How Squid and Octopuses Change Color and Shift Appearances

As squid and octopuses journey across sand and coral, their colors shift and change. Patterns emerge before disappearing just as quickly, fading into solid colors. Now, researchers have learned a little bit more about how these creatures manage to change their appearance, revealing how squid and octopuses shift their pigmentation.

Studies in the past have examined how the common market squid, Doryteuthis opalscens, manages to change color with a neurotransmitter. Known as acetylcholine, the neurotransmitter sets in motion a cascade of events that culminate in the addition of phosphate groups to a family of unique proteins called reflectins. In this latest study, though, researchers delved a bit further into this color-changing mechanism.

"We know cephalopods use their tunable iridescence for camouflage so that they can control their transparency or in some cases match the background," said Daniel E. Morse, co-author of the new study, in a news release. "They also use it to create confusing patterns that disrupt visual recognition by a predator and to coordinate interactions, especially mating, where they change from one appearance to another. Some of the cuttlefish, for example, can go from bright red, which means stay away, to zebra-striped, which is an invitation for mating."

Structural colors rely on the density and shape of the material rather than its chemical properties. In order to examine this phenomenon a little more closely, the researchers created antibodies to bind specifically to the reflection proteins. This revealed that the reflectins, which are responsible for reflecting light in a certain way, are located exclusively inside the lamellae formed by the folds in the cell membrane. In fact, the researchers found that the cascade of events culminating in the condensation of the reflectins causes the osmotic pressure inside the lamellae to change drastically due to the expulsion of water, which shrinks and dehydrates the lamellae and reduces their thickness and spacing.

In other words, the color-changing properties all rely on how hydrated or dehydrated the lamellae are. The thickness changes the wavelength of light that's reflected and "tunes" the color change over the entire visible spectrum.

"The precision of this tuning by regulating the nanoscale dimensions of the lamellae is amazing," said Morse.

The findings reveal a little bit more about how these animals change their colors to reflect their mood or their environment. Yet they may also have practical applications. The findings could be applicable to communications carried by light--telecommunications--in the future.

The findings are published in the journal Proceedings of the National Academy of Sciences.