Ultrastructure of spermiogenesis, sperm, and the spermatheca in Terebrasabella heterouncinata (Polychaeta: Sabellidae: Sabellinae)

Abstract

The sabellid polychaete, Terebrasabella heterouncinata, forms burrows in gastropod shells. It is a small, intratubular brooder that breeds semi-continuously. It has been shown to self-fertilize, but its reproductive biology suggests that some form of sperm transfer must occur between individuals. To gain an understanding of its fertilization biology, the ultrastructure of spermiogenesis and the sperm in T. heterouncinata was described, and the animal examined for sperm storage structures. Spermiogenesis occurs in clusters of eight spermatids. The mature sperm has an elongate nucleus and a bilaterally symmetrical acrosome with twisted subacrosomal spaces. The midpiece is short, with three crescent-shaped mitochondria, and forms a tight sheath around the axoneme. A single spermatheca, which opens on the inner ventral part of the crown near the buccal region, is present. It is a simple blind-ending duct that runs below the ventral nerve cord and is longer than 100 µm. This is the first record of a single spermatheca in Sabellidae. The shape of the sperm and the presence of a spermatheca confirm that individuals of T. heterouncinata produce ent-aquasperm and would normally cross-fertilize.