The genus Trichoplax includes just a single described species, T. adhaerens, which is also the only described species in the phylum Placozoa, although recent work suggests that there may be many unrecognized placozoan species (Eitel and Schierwater 2010 and references therein). Trichoplax adhaerens is the structurally simplest metazoan (multicellular animal). It is quite small (just 2 to 3 mm in diameter) and consists of several thousand cells arranged as a double-layered plate. It lacks anterior-posterior polarity and symmetry. However, the cells of the upper and lower layers differ in shape and there is a consistent dorsal-ventral orientation of the body relative to the substrate. Trichoplax moves by ciliary gliding, changing its shape along the edges as it moves, like an amoeba. Very small (presumably young) individuals can swim, but larger individuals crawl. It appears that Trichoplax feeds by phagocytosis of organic detritus. (Brusca and Brusca 2003)
Although the discovery and description of Trichoplax adhaerens in 1883 (in saltwater tanks in Austria) sparked controversy among zoologists about what its significance might be with respect to inferring the characteristics of early metazoans, interest faded away as the (incorrect) idea took hold that these enigmatic organisms were simply abnormal larvae of hydrozoan cnidarians. Although strong evidence against this view was published in 1912 and 1914, there was no more discussion of Trichoplax in the zoological literature--nor citation of the key critiques of 1912 and 1914-- for more than half a century (although the misconception that it was a cnidarian was repeated in textbooks). Beginning in the 1960s, several researchers re-focused attention on Trichoplax, demonstrating that it is an adult form of a new phylum dubbed, in 1971, Placozoa. It became clear that placozoans could be found worldwide in the shallow waters of subtropical and tropical regions (Pearse 1989; Eitel and Schierwater 2010). (Syed and Schierwater 2002) Recent data all indicate that the Placozoa represent one of the earliest branching lineages in the metazoan tree, but the exact placement of this branch remains uncertain (Schierwater et al. 2009; Ball and Miller 2010 and references therein).
In recent years it has become clear that placozoans can be found worldwide in the shallow waters of subtropical and tropical regions (Pearse 1989; Eitel and Schierwater 2010).
Trichoplax adhaerens, the only described species in the phylum Placozoa, is the structurally simplest metazoan (multicellular animal). It is quite small (just 2 to 3 mm in diameter) and consists of several thousand cells arranged as a double-layered plate. It lacks anterior-posterior polarity and symmetry. However, the cells of the upper and lower layers differ in shape and there is a consistent dorsal-ventral orientation of the body relative to the substrate. Trichoplax moves by ciliary gliding, changing its shape along the edges as it moves, like an amoeba. Very small (presumably young) individuals can swim, but larger individuals crawl.
(Brusca and Brusca 2003)
It appears that Trichoplax adhaerens, the only described species in the phylum Placozoa, feeds by phagocytosis of organic detritus (Brusca and Brusca 2003).
Although the discovery and description of Trichoplax adhaerens in 1883 (in saltwater tanks in Austria) sparked controversy among zoologists about what its significance might be with respect to inferring the characteristics of early metazoans, interest faded away as the (incorrect) idea took hold that these enigmatic organisms were simply abnormal larvae of hydrozoan cnidarians. Although strong evidence against this view was published in 1912 and 1914, there was no more discussion of Trichoplax in the zoological literature--nor citation of the key critiques of 1912 and 1914-- for more than half a century (although the misconception that it was a cnidarian was repeated in textbooks). Beginning in the 1960s, several researchers re-focused attention on Trichoplax, demonstrating that it is an adult form of a new phylum dubbed, in 1971, Placozoa. Recent data all indicate that the Placozoa represent one of the earliest branching lineages in the metazoan tree, but the exact placement of this branch remains uncertain. Trichoplax adhaerens is the only described species in the phylum Placozoa.
(Syed and Schierwater 2002; Schierwater et al. 2009; Ball and Miller 2010 and references therein)
Although recent work (Eitel and Schierwater 2010 and references therein) suggests that there may be many unrecognized placozoan species, as of 2010 there was still just a single described species in the phylum Placozoa, Trichoplax adhaerens (see this species page for additional information).
