Fine Structure of the Gametes in Rhynchospio glandulosa (Annelida: Spionidae), a Hermaphrodite Brooding Larvae on the Parent’s Dorsum

Rhynchospio glandulosa is a common polychaete living in silty tubes in soft sediments in temperate shallow waters in the Northwest Pacific. Worms are simultaneous hermaphrodites. Spermatogenesis occurs in the coelomic cavity. Spermatids are joined in 16-cell clusters. The spermatozoa have a dome-shaped acrosome 1.2 µm long, an elongated nucleus 2.9 µm long, midpiece 2.4 µm long with subspherical mitochondria, and a flagellum about 44 µm long. The acrosome is a complex structure with five internal parts. The nucleus is concave anteriorly and has a short fossa posteriorly which accommodates the basal body of the axoneme. The flagellum is strengthened by two circles of microtubules in addition to the central axoneme. Oogenesis is intraovarian. Developed oocytes are about 130 µm in diameter, with an envelope about 0.8 µm thick, consisting of three layers of extracellular matrix: the thickest basal layer penetrated by unbranched microvilli each about 0.4 µm long, a homogenous intermediate layer with the highest electron density, and the outer brush-like layer with numerous extensions each about 0.3 µm long. The oocytes are spawned to the parent’s dorsum where they are loosely held by flat branchiae and long dorsal capillaries. In this “hatchery” larvae develop until the four-segment stage, then leave the parent and continue development in sea water. The general morphology of long-headed spermatozoa and thin-envelope oocytes of R. glandulosa is similar to that of other brooding spionids, but the details of their gamete ultrastructure are different.