More than fifty metres below the sea off the coast of Western Australia lives the ruby seadragon. It's beautifully coloured, hard to spot, and no one has ever seen one alive … until now.

While it’s difficult to see here, the ruby seadragon's (Phyllopteryx dewysea) skin is a bright crimson. Image: Rouse et al./Scripps Institution of Oceanography

This footage was captured by researchers from the Scripps Institution of Oceanography in San Diego and the Western Australian Museum during an expedition to the Recherche Archipelago in April 2016. It was playfully called the "Hunt for the Ruby Seadragon", and it couldn't have happened much earlier – that's because, before 2015, no one even knew these fish existed.

Seadragons are close cousins of seahorses and pipefish. They live only in southern and western Australia, and there are two long-recognised species: the colourful common weedy seadragon and the ornately decorated leafy seadragon, both equipped with gorgeous appendages that help them blend in with kelp and seagrass.

The ruby seadragon lacks the leafy decorations of the leafy seadragon (left) and the common weedy seadragon (right). Images: James Rosindell and Richard Ling via Wikimedia Commons

But when Scripps Oceanography graduate student Josefin Stiller studied the genetics of the seadragons in 2015, she was surprised to discover the DNA signatures of not two, but three species.

The new species, known only from museum specimens, had been mistaken for a long time for the common seadragon, but close inspection revealed a unique skeletal structure, a striking red colouration and – most surprisingly – no leafy appendages.

Greg Rouse, Scripps Oceanography marine biologist and co-author on these studies, credits scientific collections for the amazing find. "This discovery was made thanks to the great benefit of museum collections," he said in a press release.   

And so the ruby seadragon was named. But at the time, it was still known only from dead specimens that had washed up on the beach or been pulled up in trawls. What was its habitat? How did it behave? Did it really lack those characteristic camouflage appendages, or had they merely broken off from the washed-up specimens? These questions could only be answered by going out and finding one alive.

There was just one problem: based on the fish's colour, the scientists suspected it lived deeper under water, beyond the reach of divers and where red light doesn't quite penetrate, making the dragons hard to spot. The solution? A remote-controlled vehicle equipped with lights and cameras. The Hunt for the Ruby Seadragon was on!

On the fourth day of the search, the ROV found its target. For almost 30 minutes, the vehicle followed two wild ruby seadragons – the first ever recorded. It even caught footage of one snapping up prey.

The discovery confirmed many things the researchers had suspected. Incredibly, the ruby species does not have leafy appendages at all; instead of living among kelp, it spends its time surrounded by sponges in the depths, where its colour does the camouflage work. The team also observed that it has a prehensile tail, something not seen in the other seadragons, but common in seahorses and pipefish.

"It was really quite an amazing moment," said Stiller. "It never occurred to me that a seadragon could lack appendages because they are characterised by their beautiful camouflage leaves."

And there's no doubt that there are more hidden wonders in the rich waters the seadragons call home. "There are so many discoveries still awaiting us in southern Australia," said study coauthor Nerida Wilson of the Western Australia Museum. "Western Australia has such a diverse range of habitats, and each one is deserving of attention."