Trichoplax was discovered in 1883 by the German zoologistFranz Eilhard Schulze, in a seawater aquarium at the Zoological Institute inGraz, Austria.[10][16] The generic name is derived from the classical Greekθρίξ (thrix), meaning "hair", andπλάξ (plax), "plate". The specific epithetadhaerens is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination.[17] Schulze realized that the animal could not be a member of any existing phyla, and based on the simple structure and behaviour, concluded in 1891 that it must be an early metazoan. He also observed the reproduction by fission, cell layers and locomotion.[18]
In 1893, Italian zoologistFrancesco Saverio Monticelli described another animal which he namedTreptoplax, the specimens of which he collected from Naples. He gave the species nameT. reptans in 1896.[19] Monticelli did not preserve them and no other specimens were found again, as a result of which the identification is ruled as doubtful, and the species rejected.[20][21]
Schulze's description was opposed by other zoologists. For instance, in 1890, F.C. Noll argued that the animal was a flat worm (Turbellaria).[22] In 1907, Thilo Krumbach published a hypothesis thatTrichoplax is not a distinct animal but that it is a form of the planula larva of theanemone-likehydrozoanEleutheria 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 aboutTrichoplax until the 1960s.[17]
The development ofelectron microscopy in the mid-20th century allowed in-depth observation of the cellular components of organisms, following which there was renewed interest inTrichoplax starting in 1966.[23] The most important descriptions were made byKarl Gottlieb Grell at the University of Tübingen since 1971.[24][25] That year, Grell revived Schulze's interpretation that the animals are unique and created a new phylum Placozoa.[26][17] Grell derived the name from the placula hypothesis,Otto Bütschli's notion on theorigin of metazoans.[27]
Placozoans do not have well-defined body plans, much likeamoebas, unicellular eukaryotes. As Andrew Masterson reported: "they are as close as it is possible to get to being simply a little living blob."[28] An individual body measures about 0.55 mm in diameter.[29] 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."[30] Animals studied in laboratories have bodies consisting of everything from hundreds to millions of cells.[31]
Placozoans have only three anatomical parts as tissue layers inside its body: the upper, intermediate (middle) and lowerepithelia. There are at least six different cell types.[32] The upper epithelium is the thinnest portion and essentially comprises flat cells with their cell body hanging underneath the surface, and each cell having acilium.[33] Crystal cells are sparsely distributed near the marginal edge. A few cells have unusually large number ofmitochondria.[32] The middle layer is the thickest made up of numerous fiber cells, which contain mitochondrial complexes, vacuoles andendosymbiotic bacteria in theendoplasmic 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 asecretory granule.[34][33]
The body axes ofHoilungia andTrichoplax are overtly similar to the oral–aboral axis ofcnidarians,[35] animals from another phylum with which they are most closely related.[36] Structurally, they can not be distinguished from other placozoans, so that identification is purely on genetic (mitochondrial DNA) differences.[37] 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-celledamoeba, 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 simpleepithelium enclosing a loose sheet of stellate cells resembling themesenchyme of some more complex animals. The epithelial cells bearcilia, which the animal uses to help it creep along the seafloor.[11]
The lower surface engulfs small particles of organic detritus, on which the animalfeeds. All placozoans can reproduce asexually, budding off smaller individuals, and the lower surface may also bud off eggs into themesenchyme.[11]Sexual reproduction has been reported to occur in oneclade of placozoans,[38][39] whose strain H8 was later found to belong to genusCladtertia,[2] whereintergenic recombination was observed as well as other hallmarks of sexual reproduction.
SomeTrichoplax species containRickettsiales bacteria asendosymbionts.[40]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 describedMargulisbacteria, that lives inside cells used foralgal digestion. It appears to eat the fats and other lipids of the algae and provide its host with vitamins and amino acids in return.[41][42]
Studies suggest that aragonite crystals in crystal cells have the same function as statoliths, allowing it to use gravity forspatial orientation.[43]
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.[44][45]
The Placozoa show substantial evolutionary radiation in regard tosodium channels, of which they have 5–7 different types, more than any other invertebrate species studied to date.[47]
Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies.[48]
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.[49]
There is no convincing fossil record of the Placozoa, although theEdiacaran biota (Precambrian,550 million years ago) organismDickinsonia appears somewhat similar to placozoans.[50] Knaust (2021) reported preservation of placozoan fossils in a microbialite bed from theMiddle TriassicMuschelkalk (Germany).[1]
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 and thus 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 thesponges and theEumetazoa.[51] In such a feature-poor phylum, molecular data are considered to provide the most reliable approximation of the placozoans' phylogeny.
Their exact position on thephylogenetic 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 asister group; once in theCtenophora, once in theCnidaria and once in theBilateria. If they branched off before the Cnidaria and Bilateria split, the neurons and muscles would have the same origin in the two latter groups.
The Placozoa descending side by side with the sponges, cnidarians and ctenophores from a gallertoid by processes of differentiation A placozoan is a small, flattened animal, typically about one mm across and about 25 μm thick. Like theamoebae they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form which may facilitate movement.Trichoplax lacks tissues and organs. There is no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers.[52]
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:
Metazoa
Placozoa
Sponges (Porifera)
Animals with tissues (Eumetazoa)
According to a functional-morphology model, all or most animals are descended from agallertoid, a free-living (pelagic) sphere in seawater, consisting of a singleciliated layer of cells supported by a thin, noncellular separating layer, thebasal lamina. The interior of thesphere is filled with contractile fibrous cells and a gelatinousextracellular matrix. Both the modern Placozoa and all other animals then descended from this multicellular beginning stage via two different processes:[53]
Infolding of theepithelium led to the formation of an internal system of ducts and thus to the development of a modified gallertoid from which the sponges (Porifera),Cnidaria andCtenophora subsequently developed.
