Editing Placozoa (section)

==Biology==
[[File:Placozoan anatomy.svg|thumb|upright=1.5|right| {{center|'''Trichoplax body structure in cross section'''<br />1 - lipid drop, 2 - cilium, 3 - dorsal layer of cells, 4 - vacuole,<br />5 - fibrous syncytium, 6 - glandular cell, 7 - vacuole,<br />8 - ventral layer of cells, 9 - zones of intercellular contacts}}]]
{{main|Trichoplax}}

Placozoans do not have well-defined body plans, much like [[amoebas]], unicellular eukaryotes. As Andrew Masterson reported: "they are as close as it is possible to get to being simply a little living blob."<ref>{{Cite web |last=Masterson |first=Andrew |date=2018-08-01 |title=Simple organisms not so simple, after all |url=https://cosmosmagazine.com/science/biology/simple-organisms-not-so-simple-after-all/ |access-date=2023-06-02 |website=Cosmos Magazine |language=en-AU}}</ref> An individual body measures about 0.55&nbsp;mm in diameter.<ref name=":12">{{Cite journal |last1=Eitel |first1=Michael |last2=Francis |first2=Warren R. |last3=Varoqueaux |first3=Frédérique |last4=Daraspe |first4=Jean |last5=Osigus |first5=Hans-Jürgen |last6=Krebs |first6=Stefan |last7=Vargas |first7=Sergio |last8=Blum |first8=Helmut |last9=Williams |first9=Gray A. |last10=Schierwater |first10=Bernd |last11=Wörheide |first11=Gert |display-authors=8 |date=2018 |title=Comparative genomics and the nature of placozoan species |journal=PLOS Biology |volume=16 |issue=7 |pages=e2005359 |doi=10.1371/journal.pbio.2005359 |pmc=6067683 |pmid=30063702 |doi-access=free }}</ref> There are no body parts; as one of the researchers Michael Eitel described: "There's no mouth, there's no back, no nerve cells, nothing."<ref name=":02">{{Cite web |last=Wood |first=Charlie |date=2018-10-06 |title=Simplest Animal Reveals Hidden Diversity |url=https://www.scientificamerican.com/article/worlds-simplest-animal-reveals-hidden-diversity/ |access-date=2023-06-02 |website=Scientific American |language=en |via=Quanta Magazine}}</ref> Animals studied in laboratories have bodies consisting of everything from hundreds to millions of cells.<ref>[https://news.stanford.edu/press-releases/2018/10/15/new-mechanism-animal-cells-stay-intact/ Stanford researchers reveal a new mechanism for how animal cells stay intact]</ref> 

Placozoans have only three anatomical parts as tissue layers inside its body: the upper, intermediate (middle) and lower [[Epithelium|epithelia]]. There are at least six different cell types.<ref name=":32">{{Cite journal |last1=Romanova |first1=Daria Y. |last2=Varoqueaux |first2=Frédérique |last3=Daraspe |first3=Jean |last4=Nikitin |first4=Mikhail A. |last5=Eitel |first5=Michael |last6=Fasshauer |first6=Dirk |last7=Moroz |first7=Leonid L. |date=2021 |title=Hidden cell diversity in Placozoa: ultrastructural insights from Hoilungia hongkongensis |journal=Cell and Tissue Research |volume=385 |issue=3 |pages=623–637 |doi=10.1007/s00441-021-03459-y |pmc=8523601 |pmid=33876313}}</ref> The upper epithelium is the thinnest portion and essentially comprises flat cells with their cell body hanging underneath the surface, and each cell having a [[cilium]].<ref name=":122">{{Cite journal |last1=Eitel |first1=Michael |last2=Francis |first2=Warren R. |last3=Varoqueaux |first3=Frédérique |last4=Daraspe |first4=Jean |last5=Osigus |first5=Hans-Jürgen |last6=Krebs |first6=Stefan |last7=Vargas |first7=Sergio |last8=Blum |first8=Helmut |last9=Williams |first9=Gray A. |last10=Schierwater |first10=Bernd |last11=Wörheide |first11=Gert |display-authors=8 |date=2018 |title=Comparative genomics and the nature of placozoan species |journal=PLOS Biology |volume=16 |issue=7 |pages=e2005359 |doi=10.1371/journal.pbio.2005359 |pmc=6067683 |pmid=30063702 |doi-access=free }}</ref> Crystal cells are sparsely distributed near the marginal edge. A few cells have unusually large number of [[mitochondria]].<ref name=":32" /> The middle layer is the thickest made up of numerous fiber cells, which contain mitochondrial complexes, vacuoles and [[Endosymbiont|endosymbiotic bacteria]] in the [[endoplasmic reticulum]]. The lower epithelium consists of numerous monociliated cylinder cells along with a few endocrine-like gland cells and lipophil cells. Each lipophil cell contains numerous middle-sized granules, one of which is a [[secretory granule]].<ref name=":222">{{Cite journal |last1=Tessler |first1=Michael |last2=Neumann |first2=Johannes S. |last3=Kamm |first3=Kai |last4=Osigus |first4=Hans-Jürgen |last5=Eshel |first5=Gil |last6=Narechania |first6=Apurva |last7=Burns |first7=John A. |last8=DeSalle |first8=Rob |last9=Schierwater |first9=Bernd |date=2022-12-08 |title=Phylogenomics and the first higher taxonomy of Placozoa, an ancient and enigmatic animal phylum |journal=Frontiers in Ecology and Evolution |volume=10 |doi=10.3389/fevo.2022.1016357 |issn=2296-701X |doi-access=free }}</ref><ref name=":122" />

