Defensive relationships between Caulerpa prolifera and its shelled sacoglossan predators

Abstract

The prey-predator relationship between the green alga Caulerpa prolifera and three shelled sacoglossans, Oxynoe olivaceca, Lobiger serradifalci and Cylindrobulla fragilis, have been investigated in order to clarify the role of the chemical molecules in the defense of both alga and molluscs. Caulerpa prolifera contains as main component, among the secondary metabolites, caulerpenyne (1), a small molecule displaying a sesquiterpenoid skeleton. A defensive role, due to the presence of a protected 1,4-dialdehyde, was previously suggested for caulerpenyne (1). In spite of this protection, three sacoglossans, the infaunal C. fragilis and the epifaunal L. serradifalci and O. olivacea, live closely associated 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 O. olivacea from Spanish coasts and by populations of C. Fragilis and L. serradifalci from S W Italy. The biotransformation of caulerpenyne (1) leads first to oxytoxin-1 (2), a monoaldehyde, which is compartmentalized into the parapodia of L. serradifalci and into the tail of O. olivacea and then, in O. olivacea and C. fragilis, to oxytoxin-2 (3) which displays the 1,4-conjugated dialdehyde partial moiety, present also in the structure of the potent antifeedant polygodial (4). The presence of the organic molecules has been detected by chemical methods, in particular by NMR (Nuclear Magnetic Resonance) of the extracts of the fresh tissues of the molluscs with deuterated solvents, avoiding any Chromatographie step and chemical treatment.