Ca(2+)-dependent bioluminescence photoprotein. Displays an emission peak at 495 nm (blue light). Trace amounts of calcium ion trigger the intramolecular oxidation of the chromophore, coelenterazine into coelenteramide and CO(2) with the concomitant emission of light. Exhibits a smaller absorbance peak at 460 nm and a shoulder at approximately 310 nm. The calcium-discharged protein loses visible absorption but exhibits a new absorption maximum at 343 nm. Exhibits a blue bioluminescence (lambda(max) = 495 nm). In contrast, the fluorescence of the calcium-discharged protein is yellow-green (lambda(max) = 520 nm, excitation at 340 nm). Belongs to the aequorin family.