The swimming crab , an important marine economic animal widely distributed in China, belongs to Crustacea, Decapoda, Decapoda, is an aquatic crustacean with wide tolerance to salinity (13.7-47.7). Salinity is one of the most important environmental factors affecting the growth and development of . Ingestion, molting, growth, metabolism, immunity, and other processes in are greatly affected by salinity. Therefore, it is important to study the mechanism of salinity adaptation to breed improved salt-tolerant varieties of . In this study, random amplification of cDNA ends technology was used to clone the crustacean cardioactive peptide (CCAP) gene. The full-length CCAP gene was 606 base pairs (bp) long, including an open reading frame of 426 bp, 5' untranslated region of 72 bp, and 3' untranslated region of 108 bp, with a poly A structure. Amino acid sequence analysis showed that the CCAP gene encodes 141 amino acids with a predicted molecular weight of 15.6 kD and isoelectric point of 9.55. Homology analysis showed that the homology between CCAP of Callinectes sapidus was higher at 85% and 82%, respectively. Phylogenetic analysis showed that were clustered first, followed by . Tissue expression analysis revealed that the relative expression of the CCAP gene was highest in the in thoracic ganglia, followed by in the brain and eye stalk. Analysis of the expression pattern of the CCAP gene during low-salinity stress showed that low salinity significantly altered the expression pattern of CCAP in the thoracic ganglia; CCAP gene expression in the experimental group was significantly higher than that in the control group at 24, 48, and 72 h ( < 0.05) with values 1.73-, 2.16-, and 2.19-fold higher than that in the control group, respectively. P. trituberculatus under low-salinity conditions and the activities of Na⁺/K⁺-ATPase and V-ATPase were significantly increased. The results demonstrate that CCAP plays a protective role in under low-salinity conditions by regulating osmotic pressure and may be associated with increased Na⁺/K⁺-ATPase and V-ATPase activities to modulate the osmotic pressure balance in .