Histological study of the development of the embryo and early larva of Oreochromis niloticus (Pisces : Cichlidae)

The developmental stages of Oreochronis niloticus are similar to those desc ribed in other mouth-breeding tilapias except that, as in zebrafish, no cav ity was found in the blastula. Variation in the rate of development of the embryo and larva of O. niloticus was found within a clutch of eggs as well as between clutches. Hatching glands are described for the first time in ti lapias. They are widely distributed within the ectoderm covering the head, body, tail, and surface of the yolk sac near its attachment to the embryo. Timing of larval development is similar to that in other mouthbrooding tila pias, but is slower than that found in substrate-spawning tilapias. A pneum atic duct connects the swimbladder to the digestive tract and swimbladder i nflation and initiation of feeding occurs at about the same time. The diges tive tract of the larva 8 and 9 days after fertilization is similar to that found in the adult, except that there are no digestive glands. An endocrin e pancreatic islet was first seen 76 h after fertilization. A prominent thy mus gland is present at 100 h. Hematopoietic tissue develops in the vicinit y of the pronephros during early larval development. A spleen develops late r, 7 days after fertilization. (C) 2001 Wiley-Liss. Inc.