A newly discovered species of seadragon, which has never before been seen in the wild, has been spotted off the south coast of Western Australia.

The ruby seadragon, Phyllopteryx dewysea, was identified last year from preserved specimens but has never been documented in the wild until now.

Researchers from the Scripps Institution of Oceanography in the US and the Western Australian Museum embarked on a five-day expedition to WA's Recherche Archipelago off the coast of Esperance to try and spot the new species and learn more about it.

The ruby seadragon differs from other seadragons because it lacks ornate leaf-like appendages that were previously thought to be a characteristic of all seadragons.

Co-author of the study and researcher at Scripps Oceanography Josefin Stiller said seeing the seadragons in the wild for the first time was an amazing moment.

"The discovery showed us that we can still find big, charismatic, bright red fish that no one has ever seen before," she said.

Ms Stiller said the research shows there are so many discoveries still to be made in southern Australia.

"It never occurred to me that a seadragon could lack appendages because they are characterised by their beautiful camouflage leaves."

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The ruby seadragon also has a curled tail, similar to a seahorse, which researchers think could help this fish hold on to objects in high-surge waters.

The creature lives in deeper waters than other members of the seadragon family and the researchers used a drop-down camera to film the creatures in waters more than 50 metres deep.

Video from the mission has been used to uncover the new facts about the new species and the findings have been published in today's issue of the Marine Biodiversity Records journal.