Time for a correction. On September 7, 2014, I put up a post about a weird new creature, Dendrogramma, two species of which were dredged up from the deep seas of Australia. Here’s one of them:
Thse species, which had stalks and inflexible disks, weren’t considered members of existing phyla like ctenophores (comb jellies) because, as the original paper (Just et al., reference and link below) noted, they lack features present in other similar phyla (my emphasis in this original quote):
Dendrogramma shares a number of similarities in general body organisation with the two phyla, Ctenophora and Cnidaria, but cannot be placed inside any of these as they are recognised currently. We can state with considerable certainty that the organisms do not possess cnidocytes, tentacles, marginal pore openings for the radiating canals, ring canal, sense organs in the form of e.g., statocysts or the rhopalia of Scyphozoa and Cubozoa, or colloblasts, ctenes, or an apical organ as seen in Ctenophora. No cilia have been located. We have not found evidence that the specimens may represent torn-off parts of colonial Siphonophora (e.g., gastrozooids). Neither have we observed any traces of gonads, which may indicate immaturity or seasonal changes. No biological information on Dendrogramma is available.
DNA data, which would have been very useful, weren’t available for these specimens as they were collected in 1986 and fixed in formalin, which destroys DNA. While the authors didn’t name a new phylum, they suggested that these two species were indeed representatives of a new phylum, and that caused a lot of excitement. (New phyla aren’t often described.)
However, a 2015 expedition, whose results are described in a new paper in Current Biology (O’Hara et al., reference and free link below), produced RNA that could be sequenced. And that RNA shows that Dendrogramma isn’t a new phylum at all, but a siphonophore. Siphonophores are well known, an order that falls in the class Hydrozoa, itself in the phylum Cnidaria. Siphonophores are a bizarre group consisting of specialized individual animals that band together as a group to form a “superorganism”; the most familiar member is the Portuguese man of war, and here’s another, the pelagic (free swimming) siphonophore Marrus orthocanna:
As the new paper notes:
Siphonophores are bizarre pelagic colonial cnidarians in the class Hydrozoa. They are complex elongate or spherical organisms with specialised locomotive and feeding zooids, and a net of tentacles that can be extended to catch prey or attach to the seafloor. There are 175 described species, living in a range of habitats from the sea surface (e.g., Physalia physalis, the Portuguese Man O’War) to the deep-sea. Larger, more delicate species have been found mainly in the non-turbulent mesopelagic (300–1000 m) or bathypelagic zones (1000–3000 m).
The RNA analysis places Dendrogramma (probably just one species, not two), firmly in the siphonophores: it’s the red species in the phylogny below.Finally, the authors hypothesize that the “animal” Dendrogramma in the first picture above is really part of a more complex colony, and that the discoid things with stalks are cormidial bracts. The figure below shows those bracts in an entire siphonophore:
So, move along folks, nothing more to see here. It’s just the usual advance of science, when we can better identify a bizarre form using DNA—or in this case, RNA. The earlier speculations that Dendrogramma may be a living remnant of the bizarre Ediacaran fauna that went extinct about 540 million years ago is no longer tenable.
h/t: Matthew Cobb, Casey Dunn
Just, J., R. M. Kristensen, and J. Olesen. 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 DOI: 10.1371/journal.pone.0102976
O’Hara, T. D. et al. 2016. Dendrogramma is a siphonophore. Current Biol. 26: R457-458.