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''Trichoplax adhaerens'' is one of the three named species in the
phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...
Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical ''Trichoplax adhaerens'', ''Hoilungia hongkongensis'', an ...
. The others are ''
Hoilungia hongkongensis ''Hoilungia hongkongensis'' is a species in the phylum Placozoa. The organism appears superficially similar to ''Trichoplax adhaerens'', but genetic analysis of its mitochondrial DNA shows numerous differences. It was discovered in brackish wat ...
'' and ''
Polyplacotoma mediterranea ''Polyplacotoma mediterranea'' is a species in the phylum Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: ...
''. The Placozoa is a basal group of multicellular
animal Animals are multicellular, eukaryotic organisms in the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, are Motilit ...
s (metazoa). ''Trichoplax'' are very flat organisms around a millimetre in diameter, lacking any
organs In biology, an organ is a collection of tissues joined in a structural unit to serve a common function. In the hierarchy of life, an organ lies between tissue and an organ system. Tissues are formed from same type cells to act together in a f ...
or internal structures. They have two cellular layers: the top epitheloid layer is made of ciliated «cover cells» flattened toward the outside of the organism, and the bottom layer is made up of cylinder cells that possess
cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
used in locomotion, and gland cells that lack cilia. Between these layers is the fibre
syncytium A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...
, a liquid-filled cavity strutted open by star-like fibres. ''Trichoplax'' feed by absorbing food particles—mainly
microbe A microorganism, or microbe,, ''mikros'', "small") and ''organism'' from the el, ὀργανισμός, ''organismós'', "organism"). It is usually written as a single word but is sometimes hyphenated (''micro-organism''), especially in olde ...
s—with their underside. They generally reproduce asexually, by dividing or
budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is know ...
, but can also
reproduce sexually Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote tha ...
. Though ''Trichoplax'' has a small genome in comparison to other animals, nearly 87% of its 11,514 predicted protein-coding genes are identifiably similar to known genes in other animals.


Discovery

''Trichoplax'' was discovered in 1883 by the German zoologist
Franz Eilhard Schulze Franz Eilhard Schulze (22 March 1840 – 2 November 1921) was a German anatomist and zoologist born in Eldena, near Greifswald. Biography He studied at the Universities of Bonn and Rostock. In 1863, he received his doctorate from Rostock, wher ...
, in a seawater aquarium at the Zoological Institute in
Graz, Austria Graz (; sl, Gradec) is the capital city of the Austrian state of Styria and second-largest city in Austria after Vienna. As of 1 January 2021, it had a population of 331,562 (294,236 of whom had principal-residence status). In 2018, the popula ...
. The generic name is derived from the classical Greek ('), "hair", and ('), "plate". The specific epithet ''adhaerens'' comes from Latin "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination. Although from the very beginning most researchers who studied ''Trichoplax'' in any detail realized that it had no close relationship to other animal phyla, the zoologist Thilo Krumbach published a hypothesis that ''Trichoplax'' is a form of the planula larva of the
anemone ''Anemone'' () is a genus of flowering plants in the buttercup family Ranunculaceae. Plants of the genus are commonly called windflowers. They are native to the temperate and subtropical regions of all continents except Australia, New Zealand an ...
-like
hydrozoan Hydrozoa (hydrozoans; ) are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the specialize ...
''Eleutheria krohni'' in 1907. 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. In the 1960s and 1970s a new interest among researchers led to acceptance of
Placozoa The Placozoa are a basal form of marine free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical ''Trichoplax adhaerens'', ''Hoilungia hongkongensis'', an ...
as a new animal phylum. Among the new discoveries was study of the early phases of the animals' embryonic development and evidence that the animals that people had been studying are adults, not larvae. This newfound interest also included study of the organism in nature (as opposed to aquariums).


