The biosynthesis of sesquiterpene isocyanides and isothiocyanates in the marine sponge Acanthella cavernosa (Dendy); Evidence for dietary transfer to the dorid nudibranch Phyllidiella pustulosa

The tropical marine sponge Acanthella cavernosa (Dendy) converts potassium [¹⁴C] cyanide to axisonitrile-3 (1); this precursor is also used for the synthesis of axisothiocyanate-3 (2) suggesting that isocyanides are precursors to isothiocyanates in A. cavernosa. Likewise, potassium [¹⁴C] thiocyanate is used for the synthesis of axisothiocyanate-3; unexpectedly this precursor also labelled axisonitrile-3. These results demonstrate either an interconversion between cyanide and thiocyanate prior to secondary metabolite formation or that the secondary metabolites can themselves be interconverted. Specimens of the dorid nudibranch Phyllidiella pustulosa, preadapted to a diet of A. cavernosa, fed on ¹⁴C-labelled sponges and were subsequently found to contain the radioactive terpenes (1) and (2). Specimens of P. pustulosa, which had not expressed a dietary preference for A. cavernosa in the field, did not generally feed in aquarium test with ¹⁴C-labelled sponges and, therefore, provided non-radioactive extracts. Since control experiments demonstrated the inability of P. pustulosa to synthesise the metabolites de novo, we therefore conclude that P. pustulosa acquires secondary metabolites by dietary transfer from A. cavernosa.