A study published in Cell described what Japan's Riken Brain Science Institute researchers named UnaG, the first glow-in-the-dark protein found to have come from a vertebrate, Christian Science Monitor reported Friday.
The researchers said they believe that the protein in the meat-eating unagi's muscles helps when it migrates.
The unagi, known as Anguilla japonica, is born in the sea. As eggs and larvae, the eels are carried by ocean currents to rivers, lakes and estuaries in Japan, China, Taiwan, Korea, and Vietnam, where they live until returning to the ocean to spawn.
Researchers said they were surprised to find UnaG, a fatty-acid-binding protein, glowed only when activated by bilirubin, a chemical breakdown product of molecules found in blood, the Monitor said.
The scientists said they also found protein in the American and European species of the eel.
In February, the Japanese government declared the Anguilla japonica "likely to be extinct in the wild in the near future" and added the popular delicacy to its endangered species list, the Monitor said.
The researchers said they hope this discovery will aid conservation efforts.