Other gallertoids, according to this model, made the transition over time to abenthic mode of life; that is, their habitat has shifted from the open ocean to the floor (benthic zone). This results naturally in aselective advantage for flattening of the body, as of course can be seen in many benthic species.
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 thesubstrate, 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 termedplaculoid.
Three different life strategies have accordingly led to three different possible lines of development:
Animals that live interstitially in the sand of the ocean floor were responsible for the fossil crawling traces that are considered the earliest evidence of animals; and are detectable even prior to the dawn of theEdiacaran Period ingeology. These are usually attributed tobilaterally symmetrical worms, but the hypothesis presented here views animals derived from placuloids, and thus close relatives ofTrichoplax adhaerens, to be the producers of the traces.
Animals that incorporatedalgae as photosynthetically activeendosymbionts, i.e. primarily obtaining their nutrients from their partners insymbiosis, were accordingly responsible for the mysterious creatures of the Ediacara fauna that are not assigned to any modern animal taxon and lived during the Ediacaran Period, before the start of thePaleozoic. However, recent work has shown that some of the Ediacaran assemblages (e.g.Mistaken Point) were in deep water, below thephotic zone, and hence those individuals could not dependent on endosymbioticphotosynthesisers.
Animals that grazed onalgal mats would ultimately have been the direct ancestors of the Placozoa. The advantages of an amoeboid multiplicity of shapes thus allowed a previously present basal lamina and a gelatinousextracellular matrix to be lostsecondarily. Pronounced differentiation between the surface facing the substrate (ventral) and the surface facing away from it (dorsal) accordingly led to the physiologically distinct cell layers ofTrichoplax adhaerens that can still be seen today. Consequently, these areanalogous, but nothomologous, toectoderm andendoderm – the "external" and "internal" cell layers in eumetazoans – i.e. the structures corresponding functionally to one another have, according to the proposed hypothesis, no common evolutionary origin.
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 theEdiacaran fauna, or even the pre-Ediacara fauna. Although very successful in theirecological niche, due to the absence of extracellular matrix andbasal lamina, the development potential of these animals was of course limited, which would explain the low rate of evolution of theirphenotype (their outward form as adults) – referred to asbradytely.[citation needed]
This hypothesis was supported by a recent analysis of theTrichoplax adhaerensmitochondrialgenome in comparison to those of other animals.[54] The hypothesis was, however, rejected in a statistical analysis of theTrichoplax adhaerens whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species, but only atthep = 0.07 level, which indicates a marginal level of statistical significance.[51]
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 theEpitheliozoa, is itself construed to be a sister group to the sponges (Porifera):
The above view could be correct, although there is some evidence that thectenophores, traditionally seen asEumetazoa, may be the sister to all other animals.[55]This is now a disputed classification.[56] Placozoans are estimated to have emerged 750–800 million years ago, and the first modern neuron to have originated in the common ancestor of cnidarians and bilaterians about 650 million years ago (many of the genes expressed in modern neurons are absent in ctenopheres, although some of these missing genes are present in placozoans).[57][58]
The principal support for such a relationship comes from special cell to cell junctions – beltdesmosomes – that occur not just in the Placozoa but in all animalsexcept 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 evolutionarily derived features (apomorphies), and thus form the basis of a common taxon for all animals that possess them.[citation needed]
One possible scenario inspired by the proposed hypothesis starts with the idea that the monociliated cells of the epitheloid inTrichoplax adhaerens evolved by reduction of the collars in the collar cells (choanocytes) of sponges as the hypothesized 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.[citation needed]
In contrast to the model based on functional morphology described earlier, in the Epitheliozoa hypothesis, 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 digestivegastrodermis in the Cnidaria or the gut epithelium in the bilaterally symmetrical animals (Bilateria) may have developed from endoderm, whereas ectoderm is the precursor to the external skin layer (epidermis), among other things. The interior space pervaded by a fiber syncytium in the Placozoa would then correspond to connective tissue in the other animals. It is unclear whether the calcium ions stored in the syncytium would be related to the lime skeletons of many cnidarians.[citation needed]
As noted above, this hypothesis was supported in a statistical analysis of theTrichoplax adhaerens whole genome sequence, as compared to the whole-genome sequences of six other animals and two related non-animal species.[51]
A third hypothesis, based primarily on molecular genetics, views the Placozoa as highly simplifiedeumetazoans. 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 theepithelium, were accordingly lost more recently by radical secondary simplification.[59]
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 theCnidaria, while in another they would be a sister group to theCtenophora, and occasionally they are placed directly next to theBilateria. Currently, they are typically placed according to the cladogram below:[60]
In this cladogram theEpitheliozoa and Eumetazoa are synonyms to each other and to theDiploblasts, and theCtenophora 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 only known for parasitic organisms, but would be difficult to explain functionally in a free-living species likeTrichoplax adhaerens.[citation needed]
This version is supported by statistical analysis of theTrichoplax 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 placozoans outside of the sampled Eumetazoans.[51][61]
DNA comparisons suggest that placozoans are related toCnidaria, derived fromplanula larva (as seen in some Cnidaria).[62] TheBilateria also are thought to be derived from planuloids.[63][64][65][66][67][68][69][70] The Cnidaria and Placozoa body axis are overtly similar, and placozoan and cnidarian cells are responsive to the sameneuropeptideantibodies despite extant placozoans not developing any neurons.[71][72]
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