Placozoa (/plækəˈzoʊə/, "flat animals"[2]) is a phylum of the simple animals[3] that are marine and free-living (non-parasitic).[4] Placozoans are simply blob-like animals without any body part or organ, and are merely aggregates of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, they are described as "the simplest animals on Earth."[5] Structural and molecular analyses have supported them as among the most basal animals,[6][7] thus, constituting the most primitive metazoan phylum.[8]
The first known placozoan, Trichoplax adhaerens, was discovered in 1883 by the German zoologist Franz Eilhard Schulze (1840–1921).[9][10] Describing the uniqueness, another German, Karl Gottlieb Grell (1912–1994), erected a new phylum, Placozoa, for it in 1971. Remaining a monotypic phylum for over a century,[11][12] new species began to be added since 2018. So far, three other species have been described: Hoilungia hongkongensis (in 2018), Polyplacotoma mediterranea (in 2019), which is the most basal,[13] and Cladtertia collaboinventa (in 2022).[14]
Trichoplax was discovered in 1883 by the German zoologist Franz Eilhard Schulze, in a seawater aquarium at the Zoological Institute in Graz, Austria.[9][15] The generic name is derived from the classical Greek θρίξ (thrix), meaning "hair", and πλάξ (plax), "plate". The specific epithet adhaerens is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination.[16] Schulze realized that the animal could not be a member of any existing phyla, and based on the simple structure and behavour, concluded in 1891 that it must be an early metazoan. He also observed the reproduction by fission, cell layers and locomotion.[17]
In 1893, Italian zoologist Francesco Saverio Monticelli described another animal which he named Treptoplax, the specimens of which he collected from Naples. He gave the species name T. reptans in 1896.[18] Monticelli did not preserve them and no other speciems were found again, as a result of which the indentification is ruled as doubful, and the species rejected.[19][20]
Schulze's description was opposed by other zoologists. For instance, in 1890, F.C. Noll argued that the animal was a flat worm (Turbellaria).[21] In 1907, Thilo Krumbach published a hypothesis that Trichoplax is not a distinct animal but that it is a form of the planula larva of the anemone-like hydrozoan Eleutheria krohni. Although this was refuted in print by Schulze and others, Krumbach's analysis became the standard textbook explanation, and nothing was printed in zoological journals about Trichoplax until the 1960s.[16]
The development of electron microscopy in the mid-20th allowed in-depth observation of the cellular components of organism, following which there was renewed interest in Trichoplax since 1966.[22] The most important descriptions were made by Karl Gottlieb Grell at the University of Tübingen since 1971.[23][24] That year, Grell revived Schulze's interpretation that the animals are unique and created a new phylum Placozoa.[25][16] Grell derived the name from the placula hypothesis, Otto Bütschli's notion on the origin of metazoans.[26]
Placozoans do not have well-defined body plan 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."[27] An individual body measures about 0.55 mm in diameter.[28] 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."[29]
Placozoans have only three anatomical parts as tissue layers inside its body: the upper, intermediate (middle) and lower epithelia. There are at least six different cell types.[30] 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.[31] Crystal cells are sparsely distributed near the marginal edge. Few cells have unusually large number of mitochondria.[30] The middle layer is the thickest made up of numerous fiber cells, which contain mitochondrial complexes, vacuoles and 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.[32][31]
The body axes of Hoilungia and Trichoplax are overtly similar to the oral–aboral axis of cnidarians,[33] animals from another phylum with which they are most closely related.[34] Structurally, they can not be distinguished from other placozoans, so that identification is purely on genetic (mitochondrial DNA) differences.[35] Genome sequencing has shown that each species has a set of unique genes and several uniquely missing genes.[14]
Trichoplax is a small, flattened, animal around 1 mm (0.039 in) across. An amorphous multi-celled body, analogous to a single-celled 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.[10]
The lower surface engulfs small particles of organic detritus, on which the animal feeds. All placozoa can reproduce asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the mesenchyme.[10] Sexual reproduction has been reported to occur in one clade of placozoans,[36][37] whose strain H8 was later found to belong to genus Cladtertia,[1] where intergenic recombination was observed as well as other hallmarks of sexual reproduction.
