Chemical aspects of mass spawning in corals. 2. (-)-Epi-thunbergol, the sperm attractant in the eggs of the soft coral Lobophytum crassum (Cnidaria: Octocorallia)

The eggs of Lobophytum crassum collected at Magnetic Island (19 degree 10'S; 146 degree 52'E) in October or November between 1983 and 1993, contained significant amounts (6% dry weight) of (-)-epi-thunbergol, in addition to other terpenoid metabolites also present in the parent colony. (-)-Epi-thunbergol was not present in the tissues of the releasing colony. Using fluorocarbon droplets impregnated with the chemotactic molecules and a videomicroscopic technique for the direct observation of sperm under the influence of attractant molecules, we showed that (-)-epi-thunbergol, whether isolated from L. crassum or from a gorgonian octocoral (Briareum), significantly attracts sperm from L. crassum colonies. Attraction could be detected using direct observation at concentrations as low as 3.25 mu g/ml. This is the first evidence for sperm chemotaxis in the Alcyonacea. Eggs from L. compactum, a common alcyonacean coral at Orpheus Island (18 degree 36'S; 146 degree 29'E) contained (-)-thunbergol as the egg-specific compound. L. compactum was not found at Magnetic Island. (-)-Thunbergol from L. compactum and (+)-thunbergol from a Douglas fir tree both showed levels of attraction similar to (-)-epi-thunbergol against L. crassum sperm. Although L. crassum sperm were apparently neither stereo- nor enantio-specific in their selectivity for chemotactic molecules, (-)-epi-thunbergol was identified as the natural sperm attractant in the eggs of L. crassum.