Morphology

''Trichoplax'' generally has a thinly flattened, plate-like body in cross-section around half a millimetre, occasionally up to two or three millimetres. The body is usually only about 25 µm thick. These colorlessly gray organisms are so thin they are transparent when illuminated from behind, and in most cases are barely visible to the naked eye. Like the single-celled
amoebae An amoeba (; less commonly spelled ameba or amœba; plural ''am(o)ebas'' or ''am(o)ebae'' ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and retracting pseudopo ...
, which they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form. These may facilitate movement to new habitats. ''Trichoplax'' lacks tissues and organs; there is also 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: dorsal epithelia cells and ventral epithelia cells, each with a single cilium ("monociliate"), ventral gland cells, syncytial fiber cells, lipophils, and crystal cells (each containing a birefringent crystal, arrayed around the rim). Lacking sensory and muscle cells, it moves using
cilia The cilium, plural cilia (), is a membrane-bound organelle found on most types of eukaryotic cell, and certain microorganisms known as ciliates. Cilia are absent in bacteria and archaea. The cilium has the shape of a slender threadlike projecti ...
on its external surface.


Signal processing

There are no neurons present, but in the absence of a nervous system the animal use short chains of amino acids known as
peptide Peptides (, ) are short chains of amino acids linked by peptide bonds. Long chains of amino acids are called proteins. Chains of fewer than twenty amino acids are called oligopeptides, and include dipeptides, tripeptides, and tetrapeptides. A ...
s for cell communication, in a manner resembling the way animals with neurons use
neuropeptide Neuropeptides are chemical messengers made up of small chains of amino acids that are synthesized and released by neurons. Neuropeptides typically bind to G protein-coupled receptors (GPCRs) to modulate neural activity and other tissues like the ...
s for the same purpose. Individual cells contain and secrete a variety of small peptides, made up of between four and 20 amino acids, which are detected by neighbouring cells. Each peptide can be used individually to send a signal to other cells, but also sequentially or together in different combinations, creating a huge number a different types of signals. This allows for a relatively complex behavioural repertoire, including behaviours such as "crinkling", turning, flattening, and internal "churning".


Epitheloid

Both structurally and functionally, it is possible to distinguish a back or dorsal side from a belly or ventral side in ''Trichoplax adhaerens''. Both consist of a single layer of cells coated on the outside with slime and are reminiscent of
epithelial tissue Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellu ...
, primarily due to the junctions—belt
desmosome A desmosome (; "binding body"), also known as a macula adherens (plural: maculae adherentes) (Latin for ''adhering spot''), is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like adh ...
s—between the cells. In contrast to true
epithelium Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellul ...
, however, the cell layers of the Placozoa possess no ''
basal lamina The basal lamina is a layer of extracellular matrix secreted by the epithelial cells, on which the epithelium sits. It is often incorrectly referred to as the basement membrane, though it does constitute a portion of the basement membrane. The ba ...
'', which refers to a thin layer of extracellular material underlying epithelium that stiffens it and separates it from the body's interior. The absence of this structure, which is otherwise to be found in all animals except the sponges, can be explained in terms of function: a rigid separating layer would make the amoeboid changes in the shape of ''Trichoplax adhaerens'' impossible. Instead of an epithelium, therefore, we speak of an ''epitheloid'' in the Placozoa. A mature individual consists of up to a thousand cells that can be divided into four different cell types. The monociliated cells of the dorsal epitheloid are flattened and contain
lipid Lipids are a broad group of naturally-occurring molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include ...
bodies. The cells on the ventral side likewise possess a single cilium, while their elongated columnar shape, with a small cross section at the surface, packs them very closely together, causing the cilia to be very closely spaced on the ventral side and to form a ciliated "crawling sole". Interspersed among these ventral epithlioid cells are unciliated gland cells thought to be capable of synthesizing
digestive enzyme Digestive enzymes are a group of enzymes that break down polymeric macromolecules into their smaller building blocks, in order to facilitate their absorption into the cells of the body. Digestive enzymes are found in the digestive tracts of anim ...
s.


