The Sea Mouse: this marine worm's body is fringed by photonic crystals that emit colorful displays of light
The scientific name of this species is Aphrodita aculeata, which is a reference to the Greek goddess of love and beauty, Aphrodite. It's also known as a sea mouse, thanks to its fluffy, almost mammalian appearance, but it's actually a type of marine worm.
Above: the photonic crystals have a color-shifting effect that makes them look almost like fiber-optic threads
The sea mouse is equipped with tufts of hollow, glassy bristles that produce a vibrant iridescence; these structures are known as photonic crystals, and Aphrodita aculeata is one of the few living organisms that is known to produce them.
Above: Aphrodita aculeata
This article explains how the colorful effect is produced:
Each sea mouse is covered in a dense felt of bristles that keep silty sediments away from the gills. But these "hairs" aren't hairs at all. The structures, known as chaetae, are actually modified scales, made largely of chitin, the same material that gives insects like jewel beetles their iridescence.
Each spine is essentially a hollow tube, and the wall of that tube contains 88 perfectly hexagonal chitin cylinders. Acting together, these cylinders form what is known as a "complete spectrum photonic crystal", the first-ever documented in a living organism. Photonic crystals are nanostructures that affect the motion of light travelling through them, and the sea mouse's crystalline spines are the most efficient in nature.
Above: close-ups of the iridescent bristles
According to the same article:
As light hits the system of chitin tubes at different angles, it is spilt, scrambled and bounced back, producing the ever-changing rainbow you see. Light that comes in perpendicular to the spine reflects a deep red. Light coming in from off-axis angles, however, results in brilliant blues and greens.
Amazingly, the spines' ability to bend light surpasses even the most brilliant non-living photonic crystal we know of, the fire opal.
Above: the underbelly of a sea mouse, with a close-up of the "feet" (parapodia) along the sides of each body segment
The purpose of this colorful display is still unclear. Many researchers believe that it is used to deter predators, while others argue that it might help to obscure the sea mouse's silhouette as it moves along the bottom of the sea, making it harder for predators to pinpoint the exact location of its body.
Above: the photonic crystals shimmering as the sea mouse moves around
The genus Aphrodita contains several other species that are commonly known as sea mice, but Aphrodita aculeata is the only one that can produce this iridescence.
Above: the colorful effect is more vibrant when viewed underwater, but it's still visible even on land
Aphrodita aculeata has an average length of about 7-15cm, and it can be found in certain parts of the North Atlantic, the North Sea, the Baltic, the Arctic, and the Mediterranean.
Marine Life Information Network: Aphrodita aculeata
BBC Wildlife: The Sea Mouse is a Psychedelic Fuzz of Fur and Spine, and a Master of Photonic Communications
Acta Zoologica: Hairy and Iridescent Chaetae of the Sea Mouse
The Wildlife Trusts: Sea Mouse
Earth Touch News Network: This Rainbow Wonder is One of the World's Weirdest Worms
The Quekett Microscopial Club: Sea Mouse Hairs
BBC: Sea Mouse Promises Bright Future
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