Morphological Characteristics of Postembryonic Early Development of the Thamnaconus septentrionalis

Abstract

The morphological characteristics, ecological habits, and physiological structures of fish vary rapidly in the early growth and development stage. Understanding the changes in the postembryonic early development morphology of organs or tissues could provide basic information for the classification or identification of fish and the investigation or utilization of early resources. Morphological characteristic observation is one of the most commonly used methods. The existing studies on the early development of a certain trait in the late larval and juvenile stage are unclear. Most of the digital images are side views, resulting in a lack of dorsal views, ventral views, or local images, which is not conducive to the understanding of larval and juvenile. In this study, the variations in observable, measurable, and countable traits in postembryonic early development of Thamnaconus septentrionalis were explored using optical microscopy and stereomicroscopy. The observable traits included the developmental changes of yolk sac, scales, and notochord curvature. The measurable traits included the growth changes of total length, body length, body height, and oral fissure. The countable characters included the number of fins in the second dorsal fin, anal fin, caudal fin, and the number of pterygiophore. The results showed that at (23.0±1.0) ℃, the newly hatched larvae moved up and down in the water, with a phenomenon of convergent clustering under light irradiation; the pectoral fin appeared at 1 d, and the heart rate was approximately 50 beats/min; the optic capsule was completely black, the maxillar appeared, the oral fissure extended below the eyes, the yolk sac disappeared, and the first dorsal fin base appeared at 4 d; the girdle was single and colorless and began from the front edge of the belly, the oil globule disappeared, and the pelvic fin base appeared at 7 d; the first dorsal fin base and pelvic fin base broke membrane at 8 d; the first dorsal fin and pelvic fin were significantly longer than before, transparent rounded scales were observed at 9 d; swimming ability was improved, indicated by hovering back and forth gnawing pool wall, transversally conical barbs appeared at the base of the first dorsal fin at 11 d; the swim bladder and postcleithrum on both sides of the belly were clearly visible at 12 d; the second dorsal fin and anal fin pterygiophore appeared at 14 d, the second dorsal fin pterygiophore spans seven sarcomeres and the anal fin pterygiophore spans six sarcomeres. There are 10~12 melanin masses in the second dorsal fin and 5~6 in the anal fin, the conical teeth appeared on the upper and lower jaws at 16 d; the second dorsal fin and anal fin were differentiated at 19 d; the swim bladder fills about 25% of the abdominal cavity; notochord curves upward clearly and the stalk of caudal fin came out at 20 d; the fin strips of the second dorsal fin, anal fin, and caudal fin were well-developed at 24 d; the open mouth was round and changed from swallowing to sucking at 30 d, and the rainbow cells were colorful and the body surface near the gill was the most dense; the abdominal cavity has a strong metallic texture, the hard spine of the pelvic fin was degraded, close to the girdle at 39 d; 4~5 black bands were arranged regularly on the body, and the lateral line was consistent with the vertebral column at 50 d. The R2 values of the curve equations of measurable traits were all greater than 0.90. The height of the first dorsal fin increased exponentially 4~30 d. The visual sac increased linearly from 0~11 d to 11~20 d, and the increment of 0~11 d was less than that of 11~20 d, while the increment of 21~30 d was exponential. The oral fissure in 4~13 d and 22~30 d increased linearly, while that in 13~22 d decreased linearly. The total length, body length, and body height increased linearly before the inflection point and exponentially after the inflection point, which was concentrated in 19~23 d. The number of fin pterygiophore of the second dorsal fin was 31~36, while that in the anal fin was 33~35. The number of pectoral fins and caudal fins were 15~18 and 12, respectively. In conclusion, metamorphosis occurs in the postembryonic early development of T. septentrionalis, showing an allometric growth pattern, with a slow linear growth in the rotifer-feeding stage and a fast exponential growth in the artemia-feeding stage. The changes in the first dorsal fin and pelvic fin were correlated with age in days. The changes in the second dorsal and anal fins were correlated with the total length. This study enriches the early biological theory of T. septentrionalis and provides basic data for the investigation or utilization of its seedling breeding and resources. The partial enlarged detail is helpful for scholars to further understand the larval or juvenile fish and perform deep research.

Keywords

• thamnaconus septentrionalis
• larvae
• juvenile
• early development
• morphological characteristics