During field surveys and culture procedures, large growth disparities in Anguilla japonica have been observed. However, the potential causes are unknown. This study explored differences in digestive ability, metabolic levels, and transcriptomic profiles of appetite-related genes between growth-retarded eel (GRE) and normal-growing eel (NGE) under the same rearing conditions. The results showed that growth hormone (gh) mRNA expression in GREs was considerably lower than NGEs. The levels of total protein (TP), total cholesterol (T-CHO), triglyceride (TG), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C), blood ammonia (BA), blood urea nitrogen (BUN), and alkaline phosphatase (ALP) in GREs were significantly lower than in NGEs. Conversely, levels of glucose (GLU), alanine aminotransferase (ALT), and aspartate transaminase (AST) were higher in GREs. The activities of SOD, CAT, and T-AOC levels were also significantly lower in GREs, as were the activities of glucose-related enzymes including hexokinase (HK), pyruvate kinase (PK), phosphoenolpyruvate carboxykinase (PEPCK), and glucose-6-phosphatase (G6PASE). Additionally, orexigenic genes (npy and ghrelin) were dramatically downregulated, whereas anorexigenic genes (crh and pyy) were significantly upregulated in GREs. These findings suggested that variances in growth hormone, metabolic activities, and appetite level could be associated with the different growth rates of A. japonica. The present research not only revealed the characteristics of the growth, metabolism, and appetite of GREs but also offered new perspectives into the substantial growth discrepancies in A. japonica, providing novel ideas for enhancing fish growth.

Significant growth heterogeneity is prevalent in farmed Japanese eels ( Anguilla japonica ), whereas its underlying causes remain poorly understood. The study investigated the metabolic, digestive, and behavioral phenotypic differences between fast‐ and slow‐growing A. japonica . A total of 100 elvers (22.88 ± 1.11 g) were randomly allocated into four net cages (25 individuals per cage). After 8 weeks of rearing, the three heaviest and three lightest individuals from each cage were selected and designated as fast‐growing eel (FGE) and slow‐growing eel (SGE), respectively. Plasma metabolic profiling revealed that SGE showed significantly lower levels of total cholesterol (TC), low‐density lipoprotein cholesterol (LDL‐C) and lactate compared to FGE, while aspartate aminotransferase (AST) was higher ( p < 0.05). Moreover, oxygen consumption rate (OR) and ammonia excretion rate (AR) in SGE were significantly higher than FGE. Significant differences in digestive and metabolic enzyme activities were observed, with SGE displaying lower amylase, trypsin, Na ⁺ ‐K ⁺ ‐ATPase, and ATPase levels than FGE ( p < 0.05). In behavioral phenotypes, SGE exhibited a longer latency time, along with reduced total feed intake and amount of feces than FGE, while no significant differences in relative feed intake or relative amount of feces. Additionally, compared to FGE, the expression levels of neuropeptide Y ( npy ) and ghrelin were significantly downregulated in SGE, whereas corticotropin‐releasing hormone ( crh ) mRNA upregulated ( p < 0.05). Multivariate analysis identified that AST and crh may serve as primary discriminators for SGE classification. In conclusion, the differences in feeding behavior, metabolic level, and digestive capacity were related to growth disparity between individual eels. These findings advanced our mechanistic understanding of growth variation in A. japonica and provided valuable insights for promoting uniform growth.

The gonadal development of Japanese eels (Anguilla japonica) plays a crucial role in the success of artificial breeding. Soy isoflavones, a class of phytoestrogens commonly found in aquafeeds, have shown potential in enhancing gonad development in fish. The present study evaluated the effects of dietary soy isoflavones on gonadal development, growth performance, histology, sex hormone levels, vitellogenin content, and expression of related genes in female Japanese eel broodstock. A 4-week feeding trial was conducted with 120 two-year-old female eels randomly assigned to four groups and fed diets containing 0 (C), 0.1 (L), 0.5 (M), and 0.9 (H) mg/g of soy isoflavones. The results indicated that gonadal development was enhanced in the M and H groups, as evidenced by a significantly higher gonadosomatic index (GSI) and increased oocyte cross-sectional area (CSA) in M group, and greater nutrient accumulation in both the M and H groups. The expression of er and cyp19a genes in the ovary was downregulated in the treatment groups, leading to decreased serum estradiol (E2) and increased testosterone levels. Furthermore, hepatic vtg gene expression was upregulated in the M and H groups, though VTG protein content remained unchanged, suggesting an initiation of vitellogenesis at the transcriptional level. In conclusion, dietary soy isoflavones at 0.5–0.9 mg/g provide an effective pretreatment strategy to enhance early ovarian development in Japanese eel broodstock, potentially improving their responsiveness to subsequent hormonal induction in artificial breeding programs.

This study examined the effects of different food deprivation periods (0, 7, 14, 28, 42, and 84 days) on physical indexes, appetite gene expression, physiological and biochemical levels and intestinal histology in Japanese eel ( Anguilla japonica ). A total of 240 elvers (3.87 ± 0.06 g) were randomly divided into a starvation group (SG) and a feeding group (FG). The results showed that SG exhibited significant declines in body weight, condition factor, and hepatosomatic index with prolonged starvation. During the starvation period, appetite‐related genes exhibited dynamic regulation. Compared to FG, the mRNA expression of neuropeptide Y ( npy ) and ghrelin in SG increased significantly at days 7, 28, and 84, while corticotropin‐releasing hormone ( crh ), pro‐opiomelanocortin ( pomca ), cholecystokinin ( cck ), and peptide YY ( pyy ) decreased significantly at days 7, 14, and 84. Superoxide dismutase and catalase activities in SG peaked at days 7 and 14, respectively, significantly higher than FG, but declined below FG by day 84. Additionally, malondialdehyde levels increased by 85.46% and total antioxidant capacity decreased by 62.88% in SG at day 84 compared with FG. Intestinal histology revealed villus atrophy, goblet cell loss, and structural damage during prolonged food deprivation, while digestive enzymes (protease, amylase, and lipase) in SG were significantly lower than FG. Metabolically, SG exhibited significantly lower glucose and serum triglyceride levels than FG at the 84th day, whereas hepatic cholesterol content was markedly elevated. Furthermore, liver glycogen and muscle glycogen in SG gradually declined, reaching the lowest level at the 84th day. These findings highlighted a specific adaptation strategy, characterized by glycogenolysis and antioxidant activation during short‐term starvation, while long‐term food deprivation led to metabolic collapse, oxidative damage, and intestinal dysfunction. This study elucidated survival strategies under nutritional stress, offering insights for optimizing feeding regimes in A. japonica farming.