Fibre syncytium

Between the two layers of cells is a liquid-filled interior space, which, except for the immediate zones of contact with the ventral and dorsal sides, is pervaded by a star-shaped fibre
syncytium A syncytium (; plural syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell which can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus) ...
: a fibrous network that consists essentially of a single cell but contains numerous nuclei that, while separated by internal crosswalls (
septa The Southeastern Pennsylvania Transportation Authority (SEPTA) is a regional public transportation authority that operates bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly 4 million people in five coun ...
), do not have true
cell membrane The cell membrane (also known as the plasma membrane (PM) or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of all cells from the outside environment ( ...
s between them. Similar structures are also found in the sponges ( Porifera) and many
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...
. On both sides of the septa are liquid-filled capsules that cause the septa to resemble
synapse In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target effector cell. Synapses are essential to the transmission of nervous impulses from ...
s, i.e. nerve-cell junctions that occur in fully expressed form only in animals with tissues ( Eumetazoa). Striking accumulations of calcium ions, which may have a function related to the propagation of stimuli, likewise suggest a possible role as ''protosynapses''. This view is supported by the fact that fluorescent antibodies against cnidarian neurotransmitters, i.e. precisely those signal carriers that are transferred in synapses, bind in high concentrations in certain cells of ''Trichoplax adhaerens'', and thus indicate the existence of comparable substances in the Placozoa. The fibre syncytium also contains molecules of
actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of over ...
and probably also of
myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility. The first myosin ...
, which occur in the muscle cells of eu
metazoan Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
s . In the placozoans, they ensure that the individual fibres can relax or contract and thus help determine the animals' shape. In this way, the fibre syncytium assumes the functions of nerve and muscle tissues. Moreover, at least a portion of digestion occurs here. On the other hand, no gelatinous extracellular matrix exists of the kind observed, in ''
mesoglea Mesoglea refers to the extracellular matrix found in cnidarians like coral or jellyfish that functions as a hydrostatic skeleton. It is related to but distinct from mesohyl, which generally refers to extracellular material found in sponges. Desc ...
'', in
cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...
ns and ctenophores. ''
Pluripotent Pluripotency: These are the cells that can generate into any of the three Germ layers which imply Endodermal, Mesodermal, and Ectodermal cells except tissues like the placenta. According to Latin terms, Pluripotentia means the ability for many thin ...
'' cells, which can differentiate into other cell types, have not yet been demonstrated unambiguously in ''T. adhaerens'', in contrast to the case of the Eumetazoa. The conventional view is that dorsal and ventral epithelioid cells arise only from other cells of the same type.


