New deep-sea species

UNCLASSIFIED

A handful of strange mushroom-shaped animals discovered in the deep sea off Australia in the 1980s have finally been named by scientists. The organisms are so unique that they may rearrange the earliest branches of the animal family tree.The animals, described for the first time in the scientific journal PLOS ONE, cannot be classified to any existing animal group, though they resemble a few long-extinct species. “It’s a very interesting surprise, and it poses lots and lots of questions,” says Simon Conway Morris, a biologist at the University of Cambridge who studies early animal evolution.

Leonid Moroz, a neurobiologist at the University of Florida’s Whitney Laboratory for Marine Bioscience in St Augustine, says that if the new species turn out to be descendants of early animals, the find could “completely reshape the tree of life, and even our understanding of how animals evolved, how neurosystems evolved, how different tissues evolved,” he says. “It can rewrite whole textbooks in zoology.”

Scientists have recently been debating the positions of the animals at the base of the family tree, and the new oddballs might be a contender for one of the earliest branches. If the new animals are directly related to the ancient organisms, the find would be reminiscent of the discovery of the coelacanth fish, which had long been thought extinct, off the coast of South Africa in 1938. The new discovery also highlights how much of Earth’s submarine realm remains unexplored, scientists say.
The tiny animals, less than an inch long when alive, are translucent and look superficially similar to chanterelle mushrooms. But the relationship ends there. What looks like a mushroom’s stalk on Dendrogramma has a mouth at the base leading to a digestive canal that forks repeatedly once it reaches a disk, which looks like a mushroom cap.

The animals’ lifestyle is as mysterious as their appearance. None of the specimens showed signs of having been torn from something else, leading researchers to think the animals are free-living, rather than attaching to a surface or each other.
But the organisms don’t seem to be able to swim, as their disk appears to be inflexible and they lack other obvious means of propulsion. Because the mouth is small and simple, researchers speculate that the animals may feed by ensnaring microbes in mucus secreted by the lobes surrounding their mouths.

Evolutionary oddballs
Biologist Jean Just, a crustacean expert by training, first found the odd creatures in 1986 in a sample of Australian seabed. In the nearly 30 years since, Just has not found any more such animals. And after consulting many experts, he still hasn’t found any record or recollection of anything remotely similar.

“It’s still amazing that no one has come back and said at least, ‘I’ve seen things like this,’ even if they haven’t published it,” says Just, who is retired from the Natural History Museum of Denmark.

When they were first brought up from the ocean depths, the animals were preserved in formalin and then ethanol, making genetic analysis difficult or impossible. Just and his coworkers hope more specimens will be found to permit such analysis, but a later trip to the area did not turn up any more.

Without genetic information, it’s difficult to know how the new species are related to other animals. But based on their physical form, Just says that they may represent a very early branch of animal life. Some scientists have recently suggested that gelatinous animals called comb jellies may have been the first to branch from the rest at the base of the animal family tree. Traditionally, sponges, with their loose affiliation of cells and lack of tissues or nervous system, have held that position.
Dendrogramma show some general similarities in body structure to comb jellies (ctenophores) and to cnidarians, which include jellyfish, corals and sea anemones.

Like these animals, the new species also appear to have only a single digestive opening where food enters and waste leaves. But they also seem to lack many key characteristics that define ctenophores and cnidarians, including cnidarians’ stinging cells. It remains possible that Dendrogramma are highly modified ctenophores or cnidarians, but no traits unique to either group that could suggest a close relationship have been found.

Ancient throwbacks?
Though no living animal closely resembles Dendrogramma, at least three fossils bear a striking resemblance. Albumares, Anfesta, and Rugoconites appear to also to have possessed a disk laced with forking, radiating channels. Those enigmatic organisms have long captivated biologists with mysterious forms that look like whirls, fronds, and shrubs, and it’s still debated whether they should be classified as animals. The life-forms are thought to have vanished more than 540 million years ago at the end of the Ediacaran period, just before a time of rapid animal evolution called the Cambrian explosion.

It is possible that Dendrogramma independently evolved a similar structure as a response to the same conditions as the three extinct species, a common phenomenon called convergent evolution.

“There is this most intriguing similarity to certain Ediacaran forms,” Conway Morris says. “But I think the similarities are exactly that. They are intriguing rather than compelling.”

“If this is true,” says study co-author Reinhardt Kristensen, an invertebrate zoologist at the University of Copenhagen, “then we have discovered animals which we’d expect to be extinct around 500 million years ago.”