Coral skeleton generally consists of aragonite crystals, but by changing Mg/Ca in seawater, calcite crystal appears in coral skeleton. In particular, corals in the early growth stage is susceptible to the Mg/Ca ratio in seawater and they obviously formed calcite crystals in low Mg/Ca seawater. To clarify how aragonite or calcite crystals are formed in corals tissue, we conducted RNA-seq analysis using calcite formed corals and aragonite formed corals in the early growth stage.