If you have walked a beach on the east coast of Australia after a few days of onshore wind, you have seen bluebottles. Blue balloons on the wet sand, tentacles trailing, everyone stepping around them.

What you almost certainly did not see is the thing that eats them.

Somewhere in that same line of blue, usually smaller than your thumbnail, was a sea slug that looks like it was designed by someone who had never been told what a slug is supposed to look like. Six fans of silver-blue fingers spread from a slender body. A dark stripe runs from head to tail. It is the colour of the water on a clear day, and that is not an accident.

Its name is Glaucus atlanticus. People who know it call it the blue dragon.

It lives upside down on the surface of the open ocean

Most sea slugs crawl along the bottom. The blue dragon does not. It swallows a bubble of air, holds it in its stomach, and floats belly-up against the underside of the surface, carried by wind and current for its whole life. It cannot swim against anything. Where the water goes, it goes.

That is why the colouring is reversed from almost every other animal in the sea. The side that faces the sky is the blue side, so a bird looking down sees water. The side that faces down is silver-grey, so a fish looking up sees sunlight. It is camouflaged in both directions at once, and the same blue is thought to reflect the ultraviolet light it spends its life lying in.

It hunts the bluebottle. Then it keeps the sting.

The bluebottle is armed with millions of stinging cells. Almost nothing in the ocean will touch it. The blue dragon eats it, and it does not just survive the meal.

As it feeds, it sorts through the stinging cells, swallows them without triggering them, and moves the most powerful ones out to the tips of its own fingers, where they sit in small sacs, loaded and ready. It has no venom of its own. Every weapon it carries was taken from something that tried to kill it first, and because it stores only the strongest cells, a blue dragon can deliver a worse sting than the animal it took them from.

This is also why you should not pick one up. The cells keep working after the animal is dead. A blue dragon on the sand can still sting like a bluebottle.

It is smaller than you think, and it travels in a fleet

A big blue dragon is about three centimetres, roughly the length of a paperclip. Most are smaller. It drifts with a whole community of surface animals that scientists call the blue fleet: bluebottles, by-the-wind sailors, blue buttons and the violet snail, all of them the same shade of blue, all of them going wherever the wind sends them. When the wind turns onshore, the fleet comes in together, which is why you find dragons in the same tide line as the bluebottles they eat.

Every blue dragon is both male and female. Two of them meet at the surface, mate, and lay strings of eggs on the floating remains of whatever they last ate. The eggs hatch into larvae, the larvae become dragons, and the whole thing is over inside a year.

Where to find one

Blue dragons live in warm and temperate open ocean around the world, but the Australian east coast is one of the places they are seen most, from southern Queensland down to northern Victoria. They do not come to the beach on purpose. They arrive when a run of onshore wind pushes the fleet in, which on the Gold Coast and in Sydney tends to happen in the warmer months, the same days the beaches put up bluebottle warnings.

In the past few years they have started turning up in places that had rarely recorded them: beaches in Texas, the Spanish Mediterranean, Thailand. Each time, the photos go around the world and a few million more people learn that this animal exists. It was described in 1777. Most people are only finding out now.

Why people who find one never forget it

There are prettier animals in the ocean and there are more dangerous ones. What makes this one stick is the combination. It looks fragile and it is not. It looks like it would be helpless and it takes weapons off the most feared thing on the beach. It floats where anyone could see it, and almost no one does.

The people who stop and crouch on the sand tend to be a particular kind of person. They are the ones who look at the tide line instead of walking past it. When they find something like this, they photograph it, look it up, and then tell everyone they know. The blue dragon has a way of picking its people.

A note on who wrote this

This page is published by Atlantica, a small Australian jewellery label. We grew up watching blue dragons scatter across the sand after an onshore wind, and we spent years wanting a way to keep one. Eventually we designed it: the Glaucus atlanticus, sculpted in metal, at close to its real size, with the fanned fingers and the long tail intact.

There are three versions. One in polished stainless steel, 20 mm across, that will not tarnish, rust or turn skin green, for $70. One in stainless steel set with blue and green lab-created opal that shifts in the light, 32 mm across, roughly the size of the animal itself. And one in gold over 925 sterling silver with an opal centre, 15 mm across, which is the one in the photo above. All three are shower and swim safe. All three ship free with tracking, and you have 30 days to change your mind if it arrives and is not what you hoped.

“Even better than the pics. The colours move in the light for the gold and opal.”

If you are the kind of person who stops at the tide line, you already know whether this is for you.

See the Opal and Gold Dragon Necklace See all three