Light-reflecting Proteins Help Disguise Squid

The Hawaiian bobtail squid uses a family of unusual proteins to disguise itself from nocturnal predators.

The squid has a light-producing organ on its underside that is powered by luminescent bacteria. The light emitted by these bacteria is reflected downward by stacks of silvery platelets located behind the bacteria-containing tissue.

By beaming this light downward, scientists think the squid avoids casting a shadow and forming a silhouette in the moonlit waters, thus camouflaging itself against predators at lower depths that are looking upward for prey.

Most reflector platelets in aquatic animals are made of purine crystals. But in the bobtail squid, these platelets have now been found to consist of a previously undescribed group of proteins that have been dubbed "reflectins" by the discovery team led by biologist Margaret J. McFall-Ngai of Kewalo Marine Laboratory at the University of Hawaii at Manoa, in Honolulu [Science, 303, 235 (2004)].

"The proteins have a very unusual composition," the researchers point out: Four relatively rare residues (tyrosine, methionine, arginine, and tryptophan) make up about 57% of a reflectin, and several common residues occur in none of the reflectins.

This discovery, the researchers suggest, may inspire nanotechnologists to include such protein-based reflectors in optical nanodevices.