Ontogeny of the immune system in rock bream Oplegnathus fasciatus

Histogenesis of the immune system and specific activity of superoxide dismutase (SOD) were studied in rock bream Oplegnathus fasciatus from fertilization to 50 days after hatching (DAH). The pronephric tubule primordium developed in the embryo, 14 h 30 min post fertilization. The spleen anlage was observed between the swim bladder and the intestine at 5 DAH, and the thymus was formed as a paired structure under the pharyngeal epithelium above the gill arch at 10 DAH. The order of the immune organs becoming lymphoid was the pronephric kidney (10 DAH), thymus (15 DAH) and spleen (21 DAH). As the embryo developed, the specific activity of SOD gradually increased until hatching, but subsequently SOD activity continuously decreased to a minimum at 14 DAH. After the spleen became lymphoid, the specific activity of SOD was relatively stable. It is suggested that the immaturity of the lymphoid organs and low specific activity of SOD was the cause of the high mortality of fingerlings 12 to 16 DAH.