ABSTRACT. Calmodulin (CaM) is a multifunctional intracellular calcium ion receptor protein that participates in a range of cellular processes, including calcium metabolism in mussels. To investigate the role of CaM in freshwater mollusk shell calcium metabolism, the full-length CaM cDNA was isolated from the freshwater pearl mussel, Hyriopsis schlegelii (referred to as hsCaM) using SMART RACE technology. The full-length hsCaM was 855 bp in size, containing a 70-bp 5ꞌ-untranslated sequence, a 447-bp open reading frame, a 309-bp 3ꞌ-untranslated sequence, and a 26-nucleotide long poly(A) tail. The hsCaM mRNA expression in different mussel tissues was examined using real-time PCR. The hsCaM mRNA was found to be ubiquitously expressed, but far more abundant in the gill, foot, and mantle than in the posterior adductor muscle. Real-time PCR was also used to determine hsCaM mRNA expression levels in mantle tissues of H. schlegelii at different ages. No significant differences between one-, two-, and threeyear-old mussels were detected, but expression increased in four-yearold mussels and then decreased in five-year-old mussels. CaM appears to be involved in calcium regulation of the mantle in four-year-old 43 ©FUNPEC-RP www.funpecrp.com.br Genetics and Molecular Research 11 (1): 42-52 (2012) Calmodulin cDNA in Hyriopsis schlegelii mussels, which may secrete more mother of pearl during pearl culture.