Some Trichoplax species contain Rickettsiales bacteria as endosymbionts.[38] One of the at least 20 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 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.[39]
The Placozoa show substantial evolutionary radiation in regard to sodium channels, of which they have 5–7 different types, more than any other invertebrate species studied to date.[41]
Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies. [42]
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.[43]
There is no convincing fossil record of the placozoa, although the Ediacaran biota (Precambrian, 550 million years ago) organism Dickinsonia appears somewhat similar to placozoans.[44] Knaust (2021) reported preservation of placozoan fossils in a microbialite bed from the Middle Triassic Muschelkalk (Germany).[45]
Traditionally, classification was based on their level of organization, i.e., they possess no tissues or organs. However this may be as a result of secondary loss, so is inadequate to exclude them from relationships with more complex animals. More recent work has attempted to classify them based on the DNA sequences in their genome; this has placed the phylum between the sponges and the eumetazoa.[46] In such a feature-poor phylum, molecular data are considered to provide the most reliable approximation of the placozoans' phylogeny.
Their exact position on the phylogenetic tree would give important information about the origin of neurons and muscles. If the absence of these features is an original trait of the Placozoa, it would mean that a nervous system and muscles evolved three times should placozoans and cnidarians be a sister group; once in the Ctenophora, once in the Cnidaria and once in the Bilateria. If they branched off before the Cnidaria and Bilateria split, the neurons and muscles would have the same origin in the two latter groups.
On the basis of their simple structure, the Placozoa were frequently viewed as a model organism for the transition from unicellular organisms to the multicellular animals (Metazoa) and are thus considered a sister taxon to all other metazoans:
MetazoaPlacozoa
Sponges (Porifera)
Animals with tissues (Eumetazoa)
According to a functional-morphology model, all or most animals are descended from a gallertoid, a free-living (pelagic) sphere in seawater, consisting of a single ciliated layer of cells supported by a thin, noncellular separating layer, the basal lamina. The interior of the sphere is filled with contractile fibrous cells and a gelatinous extracellular matrix. Both the modern Placozoa and all other animals then descended from this multicellular beginning stage via two different processes:[48]
While the probability of encountering food, potential sexual partners, or predators is the same in all directions for animals floating freely in the water, there is a clear difference on the seafloor between the functions useful on body sides facing toward and away from the substrate, leading their sensory, defensive, and food-gathering cells to differentiate and orient according to the vertical – the direction perpendicular to the substrate. In the proposed functional-morphology model, the Placozoa, and possibly several similar organisms only known from the fossils, are descended from such a life form, which is now termed placuloid.
Three different life strategies have accordingly led to three different possible lines of development:
Should any of the analyses presented above turn out to be correct, Trichoplax adhaerens would be the oldest branch of the multicellular animals, and a relic of the Ediacaran fauna, or even the pre-Ediacara fauna. Although very successful in their ecological niche, due to the absence of extracellular matrix and basal lamina, the development potential of these animals was of course limited, which would explain the low rate of evolution of their phenotype (their outward form as adults) – referred to as bradytely.
This hypothesis was supported by a recent analysis of the Trichoplax adhaerens mitochondrial genome in comparison to those of other animals.[49] The hypothesis was, however, rejected in a statistical analysis of the Trichoplax adhaerens whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species, but only at the p = 0.07 level, which indicates a marginal level of statistical significance.[46]
A concept based on purely morphological characteristics pictures the Placozoa as the nearest relative of the animals with true tissues (Eumetazoa). The taxon they share, called the Epitheliozoa, is itself construed to be a sister group to the sponges (Porifera):
MetazoaPorifera
EpitheliozoaPlacozoa
Eumetazoa
The above view could be correct, although there is some evidence that the ctenophores, traditionally seen as Eumetazoa, may be the sister to all other animals.[50] This is now a disputed classification.[51]
The principal support for such a relationship comes from special cell/cell junctions, the belt desmosomes, that occur not just in the Placozoa but in all animals except the sponges; they enable the cells to join together in an unbroken layer like the epitheloid of the Placozoa. Trichoplax adhaerens also shares the ventral gland cells with most eumetazoans. Both characteristics can be considered apomorphies, i.e. evolutionarily derived features, and thus form the basis of a common taxon for all animals that possess them.