The body axes of ''Hoilungia'' and ''Trichoplax'' are overtly similar to the oral–aboral axis of [[cnidarians]],<ref name="DuBuc_2019">{{cite journal |vauthors=DuBuc TQ, Ryan JF, Martindale MQ |date=May 2019 |title="Dorsal-Ventral" Genes Are Part of an Ancient Axial Patterning System: Evidence from Trichoplax adhaerens (Placozoa) |journal=Molecular Biology and Evolution |volume=36 |issue=5 |pages=966–973 |doi=10.1093/molbev/msz025 |pmc=6501881 |pmid=30726986}}</ref> animals from another phylum with which they are most closely related.<ref>{{Cite journal |last1=Laumer |first1=Christopher E. |last2=Fernández |first2=Rosa |last3=Lemer |first3=Sarah |last4=Combosch |first4=David |last5=Kocot |first5=Kevin M. |last6=Riesgo |first6=Ana |last7=Andrade |first7=Sónia C. S. |last8=Sterrer |first8=Wolfgang |last9=Sørensen |first9=Martin V. |last10=Giribet |first10=Gonzalo |date=2019-07-10 |title=Revisiting metazoan phylogeny with genomic sampling of all phyla |journal=Proceedings. Biological Sciences |volume=286 |issue=1906 |pages=20190831 |doi=10.1098/rspb.2019.0831 |issn=1471-2954 |pmc=6650721 |pmid=31288696}}</ref> Structurally, they can not be distinguished from other placozoans, so that identification is purely on genetic (mitochondrial DNA) differences.<ref name=":1222">{{Cite journal |last1=Eitel |first1=Michael |last2=Francis |first2=Warren R. |last3=Varoqueaux |first3=Frédérique |last4=Daraspe |first4=Jean |last5=Osigus |first5=Hans-Jürgen |last6=Krebs |first6=Stefan |last7=Vargas |first7=Sergio |last8=Blum |first8=Helmut |last9=Williams |first9=Gray A. |last10=Schierwater |first10=Bernd |last11=Wörheide |first11=Gert |display-authors=8 |date=2018 |title=Comparative genomics and the nature of placozoan species |journal=PLOS Biology |volume=16 |issue=7 |pages=e2005359 |doi=10.1371/journal.pbio.2005359 |pmc=6067683 |pmid=30063702 |doi-access=free }}</ref> Genome sequencing has shown that each species has a set of unique genes and several uniquely missing genes.<ref name=":2222">{{Cite journal |last1=Tessler |first1=Michael |last2=Neumann |first2=Johannes S. |last3=Kamm |first3=Kai |last4=Osigus |first4=Hans-Jürgen |last5=Eshel |first5=Gil |last6=Narechania |first6=Apurva |last7=Burns |first7=John A. |last8=DeSalle |first8=Rob |last9=Schierwater |first9=Bernd |date=2022-12-08 |title=Phylogenomics and the first higher taxonomy of Placozoa, an ancient and enigmatic animal phylum |journal=Frontiers in Ecology and Evolution |volume=10 |doi=10.3389/fevo.2022.1016357 |issn=2296-701X |doi-access=free }}</ref>

''Trichoplax'' is a small, flattened, animal around {{convert|1|mm|abbr=on}} across. An amorphous multi-celled body, analogous to a single-celled ''[[Amoeba (genus)|amoeba]]'', it has no regular outline, although the lower surface is somewhat concave, and the upper surface is always flattened. The body consists of an outer layer of simple [[epithelium]] enclosing a loose sheet of stellate cells resembling the [[mesenchyme]] of some more complex animals. The epithelial cells bear [[cilia]], which the animal uses to help it creep along the seafloor.<ref name="IZ">
{{cite book
 |last=Barnes  |first=Robert D.
 |year=1982
 |title=Invertebrate Zoology
 |pages=84–85
 |publisher=Holt-Saunders International
 |location=Philadelphia
 |isbn= 978-0-03-056747-6
}}
</ref>

