Mushroom-shaped deep-sea animal Dendrogramma baffles scientists

A mushroom-shaped animal that defies current classification systems has been found living in the dark depths of the Tasman Sea between Australia and New Zealand.

The animal could be a "living fossil" from the pre-Cambrian period more than 450 million years ago, but further studies would be needed to confirm this, say the researchers in the journal PLOS ONE.

• Read the full paper in PLOS ONE

"We've basically discovered an animal which we cannot place in the current animal system that science operates with," says lead author Jean Just of the Natural History Museum of Denmark.

Just and colleagues found the new organism, called Dendrogramma, while analyzing organisms collected at depths of 400 and 1000 metres on the south-east Australian continental slope, east of Bass Strait.

It is about 1.5 centimetres tall, a little over one centimetre wide and shaped like a mushroom.

The animal has a dense layer of jelly-like material between its outer skin and inner stomach cell layers.

And it has a combined mouth and anus in its "stalk'" that Just and colleagues suspect points down into the ocean floor sediments.

From the several specimens collected, two new species were identified in a new genus, Dendrogramma enigmatica and Dendrogramma discoides, in the new family, Dendrogrammatidae.

The researchers say Dendrogramma may be related to members of the Cnidaria (jellyfish) group of animals or another primitive group called Ctenophora (comb jellies).

But the Dendrogramma lacks the stinging cells that define Cnidaria and the tentacles that define Ctenophora, says Just.

Living fossil?

The newly-discovered creature has similarities to extinct life forms from the Ediacaran period, 540 to 580 million years before the present, the researchers found.

They point to branches in the digestive system, that give Dendrogramma its name, and which are visible in the "disc" of the mushroom shape.

Just says it's a "long shot" but Dendrogramma could be a descendent of pre-Cambrian life, although molecular studies are needed to provide more definitive answers on its relationship to the tree of life.

Australian palaeontologist Jim Gehling from the South Australian Museum agrees.

"It is a pity that the molecular affinities of Dendrogramma cannot be determined until new specimens are collected, due to the initial preservation in formalin that damaged the nucleic acids of these organisms," he says.

Gehling says while there are interesting similarities between Dendrogramma and some Ediacara fossils there are also significant differences.

"Many weird Ediacaran forms remain a biological mystery. Some of these defy comparison with animals or plants that populate both ancient and modern marine settings," he says.

"The tiny mushroom-shaped Dendrogramma species share some patterns familiar in the so-called Trilobozoans of Ediacara," says Gehling.

"Unfortunately, biological body patterns are often repeated in unrelated organisms if only because there are limited ways of maximizing surface area within body spaces."

However, gathering fresh specimens for molecular studies won't be easy.

"A thousand metres is a long way down and to hit the right spot can take a long time," says Just.