September 3, 2014 | 3
It’s not every day a new animal is discovered that could shake up the roots of animal taxonomy and simultaneously form its own new phylum, the top classification in the Animal Kingdom, but today is such a day. The new animals – Dendrogramma enigmatica and Dengrogramma discoides – are not yet certain to represent a precedent-breaking form of life. They could be some sort of odd cnidarians or comb-jelly, or could be fragments of some larger, as-yet unidentified animal. But their appearance and resemblance to several long-thought-extinct Ediacaran lifeforms has raised hopes that at last we could have a living representative of one of Earth’s foundational and enigmatic communities. The discovery is also emblematic, scientists say, of how much of Earth beneath the waves still lies unexplored, with wonders untold awaiting discovery.
“We can go to the moon but we don’t know about the deep sea still,” said Reinhardt Kristensen, co-author of the new paper describing the animals. It’s a sentiment I couldn’t agree with more.
As usual, I didn’t have room to tell everything I learned, so here are a few extra goodies.
Marooned on a Desk
Jean Just, lead author of the new study published today in PLOS ONE and a crustacean expert by training, was foisted into his role as unlikely caretaker of these creatures after he discovered them unexpectedly in 1986 in a bulk seabed sample hauled up by a Woods Hole Oceanographic Institute epibenthic sled pulled by the Australian ship ORV Franklin at two locations 400 and 1000 m deep during a trawl in the waters of the Bass Strait between Australia and Tasmania. The seabed in the strait is known to be unique. Unlike most deep seafloor which is made of soft sediment or mud, the seabed here is made of coarse bits of shell and sand mixed with mud that is hard on collecting gear. Possibly as a result, it also is known to harbor unusual animals from other groups like crustaceans, said Kristensen.
By the time Just got to the samples, they had already been fixed in formalin followed by ethanol, rending them apparently unsuitable for DNA sampling. Still, no one he has showed them to — and that includes seven experts in the field, at last count — had seen anything like them.
Their forms were so startling that even without molecular analysis, he and his colleagues pushed for publication even after they’d spent nearly three decades sitting in alcohol on his desk (earlier attempts at publication had failed for lack of additional evidence — this time they may have succeeded because they took better photos and new scanning electron micrographs), and the two other scientists I interviewed for this story said there were extremely glad they did.
Not Dead Yet?
Dendrogramma shares general similarities to gelatinous animals bearing rows of glittering, beating cilia called comb jellies, or ctenophores, and the group known as cnidarians made up of jellyfish, corals, sea anemones, and sea pens, but it cannot be classified (at least for the moment) into either of these or, in fact, any existing group of animals.
Like both groups of organisms, Dendrogramma seems to be made of only two major embryonic layers of cells (i.e., it’s “diploblastic“, most animals form from three and are “triploblasts“) which encase a jelly-like fluid called mesoglea between them. Like these animals, they also appear to have only a single digestive/vascular opening. But they also lack many of the key characteristics that define either ctenophores or cnidarians: stinging cells called cnidocytes or sticking cells called colloblasts, ring canals, special sense organs, or propulsive ridges bristling with basally fused cilia.
Just also recgonized the animals bore a striking resemblance to certain fossils from the Ediacaran biota — a group of odd, large multicellular organisms that flourished from about 635 to 545 million years ago then seemingly vanished when the ancestors of most modern animal groups suddenly appeared in the Cambrian Explosion. In the figure below, Dendrogramma is shown drawn from a freshly fixed specimen at left (A). D shows the trace of an Ediacaran fossil called Albumares, and E the trace of Ediacaran Rugoconites.
It may also mean, if Dendrogramma turns out to be fairly closely related to Ctenophores or Cnidarians (still a significant possibility), that the ancestors of these groups had already evolved by the Ediacaran, and that many “Ediacarans” are alive today in the form of coral reefs and jellyfish. Or it could all be a coincidence. Convergent evolution — when unrelated organisms evolve to resemble each other through adaptation to similar environments — is common on Earth.
And there are true cnidarians and ctenophores both that have evolved in some pretty radical ways. Benthic ctenophores look like thin, sometimes brightly colored films tailing streamers and are
parasites commensals (organisms that live on or in another without benefitting or harming their host) on other invertebrates. Buddenbrockia looks like a worm though its DNA reveals it to be a parasitic cnidarian. Biologist Simon Conway Morris, who is renowned for his study of early animals in general and the animals of the Cambrian Explosion in particular and whom I interviewed for my National Geographic story, feels it’s most likely that Dendrogramma will prove to be some sort of aberrant, though not necessarily parasitic, cnidarian.
On the other hand, a strange worm called Xenoturbella was classified as a flatworm and a mollusc before finally being revealed by DNA analysis to be an important and unique group of its own that is the closest living relative of the large group of animals called Deuterostomes, said Leonid Moroz, who also studies the evolution of early animals and was another interview subject. So there’s definitely precedent for the survival of something truly remarkable.
But before we can pop the champagne bottles, someone will need to find more of these organisms so we can look at their DNA, which everyone I interviewed agreed would settle the matter. Just said that though he’s hoping an opportunity will arise for someone to look for these creatures again, it will be difficult to find them. In addition to the difficulties inherent in securing the services of a large ocean-going research vessel capable of trawling the seabed 3000 deep in harsh sampling conditions, the creatures themselves must be rare and scattered because otherwise scientists would have found them long ago. Still, the authors are hoping either someone will come forward on this paper’s publication to either say they have seen these creatures before (bonus: they still have some sitting in a jar on their desk), or that they have a fully provisioned research ship parked with its engine running ready to sail for Australia. I wish them luck.
If you want to read the rest of the story, head on over to National Geographic.
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