Genetics

The ''Trichoplax'' genome contains about 98 million
base pair A base pair (bp) is a fundamental unit of double-stranded nucleic acids consisting of two nucleobases bound to each other by hydrogen bonds. They form the building blocks of the DNA double helix and contribute to the folded structure of both DNA ...
s and 11,514 predicted protein-coding genes. All nuclei of placozoan cells contain six pairs of
chromosome A chromosome is a long DNA molecule with part or all of the genetic material of an organism. In most chromosomes the very long thin DNA fibers are coated with packaging proteins; in eukaryotic cells the most important of these proteins are ...
s that are only about two to three micrometres in size. Three pairs are ''metacentric'', meaning that the
centromere The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers a ...
, the attachment point for the spindle fibers in cell division, is located at the center, or ''acrocentric'', with the centromere at an extreme end of each chromosome. The cells of the fiber syncytium can be ''
tetraploid Polyploidy is a condition in which the cells of an organism have more than one pair of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two sets of chromosomes, where each set contains ...
'', i.e. contain a quadruple complement of chromosomes. A single complement of chromosomes in ''Trichoplax adhaerens'' contains a total of fewer than fifty million base pairs and thus forms the smallest animal genome; the number of base pairs in the intestinal bacterium ''
Escherichia coli ''Escherichia coli'' (),Wells, J. C. (2000) Longman Pronunciation Dictionary. Harlow ngland Pearson Education Ltd. also known as ''E. coli'' (), is a Gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus ''Escher ...
'' is smaller by a factor of only ten. The genetic complement of ''Trichoplax adhaerens'' has not yet been very well researched; it has, however, already been possible to identify several genes, such as ''
Brachyury T-box transcription factor T, also known as Brachyury protein, is encoded for in humans by the ''TBXT'' gene. Brachyury functions as a transcription factor within the T-box family of genes. Brachyury homologs have been found in all bilaterian a ...
'' and ''
TBX2 T-box transcription factor 2 Tbx2 is a transcription factor that is encoded by the ''Tbx2'' gene on chromosome 17q21-22 in humans. This gene is a member of a phylogenetically conserved family of genes that share a common DNA-binding domain, the T ...
/
TBX3 T-box transcription factor TBX3 is a protein that in humans is encoded by the ''TBX3'' gene. T-box 3 (TBX3) is a member of the T-box gene family of transcription factors which all share a highly conserved DNA binding domain known as the T-box. Th ...
'', which are homologous to corresponding base-pair sequences in eumetazoans. Of particular significance is ''Trox-2'', a placozoan gene known under the name ''Cnox-2'' in cnidarians and as ''Gsx'' in the bilaterally symmetrical
Bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
. As a homeobox or
Hox gene Hox genes, a subset of homeobox genes, are a group of related genes that specify regions of the body plan of an embryo along the head-tail axis of animals. Hox proteins encode and specify the characteristics of 'position', ensuring that the cor ...
it plays a role in organization and differentiation along the axis of symmetry in the embryonic development of eumetazoans; in cnidarians, it appears to determine the position of mouth-facing (oral) and opposite-facing (aboral) sides of the organism. Since placozoans possess no axes of symmetry, exactly where the gene is transcribed in the body of ''Trichoplax'' is of special interest. Antibody studies have been able to show that the gene's product occurs only in the transition zones of the dorsal and ventral sides, perhaps in a fifth cell type that has not yet been characterized. It is not yet clear whether these cells, contrary to traditional views, are
stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...
s, which play a role in cell differentiation. In any case, ''Trox-2'' can be considered a possible candidate for a proto-Hox gene, from which the other genes in this important family could have arisen through gene duplication and variation. Initially, molecular-biology methods were applied unsuccessfully to test the various theories regarding Placozoa's position in the Metazoa system. No clarification was achieved with standard markers such as 18S rDNA/RNA: the marker sequence was apparently "garbled", i.e. rendered uninformative as the result of many mutations. Nevertheless, this negative result supported the suspicion that ''Trichoplax'' might represent an extremely primitive lineage of metazoans, since a very long period of time had to be assumed for the accumulation of so many mutations. Of the 11,514 genes identified in the six chromosomes of ''Trichoplax'', 87% are identifiably similar to genes in cnidarians and bilaterians. In those ''Trichoplax'' genes for which equivalent genes can be identified in the
human genome The human genome is a complete set of nucleic acid sequences for humans, encoded as DNA within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. These are usually treated separately as the n ...
, over 80% of the
introns An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of the cistron .e., gene. ...
(the regions within genes that are removed from RNA molecules before their sequences are translated in protein synthesis) are found in the same location as in the corresponding human genes. The arrangement of genes in groups on chromosomes is also conserved between the ''Trichoplax'' and human genomes. This contrasts to other model systems such as fruit flies and soil nematodes that have experienced a paring down of non-coding regions and a loss of the ancestral genome organizations.


Relationship with animals

The phylogenetic relationship between ''Trichoplax'' and other animals has been debated for some time. A variety of hypotheses have been advanced based on the few morphological characteristics of this simple organism that could be identified. More recently, a comparison of the ''Trichoplax''
mitochondria A mitochondrion (; ) is an organelle found in the Cell (biology), cells of most Eukaryotes, such as animals, plants and Fungus, fungi. Mitochondria have a double lipid bilayer, membrane structure and use aerobic respiration to generate adenosi ...
l genome suggested that ''Trichoplax'' is a basal
metazoa Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia. With few exceptions, animals consume organic material, breathe oxygen, are able to move, can reproduce sexually, and go through an ontogenetic stage in ...
n—less closely related to all other animals including sponges than they are to each other. This implies that the Placozoa would have arisen relatively soon after the evolutionary transition from unicellular to
multicellular A multicellular organism is an organism that consists of more than one cell, in contrast to unicellular organism. All species of animals, land plants and most fungi are multicellular, as are many algae, whereas a few organisms are partially uni- ...
forms. But an even more recent analysis of the much larger ''Trichoplax'' nuclear genome instead supports the hypothesis that ''Trichoplax'' is a basal eumetazoan, that is, more closely related to
Cnidaria Cnidaria () is a phylum under kingdom Animalia containing over 11,000 species of aquatic animals found both in freshwater and marine environments, predominantly the latter. Their distinguishing feature is cnidocytes, specialized cells that th ...
and other animals than any of those animals are to sponges. This is consistent with the presence in ''Trichoplax'' of cell layers reminiscent of epithelial tissue (see above).


