Deep-sea clam Phreagena okutanii harbor thioautotrophic symbiotic bacteria in their gill epithelial cells and nutritionally depend on their symbionts because their digestive tracts are vestigial. The symbiont fixes inorganic carbon to synthesize organic compounds. However, the major forms of inorganic carbon in the seawater are bicarbonate which is not permeable through the biological membrane. Therefore, a question how inorganic carbon is taken up by the host gill tissue has arisen. To elucidate the mechanism of inorganic carbon uptake in the gill tissue, we constructed normalized cDNA libraries from gill tissue of Phreagena okutanii and sequenced them with 454 FLX Titanium.