**5. Conclusions**

In summary, the mRNA expression levels of rhythm and immune- and metabolismrelated genes in the liver tissue of mackerel tuna were significantly changed under sunny and cloudy conditions, and some genes showed significant daily rhythmicity. Immune and metabolic genes can be regulated by rhythm genes and external factors through different signaling pathways and are jointly involved in regulating immunity and energy metabolism in mackerel tuna. This study provides practical implications for further understanding the regulation of lipid metabolism and immunity in mackerel tuna.

**Author Contributions:** Conceptualization, G.Y. and J.H.; Methodology, J.H.; Software, J.H.; Validation, J.H. and W.W.; Formal analysis, G.Y.; Investigation, Z.F.; Resources, Z.M.; Data curation, W.W.; Writing—original draft preparation, W.W.; Writing—review and editing, Z.M. and Z.F.; Visualization, J.H.; Supervision, G.Y.; Project administration, Z.M.; Funding acquisition, Z.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was supported by the Guangxi Innovation Driven Development Special Fund Project (grant no. Guike AA18242031), the Central Public-Interest Scientific Institution Basal Research Fund, CAFS (2020TD55), and the Central Public-Interest Scientific Institution Basal Research Fund South China Sea Fisheries Research Institute, CAFS (2021SD09).

**Institutional Review Board Statement:** The animal study protocol was approved by the Institutional Review Board (or Ethics Committee) of Animal Care and Use Committee of South China Sea fisheries Research Institute, Chinese Academy of Fishery Sciences (BIOL5312, 5 July 2021).

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The original contributions presented in the study are included in the article. Further inquiries can be directed to the corresponding authors.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Ethics Statement:** The experiment complied with the regulations and guidelines established by the Animal Care and Use Committee of the South China Sea fisheries Research Institute, Chinese Academy of Fishery Sciences.
