Discodoris indecora shows a perfect camouflage on its prey, the dictyoceratid sponge Ircinia variabilis. Shape and colour of the nudibranch are remarkably similar to that of the sponge which is widespread in the shallow waters of the Mediterranean Sea. The metabolite pattern of I. variabilis is dominated by unusual molecules, the sesterterpenoids palinurin and variabilin, containing 25 carbons and displaying at the ends a β-substituted furan ring and a tetronic acid. However, a chemical polymorphism has been recently described for I. variabilis. Some specimens contain either palinurin or variabilin whereas others possess a mixture of both sestertepenoids. During the study of Mediterranean sponges, a linear sestertepenoid related to palinurin and variabilin was found in Ircinia fasciculata, that from a taxonomic point of view is closely related to Ircinia variabilis. Discodoris indecora shows a typical defence behaviour when it is disturbed, secreting a copious white slime that contains large amounts of palinurin and variabilin. Most likely the animal is able to transfer the sponge metabolites from the digestive glands to mantle glands. Electron micrographs of the nudibranch mantle showed the presence of unicellular glands characterized by a cylindrical vacuole with a big nucleus on the basis. The glands were concentrated in the most conspicuous tubercles of the dorsal mantle. Probably, the sponge metabolites are stored in these glands. Comparative chemical analysis of mucous secretions, dorsal tubercles, mantle, foot, digestive glands and hepatopancreas of D. indecora confirmed this hypothesis. Palinurin and fasciculation were, of course, present in the digestive glands but they were also found in the mucus and in the mantle sections enriched by the above described unicellular glands. The defensive role of palinurin and variabilin, strongly suggested by their anatomical location, was supported by recording the induction of feeding-deterrence in the fresh-water fish Carassius auratus and in the marine fish Chromis chromis and Sparus auratus.