Abstract: This study investigates the differences in dorsal muscle morphology and organ physiology among subgroups with different growth performances within the cohort of juvenile black seabream (Sparus macrocephalus). The objective is to uncover the connections between growth performance, metabolic strategies, and feeding mechanisms. This information is crucial for accurately selecting high-quality breeding populations of juvenile black seabream and for scientifically guiding efficient and intensive cultivation. A total of 3000 individuals from the same cohort of juvenile black seabream which cultivated in cage culture by Ningbo Xiangshan Harbor Aquatic Seedling Co, Ltd. in the Xi Hu Harbor area were chosen. Following a 1d fasting period, they were categorized based on body mass into five subgroups: A body mass (8.2±1.5) g, frequency 5%, B body mass (5.3±0.9) g, frequency 20.6%, C body mass (3.8±0.6) g, frequency 48.3%, D body mass (2.4±0.4) g, frequency 21%, and E body mass (1.7±0.5) g, frequency 5.1%. Comprehensive studies were conducted on the differences in oxygen consumption rate, critical oxygen level, and enzyme activity in gill tissues and internal organs based on measurements of dorsal muscle morphology and organ mass ratios. The findings indicate: (1) Subgroups in this study can be distinctly differentiated based on the clustering characteristics of organ ratio traits. (2) Among the traits of dorsal muscle morphology and oxygen consumption rate, only chew resistance and daily oxygen consumption rate showed a monotonic increase with enhanced growth performance. The diurnal rhythm of oxygen consumption for subgroup C was roughly equal during day and night (P>0.05), while for other subgroups, nighttime rates exceeded daytime rates (P<0.05). (3) The critical oxygen level in water exhibited a stepwise decline as growth performance improved, with only subgroups A and B showing significant differences (P<0.05) from subgroup E, following the pattern E>A≈B. (4) Activities of internal organ enzymes such as amylase, protease, AKP, ACP, and ATP displayed a stepwise decline with enhanced growth performance, while SOD, CAT, and POD enzyme activities showed a stepwise increase, and lipase and LHD enzyme activities demonstrated an initial decline followed by an increase. (5) Gill tissue enzyme activities for SOD, POD, Na⁺/K⁺-ATPase, and Ca²⁺/Mg²⁺-ATPase all exhibited a stepwise increase with enhanced growth performance, while CAT enzyme activity showed an initial decline followed by an increase. The research provides a scientific foundation for constructing growth performance evaluation systems for black seabream, exploring fast-growing germplasm, and guiding the selection and breeding of fast-growing varieties (strains).