Ultrastructure of oogenesis in the abalone pest, Terebrasabella heterouncinata (Polychaeta: Sabellidae: Sabellinae)

Summary

Terebrasabella heterouncinata is a small, semi-continuous breeder that has become a pest on cultured abalone. Its success on the farms may, in part, be related to an increase in fecundity and the rate at which eggs are produced in response to increased food availability. In this study, the ultrastructure of vitellogenesis was described to determine how this relates to the life history of this animal. A single pair of ovaries is attached to the anterior septum of segment nine. Early oocytes in the ovary are surrounded by follicle cells that contain few mitochondria and free ribosomes. The follicle cells associated with early vitellogenic oocytes in the ovary contain glycogen granules and dense bodies that resemble the putative yolk bodies present in these oocytes. Oogenesis is asynchronous, and most of the yolk bodies accumulate in the oocytes after their release into the coelom. Endocytotic activity (which presumably represents the rapid uptake of high molecular weight extracellular yolk precursors) is observed when the solitary oocytes are released into the coelom. During vitellogenesis, lipid droplets and glycogen granules also accumulate in the ooplasm. The yolk bodies and lipid droplets fill the ooplasm from the cortex towards the centre of the oocyte, with larger bodies accumulating in the centre. The results suggest that the bulk of the yolk bodies are produced heterosynthetically, but this could not be confirmed. The pattern of oogenesis demonstrated in T. heterouncinata is similar to that described for some semelparous or annual iteroparous sabellids.

Key words:

• Semi-continuous breeder
• extraovarian
• solitary vitellogenesis