Distribution and habitat

''Trichoplax'' was first discovered on the walls of a marine aquarium, and is rarely observed in its natural habitat. ''Trichoplax'' has been collected, among other places, in the Red Sea, the Mediterranean, and the Caribbean, off Hawaii, Guam, Samoa, Japan, Vietnam, Brazil, and Papua New Guinea, and on the Great Barrier Reef off the east coast of Australia. Field specimens tend to be found in the coastal
tidal zone The intertidal zone, also known as the foreshore, is the area above water level at low tide and underwater at high tide (in other words, the area within the tidal range). This area can include several types of habitats with various species o ...
s of tropical and subtropical seas, on such substrates as the trunks and roots of mangroves, shells of molluscs, fragments of stony corals or simply on pieces of rock. One study was able to detect seasonal population fluctuations, the causes of which have not yet been deduced.


Feeding and symbionts

''Trichoplax adhaerens'' feeds on small algae, particularly on green algae (
Chlorophyta Chlorophyta or Prasinophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it refers to ...
) of the genus ''
Chlorella ''Chlorella'' is a genus of about thirteen species of single-celled green algae belonging to the division Chlorophyta. The cells are spherical in shape, about 2 to 10 μm in diameter, and are without flagella. Their chloroplasts contain the ...
'', cryptomonads (
Cryptophyta The cryptophyceae are a class of algae, most of which have plastids. About 220 species are known, and they are common in freshwater, and also occur in marine and brackish habitats. Each cell is around 10–50 μm in size and flattened in shape, ...
) of the genera ''Cryptomonas'' and ''Rhodomonas'', and blue-green bacteria (
Cyanobacteria Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
) such as ''Phormidium inundatum'', but also on detritus from other organisms. In feeding, one or several small pockets form around particles of nutrients on the ventral side, into which digestive enzymes are released by the gland cells; the organisms thus develop a temporary "external stomach", so to speak. The enclosed nutrients are then taken up by
pinocytosis In cellular biology, pinocytosis, otherwise known as fluid endocytosis and bulk-phase pinocytosis, is a mode of endocytosis in which small molecules dissolved in extracellular fluid are brought into the cell through an invagination of the cell me ...
("cell-drinking") by the ciliated cells located on the ventral surface. Entire single-celled organisms can also be ingested through the upper epitheloid (that is, the "dorsal surface" of the animal). This mode of feeding could be unique in the animal kingdom: the particles, collected in a slime layer, are drawn through the intercellular gaps (cellular interstices) of the epitheloid by the fibre cells and then digested by
phagocytosis Phagocytosis () is the process by which a cell uses its plasma membrane to engulf a large particle (≥ 0.5 μm), giving rise to an internal compartment called the phagosome. It is one type of endocytosis. A cell that performs phagocytosis is ...
("cell-eating"). Such "collecting" of nutrient particles through an intact tegument is only possible because some "insulating" elements (specifically, a basal lamina under the epitheloid and certain types of cell-cell junctions) are not present in the Placozoa. Not all bacteria in the interior of Placozoa are digested as food: in the endoplasmic reticulum, an
organelle In cell biology, an organelle is a specialized subunit, usually within a cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as organs are to the body, hence ''organelle,'' the ...
of the fibre syncytium, bacteria are frequently found that appear to live in
symbiosis Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
with ''Trichoplax adhaerens''.Driscoll T, Gillespie JJ, Nordberg EK, Azad AF, Sobral BW (2013) Bacterial DNA sifted from the ''Trichoplax adhaerens'' (Animalia:Placozoa) genome project reveals a putative rickettsial endosymbiont. Genome Biol Evol