One possible scenario inspired by the proposed hypothesis starts with the idea that the monociliated cells of the epitheloid in Trichoplax adhaerens evolved by reduction of the collars in the collar cells (choanocytes) of sponges as the ancestors of the Placozoa abandoned a filtering mode of life. The epitheloid would then have served as the precursor to the true epithelial tissue of the eumetazoans.
In contrast to the model based on functional morphology described earlier, in the Epitheliozoa concept the ventral and dorsal cell layers of the Placozoa are homologs of endoderm and ectoderm, the two basic embryonic cell layers of the eumetazoans — the digestive gastrodermis in the Cnidaria or the gut epithelium in the bilaterally symmetrical Bilateria may have developed from endoderm, whereas ectoderm is, among other things, the precursor to the external skin layer (epidermis). The interior space pervaded by a fiber syncytium in the Placozoa would then correspond to connective tissue in the other animals. It is uncertain whether the calcium ions stored in the syncytium are related to the lime skeletons of many cnidarians.
As noted above, this hypothesis was supported in a statistical analysis of the Trichoplax adhaerens whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species.[46]
A third hypothesis, based primarily on molecular genetics, views the Placozoa as highly simplified eumetazoans. According to this, Trichoplax adhaerens is descended from considerably more complex animals that already had muscles and nerve tissues. Both tissue types, as well as the basal lamina of the epithelium, were accordingly lost more recently by radical secondary simplification.[52]
Various studies in this regard so far yield differing results for identifying the exact sister group: in one case the Placozoa would qualify as the nearest relatives of the Cnidaria, while in another they would be a sister group to the Ctenophora, and occasionally they are placed directly next to the Bilateria. Currently, they are typically placed according to the cladogram below:
Metazoa Eumetazoa / Diploblasts / Epitheliozoa ParaHoxozoaPlacozoa
PlanulozoaIn this cladogram the Epitheliozoa and Eumetazoa are synonyms to each other and to the Diploblasts, and the Ctenophora are basal to them.
An argument raised against the proposed scenario is that it leaves morphological features of the animals completely out of consideration. The extreme degree of simplification that would have to be postulated for the Placozoa in this model, moreover, is known only for parasitic organisms but would be difficult to explain functionally in a free-living species like Trichoplax adhaerens.
This version is supported by statistical analysis of the Trichoplax adhaerens whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species. However, ctenophora was not included in the analyses, placing the placozoas outside of the sampled Eumetazoans.[46][53]
DNA comparisons suggest that placozoans are related to Cnidaria, derived from planula larva (as seen in some Cnidaria).[54] The Bilateria also are thought to be derived from planuloids.[55][56][57][58][59][60][61][62] The Cnidaria and Placozoa body axis are overtly similar, and Placozoan and Cnidarian cells are responsive to the same neuropeptide antibodies despite extant Placozoa not developing any neurons.[63][64]
Choanozoa Animalia Eumetazoa ParaHoxozoaPlacozoa 
Bilateria/Triploblasts 
{{cite journal}}: CS1 maint: url-status (link) {{cite press release}}: CS1 maint: url-status (link) Placozoa (/plækəˈzoʊə/, "flat animals") is a phylum of the simple animals that are marine and free-living (non-parasitic). Placozoans are simply blob-like animals without any body part or organ, and are merely aggregates of cells. Moving in water by ciliary motion, eating food by engulfment, reproducing by fission or budding, they are described as "the simplest animals on Earth." Structural and molecular analyses have supported them as among the most basal animals, thus, constituting the most primitive metazoan phylum.
The first known placozoan, Trichoplax adhaerens, was discovered in 1883 by the German zoologist Franz Eilhard Schulze (1840–1921). Describing the uniqueness, another German, Karl Gottlieb Grell (1912–1994), erected a new phylum, Placozoa, for it in 1971. Remaining a monotypic phylum for over a century, new species began to be added since 2018. So far, three other species have been described: Hoilungia hongkongensis (in 2018), Polyplacotoma mediterranea (in 2019), which is the most basal, and Cladtertia collaboinventa (in 2022).
