Ontogenesis from Embryo to Juvenile in Threadsail Filefish, .

The threadsail filefish, , is an economically important marine species. However, wild catches have sharply decreased over the past 20 years, causing to be added to the IUCN Red List of Threatened Species. Accordingly, this study seeks to promote technological development for artificial breeding and early life-stage farming by defining the morphological characteristics of ontogenesis. The fertilized eggs, with a diameter of 0.62 ± 0.01 mm, were spherical and sticky and contained multiple oil globules of varying sizes. The embryonic development was observed and divided into eight phases, which were cleavage, blastocyst, gastrula, neurula, organogenesis, muscular contraction, heart pulsation, and hatching. At 3 days post-hatching (dph), the yolk sac was completely absorbed. The eye developed rapidly, and the mouth fissure and anus initially formed. Some larvae were fed on S-rotifers (). At 6-8 dph, the upper and lower jaws of larvae were gradually covered by leathery skin, and the head-to-body proportion increased. At 14-16 dph, the fin differentiation occurred in the dorsal, anal, and pectoral fins, with widespread distribution of yellow and melanin on the body surface. Swim bladder was clear. The swimming ability of larva was enhanced, resulting in an obvious clustering phenomenon. At 22-25 dph, the end of the notochord continued to tilt upwards, forming a tail fin. The trunk was evenly distributed with protruding circular punctate scales. The snout was covered with leathery epidermis, and the mouth began to round. At 40-45 dph, the juvenile completed metamorphosis, with horizontal dark stripes appearing on the trunk. Pigmented spots appeared on the tail fins. The counts of dorsal and anal fin spines were 34-36 and 32-34 dph, respectively. During the development of larvae and juveniles, the growth parameters, such as total length, standard length, body height, and body weight, were made as growth curves. The slopes of growth curves were calculated. We found two inflexion points occurring in the growth curves, which may be associated with metamorphosis and transitions in feeding habits. These results enrich the biological understanding of filefish species while providing guidance for artificial propagation and fry production in .