Formation of the early scales and development of the newborn scales in Hapalogenys mucronatus

Fish scales are specialized integumentary structures unique to the body surface of fish, originating from the directed and specific differentiation of primordial stem cells in the skin. They not only effectively protect the fish from pathogenic infections, but also reduce friction with the surrounding water and enhance swimming efficiency. The morphological characteristics of scales serve as important criteria for fish taxonomy and represent key indicators for age determination and growth status analysis. Investigating the process of scale formation contributes to evaluating larval growth conditions and hatchery rearing effectiveness, while providing scientific references for the timely adjustment of rearing protocols. In this study, the scale development of the Hapalogenys mucronatus under artificial rearing conditions water temperature (25.0±0.5) °C was examined in detail during early developmental stages using alizarin red staining, with comprehensive observations and measurements conducted. The results showed that under artificial breeding conditions, with water temperature maintained at (25.0±0.5) °C, scales began to appear in the H. mucronatus at 13 days of age, when the total length reached (4.74-5.30) mm. The earliest scales originated from a single center located on the ventral side between the pelvic and anal fins. As the fish continued to develop, scales grew forward, backward, and upward from this ventral center, and scales also began to appear near the lateral line. These scales gradually expanded until they covered the entire body, with complete scale coverage occurring when the fish reached a total length of 12.27 to 19.61 mm at 31 to 37 days post-hatching. Further observation revealed that the primary scales were the first to develop ctenii, followed by the formation of the flat part of the scale focus and scale ridges. Based on the observation of scale development, the formation process of primary scales was divided into five developmental stages, with each stage's characteristics described. The study demonstrated that the scales of the H. mucronatus are of the typical ctenoid type and remain as ctenoid scales throughout their life cycle, without transitioning from cycloid to ctenoid scales. This research not only fills a gap in the basic biology and morphological taxonomy of the H. mucronatus in its early developmental stages but also provides important data for further exploration of the mechanisms of fish scale development. It has significant implications for understanding the morphological changes in the early developmental stages of fish, as well as for future ecological research and species conservation strategies.