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Muscles behind the iridescent display of blue-ringed octopus

A blue ring octopus flashing its rings off the coast of Melborne, Australia (Image: Suspotato on Flikr)

They might be tiny and docile, but the blue-ringed octopus (Hapalochlaena lunulata) is one of the most venomous animals in the world. While resting, this golf ball-sized cephalopod is a pale brownish yellow to blend into its surroundings. But when agitated, the blue-ringed octopus lives up to its name by flashing 50-60 iridescent blue rings in a third of a second as a warning signal. A new study published in the Journal of Experimental Biology reveals the octopuses have a unique way of producing these colorful flashes: by flexing their muscles.

Cephalopods (such as octopuses, squid, and cuttlefish) tend to be colorful, and many can change their appearance for camouflage or communication purposes. This can be accomplished in two ways. They can use chromatophores, small sacs of pigment that can be compressed or stretched by muscles, or they can use light reflectors called iridophores, which are made of stacked thin plates that reflect light to create iridescent colors. The luminous warning signal of the blue-ringed octopus is produced by iridophores.

In other cephalopods, iridophores can be switched on and off by chemical signaling of neurotransmitters. But when Lydia Mäthger and colleagues at Brown University and the Marine Biological Laboratory in Woods Hole, MA, bathed blue-ringed octopus skin samples in a range of neurotransmitters known to affect chromatophores and iridophores in other marine animals, they saw no changes in the intensity of the iridescent rings. Despite the fact that the octopuses are able to make the rings visible or invisible, the rings had no chemical "off" switch.

Next, Mäthger and her colleagues investigated the possibility that the octopuses could temporarily hide their iridophores by adjusting nearby chromatophores, as some other cephalopods do. But the blue-ringed octopus proved to be unusual in this way, too: they lack a layer of chromatophores above their iridophores, in the position where they could modify iridescence.  

If the blue rings cannot be turned off chemically or modified by chromatophores, why aren't they constantly visible? It turns out the iridophores are tucked into modified skin folds that conceal their iridescence when the octopus is relaxed. When the octopus feels threatened, it relaxes one set of muscles and contracts another to move the skin folds out of the way and expose its iridescent warning.

This mechanism of producing iridescent signals has never been seen before. The blue-ringed octopus is the first reported cephalopod to employ this mechanism, though it may not be the only one. Mäthger and her colleagues note the mechanisms of color change are known for only a fraction of the over 700 species of cephalopods worldwide. And the blue-ringed octopus' elastic, muscular skin is a highly effective tool for creating a fast and flamboyant warning to predators.

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Representative Image Caption
A blue ring octopus flashing its rings off the coast of Melborne, Australia (Image: Suspotato on Flikr)
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