In order to more efficiently utilize the abundant cellulose resources in nature, increase the utilization rate of cellulose in aquaculture, implement precise feeding and save aquaculture costs, we have conducted research on cellulase genes related to the spotted knifejaw (Oplegnathus punctatus). Cellulose, as the most abundant renewable resource, is a cornerstone in the intricate ecological balance of diverse ecosystems. While herbivorous fish are recognized for their utilization of proteins, sugars, and fats, the extent of cellulose utilization by carnivorous and omnivorous fish remains an enigma. Here, through field sampling and behavioural observations, O. punctatus' omnivorous diet has been demonstrated (stomach contents contain approximately several species of algae in the Bacillariophyta (1.12 %), Streptomyces (0.55 %), Chlorophyta (0.35 %), Rhodophyta (0.16 %), and Euglenophyta (0.19 %) phylum). Additionally, the high cellulase activity in the intestine of O. punctatus has been detected first discovery (enzyme activity up to 4800.15 U/g), indicating its ability to digest cellulose. By employing whole-genome scanning and high-throughput sequencing, a single cellulase gene (β-glucosidase) within the genome of O. punctatus, suggesting the absence of a complete cellulose digestive system. However, microbiological analysis revealed the three crucial role of microorganisms, including Actinobacteria (25.80 %), Bacteroidetes (18.93 %), and Firmicutes phylum (0.82 %), were found to play a crucial role in the decomposition of plant cell walls, thereby facilitating plant material digestion to help the host to complete the process of cellulose digestion. Expression patterns and proteomic analysis of the β-glucosidase were notably high in the gonads. In situ hybridization confirmed the expression of the β-glucosidase gene in the intestinal contents and gonads, highlighting its role in supplying energy of gonads. These discoveries shed light on the dietary habits of O. punctatus and its cellulose utilization, offering insights that can inform the development of customized feeding strategies to enhance aquaculture sustainability and minimize resource expenditure. Display omitted •Anatomy of gastric contents, DNA barcoding, and behavioral studies confirmed the spotted knifejaw's omnivorous feeding habits.•Cellulase was found in its intestines, confirming its ability to digest plant-based prey.•Spotted knifejaw's digestion of cellulose involved collaboration between the host (β-glucosidase) and intestinal microorganisms.•New insights into the diverse functionality of the β-glucosidase gene in the energy supply mechanisms of the testes and ovaries.