Changing charges make for squid rainbow

Study finds how proteins self assemble to reflect different wavelengths of light

A bit of flash usually makes someone stand out, but Loligo squids disguise themselves with shine. Now scientists have illuminated how these squids alter the charges of their reflective proteins, allowing the iridescent animals to hide in open water.

RAINBOW HIDEOUT This close-up of Loligo squid tissue reveals the rainbow of colors created with pigment cells and reflective proteins. Scientists have untangled some of the steps that lead to the on-the-spot assembly of these reflective proteins. Danny DeMartini

This charge change spurs the specialized proteins to self-assemble, shifting the shapes of cells and shifting the color of reflected light, scientists report online September 22 in the Journal of the Royal Society Interface.

Reflecting light can help attract mates, scare predators and, in the squid’s case, provide camouflage in the open ocean. And by understanding the squids’ hiding skills, scientists may be able to design new approaches for other self-assembling proteins, which may be used in materials such as thin films and invisibility fabrics.

Scientists had already identified and described some of these unusual reflective proteins—dubbed reflectins—when they were found in the Hawaiian bobtail squid, Euprymna scolopes, and in another species of Loligo. But these reflectins were long-lasting proteins that, once assembled, stay put. In the new study, the team investigated reflectors that Loligo can break down and rebuild on demand.

Loligo squids dwell in the top 50 feet of ocean water and make themselves invisible by matching the wave-bending features of the water, says study coauthor Alison Sweeney of the University of California, Santa Barbara. “The light there is really dynamic and complex in terms of wavelength and direction. There are going to be beams of light bouncing in really complicated and nonintuitive ways.” 

To match their constantly changing surroundings, these squid turn their shine on at will and tune the shine to reflect different colors, Sweeney says.

The animals keep special proteins at the ready, which, with the right chemical signal, self-assemble. Instead of loosely flopping about in the cell, the proteins aggregate into reflective plates. As the proteins become more and more tightly packed, they reflect shorter and shorter wavelengths of light. Soon the squid is dappled in red or violet.

Scientists suspected that the nervous system could be in charge of kicking reflectins into gear. When the researchers dripped the common nerve messenger acetylcholine onto squid tissue, for example, the chemical triggered a burst of iridescence but it was unclear how the phenomenon worked.

Guided by DNA sequences in bobtails, the team detected three previously unknown but similar reflectins in the Loligo squid. Further analyses revealed that phosphate groups eventually attached to these proteins in response to acetylcholine, the researchers report.

When the reflectins are unassembled, they are highly charged, notes Sweeney. But in tissue in which acetylcholine caused iridescence to increase, the scientists found phosphate groups attached to two of the new proteins. This neutralized the charges of these reflectins and allowed the proteins to aggregate. Sweeney speculates that the change in charges also affect the proteins’ interactions with the cell membranes, and this may also affect the squid’s shine.

The work is very well done and an important contribution, says Margaret McFall-Ngai of the University of Wisconsin–Madison. There is still a lot to learn about reflectins, which are probably present in all cephalopods including cuttlefish and octopus, she notes. For example, in the bobtail squid, these proteins create a permanent mirror that reflects light given off by luminescent bacteria living inside the squid. Creatures that are active in daylight, however, such as octopus, probably have the dynamic switchable version of the proteins found in the Loligo. “Their camouflage is amazing, and their colors change for signaling as well,” she says. “It’s like a mood sort of thing.”

Loligo squids sometimes form schools, as do geese or fish, and their dynamic proteins may also help the squids maintain their place in the formation, Sweeney speculates. She hopes the work will lead to new materials. Scientists are already trying to mimic the self-assembling properties of the squid’s reflectors. But finding a way to change a both material’s geometry and reflective powers remains a challenge. “There are no devices in the world that work like the squid cells do, and the squid cells have all the advantages,” she says.

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