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 engulfment, and reproducing by fission or budding, placozoans are described as "the simplest animals on Earth". Structural and molecular analyses have supported them as among the most basal animals, thus constituting a primitive metazoan phylum.
The first known placozoan, Trichoplax adhaerens, was discovered in 1883 by the German zoologist Franz Eilhard Schulze (1840–1921).F. E. Schulze "Trichoplax adhaerens n. g., n. s.", Zoologischer Anzeiger (Elsevier, Amsterdam and Jena) 6 (1883), p. 92. Recognizing its distinctive characteristics, the German zoologist Karl Gottlieb Grell (1912–1994) erected a new phylum, Placozoa, for it in 1971. Remaining a monotypic phylum for over a century, new species began to be added in 2018. So far, three other extant species have been described, in two distinct classes: Uniplacotomia (Hoilungia hongkongensis in 2018 and Cladtertia collaboinventa in 2022) and Polyplacotomia (Polyplacotoma mediterranea, the most basal, in 2019). A single putative fossil species is known, the Middle Triassic Maculicorpus microbialis.
History Trichoplax was discovered in 1883 by the German zoologist Franz Eilhard Schulze, in a seawater aquarium at the Zoological Institute in Graz, Austria. The generic name is derived from the classical Greek (), meaning "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. 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.
In 1893, Italian zoologist Francesco Saverio Monticelli described another animal which he named Treptoplax, the specimens of which he collected from Naples. He gave the species name T. reptans in 1896. 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.
Schulze's description was opposed by other zoologists. For instance, in 1890, F.C. Noll argued that the animal was a flatworm (Turbellaria). In 1907, Thilo Krumbach published a hypothesis that Trichoplax is not a distinct animal but that it is a form of the planula larva of the anemone-like hydrozoan Eleutheria krohni. Although this was refuted in print by Schulze and others, Krumbach's analysis became the standard textbook explanation, and nothing was printed in zoological journals about Trichoplax until the 1960s.
The development of electron microscopy in the mid-20th century allowed in-depth observation of the cellular components of organisms, following which there was renewed interest in Trichoplax starting in 1966. The most detailed descriptions were made by Karl Gottlieb Grell at the University of Tübingen beginning in 1971. That year, Grell revived Schulze's interpretation that the animals are unique and created a new phylum Placozoa. Grell derived the name from the placula hypothesis, Otto Bütschli's notion on the origin of metazoans.
Biology
Anatomy
Placozoans lack a well-defined body plan and have a simple, flattened body. As Andrew Masterson reported: "they are as close as it is possible to get to being simply a little living blob." An individual body measures about 0.55 mm in diameter. 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." Animals studied in laboratories have bodies consisting of everything from hundreds to millions of cells.Stanford researchers reveal a new mechanism for how animal cells stay intact
Placozoans have only three anatomical parts as tissue layers inside its body: the upper, intermediate (middle) and lower epithelia. There are at least six different cell types. (A 2023 analysis of 4 species across 3 genera found 8, one with an unknown role.) The upper epithelium is the thinnest portion and essentially comprises flat cells with their cell body hanging underneath the surface, and each cell having a cilium. Crystal cells are sparsely distributed near the marginal edge. A few cells have unusually large number of mitochondria. The middle layer is the thickest made up of numerous fiber cells, which contain mitochondrial complexes, vacuoles and endosymbiotic bacteria in the endoplasmic reticulum. The lower epithelium consists of numerous monociliated cylinder cells along with a few endocrine-like gland cells and lipophil cells. Each lipophil cell contains numerous middle-sized granules, one of which is a secretory granule.
The body axes of Hoilungia and Trichoplax are similar to the oral–aboral axis of cnidarians, animals from another phylum with which they are most closely related. Structurally, they can not be distinguished from other placozoans, so that identification is purely on genetic (mitochondrial DNA) differences. Genome sequencing has shown that each species has a set of unique genes and several uniquely missing genes.
Trichoplax is a small, flattened, animal around across. An amorphous multi-celled body, analogous to a single-celled amoebas, it has no regular outline, although the lower surface is somewhat concave, and the upper surface is always flattened. The body consists of an outer layer of simple epithelium enclosing a loose sheet of stellate cells resembling the mesenchyme of some more complex animals. The epithelial cells bear cilia, which the animal uses to help it creep along the seafloor.
The lower surface engulfs small particles of organic detritus, on which the animal feeds.
