The mysterious sea creature Dendrogramma was rated one of the top-ten new species of 2015. There was speculation it might even represent a new invertebrate phylum, a tremendously rare event. Further specimens have revealed the organism was so hard to classify because the specimens are not complete animals, as appeared, but appendages. Nevertheless, many questions remain unanswered.

In 1986, two mushroom-looking specimens were captured near the bottom of Australia's Bass Strait. Taxonomists being few and over-stretched for the abundance of unknown Australian species, the items were preserved in formalin and neglected for decades.

In 2014, Danish scientists concluded these represented no mere new species, a common enough occurrence, but a new phylum. The blog Deep Sea News compared the claim to finding a vertebrate that was neither mammal, bird, fish, or reptile. Huge news if true.

The name Dendrogramma was given for the way the creature's branching veins resemble the tree of life, on which it appeared unplaceable. The specimens lacked the stinging cells that define true jellyfish and the rows of cilia comb jellyfish use to move. Many marine biologists were skeptical of the phylum claim, but no one knew what Dendrogramma was. The closest resemblance was to 560-million-year-old fossils.

Last year, Museum Victoria's Dr. Hugh MacIntosh pulled a fresh specimen out of the waters of the Great Australian Bite. “It was a ‘eureka’ moment! Holding one up to the light, the distinctive forked veins shimmered through the transparent body, and it suddenly dawned on me that we had rediscovered the elusive Dendrogramma,” MacIntosh said.

MacIntosh and colleagues have published what they have learned in Current Biology. DNA conclusively indicated Dendrogramma is a cnidarian, related to true jellyfish. The stinging cells were absent because the scientists lacked the complete organism. Instead, what they had was a bract, an easily detachable part used for defense. It was not previously recognized as such because bracts are usually smaller and less elaborate.

Although MacIntosh told IFLScience he collected “around 80” Dendrogramma bracts on the voyage, there still isn't a complete specimen. In some cases, MacIntosh told IFLScience, the main body remains attached, but these are surprisingly small and “may be juveniles.”

The list of unanswered questions is long. “We don't know why they have such big bracts, we don't know why we collected so many bracts without complete samples and we don't know how many bracts fit on each,” MacIntosh said.

Nevertheless, he added: “They're probably like other deep sea jellies, floating just off the sea floor, anchored with small tentacles and using stinging cells to feed on crustaceans on the sea floor.” The bracts, while tenuous, provide protection against predators, or even against the fragile main Dendrogramma body.

The survey took place to assess the little-known biology of the Bite, where oil companies are seeking to drill. MacIntosh couldn't comment on whether the discovery of such an unusual species will interfere with these plans.

https://www.iflscience.com/deep-sea-species-mystery-partially-explained-36142