The Hawaiian bobtail squid is a pipsqueak half the size of your thumb that jets around the coastal waters of Hawaii. Although it's tiny, it has a feature few can match: A flashlight shines out of its underside. In the 9 January issue of Science, researchers describe the unusual proteins the squid uses to focus its glow.
Squids and octopuses have an unrivaled ability to change their look. Many of them can change not only color and pattern, but how much light they reflect. Some even emit light, thanks to glowing bacteria that live in their skin. These talents help the cephalopods hide from predators and communicate with each other, but biologists know little about the underlying mechanisms.
Biologist Wendy Crookes of the University of Hawaii, Manoa, and colleagues stumbled on a clue while looking at how the presence of symbiotic bacteria affects gene expression in bobtail squid. The team noticed that some proteins produced by the squid had a bizarre amino acid composition that included many copies of six normally rare amino acids, particularly tyrosine and methionine, and none at all of four common ones. The researchers soon realized that the proteins were found in the reflective tissues of the squid. A set of follow-up experiments revealed that the proteins, which the team dubbed reflectins, reflect light: A stack of just 10 reflectins acts as a perfect mirror.
"These animals have possibly the most extraordinary skin on Earth," says Roger Hanlon, a marine biologist at the Marine Biological Laboratory in Woods Hole, Massachusetts. This is the first time anyone has characterized a reflective protein in squids or octopuses, he says. Hanlon thinks the protein may help the squid reflect light downwards to match the intensity of light shining from above, effectively disguising their silhouette from predators below them.