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''Trichoplax adhaerens'' is one of the four named species in the
phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...
Placozoa Placozoa ( ; ) is a phylum of free-living (non-parasitic) marine invertebrates. They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion, eating food by Phagocytosis, engulfment, reproducing by Fission (biol ...
. The others are '' Hoilungia hongkongensis'', ''
Polyplacotoma mediterranea ''Polyplacotoma mediterranea'' is a species in the phylum Placozoa, only representative of the genus ''Polyplacotoma'', and was discovered in the Mediterranean Sea The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surro ...
'' and '' Cladtertia collaboinventa''. Placozoa is a basal group of multicellular
animal Animals are multicellular, eukaryotic organisms in the Biology, biological Kingdom (biology), kingdom Animalia (). With few exceptions, animals heterotroph, consume organic material, Cellular respiration#Aerobic respiration, breathe oxygen, ...
s, possible relatives of
Cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
. ''Trichoplax'' are very flat organisms commonly less than 4 mm in diameter, lacking any
organs In a multicellular organism, 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 a ...
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 (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
used in locomotion, and gland cells that lack cilia. Between these layers is the fibre
syncytium A syncytium (; : syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell that can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus), i ...
, a liquid-filled cavity strutted open by star-like fibres. ''Trichoplax'' feed by absorbing food particles—mainly
microbe A microorganism, or microbe, is an organism of microscopic size, which may exist in its single-celled form or as a colony of cells. The possible existence of unseen microbial life was suspected from antiquity, with an early attestation in ...
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 kno ...
, but can also
reproduce sexually Sexual reproduction is a type of reproduction that involves a complex Biological life cycle, 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 p ...
. 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 () is the capital of the Austrian federal state of Styria and the second-largest city in Austria, after Vienna. On 1 January 2025, Graz had a population of 306,068 (343,461 including secondary residence). In 2023, the population of the Gra ...
. The generic name is derived from the classical Greek ('), "hair", and ('), "plate". The specific epithet ''adhaerens'' is Latin meaning "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-like
hydrozoan Hydrozoa (hydrozoans; from Ancient Greek ('; "water") and ('; "animals")) is 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 sp ...
''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 Placozoa ( ; ) is a phylum of free-living (non-parasitic) marine invertebrates. They are blob-like animals composed of aggregations of cells. Moving in water by ciliary motion, eating food by Phagocytosis, engulfment, reproducing by Fission (biol ...
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. Because they are so thin and fragile, and because the cilia which they use for locomotion are only loosely coordinated, they are constantly being split into two or three separate clones when their cilia moves in opposite directions, causing microfractures in the animal’s epithelium. One hypothesis is that the larger a motile animal lacking a nervous system is, the less coordinated its locomotion becomes, placing an upper limit on their possible size. 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; : amoebas (less commonly, amebas) or amoebae (amebae) ), often called an amoeboid, is a type of cell or unicellular organism with the ability to alter its shape, primarily by extending and r ...
, 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 (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...
on its external surface. The collective movements of the cilia are completely coordinated by mechanical interactions.


Signal processing

There are no neurons present, but in the absence of a nervous system the animal uses short chains of amino acids known as
peptide Peptides are short chains of amino acids linked by peptide bonds. A polypeptide is a longer, continuous, unbranched peptide chain. Polypeptides that have a molecular mass of 10,000 Da or more are called proteins. Chains of fewer than twenty am ...
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. These specialized cells are called peptidergic cells, but unlike neurons they don't use electrical impulses and their messaging is restricted to sending signals to other nearby cells only, as they're unable to both send and receive signals. 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". The genome of Trichoplax codes for eighty-five neurotransmitter receptors, more than in any other sequenced animal.


