Study on Allometric Growth and Digestive System Development in Larvae of Largemouth Bronze Gudgeon (Coreius guichenoti)

To reveal the early developmental strategies of larvae and juveniles of Coreius guichenoti and optimize artificial rearing techniques, this study systematically investigated the allometric growth patterns, histological development of the digestive system, and dynamic changes in digestive enzyme activities in C. guichenoti from 0 to 30 days post-hatch (dph). Through morphological measurements, histological section observations, and enzyme activity assays, the results showed that: In terms of allometric growth, the total length of C. guichenoti increased exponentially with age (y = -0.1985 + 1.7883×e0·0325ˣ). Swimming organs (pectoral fin, dorsal fin, caudal fin) and eye diameter exhibited rapid growth (allometric exponent b > 1), while body depth and pre-anal length showed slow growth (b < 1). Head length grew rapidly (b = 1.7001) before reaching a total length of 1.2 cm (10–12 dph), and then shifted to slow growth (b = 0.8233). This prioritized development of swimming and sensory organs reflects its adaptation to the swift-flowing habitat and feeding requirements. Regarding digestive system development: Newly hatched larvae relied on the yolk sac (endogenous nutrition). A rudimentary digestive tract appeared at 1–2 dph, and the digestive tract was fully lumenized by 4 dph. By 5 dph, the yolk sac was completely absorbed, and the oropharyngeal cavity, esophagus (with specialized esophageal papillae), intestine, and hepatopancreas had initially functional structures, marking the transition to exogenous feeding. The digestive system was basically mature by 20 dph, with intestinal segmentation (foregut enlargement, enhanced absorptive function of the midgut, and increased mucous cells in the hindgut). The liver and pancreas fused into a diffuse hepatopancreatic complex with significant fat accumulation. For digestive enzyme activities: Amylase, lipase, trypsin, and alkaline phosphatase (ALP) activities were detectable in newly hatched larvae. Trypsin activity was the highest and increased continuously (peaking at 30 dph). Amylase activity first increased and then stabilized (peaking at 20 dph). Lipase activity remained low (slightly increased at 30 dph). ALP activity showed a trend of first increasing and then decreasing (peaking at 20 dph). This study indicates that larvae and juveniles of C. guichenoti adapt to the swift-flowing environment by prioritizing the development of swimming and sensory organs. The digestive system achieves functional transition at 5 dph and matures by 20 dph, with dynamic changes in digestive enzyme activities closely matching dietary shifts and nutritional demands. These findings provide a scientific basis for determining the initial feeding timing, optimizing feed formulations, and formulating weaning strategies in the artificial breeding of C. guichenoti.