Locomotion

Placozoa can move in two different ways on solid surfaces: first, their ciliated crawling sole lets them glide slowly across the substrate; second, they can change location by modifying their body shape, as an amoeba does. These movements are not centrally coordinated, since no muscle or nerve tissues exist. It can happen that an individual moves simultaneously in two different directions and consequently divides into two parts. It has been possible to demonstrate a close connection between body shape and the speed of locomotion, which is also a function of available food: * At low nutrient density, the spread-out area fluctuates slightly but irregularly; speed remains relatively constant at about 15 micrometres per second. * If nutrient density is high, however, the area covered
oscillates Oscillation is the repetitive or periodic variation, typically in time, of some measure about a central value (often a point of equilibrium) or between two or more different states. Familiar examples of oscillation include a swinging pendulum ...
with a stable period of about 8 minutes, in which the greatest extent reached by the organism can be as much as twice the smallest. Its speed, which remains consistently below 5 micrometres per second, varies with the same period. In this case, a high speed always corresponds to a reduced area, and vice versa. Since the transition is not smooth but happens abruptly, the two modes of extension can be very clearly separated from one another. The following is a qualitative explanation of the animal's behavior: * At low nutrient density, ''Trichoplax'' maintains a constant speed in order to uncover food sources without wasting time. * Once such a source is identified by high nutrient density, the organism increases its area in regular increments and thereby enlarges the surface in contact with substrate. This enlarges the surface through which nutrients can be ingested. The animal reduces its speed at the same time in order to actually consume all of the available food. * Once this is nearly completed, ''Trichoplax'' reduces its area again to move on. Because food sources such as algal mats are often relatively extensive, it is reasonable for such an animal to stop moving after a brief period in order to flatten out again and absorb nutrients. Thus ''Trichoplax'' progresses relatively slowly in this phase. The actual ''direction'' in which ''Trichoplax'' moves each time is random: if we measure how fast an individual animal moves away from an arbitrary starting point, we find a linear relationship between elapsed time and mean square distance between starting point and present location. Such a relationship is also characteristic of random
Brownian motion Brownian motion, or pedesis (from grc, πήδησις "leaping"), is the random motion of particles suspended in a medium (a liquid or a gas). This pattern of motion typically consists of random fluctuations in a particle's position insi ...
of molecules, which thus can serve as a model for locomotion in the Placozoa. Small animals are also capable of swimming actively with the aid of their cilia. As soon as they come into contact with a possible substrate, a ''dorsoventral response'' occurs: the dorsal cilia continue to beat, whereas the cilia of ventral cells stop their rhythmic beating. At the same time, the ventral surface tries to make contact with the substrate; small protrusions and invaginations, the ''microvilli'' found on the surface of the columnar cells, help in attaching to the substrate via their adhesive action.


Regeneration

A notable characteristic of the Placozoa is that they can regenerate themselves from extremely small groups of cells. Even when large portions of the organism are removed in the laboratory, a complete animal develops again from the remainder. It is also possible to rub ''Trichoplax adhaerens'' through a strainer in such a manner that individual cells are not destroyed but are separated from one another to a large extent. In the test tube they then find their way back together again to form complete organisms. If this procedure is performed on several previously strained individuals simultaneously, the same thing occurs. In this case, however, cells that previously belonged to a particular individual can suddenly show up as part of another.


