Behavioural modifications imposed to the ciliate protist Euplotes crassus by caulerpenyne: The major toxic terpenoid of the green seaweed, Caulerpa taxifolia

Caulerpenyne is the most abundant, toxic terpenoid produced by Caulerpa tax ifolia, one of the few toxigenic, green seaweed endemic in the tropics. Owi ng to the bewildering high concentration of this harmful secondary metaboli te, associated with largely magnified homeostatic and reproductive potentia lities, the strain(s) of C. taxifolia invading the Mediterranean Sea are a potential risk for biodiversity and ecotoxicity. The effects of caulerpenyn e on the biology of the marine ciliate protist Euplotes crassus were studie d by means of the ethogram, that is, by the qualitative and quantitative an alysis of its creeping behaviour. Concentrations as low as 1.5 mu g/ml (= 4 mu M) sufficed to induce several clear-cut changes in the creeping of E. c rassus and 6 mu g/ml (= 16 mu M) affected significantly the entire ethogram : (a) the cell "activity" was reduced decreasing velocities as well as leng ths of the "Long Lasting Elements" (LLE) and largely increasing the percent age of motionless Euplotes; (b) the adhesion to the substrate was relaxed c ausing disappearance of the "Smooth Trajectory Change" (STC); (c) the elect rophysiological states were altered modifying the Side Step Reaction (SSR) which lasted longer (prolonged Side Step Reaction, p-SSR). It follows that the Euplotes' adaptive behaviour becomes aberrant dooming individuals to a short-term death even under conditions of largely sublethal concentrations of caulerpenyne. The complex of the foregoing induced, behavioural variatio ns is discussed in view of clues to the underlying mechanisms.