Cnidarians  -  Phylum:   Cnidaria
"Jellyfish serve as a model for bioengineers for the same reason yeast were once so valuable to geneticists: they're simple to deconstruct."
Jeffrey Kluger (b. 1954)
      With just over 15,000 species in the phylum, the Cnidaria are by no means the largest group but they are certainly among the most widespread having been recorded in marine environments all over the world.  There are even freshwater species although these are not seen as often.   They come in two forms;  as 'medusae', free-swimming bodies with tentacles suspended therefrom, or, as 'polyps' which are sessile     (= fixed in one place)  and consist mostly of a tubular body with a mouth at the top end that is surrounded by tentacles.  
      There are six, possibly seven Classes  within the phylum with the ambiguously placed  class: Polypodiozoa containing just a single species that is a parasite found inside the eggs of sturgeon and other related fishes.  iNaturalist places this particular animal in a separate Class within the the Cnidaria whereas The Catalogue of Life does not. Furthermore the genus name: Polypodium, is also used for a genus of ferns.  Fortunately the single species within this class is not a local one and so we can sefely ignore this conundrum.
      
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Image 1. Jellyfish do not fosilise often but when they do make for interesting finds.
      Cnidarians get their name from the presence of cnidocytes on their tentacles.  There are over thirty three types of cnidocytes divided into three group. Each group fires a different type of thread;  first there are nematocysts, threads of which inject venom into the animal that triggers them.    Secondly, spirocysts which fire threads with sticky hairs designed to entangle prey. And thirdly, ptychocysts, firing threads which are used by Tube Anemones in the order: Cheriantharia to construct the tube they live in.  All types are fairly complex organs and they are  'single use' only. Replacement of spent cnidocytes is therefore an energy intensive exercise.
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Image 2. What happens when a cnidocyte is triggered. A. still under the surface of the tentacle. B. Ready to fire caused by external stimuli. C. Triggered and in the process of turning itself inside out. D. Fully extended, thread usually inserted into the prey releasing its poison. (image by Antropoteuthis)
​      The Cnidaria are the oldest of the true metazoan phyla.  Molecular research suggests anything upto 741 Mya.   A fossil Hydrozoan from South Australia called Ediacara was said to be 700 million years old although the notion that is was indeed a jellyfish has been challenged.  And, while numerous fossil Cnidarians exist from the Cambrian 500 million years ago,  currently the title for the oldest Cnidarian fossil  goes to  Auroralumina attenboroughii, a recent discovery, named after the famous Sir David Attenborough.  The fossil specimens were determined to be about 560 My old.
Of the six, possibly seven classes within the phylum, species of four classes are encountered in the waters of the Northern Rivers region.
      The two classes not dealt with on this website are the Myxozoa and the Staurozoa.   Myxozoans are very small, some consisting of only a few cells and some are unicellular. The smallest living animals on earth belong to this class.  Those that are among the largest in the class may reach a whopping 2 millimetres in length.  The level of 'devolution' (= loss of features due to them becoming obsolete.  Common in parasites.) within the class is sometimes extreme. The species:  Henneguya salminicola, for instance, is the only known multicellular animal not to rely on aerobic respiration of oxygen as it no longer has the internatal structures to enable it to do so.  It is found as a parasite in the muscles of some salmon species.          The Staurozoans are a small class of nearly 50 species that are not found in our waters at all.
References  and  links:
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^      Image 1. By  Bather, Francis Arthur, 1863-1934   public domain image at the  British Museum (Natural History). Dept. of Geology
^      Image 2.​  By Antropoteuthis   CC  BY AS 4.0  via  Creative Commons Attribution-ShareAlike 4.0 International

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​^      Dunn, F.S., Kenchington, C.G., Parry, L.A. et al. "A crown-group cnidarian from the Ediacaran of Charnwood Forest, UK." 
​                    Nat Ecol Evol 6, 1095–1104 (2022).
^      Park E., Hwang D.S., Lee J.S., Song J.I., Seo T.K., Won, Y.J., " Estimation of divergence times in cnidarian evolution based on mitochondrial protein-                                  coding genes and the fossil record."   Molecular Phylogenetics and Evolution,  Volume 62, Issue 1,  2012,   Pages 329-345,  ISSN 1055-7903.
^       Yahalomi, D,  Atkinson, S. D.,  Neuhof, M.,  Chang, E. S.,  Philippe, H.,  Cartwright, P.,  Bartholomew, J. L.,  Huchon, D. (2020). "A cnidarian                                            parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome". Proceedings of the National Academy of Sciences. 117 (10): 5358-5363. ​
^      Zimmer. C., (2000).  Parasite Rex.  Free Press, New York, United States.  

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       https://www.guinnessworldrecords.com/
       https://en.wikipedia.org/wiki/Francis_Arthur_Bather
      https://en.wikipedia.org/wiki/Cnidocyte​
       https://en.wikipedia.org/wiki/David_Attenborough​
​       https://en.wikipedia.org/wiki/Devolution_(biology)
       https://en.wikipedia.org/wiki/Jellyfish#Life_history_and_behavior
      https://en.wikipedia.org/wiki/Polyp_(zoology)
      https://www.nature.com/articles/s41559-022-01807-x

Text: 
Erik Beringen.

​Header Image:  
Diana Davey.
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