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

Spermatogenesis ultrastructure was studied in a simultaneous hermaphrodite population of the solitary coral Balanophyllia europaea. In this species, s permatogenesis takes place in spermatocysts located within gametogenetic me senteries surrounded by a bilayered boundary. Spermatogonia and spermatocyt es 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 spermatocy tes often contains short lengths of free axonemes, probably derived from th e 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 hermap hroditic scleractinian, very similar to those observed in the sperm of gono choric taxa, support the hypothesis that the hermaphroditism of this popula tion is an adaptive condition.