| Dendrogramma | |
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| Dendrogramma enigmatica | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Incertae sedis |
| Family: | Dendrogrammatidae Just, Kristensen, & Olesen, 2014[1] |
| Genus: | Dendrogramma Just, Kristensen, & Olesen, 2014 |
| Type species | |
| Dendrogramma enigmatica Just, Kristensen, & Olesen, 2014 |
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| Species | |
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Dendrogramma is a genus consisting of two species, D. enigmatica and D. discoides, identified in 2014 from a collection of specimens gathered in 1986. The two species, which are small ocean-dwelling organisms, have been placed in the animal kingdom, but have not been definitively assigned to any existing phylum.[1] The two species are the only known members of the family Dendrogrammatidae.[2]
Discovery[edit]
The Dendrogramma specimens were collected off the south-east coast of Australia during a scientific expedition in 1986. They were collected at water depths of 400 metres (1,300 ft) and 1,000 metres (3,300 ft) on the continental slope near Tasmania,[3] using a sled that was dragged over the sea floor to collect bottom-dwelling animals.[4] The researchers were immediately struck by the unusual characteristics of the 18 specimens they collected. These were preserved in formaldehyde, and later in ethanol, for further study.
On returning to the sample site in 1988, they were unable to find any further specimens. It was not until 2014 that they published the discovery.[5] Jean Just of the University of Copenhagen, who carried out the trawling in 1986, has explained the long delay before publication in terms of the extraordinary nature of the discovery: "Once you think you have something really extraordinary, it takes a long time to study, read, consult left, right and centre, and convince yourself that you’ve really stumbled across something special."[6]
Naming[edit]
The genus name Dendrogramma alludes to the branching pattern of the digestive canals,[1] which resemble dendrograms, branching diagrams frequently used by biologists to illustrate the evolutionary relationships among organisms. The specific name enigmatica refers to the mysterious nature of the organisms, while discoides, alludes to the disc-like shape of the animals.[4]
Description[edit]
These organisms are roughly mushroom-shaped, and with a mostly asymmetrical structure. So far no sex organs or nervous system has been detected.[7] The body consists of a flattened, somewhat circular disc and a stalk with a mouth on the end, surrounded by lobes. The mouth on the end of the stalk leads to a digestive canal that forks repeatedly when it reaches the disc. The individuals have an outer skin, with a stomach separated from the skin by a layer of dense gelatinous material (mesoglea). The stalks measure up to 8 millimeters in length, while the disc ranges from 11 to 17 mm in diameter, although the original specimens experienced significant shrinkage in full-strength ethanol after measurements were made.[1]
The two species primarily differ in shape of the disc and proportional length of the stalk: D. discoides has a complete disc and proportionally shorter stalk (length about 10% of the disc diameter) while D. enigmatica has a notched disc and proportionally longer stalk (up to 70% of disc diameter).[1]
The animals appear to have been free-living, as they do not show evidence of having been attached to something else, whether a surface or each other. There is no obvious sign that they had a means of propulsion and they do not appear to have been capable of swimming. The small mouth is simple and their manner of feeding is unclear; however, it has been suggested that the lobes around the creature's mouth could have secreted a mucus which was used to capture microbes in the water.[6]
Relationships[edit]
Species of Dendrogramma have a body plan similar to animals in the phyla Cnidaria and Ctenophora, but lack the stinging cells that define cnidarians and the tentacles that define ctenophorans. The relationship between Cnidaria, Ctenophora, and other basal metazoa is not firmly resolved. Dendrogramma is a basal metazoan, but the sister group to these organisms is unclear.[1] They have been assigned their own genus, Dendrogramma,[3] and family, Dendrogrammatidae;[2] and the researchers even considered putting them in their own phylum. As they put it, however, "we refrain from erecting such a high-level taxon for the time being, because new material is needed to resolve many pertinent outstanding questions."[5]
The lead scientist of the identification effort, Jørgen Olesen of the University of Copenhagen, suggests that they represent "an early branch on the tree of life, with similarities to the 600 million-year-old extinct Ediacara fauna."[8] At least three genera of Ediacarans—Albumares, Anfesta, and Rugoconites—share similarities with Dendrogramma; all three appear to have possessed a disc with an internal network of forking channels.[6]
Genetic identification, which could help place the organisms on the tree of life, cannot be done using the current specimens, as they were preserved with formaldehyde and alcohol, a method that does not preserve nucleic acids. Further genetic research may have to wait until fresh specimens of the organisms can be obtained.[3]
Neurobiologist Leonid Moroz of the Whitney Laboratory for Marine Bioscience at the University of Florida said that if the new genus turns out to be directly descended from early animals, it could "completely reshape the tree of life, and even our understanding of how animals evolved, how neurosystems evolved, how different tissues evolved."[6] Simon Conway Morris of the University of Cambridge stated that the discovery is "a very interesting surprise, and it poses lots and lots of questions." He notes the "most intriguing similarity to certain Ediacaran forms," but cautions that "the similarities are exactly that. They are intriguing rather than compelling."[6] Nonetheless, if it is confirmed that Dendrogramma is a descendant of the Ediacarans, Reinhardt Kristensen of University of Copenhagen comments that then "we have discovered animals which we'd expect to be extinct around 500 million years ago."[6]
The discovery of what potentially could be an entirely new phylum of animals is an extremely rare event; as Jørgen Olesen notes, "it's maybe only happened about four times in the last 100 years."[3] Andreas Hejnol of the Sars International Centre for Marine Molecular Biology in Norway, describes tracking down a new lineage as being "like the discovery of a treasure," as survivors of lineages thought long extinct can help researchers to reconstruct evolutionary history.[4]
References[edit]
- ^ a b c d e f Just, J.; Kristensen, R.M.; Olesen, J. (2014). "Dendrogramma, New Genus, with Two New Non-Bilaterian Species from the Marine Bathyal of Southeastern Australia (Animalia, Metazoa incertae sedis) – with Similarities to Some Medusoids from the Precambrian Ediacara.". PLOS ONE 9 (9): e102976. doi:10.1371/journal.pone.0102976.
- ^ a b John Timmer (9 September 2014). "Tasmanian depths may have been hiding unknown animal phylum". Ars Technica.
- ^ a b c d Rincon, Paul (3 September 2014). "Deep sea 'mushroom' may be new branch of life". BBC News. Retrieved 4 September 2014.
- ^ a b c Skinner, Nicole (3 September 2014). "Sea creatures add branch to tree of life". Nature. doi:10.1038/nature.2014.15833.
- ^ a b Nuwer, Rachel (3 September 2014). "Deep-Sea Mushroom Creatures Found Off Australia". Smithsonian.com. Retrieved 4 September 2014.
- ^ a b c d e f Frazer, Jennifer (3 September 2014). "New Deep-Sea Animal Species Look Like Mushrooms but Defy Classification". National Geographic. Retrieved 4 September 2014.
- ^ Weird creatures may be relics from dawn of animal life
- ^ "Two unclassifiable species found off Australian coast". Press Association. 4 September 2014. Retrieved 4 September 2014.
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