The needle in the haystack Scripps Institution of Oceanography/UC San Diego

Ruby seadragons, a kind of spiny fish that lives in the deep waters off Western Australia, have been recorded alive and swimming for the first time.

They were first identified as a species in 2015 based on DNA sequencing of just four dried out specimens that had washed ashore or been caught in trawling nets between 1919 and 2007.

“We had very good details on where the fish in 2007 had been found, but very little other information to go on,” says Greg Rouse from the Scripps Institution of Oceanography at the University of California, San Diego, who led the expedition to the remote Archipelago of the Recherche.


“We knew it was like finding a needle in a haystack to go and look for this thing.”

The team used a small, remotely operated vehicle to make a sequence of dives down to 55-metres deep. During the fourth and final dive, they happened upon a ruby seadragon and followed it to another, watching them both feed and move through the strong ocean swells.

Sponge garden

The surrounding area was what Rouse calls a “sponge garden”, in contrast to the kelp-filled waters that leafy and common seadragons inhabit. These relatives of the ruby seadragon have fleshy appendages that help them move about and hide among sea kelp in the rocky reefs where they live.

Rouse says seeing the ruby seadragon alive confirms that they lack such lacy protuberances. In waters 55 metres deep, they just aren’t necessary – or even a good idea.

For starters, the strong currents that far down could catch them like a sail. “They don’t have appendages, just stumps where they would have attached,” says Rouse. “Where they live, it’s not useful for camouflage.”

Instead, their colour helps them hide. Red is the first colour seawater absorbs from sunlight, so at that depth, no red light will bounce off these seadragons, making it hard for predators to see them.

Rouse’s video also shows that the creature has a curled tail similar to that of a seahorse. This is likely to be used for gripping objects and keeping in one position in rough waters, he says.