The swimming crab,Portunus trituberculatus is a commercially important mariculture species distributed in east coast regions of China,Japan and Korea.Its high nutritional value and increasing market demands have promoted the quick development of pond-culture in east coastal waters of China since the 1990 s.However,the molting death syndrome (MDS) and other disease problems are two limiting factors for the sustainable development of this crab aquaculture.Chitinase has been reported to participate in the molting regulation and immune defence for crustacean species,and the further study of crustacean chitinase will not only enhance our understanding for the mechanism of molting activities,but also provide valuable information for the control of MDS in the aquaculture of P.trituberculatus. Unfortunately,to date,no available information could be found on the chitinase of P.trituberculatus.The present study was therefore conducted to clone full length cDNA of a chitinase (Genbank accession number:KF914663) for P.trituberculatus by transcriptome sequencing and RACE (rapid amplification of cDNA ends).Relative gene expression levels of PtChi (P.trituberculatus Chi) gene in various tissues were then detected by qRT-PCR (quantitative real-time PCR) during the molting cycle.The results showed that:(1) The full length of PtChi cDNA was 2 200 bp,including a 1 470 bp ORF (open reading frame) which encoded 489 amino acid residues,a 16 bp 5'-UTR and a 714 bp 3'-UTR,while its calculated molecular weight and isoelectric point were 53.97 ku and 4.76,respectively.(2) Homologous analysis using Blastp showed that the predicted amino acid sequence of PtChi shared 61%-96% identity with the type of Chi-3 of other crustaceans,and PtChi was clustered with crustaceans Chi-3s in phylogenetic tree.(3)PtChi had the highest gene expression levels in hepatopancreas,then decreased in the order of stomach> mandibular organ > heart> eyestalk,while the lowest expression levels were found in other tissues.(4) The gene expression patterns of PtChi were different among tissues during the molting cycle.PtChi-mRNA had the highest expression level at AB stage and the lowest expression level at C stage in hepatopancreas,which suggested PtChi may participate in the immunity and pathogen defense after ecdysis.In intestine,the highest PtChi-mRNA expression level was found at E stage while the lowest level was detected at C stage.Therefore,it was deduced that intestinal PtChi was involved in the degradation of endogenous chitin in gut peritrophic membrane and the participation in immune functions.In addition,the peak value of PtChi-mRNA expression level in stomach was found at C stage,due probably to the digestion of food chitin during this stage.These results indicated that PtChi may belong to the type of Chi-3 in crustacean family,and may be involved in the molting process,food digestion and immunity of P.trituberculatus.However,its exactly biological function requires further studies.