Metamorphosis and skeletal development of hybrid Epinephelus awoara () and Epinephelus tukula () progenies

Here, we explored the metamorphosis and skeletal development of hybrids between Epinephelus awoara (female) and Epinephelus tukula (male) (EAT). Currently, many varieties with good characters have been produced by cross breeding, which plays an important role in the development of aquaculture. However, few basic studies have focused on the developmental biology of hybrid progeny. We studied the morphological characteristics and body structure of larvae and juveniles of the hybrid and measured their body length. The results showed that the metamorphosis process of the hybrid grouper was comparable to that of other groupers. Moreover, using wholemount bone-staining technology, the ontogeny and ossification of the cranium, appendage skeleton and vertebral column were observed and described in the larvae and juveniles. Newly hatched larvae did not have a skeleton. Head cartilage and the pectoral fin developed at 3 days after hatching (DAH), and the total length (TL) was 2.74 +/- 0.06 mm. Cartilage of the dorsal fin and pelvic fin developed at 15 DAH (3.90 +/- 0.97 mm TL). The vertebral column and caudal fin cartilage appeared at 18 DAH (4.69 +/- 0.58 mm TL). Cartilage was observed in the anal fin at 22 DAH (6.16 +/- 1.76 mm TL). The second dorsal fin spine and pelvic fin spine were fully ossified between 24 and 26 DAH (8.01 +/- 1.41-9.29 +/- 1.64 mm TL). The first skeletal part that completed ossification was the pelvic fin, which occurred at the 44 DAH (24.64 +/- 3.63 mm TL). The vertebral column was fully ossified at 47 DAH (29.70 +/- 1.97 mm TL). The dorsal fin and anal fin were ossified at 51 DAH (34.64 +/- 1.92 mm TL) and the cephalic skeleton, caudal fin and pectoral fin were fully ossified at 68 DAH (41.89 +/- 2.16 mm TL). These results provide abundant biological data on the metamorphosis and skeletal development of larvae and juvenile groupers, as well as analysis of the growth and survival rate advantage of hybrid offspring.