A flash of blue.

Every spring, bright blue flashes lifht up the bay in Toyama, Japan, as billions of firefly squid gather to breed. Now Frederick I. Tsuji of the SCripps Institution of OCeanography in La Jolla, Calif., describes the biochemistry of that brilliance. The reaction, reported in the July Proceedings of the National Academy of Sciences (No. 14), is the second distinct example of luminescence that requires ATP, one of biology's most common energy-storing molecules.

The well-studied luminescence of firefiles, and of a few related insects, uses ATP, but the firefly squid (above) employs it differently. In the squid, Watasenia scintillans, the ATP reaction appears to transfer sulfate groups onto a chemical called coelenterazine. Tsuji proposes that the sulfated compound then serves as the substrate for the enzyme "luciferase" in the light-producing reaction. Previous work by other scientists has demonstrated that coelenterazine, without the sulfate groups, is the substrate of the bioluminescence reaction of some coelenterates and a decapod shrimp.