They look like little mushrooms. They float like jellyfish. And they have scientists scratching their heads.

University of Copenhagen researchers had collected hundreds of organisms in 1986 near Bass Strait at depths of 400 and 1000 metres.

Tiny, jellyfish-like Dendrogramma species could represent a major new category in the animal kingdom.

But almost 30 years later the Danes realised two types of specimens "could not be placed in any recognised group of animals".

It led them to announce two new species of what they call Dendrogramma in a study published in the journal PLOS ONE on Thursday.