calaxin is a newly identified calcium sensor gene that modulates the movement of flagella (and possibly cilia). It was first identified from the ascidian Ciona intestinalis, and its orthologs have been observed in a wide range of animals and choanoflagellates. However, no calaxin-ortholog in a Lophotrochozoa species has been reported so far. This leaves open the question of whether the modulation of ciliary motility by calaxin is conserved among animals. We report a calaxin gene from the Pacific oyster Crassostrea gigas. This gene, termed cgi-calaxin, possesses three conserved EF-hand motifs as its orthologs from chordates. A phylogenetic analysis confirmed its orthology. Expression analysis revealed high expression in typical ciliated tissues such as gill and hepatopancreas. The spatiotemporal expression of the gene during early development was investigated using whole mount in situ hybridization. The results revealed that cgi-calaxin mRNA was aggregated in ciliated tissues of early larvae such as prototroch and velum. Immunofluorescence experiment further certified the ciliary localization of Cgi-Calaxin protein in D-veligers. We prepared the recombinant protein of cgi-calaxin and proved it had the capacity to bind to calcium. These results support the conserved roles of cgi-calaxin as a calcium sensor in ciliated cells and enrich the current knowledge on regulatory mechanisms of molluscan cilia. Moreover, we found the gene to be expressed in poorly ciliated tissues, such as adductor muscle, indicating possible roles in non-ciliated cells, which merits further investigation.