Embryonic development and morphological characteristics of larvae and juveniles of cobia (Rachycentron canadum)

Cobia(Rachycentron canadum) is a highly prized recreational species worldwide as well as a promising candidate for aquaculture because of its rapid growth rate, strong disease resistance and high quality flesh. Studying the morphometrics and physiology of embryos, larvae and juveniles to obtain information about their early life history is the first step and key to the successful aquaculture of a fish species. In order to find out the characteristics and rules of the early development stages of cobia, the morphological characteristics and developmental characteristics of embryo, larvae and juvenile were studied by microscopic observation. The fertilized eggs obtained by artificial spawning were spherically shaped and buoyant, there was an oil globule in the centre, with an egg diameter of (1.245±0.065) mm and an oil globule diameter of (0.325±0.027) mm. The fertilized eggs hatched 26 h 30 min after fertilization in the sea water at (27.0±0.5)℃, salinity of 29 and pH of 8.3. The embryonic development process was divided into 7 stages, including fertilized egg stage, the cleavage stage, the blastocyst stage, the gastrula stage, the neurula stage, the organogenesis stage and the hatching stage (totally 24 developing periods). The total length (TL) of newly hatched larvae was (3.254±0.096) mm. The larvae opened their mouths and exhibited blackened eyes at 3 days post hatching (dph) with a TL of (4.453±0.267) mm, meanwhile, the yolk-sac decreased in volume by approximately 80%. The yolk-sac and oil globule were completely depleted at 5 dph (TL of 6.007 mm±0.171 mm) and 7 dph (TL of 8.173 mm±0.317 mm) respectively, and the larvae completely entered the exogenous nutritional stage. The intestine of the newly hatched larvae was thin, short, and straight, and the intestinal physiological curve formed at 9 dph when the larvae were (10.053±0.594) mm in TL. The larvae started to develop into juveniles at 14 dph (TL of 19.933 mm±1.118 mm) when the development of the dorsal fin, pectoral fin, anal fin and caudal fin was completed. At 22 dph, the juvenile had a TL of (41.140±3.779) mm, some scales on the caudal peduncle formed. The 46 dph juvenile reached (116.667±5.916) mm in TL, the whole body surface was covered with cycloid scales and their general appearance was similar to that of the adults, except for the shape of caudal fin. These results indicated that the fertilized egg diameter and newly hatched larvae size of cobia were both larger, and its early developmental characteristics had certain adaptability to the ecological behaviors. The present study could provide basic knowledge for investigating biology and artifical propagation of cobia.