Squid's Built-In Light to Inspire New Gadgets?

John Roach
for National Geographic News
January 8, 2004
A nocturnal squid that cruises the ocean around Hawaii for prey and mates uses a built-in flashlight to hide its shadow from predatory fish on the seafloor.

The unique light organ found in the Hawaiian bobtail squid (Euprymma scolopes) is composed of stacks of silvery reflector plates called reflectins that surround colonies of luminescent, symbiotic bacteria.

Scientists behind the find believe it may inspire a new generation of high-tech miniature gadgets.

"When you look at the [light organ reflector] under a microscope, it is composed of tens of thousands of these little platelets," said Wendy Crookes, a researcher at the Kewalo Marine Laboratory at the University of Hawaii in Honolulu and lead author of a study appearing in tomorrow's issue of the journal Science. "They look like little Frisbees just stacked in [columns]."

Understanding how these tiny reflectins are constructed may offer inspiration for nanotechnology designs in spectroscopy and optics, according to researchers.

Nanotechnology derives its name from a unit of measurement known as a nanometer, which is one billionth of a meter. A human hair is about 100,000 nanometers thick.

George Whitesides, a chemist at Harvard University in Cambridge, Massachusetts, who received the 2003 Kyoto Prize for pioneering advances in materials sciences related to nanostructures, said there is something to be learned from the reflectins.

Their further study may reveal "soft approaches to a world in which most of the materials science has been hard," he said.

Researchers hope to learn more about how the reflectivity of the squid's reflectins is turned on and off. "If we can figure out how that change occurs, you could create materials in which you could switch on and off the reflector," said Crookes.

Symbiotic Relationship

The ability of the bobtail squid to shine its light organ—which, in addition to the reflectins, contains a lens derived from muscle material—hinges on the luminescence of the symbiotic bacteria Vibrio fischeri.

Among the thousands of creatures swimming in the ocean, from bacteria and microorganisms to fish and other marine animals, Vibrio fishceri bacteria and bobtail squid apparently only have eyes for each other.

"The squid allows one particular bacteria to colonize it while excluding all other types. This bacteria only colonizes this one particular tissue in this animal, so it is very specific," said Crookes.

This recognition occurs anew every generation, as the light organs of newly hatched squid are not yet colonized by Vibrio fischeri.

The squid benefits from the relationship by reflecting the luminescence of the bacteria. In turn, the bacteria benefits by living in a nutrient-rich and competition-free environment, researchers believe.

Reflecting its glowing bacteria colonies, the squid can swim, forage, and search for mates at night while hiding its shadow or silhouette from predatory fish that bury themselves in the sand on the ocean bottom.

"If a predator sees a shadow or silhouette, it knows dinner has arrived," said Crookes. "So we think the light produced by the bacteria is used by the squid to kind of counter-illuminate, to mimic, the moonlight … so the squid doesn't cast a shadow and doesn't create a silhouette."

Platelet Construction

Crookes, study co-author Margaret J. McFall-Ngai, researchers at the University of California at Los Angeles, and other collaborators investigated the structure and composition of the reflectins.

In addition to finding that the reflectors are made up of stacks and stacks of Frisbee-shaped plates, they discovered that the molecules that make up each plate consist of a unique set of proteins.

Unlike most proteins, which consist of 20 different amino acid subunits, Crookes and her colleagues found that the reflectin platelets are made up predominantly of just six amino acids.

The researchers also found that the sequence of the protein is divided up into five repeating units, so that perhaps only one fifth of the protein is responsible for helping to reflect light, said Crookes.

"Not only is the protein made principally of six components, but it's possible you can break it down to smaller pieces and still have it function the same way," she said.

Fully understanding the seemingly simple construction of the reflectins should make it easier to create synthetic versions, said Crookes.

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