Walk a storm-blown strand-line and mingled with the flotsam and jetsam of humanity you’ll find a tangle of natural treasures. If you’re lucky, these will include a strange creature called the sea mouse, Aphrodita aculeata. It’s a psychedelic fuzz of fur and spine, and can look like both something and nothing.
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From one angle it resembles a sandy fur-ball. But from another, it catches the light and your eye. Pop it in some seawater and it takes on a completely new vibrance. It becomes a splash of fibre-optic brilliance.
What is a Aphrodita aculeata and how big is it?
The sea mouse simply doesn’t conform to what you might expect a ‘worm’ to be. Far from being a slim tube it is quite chunky: up to 20cm long and 5cm wide. The body is covered in a velveteen pelage, and its perimeter is trimmed with a wide skirt of long glassy hairs (setae) amongst which are embedded a palisade of much thicker, stouter spines, making the creature quite prickly.
Turn the worm and you’ll get a hint as to its place in the animal kingdom. It is a kind of segmented (annelid) worm, with its body divided up into sections. Each has a pair of fleshy ‘feet’ called parapodia, complete with a bundle of more spines at the tip. The sea mouse uses these as it ploughs through the seabed sediments, as deep as 3,000m, where it hunts other worms, molluscs and crustaceans.
Most of the sea mice washed up are dead, or lethargic at best, but if you do place one in a clear-sided vessel of seawater, you will hopefully observe its real magic. Natural light falling on the worm’s veil of fluff creates a kaleidoscopic rainbow of colour – a phenomena termed pseudo-birefringence. This chromatic shimmering of blues, reds, greens and yellows seems to emanate from the finer strands, while the thicker spines glow ember-red, making for a garish opulence rare in a creature of the deep.
Why is a sea mouse psychedelic?
The purpose of these colours has long been debated, but recent investigations into the nano-structure of the bristles and hairs have revealed the first known instance of natural photonic engineering. Under an electron microscope, each filament is shown to be a hollow tube whose walls in turn comprise some 88 layers of hollow hexagonal tubes, which run parallel to the length of the bristle. A cross-section therefore looks a little like a honeycomb.
When light hits this precise matrix of tubes, it splits and reflects back those portions in a specific part of the light spectrum. Amazingly, this structure is very similar to the artificial structures being developed by physicists in the exciting field of photonic communications, which uses light instead of electricity in telecommunication and computing. For this we need to be able to amplify and control photons. The sea mouse has been doing this all along.
Not only did this worm get there first, it exhibits almost 100 per cent reflectivity – much better than anything our technology has come up with. What’s more, it achieves this using nothing more than a simple arrangement of hexagonal crystal-like structures exuded from its skin. But how does the worm benefit?
One theory is that the light show deters other predators in the dimpsy darkness of the underwater world. If our worm used normal pigmented colours, much of the light hitting its fur would simply become absorbed. By deploying photonics the worm overcomes these limitations on the seabed.
Main image © Hans Hillewaert
