Experiments in aquaria have shown that the feeding behaviour of the sea urchin Paracentrotus lividus in presence of the green alga Caulerpa taxifolia varies during the year. In summer, the alga is strongly avoided and is not consumed. in winter and spring, although the alga is only moderately appreciated, its consumption is significant, but it is hardly or not at all absorbed. These results are confirmed by content analysis of the digestive tract of P. lividus sampled in situ in an area colonized by C. taxifolia. The sea urchins' feeding behaviour is linked to the presence of toxic and repellent secondary metabolites of C. taxifolia, caulerpenyne in particular.