Moonlighting: Reflective protein causes squid to shimmer

As the Hawaiian bobtail squid glides through the ocean on moonlit nights, when darkness alone wouldn’t cloak it, reflective materials in its tissues render the animal invisible. Biologists have long known that squid and other cephalopods such as octopuses manipulate light in this way.

“But nobody could figure out what the agent was that was helping these animals become reflective,” says Wendy Crookes at the University of Hawaii at Manoa.

Now, she and her colleagues have uncovered the squid’s secret. Embedded in the animal’s reflective tissues is a unique set of proteins that the researchers call reflectins.

In earlier studies, scientists led by Margaret McFall-Ngai at the same University of Hawaii lab showed that bobtail squid generate light using a bioluminescent organ on their undersides (SN: 9/14/96, p. 167: https://www.sciencenews.org/sn_arch/9_14_96/fob2.htm). The organ houses a population of bacteria that glow in response to changes in oxygen concentrations in the squid. Inside cells around the organ–as well as in the skin, in the ink sac, and around the eyes–are reflective, iridescent structures that resemble stacks of coins. Called platelets, these structures reflect the bacterial light.

In shallow waters, moonlight can cast a shadow of the squid onto the seafloor. This shadow would expose the animal to predators, says Crookes. To avoid having a shadow, the squid produces its own light, she says. The reflective platelets focus the light downward and modulate it to match the intensity of the moonlight hitting the ground.

In the latest study, described in the Jan. 9 Science, McFall-Ngai’s team isolated and sequenced the proteins that make up the platelets. Although most proteins consist of many of the 20 amino acids strung together in various combinations, reflectins are made up mostly of only 6 amino acids. “And the proteins completely lacked four other amino acids that are common in other proteins,” says Crookes.

Many aquatic creatures reflect light, but their platelets are typically composed of crystals of purine molecules, one of DNA’s building blocks, rather than of protein. Because the purine crystals are rigid, these organisms can’t alter their platelets’ reflectivity, says Crookes. In contrast, the flexible protein structures in a bobtail squid’s platelets can quickly change configuration.

“This is a significant step forward in understanding the structure and function of these light-reflecting molecules,” says Roger Hanlon of the Marine Biological Laboratory in Woods Hole, Mass. In previous experiments, Hanlon showed that squid induce their platelets to disperse within cells or aggregate into reflective structures. Now, he says, “it will be interesting to see if these proteins are the same ones that achieve . . . iridescence in other squid.”

These unique proteins, says Crookes, could inspire material scientists to develop more-sophisticated optical materials and devices for uses that range from camouflage to artificial photosynthesis.

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