Reproduction

The Placozoa normally propagate asexually, dividing down the middle to produce two (or sometimes, three) roughly equal-sized daughters. These remain loosely connected for a while after fission. More rarely,
budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is know ...
processes are observed: spherules of cells separate from the dorsal surface; each of these combines all known cell types and subsequently grows into an individual on its own. Sexual reproduction is thought to be triggered by excessive population density. As a result, the animals absorb liquid, begin to swell, and separate from the substrate so that they float freely in the water. In the protected interior space, the ventral cells form an ovum surrounded by a special envelope, the fertilisation membrane; the ovum is supplied with nutrients by the surrounding syncytium, allowing energy-rich yolk to accumulate in its interior. Once maturation of the ovum is complete, the rest of the animal degenerates, liberating the ovum itself. Small, unciliated cells that form at the same time are interpreted to be spermatozoa. It has not yet been possible to observe fertilisation itself; the existence of the fertilisation membrane is currently taken to be evidence, however, that it has taken place. Putative eggs have been observed, but they degrade, typically at the 32–64 cell stage. Neither embryonic development nor sperm have been observed. Despite lack of observation of sexual reproduction in the lab, the genetic structure of the populations in the wild is compatible with the sexual reproduction mode, at least for species of the analysed genotype H5. Usually even before its liberation, the ovum initiates cleavage processes in which it becomes completely pinched through at the middle. A ball of cells characteristic of animals, the
blastula Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (from ...
, is ultimately produced in this manner, with a maximum of 256 cells. Development beyond this 256-cell stage has not yet been observed. Trichoplax lack a homologue of the Boule protein that appears to be ubiquitous and conserved in males of all species of other animals tested. If its absence implies the species has no males, then perhaps its "sexual" reproduction may be a case of the above-described process of regeneration, combining cells from two separate organisms into one. Due to the possibility of its cloning itself by asexual propagation without limit, the life span of Placozoa is infinite; in the laboratory, several lines descended from a single organism have been maintained in culture for an average of 20 years without the occurrence of sexual processes.


Role as a model organism

Long ignored as an exotic, marginal phenomenon, ''Trichoplax adhaerens'' is today viewed as a potential biological model organism. In particular, research is needed to determine how a group of cells that cannot be considered full-fledged epithelial tissue organizes itself, how locomotion and coordination occur in the absence of true muscle and nerve tissue, and how the absence of a concrete
body axis Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position prov ...
affects the animal's biology. At the genetic level, the way in which ''Trichoplax adhaerens'' protects against damage to its genome needs to be studied, particularly with regard to the existence of special DNA-repair processes. ''T. adhaerens'' can tolerate high levels of radiation damage that are lethal to other animals.Fortunato A, Fleming A, Aktipis A, Maley CC. Upregulation of DNA repair genes and cell extrusion underpin the remarkable radiation resistance of Trichoplax adhaerens. PLoS Biol. 2021 Nov 17;19(11):e3001471. doi: 10.1371/journal.pbio.3001471. PMID: 34788294; PMCID: PMC8635375 Tolerance to X-ray exposure was found to depend on expression of genes involved in
DNA repair DNA repair is a collection of processes by which a cell identifies and corrects damage to the DNA molecules that encode its genome. In human cells, both normal metabolic activities and environmental factors such as radiation can cause DNA dam ...
and
apoptosis Apoptosis (from grc, ἀπόπτωσις, apóptōsis, 'falling off') is a form of programmed cell death that occurs in multicellular organisms. Biochemical events lead to characteristic cell changes (morphology) and death. These changes incl ...
including the gene ''Mdm2''. Complete decoding of the genome should also clarify the placozoans' place in evolution, which continues to be controversial. Its ability to fight cancer through a combination of aggressive DNA repair and ejection of damaged cells makes it a promising organism for cancer research. In addition to basic research, this animal could also be suitable for studying wound-healing and regeneration processes; as yet unidentified metabolic products should be researched. Finally, ''Trichoplax adhaerens'' is also being considered as an animal model for testing compounds and antibacterial drugs.


Systematics

Francesco Saverio Monticelli described another species in 1893, which he found in the waters around Naples, naming it ''Treptoplax reptans''. However, it has not been observed since 1896, and most zoologists today doubt its existence. Significant genetic differences have been observed between collected specimens matching the morphological description of ''T. adhaerens'', suggesting that it may be a
cryptic species complex In biology, a species complex is a group of closely related organisms that are so similar in appearance and other features that the boundaries between them are often unclear. The taxa in the complex may be able to hybridize readily with each oth ...
. At least eight distinct genotypes (marked from H1 to H8) have been observed. Because great genetic differences often occur between representatives of ''Trichoplax adhaerens'', differences that in other taxa would result in their being spread among different genera, it is currently unclear whether the single species, based on morphological criteria, does not actually correspond to a group of '' cryptospecies'', i.e. species that are not outwardly distinguishable from one another. Distribution of the genetic variants is not a function of geography: some variants are found in multiple regions (e.g. Pacific, Caribbean and Red Sea). At the same time, very different genetic variants can be isolated from the same habitat.


