Spermatogenesis and sperm ultrastructure in the interstitial syllidPetitia amphophthalma(Annelida, Polychaeta)

The interstitial syllid Petitia amphophthalma is gonochoristic. The mature spermatozoa accumulate in the dorsal region of a seminal vesicle that extends over several segments. TEM studies have shown them to be filiform spermatozoa of the so-called modified type (35 µm long) typical of many interstitial polychaetes with internal fertilization: the head comprises two elongated elements, the acrosome (ca. 8 µm long) and a nucleus (only half as long, ca. 3 µm) into which the basal part of the axoneme of the flagellum projects; in the adjacent midpiece of identical diameter (0.6 µm) the axoneme is surrounded by two mitochondrial complexes over 16 µm of its length; the spermatozoon terminates in a tail (ca. 10 µm long) consisting of the axoneme alone. Spermatogenesis occurs in the ventral region of the vesicle. The gametes differentiate into morulae, which form a cytophore after the second maturation division. Striking features of both primary and secondary spermatocytes is the presence of two pairs of centrioles together with a short flagellum. They are unique to the Syllidae and extremely rare within the animal kingdom.