Bryozoans    -    Phylum:   Bryozoa
"Anyone who starts looking at Bryozoans will continue to do so, for their biology is full of interest and unsolved mysteries."
John S. Ryland (b. 1936)  in  'Bryozoans',  1970.
        The name Bryozoa is derived from the Greek  'bruon' (=moss) and 'zoia'  (=animals). The name is in reference to the moss-like appearance of some species.   At times they were also known as Polyzoa and Ectoprocta but these names are now generally regarded as obsolete. The generic  names 'sea mats' and 'moss animals' are still in common use even though the number of species that builds moss- or mat-like colonies is relatively small.  By 2022 there were nearly 6,500 described species, with the fossil record containing another 15,800 species or so.  The name 'Bryozoa' was introduced by Christian Gottfried Ehrenberg, a Professor of Medicine at Berlin University, in 1831. 
      Bryozoans are minute aquatic invertebrates that form colonies*, very much like corals and sponges.  These colonies can take a wide variety of shapes such as cups, fans, trees, spirals, sheets, etc. even blobs of jelly or, like most, just plain crusts.   The individual animal is usually no more than 0.5 millimetre in size and is known as a zooid.  This to indicate they are not independent animals that could fully function by them selves.  Colonies can reach well over half a metre in size but  many are limited to about a third of that.  Most species live in salt or brackish water. Only 108 species are confined to freshwater bodies and it is these that tend to make 'jellies' rather than solid structures.
*  There is one genus found in Northern Europe that does not.  Those Bryozoans live a solitary life independently from others.
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Image 1. Ordovician bryozoans in an Estonian oil shale. By Mark A.Wilson.
       The individual zooid within the colony consists of a polypide, a body made of soft tissue and organs,  and a lophophore which is like a crown of cilia-covered tentacles with a mouth opening in the middle. The exo-skeleton of the zooid forms one of the building blocks of the colony and fixes the animal in its' place.  Not all the zooids in the colony are always the same. Some colonies produce 'autozooids' which are responsible for food and waste management throughout the colony.  Some species, particularly those in the Order: Cheilostomata, produce a wide range of such specialist zooids dedicated to certain tasks within the colony.   The zooids in the colony are connected to their neighbours by a strand of cytoplasm.  This allows the transfer of nutrients and other material.  There can easily be as many as 2,000,000 zooids in a large colony.
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Image 3. 'Budding' in yeast cells. By'Masur'.
      Bryzoans are filter feeders and as such are good indicators of an eco-systems' health as they are rather sensitive to pollution.  Reproduction can occur by both sexual and asexual means.  Colonies are started by a sexually produced 'ancestrula', the origin zooid which is a free-swimming larva.  Once the larva settles the colony grows by means of 'budding' from the ancestrula, an asexual mchanism of reproduction during which new organisms grow from an existing one and separates therefrom once development is complete.   Most Bryozoans are 'simultaneous' (= all the time) haemaphrodites.  Some are 'protandric' (= one sex when young and changing later in life) and a few  are 'dioecious' (= only male and only female individuals).
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Image 2. A generalized Bryozoan. By Mark A.Wilson.
      ​      Bryozoans have an extensive fossil record but until recently had not been discovered yet in rocks of Cambrian , or earlier, origin.  There has been a long-standing consensus that it was not until the early Ordivician (485-431 My ago) that fossils of both the marine classes of Bryozoans started to appear. It was speculated upon they may have been present in the Cambrium or earlier but for any number of reasons were not preserved.   A recent study by Zhang et al. however, has shed more light on their origins and the timeline of Bryozoan evolution has been pushed backwards by a considerable measure.   As is the case with Brachiopods, Bryozoa fossils are very common and show wonderful diversity.  They are found world-wide.
References  and  links:
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​^      Image 1.   Prof. Mark.A. Wilson,   CC0   via   commons.wikimedia.org/wiki/User:Wilson44691 ​
^      Image 2.   Prof. Mark A. Wilson,   CC0  via   commons.wikimedia.org/wiki/File:Ectoproct_generalized_01.png
​^      Image 3.  Masur,  CC0    via   https://commons.wikimedia.org/wiki/File:S_cerevisiae_under_DIC_microscopy.jpg

​^      Brusca, R., G. Brusca. 2003. Invertebrates (2nd Edition). Sunderland, MA: Sinauer Associates.
^      WoRMS (2022). Bryozoa. Accessed at: https://www.marinespecies.org/aphia.php?p=taxdetails&id=146142 on 2022-12-12
​^      Zhang, Z., Zhang, Z., Ma, J. et al. "Fossil evidence unveils an early Cambrian origin for Bryozoa." Nature 599, 251–255 (2021). https://doi.org/10.1038/s41586-021-04033-w  under  http://creativecommons.org/licenses/by/4.0/.
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​      https://animaldiversity.org/accounts/Bryozoa/
      https://www.britannica.com/science/budding-reproduction
      https://www.digitalatlasofancientlife.org/
       http://www.mesa.edu.au/bryozoa/
      https://www.nature.com/scitable/definition/cytoplasm-280/​
        https://rdcu.be/c1nzc
      https://en.wikipedia.org/wiki/Christian_Gottfried_Ehrenberg
       https://en.wikipedia.org/wiki/Lophophore
        https://en.wikipedia.org/wiki/Cilium
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Text:   Erik Beringen
Header photo:   TNRP stock photo
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