References


Further reading

* Edward E. Ruppert, R. S. Fox, R. D. Barnes: ''Invertebrate Zoology – a functional evolutionary approach.'' ch. 5. Brooks/Cole, London 2004 (7th ed.), p. 94, * Richard C. Brusca, G. J. Brusca: ''Invertebrates.'' ch. 7. Sinauer Associates, Sunderland Mass 2002 (2nd ed.), p. 210,


Scientific literature

* * V. J. Birstein: "On the Karyotype of ''Trichoplax'' sp. (Placozoa)." in: ''Biologisches Zentralblatt.'' Fischer, Jena – Stuttgart 108 (1989), p. 63, * K. G. Grell, A. Ruthmann: "Placozoa." in: F. W. Harrison, J. A. Westfall (eds.): ''Microscopic Anatomy of Invertebrates.'' Vol. 2. Wiley-Liss, New York 1991, p. 13, * W. Jakob, S. Sagasser, S. Dellaporta, P. Holland, K. Kuhn, B. Schierwater: "The Trox-2 Hox/Para Hox gene of ''Trichoplax'' (Placozoa) marks an epithelial boundary." in: ''Development Genes and Evolution.'' Springer, Berlin 214 (2004), p. 170, * Y. K. Maruyama: "Occurrence in the Field of a Long-Term, Year-Round Stable Population of Placozoa." in: ''
The Biological Bulletin ''The Biological Bulletin'' is a peer-reviewed scientific journal covering the field of biology. The journal was established in 1897 as the ''Zoological Bulletin'' by Charles Otis Whitman and William Morton Wheeler. In 1899 the title was changed to ...
''. Laboratory, Woods Hole Mass 206:1 (2004), p. 55. * T. Syed, B. Schierwater: "The evolution of the Placozoa – A new morphological model." in: ''
Senckenbergiana Lethaea This is a list of academic journals, monographic series, and other serials published by E. Schweizerbart. A B C D E F G H I J K L M N P Q R S T U V W Z {{columns-list, colwidth=30em, *''Zeitschrift ...
'' Schweizerbart, Stuttgart 82:1 (2002), p. 315, * P. Schubert: "''Trichoplax adhaerens'' (Phylum Placozoa) has cells that react with antibodies against the neuropeptide RFAmide." in: ''Acta Zoologica.'' Blackwell Science, Oxford 74:2, (1993), p. 115, * T. Ueda, S. Koga, Y. K. Marayama: "Dynamic patterns in the locomotion and feeding behaviour by the placozoan ''Trichoplax adhaerens''." in: ''BioSystems.'' North-Holland Publ., Amsterdam 54 (1999), p. 65,


First descriptions

;Placozoa * K. G. Grell: "''Trichoplax adhaerens'', F. E. Schulze und die Entstehung der Metazoen." ("''Trichoplax adhaerens'', F.E. Schulze, and the evolution of the metazoans") in: ''Naturwissenschaftliche Rundschau.'' Wiss. Verl.-Ges., Stuttgart 24 (1971), p. 160, ;''Treptoplax reptans'' * F. S. Monticelli: "''Treptoplax reptans'' n. g., n. s." in: ''Rendiconti / Accademia Nazionale dei Lincei, Roma, Classe di Scienze Fisiche, Matematiche e Naturali.'' Rome 2:5 (1893), p. 39, ;''Trichoplax adhaerens'' * F. E. Schulze: "''Trichoplax adhaerens'' n. g., n. s." in: ''Zoologischer Anzeiger.'' Elsevier, Amsterdam-Jena 6 (1883), p. 92,


External links


Tree of life





UPI: "Scientists study genome of the Trichoplax", summary of a report in ''Nature''


* ttp://tolweb.org/onlinecontributors/app?service=external/ViewImageData&sp=35907 Video of Trichoplax in motion {{Authority control Placozoa Monotypic animal genera Articles containing video clips Parazoa