Studies suggest that aragonite crystals in crystal cells have the same function as statoliths, allowing it to use gravity for spatial orientation.
Located in the dorsal epithelium there are lipid granules which release a cocktail of toxins as a means of defense, and can induce paralysis or death in some predators. Genes encoding for proteins which make up the poisonous secretions of Trichoplax have been found to resemble venom-associated genes present in the genomes of certain snakes, like the American copperhead and the West African carpet viper.Living Mysteries: Meet Earth's simplest animal
Proto-nervous system The Placozoa have a very primitive analogue of the nervous system. It has peptidergic cells that resemble neurons in that they have G protein-coupled receptors and release vesicles and neuropeptides. They are descended from progenitor cells with an expression pattern similar to that of neuron progenitor cells and themselves express genes associated with the pre-synaptic scaffold. Still, they differ from cnidarian neurons by having no projections and not expressing genes of the post-synaptic scaffold. There are a total of 14 types of these peptidergic cells.
Despite their absence of neurons, they are able to produce electrical impulses.Sodium Channels Involved in the Initiation of Action Potentials in Invertebrate and Mammalian Neurons
Reproduction All placozoans can reproduce asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the mesenchyme. Sexual reproduction has been reported to occur in one clade of placozoans, whose strain H8 was later found to belong to genus Cladtertia, where intergenic recombination was observed as well as other hallmarks of sexual reproduction.
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.
Endosymbionts Some Trichoplax species contain Rickettsiales bacteria as endosymbionts. One of the at least 20 described species turned out to have two bacterial endosymbionts; Grellia which lives in the animal's endoplasmic reticulum and is assumed to play a role in the protein and membrane production. The other endosymbiont is the first described Margulisbacteria, that lives inside cells used for algal digestion. It appears to eat the fats and other lipids of the algae and provide its host with vitamins and amino acids in return.
Evolution and population dynamics The Placozoa show substantial evolutionary radiation in regard to sodium channels, of which they have 5–7 different types, more than any other invertebrate species studied to date.
Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies.
Distribution
Internal phylogeny The internal relationships among placozoa has been studied using ribosomal DNA, mitochondrial DNA, and phylogenomics (gene content comparison). The latter methods require the sequencing of more DNA but produce more reliable results. Tessler et al. (2022) recovered the following maximum likelihood phylogenomic tree based on genomic data and reformed the inner taxonomy of the group based on these results:
Placozoa Grell, 1971 Class Polyplacotomia Tessler, Neumann, Osigus, DeSalle & Schierwater, 2022 Order Polyplacotomea Tessler, Neumann, Osigus, DeSalle & Schierwater, 2022 Family Polyplacotomidae Tessler, Neumann, Osigus, DeSalle & Schierwater, 2022 Class Uniplacotomia Tessler, Neumann, Osigus, DeSalle & Schierwater, 2022 Order Trichoplacea Tessler, Neumann, Osigus, DeSalle & Schierwater, 2022 Family Trichoplacidae Bütschli & Hatschek, 1905 Order Cladhexea Tessler, Neumann, Osigus, DeSalle & Schierwater, 2022 Suggested future new family Order Hoilungea Tessler, Neumann, Osigus, DeSalle & Schierwater, 2022 Family Cladtertiidae Tessler, Neumann, Osigus, DeSalle & Schierwater, 2022 Family Hoilungidae Tessler, Neumann, Osigus, DeSalle & Schierwater, 2022
Evolutionary relationships Although there is no convincing fossil record of the Placozoa, proarticulates from the Ediacaran biota (Precambrian, ) such as Dickinsonia have been suggested to be stem-placozoans based on some similarities in anatomy and lifestyle, feeding on algal mats through a ventral epithelium. Knaust (2021) reported preservation of placozoan fossils in a microbialite bed from the Middle Triassic Muschelkalk (Germany).
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 the sponges and the Eumetazoa. In such a feature-poor phylum, molecular data are considered to provide the most reliable approximation of the placozoans' phylogeny.
Their exact position on the phylogenetic tree would give important information about the origin of neurons and muscles. If the absence of these features is an original trait of the Placozoa, it would mean that a nervous system and muscles evolved three times should placozoans and cnidarians be a sister group; once in the Ctenophora, once in the Cnidaria and once in the Bilateria. If they branched off before the Cnidaria and Bilateria split, the neurons and muscles would have the same origin in the two latter groups.
Functional-morphology hypothesis: Sister to Porifera and Eumetazoa
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: