How Do Squids Camouflage?

Beneath the turquoise waters surrounding the Hawaiian coast live walnut-sized creatures that have mastered the art of disguise. These animals—a squid species called Euprymna scolopes—use the ability to camouflage to avoid ending up as a tasty snack for predators like lizardfishes and seals.

During the day, the squids bury themselves in the sand. When they venture out to forage for food at night, their undersides light up, making their shadows from the moonlight disappear and hiding them from predators beneath. How do the squids achieve this camouflage?

In the 1990s, Margaret McFall-Ngai, an animal physiologist and protein biochemist at the University of Southern California, and her colleague found that E. scolopes forms a symbiotic relationship with luminescent bacteria that live in a specialized reflector organ in the squid’s head.¹ The structure modulates light by preventing it from going through the squid’s topside and directing it below the animal.²

A little more than 10 years later, the researchers pinpointed this property to a family of proteins, abundantly present in the reflector tissue.³ Now, detailed characterizations of the protein have shed light on its reflective properties, drawing interest from bioengineers, including those from the military, to leverage it for developing new camouflage material.

Reflectin Proteins in Reflector Tissues Help Squids Hide

The path to discovering these reflective proteins was by no means direct. “It was sort of serendipity,” said McFall-Ngai, now at the California Institute of Technology. While studying different squid proteins, McFall-Ngai and her team observed a conspicuous polypeptide band on a gel. “I was like, ‘What in the world is that?’,” she recalled.

The researchers isolated the polypeptides from the gel and sequenced them to find an unusual amino acid composition: They largely consisted of relatively rare amino acid residues like tyrosine and tryptophan but completely lacked common ones like alanine and isoleucine. Antibodies generated to bind the protein localized to the reflector organ, hinting at the proteins’ role in camouflaging the animal, so McFall-Ngai and her team called them “reflectins.”

Prior to this discovery, scientists studying reflector tissues in fish and insect scales or animals’ glowing eyes had found that these contain flat and insoluble structures that had a high refractive index.⁴ The animals sandwich these structures between materials of a lower refractive index, resulting in light waves getting reflected and interfering with each other to create colorful patterns. Researchers had found that reflector plates in aquatic animals consisted of purine crystals, particularly guanine and hypoxanthine.

“Nobody had ever shown that a protein was a reflector,” said McFall-Ngai. “They are fascinating proteins, and they have a lot of properties that lend themselves to engineering.”

Characterizing Reflectins' Biochemistry and Their Applications

In the years since reflectins’ discovery, researchers have investigated the biochemistry behind the proteins’ reflective traits and ways to translate those characteristics for bioengineering and materials science applications.⁵

“The properties of [reflectins’] unusual amino acid content give them a high refractive index,” explained Alon Gorodetsky, a biomolecular engineer at University of California, Irvine, who develops materials inspired by the optical properties of reflectins. The refractive index of reflectins is more than 0.2, while that of crystallins, which contribute to the refractive properties of the eye’s lens, is 0.19, meaning that reflectins can bend more incident light than other proteins with the same function.⁶

Gorodetsky and others found that reflectins are partially disordered, meaning that they lack a fixed three-dimensional structure under physiological conditions.⁷ External stimuli such as pH changes can alter the protein’s configuration, which influences the outflow of water from structures in the reflector tissue, changing the refractive index.⁸ This property allows the squid to change its appearance when it needs to.

Inspired by this, Gorodetsky and his team engineered human cells to express reflectins, and they observed that they could tune the cells’ optic properties by tweaking the salt content in the culture medium.⁹ The squids’ color-changing system also motivated Gorodetsky’s team to develop materials with adjustable infrared reflectivity.¹⁰ Gorodetsky noted that this material could coat objects and hide them from infrared cameras, with potential applications in the military.

Even as researchers increasingly appreciate the value of these squid proteins in bioengineering and material sciences, Gorodetsky appreciates McFall-Ngai and her team’s basic research that led to these innovations. “Her discovery and isolation of the proteins opened up the field for everyone,” he said.