A bright red, black-eyed bony beauty, it is only the third species of these peculiar Australian marine creatures ever discovered.

All the time, it was drifting right under scientists' noses.

The ruby seadragon is believed to live in dark waters beyond normal scuba diving depth, and so escaped attention, unlike its relatives, the leafy and weedy seadragons.

Josefin Stiller, a marine biologist at Scripps Institution in California, led research that first uncovered the ruby seadragon, and described it as a new species of "mesmerising beauty" in the journal Royal Society Open Science.

The discovery of the first new seadragon species in 150 years was "highly unexpected," Ms Stiller said.

Teasing out the genetics of the two better-known species, she was sent a tissue sample from a Western Australian Museum specimen, thought to be a common seadragon, but found to have clearly different DNA.

Ms Stiller, her Scripps colleague Greg Rouse, and the Western Australian Museum's Nerida Wilson then checked on the actual specimen, which had been trawled off the Recherche Archipelago in WA in 2007.

It was a male, brooding young, as male seahorses do, about 24 centimetres long and still a vivid red when it was photographed on the research ship.

It was intact except for the waving appendages that keep seadragons hidden in their surroundings, both from predators and the tiny shrimp prey they suck up through straw-like mouths.

The scientists then scoured Australian collections and turned up several other rubies, all of them different anatomically from the leafy and common species.

"The red colour of the ruby seadragon helps to camouflage it at depth, as red light does not penetrate very deep," Ms Stiller said. "So anything that is red is effectively black."

She said the discovery underscored two things: how important long-term museum collections were, and how many more secrets still waited to be uncovered in the sea.

The ruby seadragon was found at a depth of 51 metres, just a few kilometres off the Austalian coast.

"Even at relatively shallow depth, which is much better explored than other parts of Australia's Exclusive Economic Zone, we still find new species,"  Ms Stiller said.

"This particular new species is relatively large, brightly coloured and charismatic - and so it is all the more remarkable that it escaped recognition until now.

"It is a sign of how much more there may be to find in shallow and deeper waters of Australia."

The team gave it the scientific name Phyllopteryx dewysea, naming it after  Mary "Dewy" White, an American benefactor of seadragon conservation and research.