Polyploid breeding is widely used in various marine species.Low salinity treatment is an effective method of inducing triploid of bivalve mollusks.In this study,RNA-seq was performed to determine genes and pathways involved in hyposaline adaption and cell division of Pacific oyster (Crassostrea gigas) zygotes,trying to better understand the possible molecular mechanism of hypo-osmotic induction.A total of 26965 unigenes were generated in the de novo assembly of clean Illumina reads with an average length of 934bp and N50 of 1721 bp.Of 3024 differentially expressed genes (DEGs),2501 were up-regulated and 523 were downregulated.GO (Gene Ontology) annotation and KEGG (Kyoto Encyclopedia of Genes and Genomes) pathway analysis of these DEGs revealed that these DEGs participate a variety of biological processes including osmoregulation,cytoskeleton organization,cell survival and death,and substantially modulate cell proliferation and embryonic development.In summery,RNA-seq methodology was applied for the first time to demonstrate hypotonic-induced transcriptomic alteration in oyster zygotes.Our findings not only interpreted the relatively high mortality of induced larvae,but also provided a valuable reference for further investigations on the mechanism of hyposaline induction,thus should aid to the application of low salinity in triploid induction in large scale aquaculture in future.