Osmoregulation is one of the most fundamental and delicately controlled processes in euryhaline marine species like penaeid shrimp. Here we first report a cDNA encoding for the crustacean cardioactive peptide (CCAP) precursor cloned from the intestine of Pacific white shrimp, Litopenaeus vannamei, a prominent commercial culture species. The CCAP mature peptide (PFCNAFTGC-NH2) is generated by post-translational processing and found to be highly comparable with its counterparts in insects and other crustaceans. Semi-quantitative RT-PCR reveals that L. vannamei CCAP (Lv-CCAP) mRNA is predominantly expressed in the brain and intestine. However, only the expression level of intestine-derived Lv-CCAP mRNA showed responsiveness to salinity changes in culture. The inducible expressed Lv-CCAP mRNA by transfer to high-salinity condition was continuous, while mRNA expression stimulated by transfer to low-salinity condition was transient. Functionally, the stimulatory effects of CCAP on heart rates in the shrimp cultured in low- and normal-salinity conditions were confirmed. Furthermore, injection of CCAP could up-regulate the freshwater tolerance of shrimp by increasing their survival rates. Our study, as a whole, provides new insights into hormone-regulated crustacean osmostasis and may prove instructive to the inland freshwater penaeid shrimp aquaculture.It proves instructive to freshwater shrimp aquaculture.