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 a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
, 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 ad ...
s—between the cells. In contrast to true
epithelium Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...
, 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 b ...
'', 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 organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
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 take part in the chemical process of digestion, which follows the mechanical process of digestion. Food consists of macromolecules of proteins, carbohydrates, and fats that need to be broken down chemically by digestive enzymes ...
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 (; : syncytia; from Greek: σύν ''syn'' "together" and κύτος ''kytos'' "box, i.e. cell") or symplasm is a multinucleate cell that can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus), i ...
: a fibrous network that consists essentially of a single cell but contains numerous nuclei that, while separated by internal crosswalls (
septa SEPTA, the Southeastern Pennsylvania Transportation Authority, is a regional public transportation authority that operates bus, rapid transit, commuter rail, light rail, and electric trolleybus services for nearly four million people througho ...
), do not have true
cell membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...
s between them. Similar structures are also found in the sponges (
Porifera Sponges or sea sponges are primarily marine invertebrates of the animal phylum Porifera (; meaning 'pore bearer'), a Basal (phylogenetics) , basal clade and a sister taxon of the Eumetazoa , diploblasts. They are sessility (motility) , sessile ...
) and many
fungi A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
. 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 allows a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or a target effector cell. Synapses can be classified as either chemical or electrical, depending o ...
s, i.e. nerve-cell junctions that occur in fully expressed form only in animals with tissues (
Eumetazoa Eumetazoa (), also known as Epitheliozoa or Histozoa, is a proposed basal animal subkingdom as a sister group of Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a eumetazoan ...
). 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 Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
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 ...
and probably also of
myosin Myosins () are a Protein family, family of motor proteins (though most often protein complexes) best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are adenosine triphosphate, ATP- ...
, 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, have myocytes and are able to move, can reproduce sexually, and grow from a ho ...
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 as well as ctenophores that functions as a hydrostatic skeleton. It is related to but distinct from mesohyl, which generally refers to extracellular material f ...
'', in
cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
ns and
ctenophore Ctenophora (; : ctenophore ) is a phylum of marine invertebrates, commonly known as comb jellies, that inhabit sea waters worldwide. They are notable for the groups of cilia they use for swimming (commonly referred to as "combs"), and they ar ...
s. ''
Pluripotent Cell potency is a cell's ability to differentiate into other cell types. The more cell types a cell can differentiate into, the greater its potency. Potency is also described as the gene activation potential within a cell, which like a continuum ...
'' 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 package of DNA containing part or all of the genetic material of an organism. In most chromosomes, the very long thin DNA fibers are coated with nucleosome-forming packaging proteins; in eukaryotic cells, the most import ...
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 fiber ...
, 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 two paired sets of ( homologous) chromosomes. Most species whose cells have nuclei (eukaryotes) are diploid, meaning they have two complete sets of chromosomes, one fro ...
'', 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. is a gram-negative, facultative anaerobic, rod-shaped, coliform bacterium of the genus '' Escherichia'' that is commonly fo ...
'' 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 and other apes by the ''TBXT'' gene. Brachyury functions as a transcription factor within the T-box, T-box family of genes. Brachyury Sequence homology, homo ...
'' and '' TBX2/ TBX3'', 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 Bilateria () is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal axis with a front (or "head") and a rear (or "tail") end, as well as a left� ...
. As a homeobox or
Hox gene Hox genes, a subset of homeobox, homeobox genes, are a gene cluster, group of related genes that Evolutionary developmental biology, specify regions of the body plan of an embryo along the craniocaudal axis, head-tail axis of animals. Hox protein ...
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 change into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type of cell ...
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 the DNA within each of the 23 distinct chromosomes in the cell nucleus. A small DNA molecule is found within individual Mitochondrial DNA, mitochondria. These ar ...
, over 80% of the
introns An intron is any Nucleic acid sequence, 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 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 cells of most eukaryotes, such as animals, plants and fungi. Mitochondria have a double membrane structure and use aerobic respiration to generate adenosine triphosphate (ATP), which is us ...
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, have myocytes and are able to move, can reproduce sexually, and grow from a hol ...
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 (biology), cell, unlike unicellular organisms. All species of animals, Embryophyte, land plants and most fungi are multicellular, as are many algae, whereas a few organism ...
forms. But an even more recent analysis of the much larger ''Trichoplax'' nuclear genome instead supports the hypothesis that ''Trichoplax'' is a basal
eumetazoa Eumetazoa (), also known as Epitheliozoa or Histozoa, is a proposed basal animal subkingdom as a sister group of Porifera (sponges). The basal eumetazoan clades are the Ctenophora and the ParaHoxozoa. Placozoa is now also seen as a eumetazoan ...
n, that is, more closely related to
Cnidaria Cnidaria ( ) is a phylum under kingdom Animalia containing over 11,000 species of aquatic invertebrates found both in fresh water, freshwater and marine environments (predominantly the latter), including jellyfish, hydroid (zoology), hydroids, ...
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 zones 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 is a division of green algae informally called chlorophytes. Description Chlorophytes are eukaryotic organisms composed of cells with a variety of coverings or walls, and usually a single green chloroplast in each cell. They are ...
) of the genus ''
Chlorella ''Chlorella'' is a genus of about thirteen species of single- celled or colonial green algae of the division Chlorophyta. The cells are spherical in shape, about 2 to 10 μm in diameter, and are without flagella. Their chloroplasts contain t ...
'', cryptomonads (
Cryptophyta The cryptomonads (or cryptophytes) are a Class (biology), superclass of algae, most of which have chloroplast, plastids. They are traditionally considered a Division (taxonomy), division of algae among phycologists, under the name of Cryptophyta ...
) of the genera ''Cryptomonas'' and ''Rhodomonas'', and blue-green bacteria (
Cyanobacteria Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
) 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 (biology), 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 ph ...
("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. When the concentrations of algae are high the animals are more likely to engage in social feeding behavior. 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 (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...
of the fibre syncytium, bacteria are frequently found that appear to live in
symbiosis Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biological interaction, between two organisms of different species. The two organisms, termed symbionts, can fo ...
with ''Trichoplax adhaerens''. These
endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...
s, which are no longer able to survive outside its host, are transferred from one generation to the next through both vegetative and sexual reproduction.


