Deep below the ocean's surface lives a mouse that isn't a mouse, covered in fur that isn't fur, that spends its days crawling around on legs that aren't legs. Not weird enough? No problem! It's also a living fibre-optic lamp with light-bending skills that rival the most iridescent opals on earth.
This glowing bristle brush is the sea mouse (Aphrodita aculeata), and like most of the strangest creatures on our planet, it's a worm.
The animal's Latin name, bestowed upon it by none other than famed naturalist Carl Linnaeus, is a nod to the Greek goddess of love and beauty. From the above image, it's easy to see why early naturalists were inspired to turn to mythology when picking a permanent moniker for this peculiar polychaete (marine worm). But as it turns out, these creatures don't always look beautiful.
When viewed in direct light, sea mice appear an unassuming grey-brown – stumpy, fuzzy logs that more closely resemble mouldy animal excrement (yes, we mean white dog poop) than actual animals. Change the angle of the light, however, and that fuzzy "coat" produces the worm's colourful party trick.
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.
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:
This is similar to how a Morpho butterfly gets its blue hue (though in that case, only light in the blue spectrum is bounced back.)
The spines of the sea mouse, however, manage to re-route nearly 100 percent of the light that hits them, making them some of the best light-benders ever observed. Amazingly, the spines' ability to bend light surpasses even the most brilliant non-living photonic crystal we know of, the fire opal:
The sea mouse's body pulls off the feat so well, in fact, that we've used it as the blueprint for some man-made optical communication fibres, which now use a similar system of hexagonal cylinders to transfer light and data.
Sea mouse habitats range from shallow, coastal waters all the way to some 3,000 metres (nearly two miles!) below the ocean's surface, and although they resemble many herbivorous slugs in shape, these animals are active predators. They spend their days half-buried in the seabed, scooching along on "parapodia" (stiff bristles that act as legs) in search of other worms and dead crustaceans to feed on.
But why do these "hairy" worms need such fancy fibre, you ask? Interestingly, there are several theories out there to explain those vibrant colours.
One hypothesis suggests that the bright flash of colour acts as a warning, a way to tell would-be predators to back off. Others still believe the highly reflective surface helps to camouflage the sea mouse by breaking up its silhouette against the seabed.
Either way, the spines' success depends on exploiting every glimmer of light that reaches the depths. The ocean floor is largely swathed in darkness, so in order for the spines to do their job – as an invisibility cloak or a warning – they need to do a lot with very little.
This is a far less intensive way of producing colour than through the use of pigments, and in the deep sea, where food is a hot commodity, every energy saving counts.
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UPDATE (November 01, 2017):
One of these peculiar polychaetes washed up recently on Liverpool's Crosby Beach:
This interesting creature was spotted on #Crosby beach today! But can any marine biologists tell us what it is? 👀🐡 #MySefton #BluePlanet2 pic.twitter.com/OBobsNYQyI
— Sefton Council (@seftoncouncil) October 30, 2017
Top header image: Wikimedia Commons
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