The lower surface engulfs small particles of organic detritus, on which the animal [[Detritivore|feeds]]. All placozoans can reproduce asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the [[mesenchyme]].<ref name=IZ/>
[[Sexual reproduction]] has been reported to occur in one [[clade]] of placozoans,<ref name="pmid16230622">
{{cite journal
 |last1=Signorovitch  |first1=A.Y.
 |last2=Dellaporta    |first2=S.L.
 |last3=Buss          |first3=L.W.
 |year=2005 
 |title=Molecular signatures for sex in the Placozoa
 |journal=[[Proceedings of the National Academy of Sciences of the United States of America]]
 |volume=102  |issue=43  |pages=15518–22
 |pmid=16230622  |doi=10.1073/pnas.0504031102
 |pmc=1266089  |bibcode=2005PNAS..10215518S
|doi-access=free
 }}
</ref><ref name="pmid16552431">
{{cite journal
 |last=Charlesworth  |first=D.
 |year=2006
 |title=Population genetics: Using recombination to detect sexual reproduction: The contrasting cases of Placozoa and ''C.&nbsp;elegans''
 |journal=Heredity {{small|(Edinb.)}}
 |volume=96  |issue=5  |pages=341–342
 |pmid=16552431  |doi=10.1038/sj.hdy.6800809
 |s2cid=44333533
}}
</ref> whose strain H8 was later found to belong to genus ''[[Cladtertia]]'',<ref name="Tessler et al 2022" /> where [[Genetic recombination|intergenic recombination]] was observed as well as other hallmarks of sexual reproduction.

Some ''Trichoplax'' species contain ''[[Rickettsiales]]'' bacteria as [[endosymbiont]]s.<ref>
{{Cite journal
 |first1=Kai    |last1=Kamm 
 |first2=Bernd  |last2=Schierwater
 |first3=Rob    |last3=DeSalle 
 |date=2019-01-05
 |title=Innate immunity in the simplest animals – placozoans
 |journal=BMC Genomics
 |volume=20  |issue=1  |pages=5
 |pmc=6321704  |pmid=30611207  |issn=1471-2164
 |doi=10.1186/s12864-018-5377-3
|doi-access=free 
 }}
</ref>
One of the at least 20&nbsp;described species turned out to have two bacterial endosymbionts; ''[[Grellia]]'' which lives in the animal's endoplasmic reticulum and is assumed to play a role in the protein and membrane production. The other endosymbiont is the first described ''[[Margulisbacteria]]'', that lives inside cells used for [[Algivore|algal digestion]]. It appears to eat the fats and other lipids of the algae and provide its host with vitamins and amino acids in return.<ref>
{{cite press release
 |title=Deceptively simple: Minute marine animals live in a sophisticated symbiosis with bacteria
 |date=10 June 2019
 |publisher=Max Planck Society
 |website=Phys.org
 |url=https://phys.org/news/2019-06-deceptively-simple-minute-marine-animals.html
 |access-date=2021-06-23  }}</ref>
<ref name=Harald>{{cite journal
 |first1=Harald  |last1=Gruber-Vodicka    |first2=Niko     |last2=Leisch 
 |first3=Manuel  |last3=Kleiner           |first4=Tjorven  |last4=Hinzke
 |first5=Manuel  |last5=Liebeke           |first6=Margaret |last6=McFall-Ngai
 |first7=Michael G.   |last7=Hadfield     |first8=Nicole   |last8=Dubilier
 |display-authors=6
 |year=2019
 |title=Two intracellular and cell type-specific bacterial symbionts in the placozoan ''Trichoplax'' H2
 |journal=Nature Microbiology
 |volume=4 |issue=9 |pages=1465–1474 |doi=10.1038/s41564-019-0475-9
 |pmid=31182796 |pmc=6784892
}}
</ref>

Studies suggest that aragonite crystals in crystal cells have the same function as statoliths, allowing it to use gravity for [[Gravitaxis|spatial orientation]].<ref>{{Cite journal |last1=Schierwater |first1=Bernd |last2=Osigus |first2=Hans-Jürgen |last3=Bergmann |first3=Tjard |last4=Blackstone |first4=Neil W. |last5=Hadrys |first5=Heike |last6=Hauslage |first6=Jens |last7=Humbert |first7=Patrick O. |last8=Kamm |first8=Kai |last9=Kvansakul |first9=Marc |last10=Wysocki |first10=Kathrin |last11=DeSalle |first11=Rob |date=2021 |title=The enigmatic Placozoa part 2: Exploring evolutionary controversies and promising questions on earth and in space |url=https://onlinelibrary.wiley.com/doi/10.1002/bies.202100083 |journal=BioEssays |language=en |volume=43 |issue=10 |pages=e2100083 |doi=10.1002/bies.202100083 |pmid=34490659 |issn=0265-9247|doi-access=free }}</ref>

