The diminutive blue-ringed octopus (Hapalochlaena lunulata) looks like a sweet, possibly even fantastical creature. Often measuring less than 20 centimeters long and covered with dozens of bright blue rings, it spends most of its time hiding out in shells or rocks near the beach. But don't be fooled—this little cephalopod is trouble. One small nip from its beak can inject enough powerful neurotoxin to kill an adult human in minutes.

This venomous octopus, however, does not bite without warning first: it makes its roughly 60 blue rings that cover its arms and body glow especially bright. Although we humans might not always understand what the octopus is trying to tell us, its message is probably crystal clear to other marine animals.

Octopuses—and their cephalopod cousins squid and cuttlefish—are renowned for their quick color-changing abilities. A common octopus (Octopus vulgaris) can assume a full warning display, changing its color, skin texture and posture, in roughly two seconds. That might sound fast enough to get the point across, but predators can sneak up quickly in the oceans, so rapid communication is key. The blue-ringed octopus, on the other hand, can flash its full bright-blue warning display in less than a third of a second. How does it do it?

H. lunulata with warning rings; image courtesy of Wikimedia Commons/Roy Caldwell

Lydia Mäthger, a biologist at the Marine Biological Laboratory in Woods Hole, and her colleagues set out to answer the intriguing question. To do so, they enlisted six adult H. lunulata octopuses and filmed them at super-slow speeds.

The octopus generally relies on three structures in its skin to create its elaborate displays. Chromatophores are pigment-filled sacs that are controlled by surrounding muscles. Flexing and contracting these muscles can expand or shrink the sacs, changing the overall appearance in a complicated choreography of color droplets. Beneath these are iridophores, which are firmer, iridescent sheets whose color is controlled by a shifting in the arrangement of proteins and cytoplasm to reflect different wavelengths of light or UV waves. And finally, leucophores are more passive, white reflectors that add luminosity and contrast to the overall display. The chromatophore sacs that contribute to color displays can start to change in a matter of milliseconds, but standard iridophores, which rely on physiological changes to shift color or luminosity, can take seconds or even minutes.

The researchers found that this little octopus has developed iridophores that are un-obscured by chromatophores. These rings of iridophores are lodged in pockets of muscular skin, which can quickly relax or contract, exposing more or less of the iridescent structures, respectively.

To create an even more impressive—and clear—warning signal, the chromatophores surrounding the blue rings turn a dark brown, and those on the rest of the body turn paler, creating high contrast with the glowing blue rings. The findings are in the November issue of The Journal of Experimental Biology.

Why did the animal pick blue as the hue of death? As Mäthger and her colleagues noted in their paper, the blue-green part of the spectrum is seen by most potential predators, such as fish, whales, seals, other cephalopods and even birds. Furthermore, they noted, "the blue-green part of the visible spectrum is the most prominent ambient underwater light field," so these hues will travel well and "pop."

The researchers found that unlike other animals that have iridophores, these octopuses don't use chemical signals to alter their color. "A fast, conspicuous display under muscular control is an advantage to predators, who are warned before attacking a venomous creature, and of course to the octopus itself, as it avoids being eaten," Mäthger told The Journal of Experimental Biology.

"This signaling display method has never been seen before," Mäthger said. The researchers found this tactic to be at work in two of the known three or four blue-ringed octopus species. Others have yet to be tested.

Video courtesy of Lydia Mäthger/MBL

Illustration courtesy of Ivan Phillipsen