**5. Conclusions**

In conclusion, in this study, we cloned the *PmMKK4* gene sequence of *P. monodon*, which was ubiquitously expressed in all tissues of *P. monodon* and play an important role in the innate immunity after pathogen infection and the adaptation process in low-salt environments. The role of the mitogen-activated protein kinase signaling pathway in the process of low-salt stress and the immune response in *P. monodon* was provided.

**Author Contributions:** Conceptualization, S.J. (Shigui Jiang) and F.Z.; methodology, Y.L.; software, H.F.; investigation, S.J. (Song Jiang); resources, Q.Y.; data curation, L.Y. and X.C.; writing—original draft preparation, Y.L.; writing—review and editing, W.Z.; visualization, J.H.; supervision, F.Z.; project administration, S.J. (Shigui Jiang) All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by China Agriculture Research System (CARS-48); Hainan Yazhou Bay Seed Laboratory (Project of B21Y10701 and B21HJ0701); Central Public Interest Scientific Institution Basal Research Fund, South China Sea Fisheries Research Institute, CAFS (2020ZD01, 2021SD13); Hainan Provincial Natural Science Foundation of China (320QN359, 322RC806, 320LH008); Guangdong Basic and Applied Basic Research Foundation (2020A1515110200); Guangzhou Science and Technology Planning Project (202102020208); and Hainan Provincial Association for Science and Technology of Young Science and Technology Talents Innovation Plan Project (QCQTXM202206).

**Institutional Review Board Statement:** The use of all the shrimps in these experiments was approved by the Animal Care and Use Committee at the Chinese Academy of Fishery Sciences (CAFS), and we also applied the national and institutional guidelines for the care and use of laboratory animals at the CAFS.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors have declared no conflict of interest.

#### **References**

