Gametogenesis and larval development in Sabella spallanzanii (Polychaeta: Sabellidae) from Mediterranean Sea.

Sabella spallanzanii (Gmelin) is one of the best known and widely distributed Mediterranean polychaetes, but available data on its general biology has been inferred mostly from populations recently introduced to Australia. In the present paper, data on gametogenesis and larval development of a Mediterranean population from the Gulf of Taranto (Ionian Sea, Italy) are reported. Histological and electron microscopical analysis of gametogenesis showed that oogenesis, a long process beginning soon after spawning in February, is of extraovarian type. By contrast spermatogenesis is a very fast process lasting from September to December, when the coelomic cavity is completely packed with mature gametes and almost devoid of coelomocytes. During the period of spawning, some experiments of in vitro fertilisation were performed, always without success, while some of the collected females naturally spawned fertilised eggs (100%). Particularly interesting was the discovery of a small percentage of eggs which appeared fertilised but unspawned within the coelomic cavity of female. Investigation of the S. spallanzanii life cycle, from the naturally spawned fertilised eggs to embryonic and juvenile stages, was performed by daily microscopical observations and by SEM. Despite the presence of large eggs giving rise to a lecithotrophic larva, a long pelagic phase was observed, the longest found among Sabellidae. The larval development pattern is also described. Settlement began after about 2 weeks; metamorphosis occurred 10 d after settlement, when mucus was secreted abundantly and an external tube was formed. The long larval pelagic period and the development pattern, suggesting a high potential for dispersal, support both the introduction and invasive behaviour of the species in Australian waters.