Study On The Early Ontogenesis Of Brachymystax Lenok Tsinlingensis

Qinling lenok(Brachymystax lenok tsinlingensis), which is endemic to China and distributes in the most southward of the world among salmonidae, is a typical landlocked cold water fish. The species was listed as a Second Class Protected Animal by the national government of China in1988. B. l. tsinlingensis has a narrow regional distribution, centralizing in the tributaries of the Weihe River and Yellow River, as well as in the upper Xushui River and Ziwu River which belonged to Hanjiang, a tributary of Yangtze River. Due to habitat destruction and illegal fishing, the wild population of B. l. tsinlingensis has declined markedly, Therefore, the study of species protection and stock enhancement is urgent to be done. In this paper, all of the experimental materials (fertilized eggs, embryos and larva) were collected from the artificial reproduction with the brood wild B. l. tsinlingensis which were caught in the National Reserve for B. l. tsinlingensis at Longxian county Shanxi Province. We observed the development of embryonic and larval fish as well as retina during B. l. tsinlingensis ontogeny to update the knowledge of fish to help the conservation as well as larva cultivation. The main results were as follows:1. The unfertilized matured egg of B. l. tsinlingensis which had plenty of yolk was spheroidal, yellowish, telolecithal. Its density was heavier than water. The egg was immersed in normal saline for30s, egg diameter ranged from (3.81±0.06mm) to (4.25±0.05mm). The total accumulated temperature of the embryonic development was193.35℃·d under the condition of water temperature9.31～12.80℃(Mean±SD:11.03±0.03℃). We defined six broad stages of embryogenesis (the zygote (2.5h), cleavage stage (12.5h), blastula stage (34h), gastrula stage (70h), neurula stage (24h) and organ formation stage (264h)) which included26periods. 2. B. l. tsinlingensis larvae were hatching with a body length of (9.64±1.03mm), body weight of (22.9±2mg), had transparent body, obvious sarcomere and anal tubercle. The larva after hatching had less swimming capability, and its mouth opened. The pectoral fin, tail fin and rudiment of dorsal fin emerged except adipose fin. Three days later, the larva had a little of melanin on the surface of the body and had dorsal fin, rudiment of anal fin and pelvic fin. Boundaries between pelvic fin and anal fin part obviously, amounts of melanin on the surface of the body on the5days after hatching. Some of larvae with well-developed dorsal fin, pelvic fin, anal fin and rudiment of adipose fin swim to upper water. The yolk sac began to disappear on the12days after hatching, and the larvae shifted from the endogenous to exogenous feeding. Parr marks began to appear46days after hatching, when fins of the larvae were the same as young fish.3. The diameter of eyeball of B.l. tsinlingensis at hatching was (951±50.2μm), accounted for9.87%of body length. The ratios (8.98%～10.44%) of eyeball diameter vs. body length didn’t change so much comparing with the development of larvae, the result showed that the diameter of eyeball was bigger relative to body length. Lens diameter/eyeball diameter ranged from23.03%to31.18%, showing the proportion of lens in the eyeball had a high value. The larvae of B. l. tsinlingensis grew slowly according to the trend of body length and total length at early stage, which was in accordance with the characteristic of cold water fish.4. Retina of B. l. tsinlingensis was a filmy pigment epithelium (PE) that would be to thicken as the development of retina on the1DPH. There were retinomotor responses in the retina of larvae on the14DPH. The cones (C.) and rods (R.) didn’t appear until5DPH and9DPH, respectively. The density of rods increased gradually while that of cones didn’t change so much but slightly decreased as the time went on. The density of ganglion cells (G.) declined distinctly at early development stage (9-18DPH) while reduced gently later. The degree of network aggregation of retina increased gradually accompanying with the development of B. l. tsinlingensis. Subsequently, this phenomenon became more obvious. It increased gently before31DPH in diameter of lens, then after it rose dramatically. The maximum and minimum of minimum separated angle were0.0461on the20DPH,0.0258on the56DPH, respectively. The retina structure of B. l. tsinlingensis was associated with characteristics of its habitat.