Release of sperm clusters in spheres by the black coral Cupressopathes pumila (Anthozoa, Antipatharia)

Fecund polyps from a bottlebrush-shaped colony of black coral Cupressopathes pumila (Brook 1889) from Ambon Island (Maluku Archipelago, Indonesia) were studied at the structural and ultrastructural levels. Five fragments, each 5 cm long and containing about 60 polyps, were cut from a single parental colony. The fragments underwent different treatments: two were fixed in 70% alcohol for species identification; the remaining three were reared in aerated aquaria and then processed for anatomic studies by light and transmission electron microscopy. Some polyps from one of these reared fragments displayed spheres of various sizes protruding from the mouth. Four hours after sampling, spheres settled on the bottom of the aquarium. Comparative analysis of the fecund polyps showed: (i) the differentiation of spermatocysts inside the transverse primary mesenteries that separate the lateral tentacles from one another; (ii) spermatocysts entering the gastric cavity after breakage of the mesentery wall and (iii) location of spermatocysts close to the basal part of the pharynx. Analysis of the largest spheres, both close to the external surface of the mouth and settled on the bottom of the small aerated aquaria, showed that they were irregularly bordered with residual mesentery tissue and contained clusters of spermatocysts. Gametes presented various phases of differentiation up to the final sperm, as it occurs in the spermatocysts developing inside the mesenteries of the fecund polyps. In consideration of the particular distribution of the colonies of C. pumila, which grow very far apart, the release of buoyant spheres is a particular modality of spawning which can be viewed as a strategy for successful fertilisation over long distances.