Located in the dorsal epithelium there are lipid granules called shiny spheres which release a cocktail of venoms and toxins as an anti-predator defense, and can induce paralysis or death in some predators. Genes has been found in Trichoplax with a strong resemblance to the venom genes of some poisonous snakes, like the American copperhead and the West African carpet viper.<ref>[https://www.snexplores.org/article/living-mysteries-meet-earths-simplest-animal Living Mysteries: Meet Earth’s simplest animal]</ref><ref>{{Cite journal |last=Cuervo-González |first=Rodrigo |date=September 2017 |title=Rhodope placozophagus (Heterobranchia) a new species of turbellarian-like Gastropoda that preys on placozoans |url=https://linkinghub.elsevier.com/retrieve/pii/S0044523117300694 |journal=Zoologischer Anzeiger |language=en |volume=270 |pages=43–48 |doi=10.1016/j.jcz.2017.09.005|bibcode=2017ZooAn.270...43C}}</ref>

[[File:Placozoa.svg|thumb|upright=2|left| {{center|Global distribution{{hsp}}<ref>
{{cite journal
 |last1 = Eitel    |first1 = Michael     |last2 = Osigus    |first2 = Hans-Jürgen
 |last3 = Desalle  |first3 = Rob         |last4 = Schierwater   |first4 = Bernd
 |year = 2013
 |title = Global diversity of the Placozoa
 |journal = PLOS ONE
 |volume = 8  |issue = 4  |page = e57131
 |pmid = 23565136 |pmc = 3614897 |bibcode = 2013PLoSO...857131E
 |doi = 10.1371/journal.pone.0057131  |doi-access = free
}}
</ref>}}]]

{{clear left}}

The Placozoa show substantial evolutionary radiation in regard to [[sodium channel]]s, of which they have 5–7&nbsp;different types, more than any other invertebrate species studied to date.<ref>
{{cite journal
 |last1=Romanova |first1=Daria Y.        |last2=Smirnov  |first2=Ivan V. 
 |last3=Nikitin  |first3=Mikhail A.      |last4=Kohn     |first4=Andrea B. 
 |last5=Borman   |first5=Alisa I.        |last6=Malyshev |first6=Alexey Y. 
 |last7=Balaban  |first7=Pavel M.        |last8=Moroz    |first8=Leonid L. 
 |display-authors=6
 |date=29 October 2020
 |title=Sodium action potentials in placozoa: Insights into behavioral integration and evolution of nerveless animals
 |journal=Biochemical and Biophysical Research Communications
 |volume=532  |issue=1  |pages=120–126
 |doi=10.1016/j.bbrc.2020.08.020
 |pmid=32828537 |pmc=8214824
}}
</ref>

Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies. <ref>{{cite journal| doi=10.3389/fcell.2022.823283 | volume=10 | title=Expanding of life strategies in placozoa: Insights from long-term culturing of ''Trichoplax'' and ''Hoilungia''  | year=2022 | journal=Frontiers in Cell and Developmental Biology | last1 = Romanova | first1 = Daria | last2 = Nikitin | first2 = Mikhail | last3 = Shchenkov | first3 = Sergey | last4 = Moroz | first4 = Leonid | page=823283 | pmid=35223848 | pmc=8864292 | doi-access=free }}</ref>

In addition to fission, representatives of all species produced “swarmers” (a separate vegetative reproduction stage), which could also be formed from the lower epithelium with greater cell-type diversity.<ref>{{cite journal| doi=10.3389/fcell.2022.823283 | volume=10 | title=Expanding of Life Strategies in Placozoa: Insights From Long-Term Culturing of ''Trichoplax'' and ''Hoilungia''  | year=2021 | pages=623–637 |journal=Frontiers in Cell and Developmental Biology | last1 = Romanova | first1 = Daria | last2 = Varoqueaux | first2 = Frederique | last3 = Daraspe | first3 = Jean | last4 = Nikitin | first4 = Mikhail | last5 = Eitel | first5 = Michael | last6 = Fasshauer | first6 = Dirk | last7 = Moroz | first7 = Leonid | pmid=35223848 | pmc=8864292 | doi-access=free }}</ref>