DEFENSIVE RELATIONSHIPS BETWEEN CAULERPA-PROLIFERA AND ITS SHELLED SACOGLOSSAN PREDATORS

The prey-predator relationship between the green alga Caulerpa prolife ra and three shelled sacoglossans, Oxynoe olivacea, Lobiger serradifal ci and Cylindrobulla fragilis, have been investigated in order to clar ify the role of the chemical molecules in the defense of both alga and molluscs. Caulerpa prolifera contains as main component, among the se condary metabolites, caulerpenyne (1), a small molecule displaying a s esquiterpenoid skeleton. A defensive role, due to the presence of a pr otected 1,4-dialdehyde, was previously suggested for caulerpenyne (1). In spite of this protection, three sacoglossans, the infaunal C. frag ilis and the epifaunal L. serradifalci and 0. olivacea, live closely a ssociated with Caulerpa prolifera. The Thyrrhenian O. olivacea is able to modify the algal metabolite into a couple of more toxic compounds, oxytoxin-1 (2) and -2 (3), characterized by the presence of aldehyde groups. The same defensive strategy is adopted by populations of 0. ol ivacea from Spanish coasts and by populations of C. fragilis and L. se rradifalci from SW Italy. The biotransformation of caulerpenyne (1) le ads first to oxytoxin-1 (2), a monoaldehyde, which is compartmentalize d into the parapodia of L. serradifalci and into the tail of 0. olivac ea and then, in 0. olivacea and C. fragilis, to oxytoxin-2 (3) which d isplays the 1,4-conjugated dialdehyde partial moiety, present also in the structure of the potent antifeedant polygodial (4). The presence o f the organic molecules has been detected by chemical methods, in part icular by NMR (Nuclear Magnetic Resonance) of the extracts of the fres h tissues of the molluscs with deuterated solvents, avoiding any chrom atographic step and chemical treatment.