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 is the random motion of particles suspended in a medium (a liquid or a gas). The traditional mathematical formulation of Brownian motion is that of the Wiener process, which is often called Brownian motion, even in mathematical ...
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. Using ''T. adhaerens'' as a model, 0.02–0.002 Hz oscillations in locomotory and feeding patterns were observed, and taken as evidence of complex multicellular integration, dependent on endogenous secretion of signal molecules. Evolutionarily conserved low-molecular-weight transmitters (glutamate, aspartate, glycine, GABA, and ATP) acted as coordinators of distinct locomotory and feeding patterns. Specifically, L-glutamate induced and partially mimicked endogenous feeding cycles, whereas glycine and GABA suppressed feeding. ATP-modified feeding is complex, first causing feeding-like cycles and then suppressing feeding. Trichoplax locomotion was modulated by glycine, GABA, and, surprisingly, by animals’ own mucus trails. Mucus triples locomotory speed compared to clean substrates. Glycine and GABA increased the frequency of turns.


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 kno ...
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 (fr ...
, 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

''Trichoplax adhaerens'' is a
model organism A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...
. 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
body axis Standard anatomical terms of location are used to describe unambiguously the anatomy of humans and other animals. The terms, typically derived from Latin or Greek roots, describe something in its standard anatomical position. This position provi ...
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. 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 (biology), cell identifies and corrects damage to the DNA molecules that encode its genome. A weakened capacity for DNA repair is a risk factor for the development of cancer. DNA is cons ...
and
apoptosis Apoptosis (from ) is a form of programmed cell death that occurs in multicellular organisms and in some eukaryotic, single-celled microorganisms such as yeast. Biochemistry, Biochemical events lead to characteristic cell changes (Morphology (biol ...
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. The related lineage ''Trichoplax'' sp. H2 has been suggested to be a more suitable model organism than ''T. adhaerens'', due to its abundance and ease of culture.


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'', leading scientists to suggest in 2004 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 twenty haplotypes have since been assigned based on the 16S mitochondrial DNA fragment, with ''T. adhaerens'' being equated to the lineage H1. While most haplotypes have not been formally described as species, they have been (with the exception of the morphologically distinct H0, ''
Polyplacotoma mediterranea ''Polyplacotoma mediterranea'' is a species in the phylum Placozoa, only representative of the genus ''Polyplacotoma'', and was discovered in the Mediterranean Sea The Mediterranean Sea ( ) is a sea connected to the Atlantic Ocean, surro ...
'') provisionally placed into seven distinct clades. The genus ''Trichoplax'' was redefined in 2021 as comprising clades I and II, including haplotypes H1, H2, H3 and H17. A 2022 study defined ''Trichoplax'' more restrictively as only clade I (haplotypes H1, H2 and H17), with H3 being suggested to belong to a separate undescribed genus in the family Trichoplacidae.


Haplotype H2

Placozoan haplotypes are not necessarily equivalent to species, and several haplotypes of the related placozoan genus '' Hoilungia'' have been found to belong to the same species. Nonetheless, haplotype H2 is usually considered to be a separate undescribed species, referred to as ''Trichoplax'' sp. H2. It has been reported to be more robust and abundant than ''T. adhaerens'', and easier to culture, making it a better fit for experimental research. ''Trichoplax'' sp. H2 also show differences in cell differentiation, having 29 cell types compared to H1's 28 when analyzed using graph-based 2D projections and gene expression patterns. It has a population of "concave-disk" epithelia upper-like cells unique to this haplotype, first described in 2023. It also has an additional "unknown" cell type characterized by β-secretase expression; this type is found in other Placozoans besides H1. Other relatively minor differences make up for the difference in number.


Haplotype H17

The genome of H17 has been sequenced. In a gene-content analysis it branches off first as the
sister clade In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and ...
of (H1+H17), countary to earlier rDNA-based reports H2 branching off first as the sister to (H1+H17). H17 has been imaged using mmunofluorescence imaging.


Comparative genomics

Comparative genomic studies of ''Trichoplax adhaerens'' and the Panama strain of ''Trichoplax'' sp. H2 have suggested that their genetic similarity might be due to an interbreeding event having happened in the wild at least several decades ago, with one of them being the result of hybridization between the other and a third unknown strain. Analysis of bacterial endosymbionts supports this as a possible hypothesis, as the endosymbiont found in the Panama strain is closer to the one in ''T. adhaerens'' than to the one in the Hawaii strain of ''Trichoplax'' sp. H2.


References


Further reading

* *


Scientific literature

* * * * * * * *


First descriptions

;Placozoa * ;''Treptoplax reptans'' * ;''Trichoplax adhaerens'' *


External links

* Tree of life * Observation report with image * Brief description with image * summary of a report in ''Nature'' * JGI/DOE: "Genome of Simplest Animal Reveals Ancient Lineage, Confounding Array of Complex Capabilities" * Video of ''Trichoplax'' in motion {{Portal bar, Animals, Marine life Articles containing video clips Placozoa Animal genera with one living species Taxa named by Franz Eilhard Schulze Taxa described in 1883