Chemical ecology and origin of defensive compounds in the Antarctic nudibranch Austrodoris kerguelenensis (Opisthobranchia: Gastropoda)

The common Antarctic nudibranch Austrodoris kerguelenensis (Bergh) contains diterpene diacylglycerides only present in its external body parts. These compounds provide a chemical defense against sympatric predators, such as the seastar Odontaster validus Koehler. Bioassays conducted with O. validus revealed that live nudibranchs, mantle tissue and Et2O extract of the A. kerguelenensis mantle deterred feeding by the seastar. Further bioassays testing organic fractions of the Et2O mantle extract showed that the diterpene diacylglycerides, as well as corresponding monoacylglycerides and monoacylglycerides of regular fatty acids, were responsible for the feeding deterrence in O. validus. We suggest that A. kerguelenensis derives the bioactive diacylglycerides by de novo biosynthesis rather than by sequestration from its sponge diet, since the mollusk does not contain active metabolites in the viscera, and neither the active compounds nor precursors were detected in the sponge diet. Furthermore, A. kerguelenensis did not show a strong chemodetection or feeding preference for its main diet, hexactinellid sponges, in Y-maze and food choice experiments, respectively.