Introduction
The freshwater bivalve Corbicula fluminea (Müller, 1774), commonly known as the Asian clam, is native to Southeast Asia [1]. In China, C. fluminea is widely distributed in rivers and lakes, such as the Yangtze River and the lakes Hongze (HZ), Poyang (PY) and Taihu (TH). As an important aqua-cultural shellfish, C. fluminea is currently one of the most economically important aquatic species in China with a yearly output of more than 12 million tons. C. fluminea meat is thought to have beneficial effects on the liver and is valued for its nutritional and pharmacological activity (e.g., the amelioration of hypercholesterolemia; hepato-protective, antioxidant and anticancer activities; protection against fatty liver induced by exposure to xenobiotics; and increased ethanol metabolism) [2,3]. This species underwent a massive global range expansion over the last century and is considered as an invasive species in North America and Europe because it can potentially endanger native aquatic communities [4-7].
Recently, many reports have studied this genus to analyze its growth, reproduction and immunity [8-11]. C. fluminea has also been used as a test organism in many field and laboratory studies for monitoring environmental changes and contamination [12-19]. As a benthic species, C. fluminea is a suspension feeder that filter-feeds on unicellular algae, bacteria and detrital particles and thus is extensively used as a bioindicator of water pollution [16]. Several genes related to pollution and stress response have been cloned as biomarkers [20,21]. However, the lack of genomic resources, such as a completed genome, expressed sequence tags (ESTs) or transcriptome sequences, limited research on C. fluminea. As of April 2013, there were only 12 C. fluminea ESTs available in the NCBI database and only 184 genes have been assembled and annotated (http://www.ncbi.nlm.nih.gov/). De novo transcriptome approaches are increasingly viable for organisms lacking a sequenced genome, although the sequencing of complex genomes remains expensive [22,23]. 
The fields of transcriptomics and genome characterization have developed rapidly with the advent of next-generation high-throughput sequencing technologies (such as the Illumina (Illumina), 454 (Roche) and SOLiD (ABI) platforms) in recent years [24-26]. Among the next-generation sequencing technologies, Illumina sequencing is cost-effective and yields longer sequencing reads and higher throughput than other methods in transcriptome analysis [27]. To date, next-generation sequencing methods have been applied to only a few mollusk species, such as Mytilus galloprovincialis [28], Bathymodiolus azoricus [29], Yesso scallop (Patinopecten yessoensis) [30] and Ruditapes philippinarum [31]. Most of these studies used the 454 method, and the studied species were marine bivalves. Furthermore, the data on mollusks in the NCBI database are mainly for marine bivalves, e.g., the blue mussel Mytilus edulis and the Pacific oyster Crassostrea. gigas [32]. 
In the present study, we constructed a cDNA library from the C. fluminea adult and sequenced it using the Illumina GAIIx platform. More than 67.1M Illumina paired-end reads and 5.9 G of high-quality data were generated with 134,684 unigenes were assembled. We further annotated the unigenes by matching them against Nt, Nr, Swissprot, Clusters of Orthologous Groups of Proteins (COG), Gene ontology (GO), and Kyoto Encyclopedia of Gene and Genome (KEGG). A subset of these unigenes that are related to antioxidase, cytochrome P450, gamma-aminobutyric acid (GABA) receptor and heat shock protein (HSP) genes were further annotated and partially verified because they were determined to be useful as environmental pollution biomarkers. Our database is expected to provide an invaluable resource for studies of the genome and the functional genes of C. fluminea and for future comparative genomic studies of other bivalves.