A fluorescent protein – the first ever found naturally occurring in a vertebrate – has been spotted in a freshwater eel that is common on sushi menus but becoming scarce in the wild.
Anguilla japonica, also known as unagi, is born in the sea. As eggs and larvae they are carried by ocean currents to rivers, lakes, and estuaries in Japan, China, Taiwan, Korea, and Vietnam, where they spend their lives until returning to the sea to spawn.
The carnivorous fish also possess a glow-in-the-dark protein in their muscles, as scientists at Japan's RIKEN Brain Science Institute have found. In a study published in the scientific journal Cell, researchers describe what they've named UnaG, the first fluorescent protein found to have come from a vertebrate. The researchers believe that the glowing protein aids in the species' migration.
UnaG is a fatty-acid-binding protein found in the unagi's muscle fibers. The researchers were surprised to find that it glowed only when activated by a chemical called bilirubin, a breakdown product of molecules found in blood. The scientists also identified the protein in the American and European species of Anguilla.
Bioluminescence, the production and emission of light by a living organism, can be found in many species of microbes, plants, and animals, including many other marine vertebrates. Some species, such as the anglerfish, use glowing bacteria to cast light underwater. Other species, such as the lanternfish, emit light through chemical reactions. Until this discovery, it was unknown how the eels emitted green light. Fluorescent proteins have been found naturally occurring only in some species of jellyfish.
The jellyfish genes that code proteins for bioluminescence have been successfully introduced in mammals, however. In 1998, a performance artist paired with a geneticist to produce Alba, a rabbit with glowing green fur. Scientists in Taiwan have also used gelatinous sea creatures' fluorescent genes to create glowing pigs.
Unagi for its part, remains a popular delicacy. But it is being dined to extinction. In February, the Japanese government declared Anguilla japonica, "likely to be extinct in the wild in the near future" and added it to its endangered species list. In a RIKEN press release, the researchers note that they hope this discovery will aid in conservation efforts.