Study on the Morphological Development Timeline and Growth Model of Embryos and Larvae of European Catfish ().

To systematically elucidate the chronological patterns of embryonic development and morphological changes in the larval and juvenile stages of , and provide fundamental biological insights into this species, in this study, fertilized eggs were obtained through artificial spawning induction technology. After removing adhesiveness from fertilized eggs using trypsin, a detailed developmental study was conducted. The study systematically analyzed the chronological sequence of embryonic development and the morphological change patterns of larval and juvenile fish. The results showed the following: The fertilized eggs of are yellow, spherical, and sticky, and the stickiness allows eggs to attach to spawning substrates, enhancing hatching success. The egg diameter after water absorption was (2.88 ± 0.13) mm. The embryonic development took 47 h and 55 min, with a total accumulated temperature of 1245.56 h degrees Celsius, the developmental process includes seven stages and twenty-six periods, namely the zygophase stage, cleavage stage, blastula stage, gastrula stage, neurula stage, organogenesis stage, and hatching stage. At a temperature of (26.0 ± 0.9) °C, the hatched individuals went through the pre-yolk sac larval stage, late larval stage, juvenile fry stage, and juvenile stage. In the pre-yolk sac larval stage, otoliths appeared in the bilateral otic vesicles, a pair of barbel primordia emerged under the mandible, a short and thin straight intestine formed in the abdominal cavity, and the oral fissure first appeared. In the late larval stage, the fin rays were initially formed, the intestine became thicker and longer, the oral fissure, anus, and cloaca were formed, and the larvae could float and start feeding on exogenous food. In the juvenile fry stage, the differentiation of various organs was basically complete, the nostrils became larger, and both the anal fin and caudal fin had dark black markings. In the juvenile stage, the maxillary barbels elongated, the mucus layer thickened on the body and back, the abdomen is light white, and it had the external morphological characteristics of an adult fish. By measuring and calculating the total length, body length, body height, and head length of larvae and juveniles (0-40 days), the results showed that the growth characteristics conformed to the following fish growth formula: = 0.0141 + 0.8096 + 8.2421 ( = 0.9916), where x denotes days after hatching. This study has preliminarily mastered the chronological patterns of the embryonic development, growth, and formation of the morphological characteristics in larval and juvenile , providing scientific data and laying a theoretical foundation for the division of early developmental stages, reproduction, hatching, and fry cultivation.