Ultrastructural observations of the spermatogenesis of the hermaphroditic solitary coral Balanophyllia europaea (Anthozoa, Scleractinia)

Abstract 

Spermatogenesis ultrastructure was studied in a simultaneous hermaphrodite population of the solitary coral Balanophyllia europaea. In this species, spermatogenesis takes place in spermatocysts located within gametogenetic mesenteries surrounded by a bilayered boundary. Spermatogonia and spermatocytes are large flagellate cells, densely packed at the outermost edges of the spermatocyst. Spermatids and sperm are loosely distributed near the centre of the spermatocyst. The cytoplasm of spermatogonia and primary spermatocytes often contains short lengths of free axonemes, probably derived from the reabsorption of a primitive flagellum. Maturing spermatids either contain long intracytoplasmic axonemes, that may be stages of the tail synthesis, or have a flagellum. The morphological features of the sperm of this hermaphroditic scleractinian, very similar to those observed in the sperm of gonochoric taxa, support the hypothesis that the hermaphroditism of this population is an adaptive condition.