Characteristics of the embryonic and post-embryonic development of golden trevally (Gnathannodon speciosus)

Golden trevally (Gnathanodon speciosus), a marine fish that combines ornamental value with edibility, is a suitable species for marine cage culture. At present, there are few studies on G. speciosus in China, while the studies abroad mainly focus on its adaptability to environmental factors, growth performance, feeding and nutrient requirements, as well as microsatellite sequencing. The studies on its reproductive characteristics and embryonic development is still lacking. Therefore, in this paper, the fertilized eggs of G. speciosus were obtained by artificial oxytocin and observed, in order to elucidate the characteristics of its embryonic and post-embryonic development. The hatchery conditions of the fertilized eggs were as follows : salinity 30.26±0.67, temperature 24.72±0.32 °C, pH value 7.46±0.12, dissolved oxygen 5.13±0.33 mg/L and light intensity 3000 lx. The results showed that the zygotes were transparent floating spherical eggs with an average diameter of 764.29±14.74 μm and a single oil bulb with an average diameter of 166.32±18.28 μm. Embryonic development included 8 stages i.e. fertilized egg stage, blastodisc stage, cleavage stage, blastula stage, gastrula stage, neurula stage, organo-forming stage and hatching stage, subsequently subdivided into 24 stages. The fertilized eggs were hatched out at 18 h 30 min after fertilization followed by the larval stage. According to the absorption of yolk sac, larval stage was further divided into pre-larval stage (0-3 days, not complete absorption of yolk sac) and post-larval stage (4-20 days, complete absorption of yolk sac). The total length of the hatched-out larvae was 1 520±19 μm. The yolk sac has been completely absorbed at 3 days post hatchery (DPH), and the oil globule disappeared at 6 DPH with the appearance of swimming bladder. At 20 DPH, the posterior part of spinal column was curved which was the mark of juvenile stage, meanwhile, the fins and the digestive system were fully developed with a great amount of pigments deposited on the surface of the body, which were the symbols for its ability to live independently. The larval development belonged to an allometric model with faster speed at the early stage and slower speed at the late stage. The development pattern of G. speciosus is in line with typical bony fish, and the development time was shorter than those of other species of Carangidae. The embryonic and post-embryonic development characteristics of G. speciosus were close to those in Trachinotus ovatus, which was the main species of marine ranching in Guangdong province and Hainan province in China. In the process of exploitation and industry development of G. speciosus, we could benefit a lot from T. ovatus. This study will shed a new light on the breeding of G. speciosus in the future.