Chemoecological study of the invasive alga Caulerpa taxifolia var. distichophylla from the Sicilian coast

Marine invasive species and their bioactive metabolites have become critical ecological issues in the Mediterranean Sea. In particular, the highly invasive green algae Caulerpa taxifolia and Caulerpa cylindracea are known to contain the bioactive sesquiterpene caulerpenyne (1) and the bisindolic alkaloid caulerpin (2), potentially acting as chemical stressors for native species. The recent spread of a variety of C. taxifolia, Caulerpa taxifolia var. distichophylla, also raises urgent questions about its chemical composition. Indeed, the only chemical data available for this alga are limited to the seasonal variations of caulerpenyne (1) in samples collected in the Eastern Mediterranean. In this study, we confirmed the presence of 1 also in C. taxifolia var. distichophylla collected along the Sicilian coast, while 2 was not detected in the alga. However, caulerpin (2) was found both in a Mediterranean specimen of C. taxifolia, and at a much higher level in the congeneric C. cylindracea. This suggests that C. taxifolia var. distichophylla differs from C. taxifolia in its secondary metabolism, potentially exerting dissimilar chemically mediated impacts on native biota. Further chemical investigations on the terpenoidic content of C. taxifolia var. distichophylla led to isolate and identify squalene 2,3 oxide (3), phytol (4), and plastoquinone (5), along with the two unreported sesquiterpene lactones 6 and 7. Finally, chemoecological assays clarified that caulerpenyne (1) at its natural concentration in C. taxifolia var. distichophylla elicits avoidance responses in native shrimp, although sensitization was a prerequisite to significantly induce food rejection.