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A New Phylum of Invertebrates Discovered? Meet Dendrogramma! [UPDATED; It is a siphonophore] - No Bones

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03 September 2014

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Priscila A. Grohmann

Sometimes the cnidocysts are so small, that only a powerful microscope is able to evidence them! I'd like very much to observe this animal personally! It's a pity that it's impossible!

Priscila A. Grohmann

Regarded as a likely new phylum, Dendrogramma looks, in many aspects, like a Cnidaria, Octocorallia, Pennatulacea. The same way as a Renilla, it has a stalk-like structure, an apical structure similar to a rachis, with a channel system inside (seen by transparency) and an opening at its tip. This opening could be a structure for entry and exit of water, like a special exalant siphonozooid. The absence of polyps on the rachis could be due to a simplification caused by miniaturization itself, or a modification, adaptation or even lack of these structures due to the animal’s reduced size (see Hanken & Wake, 1993. Annual Review of Ecology and Systematics vol 24 p. 505). So, the only point to clarify would be the lack of cnidocysts.

Allen Collins

Priscila Grohmann, the hypothesis you lay out seems very reasonable. Only the apparent absence of cnidocysts contradicts a cnidarian interpretation for this animal, but this then is an absence of evidence argument. Absence of evidence is not necessarily evidence of absence. Could cnidocysts have been missed? Or could they be evolutionarily lost? While I do not believe there are any known cases where a cnidarian lacks cnidocysts, losses of "defining characters" are absent in other clades. For instance, the loss of a water filtration system in carnivorous sponges, comb rows missing on benthic ctenophores, lack of chaetae in some derived lineages of annelids, etc. More data are needed on these interesting critters, and I hope it will not be long.

Priscila A. Grohmann

Regarded as a likely new phylum, Dendrogramma looks in many ways a Cnidaria, Octocorallia, Pennatulacea. The same way as a Renilla, it has a stalk-like structure, an apical structure similar to a rachis, with a channel system inside (seen by transparency) and an opening at its tip. This opening could be a structure for entry and exit of water, like a special exalant siphonozooid. The absence of polyps in the rachis could be due to a simplification caused by miniaturization itself, or a modification, adaptation or even lack of these structures due to the animal’s reduced size (see Hanken & Wake, 1993. Annual Review of Ecology and Systematics vol 24 p. 505). So, the only point to clarify would be the lack of cnidocysts!

ENT-TT

Very interesting. My first thought was, "what if it is an as-yet unknown life stage of a known critter?" So.. perhaps this might represent the sessile polyp stage of some larger, mobile organism?

Allen Collins

And yes, "manus" a name for an opening serving as both mouth and anus, is neologistic jargon, the cardinal sin of a blogger.

Allen Collins

Hello Stefano!
Yes, an oversimplification regarding sex in placozoans. Certainly the life cycle is still unknown, although there are some indications that sex could happen in cultures. Interested readers can check out this paper, also Open Access, from PLoS ONE: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0019639 "New Insights into Placozoan Sexual Reproduction and Development" by
Michael Eitel, Loretta Guidi, Heike Hadrys, Maria Balsamo, and Bernd Schierwater

Stefano Piraino

Hi Allen
thanks for your post and the great presentation of this fascinating animal. I have a couple of comments. Bob Paine's (1966) dictum warned: "thous shalt not commit jargon". At that time he was talking about species interactions and the keystone species concept, but it may apply here with reference to Meg's neologism - "manus" . This term looks intriguing to refer to a multi-task single orifice by a single word, but this one combines two terms with different roots (mouth, English, from Old German; and anus, from Latin). Also, everyone familiar with Latin may interpret "manus" as a human hand... Second, I am not sure about the lack of evidence of sexuality in placozoans. as far as I know 64-cells embryos have been documented in Trichoplax adherens cultures. Concerning this fascinating Dendrogramma mistery, phylum-diagnostic characters may be occasionally absent from representatives of a given phylum. I am thinking to the benthic ctenophores for instance, lacking comb rows, or to adult ascidians, lacking notochord. I am looking forward to reading more from future investigations, eg molecular data, histology (esp for sensory-motor integration systems). Best, Stefano

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