Placozoa

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Placozoans
Temporal range: Middle Triassic–Recent [1]
Trichoplax adhaerens
Scientific classification Edit this classification
Domain: Eukaryota
(unranked): Filozoa
Kingdom: Animalia
Phylum: Placozoa
Grell, 1971
Type species
Trichoplax adhaerens

Classes[2]

Placozoa (

metazoan phylum.[9]

The first known placozoan,

Polyplacotoma mediterranea, the most basal, in 2019[15]
).

History

Trichoplax was discovered in 1883 by the German zoologist

Graz, Austria.[10][16] The generic name is derived from the classical Greek θρίξ (thrix), meaning "hair", and πλάξ (plax), "plate". The specific epithet adhaerens is Latin meaning "adherent", reflecting its propensity to stick to the glass slides and pipettes used in its examination.[17] Schulze realized that the animal could not be a member of any existing phyla, and based on the simple structure and behaviour, concluded in 1891 that it must be an early metazoan. He also observed the reproduction by fission, cell layers and locomotion.[18]

In 1893, Italian 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.[19] Monticelli did not preserve them and no other specimens were found again, as a result of which the identification is ruled as doubful, and the species rejected.[20][21]

Schulze's description was opposed by other zoologists. For instance, in 1890, F.C. Noll argued that the animal was a flat worm (Turbellaria).

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.[17]

The development of

electron microscopy in the mid-20th century allowed in-depth observation of the cellular components of organism, following which there was renewed interest in Trichoplax since 1966.[23] The most important descriptions were made by Karl Gottlieb Grell at the University of Tübingen since 1971.[24][25] That year, Grell revived Schulze's interpretation that the animals are unique and created a new phylum Placozoa.[26][17] Grell derived the name from the placula hypothesis, Otto Bütschli's notion on the origin of metazoans.[27]

Biology

Trichoplax body structure in cross section
1 - lipid drop, 2 - cilium, 3 - dorsal layer of cells, 4 - vacuole,
5 - fibrous syncytium, 6 - glandular cell, 7 - vacuole,
8 - ventral layer of cells, 9 - zones of intercellular contacts
Main article: Trichoplax

Placozoans do not have well-defined body plan much like

amoebas, unicellular eukaryotes. As Andrew Masterson reported: "they are as close as it is possible to get to being simply a little living blob."[28] An individual body measures about 0.55 mm in diameter.[29] There are no body parts; as one of the researchers Michael Eitel described: "There's no mouth, there's no back, no nerve cells, nothing."[30] Animals studied in laboratories have bodies consisting of everything from hundreds to millions of cells.[31]

Placozoans have only three anatomical parts as tissue layers inside its body: the upper, intermediate (middle) and lower

secretory granule.[34][33]

The body axes of Hoilungia and Trichoplax are overtly similar to the oral–aboral axis of

cnidarians,[35] animals from another phylum with which they are most closely related.[36] Structurally, they can not be distinguished from other placozoans, so that identification is purely on genetic (mitochondrial DNA) differences.[37] Genome sequencing has shown that each species has a set of unique genes and several uniquely missing genes.[14]

Trichoplax is a small, flattened, animal around 1 mm (0.039 in) across. An amorphous multi-celled body, analogous to a single-celled

cilia, which the animal uses to help it creep along the seafloor.[11]

The lower surface engulfs small particles of organic detritus, on which the animal feeds. All placozoa can reproduce asexually, budding off smaller individuals, and the lower surface may also bud off eggs into the mesenchyme.[11] Sexual reproduction has been reported to occur in one clade of placozoans,[38][39] whose strain H8 was later found to belong to genus Cladtertia,[2] where intergenic recombination was observed as well as other hallmarks of sexual reproduction.

Some Trichoplax species contain Rickettsiales bacteria as endosymbionts.[40] One of the at least 20 described species turned out to have two bacterial endosymbionts;

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.[41]
[42]

Studies suggests that aragonite crystals in crystal cells have the same function as statoliths, allowing it to use gravity for spatial orientation.[43]

Located in the dorsal epithelium there are lipid granules called shiny spheres which release a cocktail of venoms and toxins as an anti-predator defense, and can induce paralysis or death in some predators. Genes has been found in Trichoplax with a strong resemblance to the venom genes of some poisonous snakes, like the American copperhead and the West African carpet viper.[44][45]

Global distribution [46]

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.[47]

Three modes of population dynamics depended upon feeding sources, including induction of social behaviors, morphogenesis, and reproductive strategies. [48]

In addition to fission, representatives of all species produced “swarmers” (a separate vegetative reproduction stage), which could also be formed from the lower epithelium with greater cell-type diversity.[49]

Evolutionary relationships

There is no convincing fossil record of the placozoa, although the Ediacaran biota (Precambrian, 550 million years ago) organism Dickinsonia appears somewhat similar to placozoans.[50] Knaust (2021) reported preservation of placozoan fossils in a microbialite bed from the Middle Triassic Muschelkalk (Germany).[1]

Traditionally, classification was based on their level of organization, i.e., they possess no tissues or organs. However this may be as a result of secondary loss and thus is inadequate to exclude them from relationships with more complex animals. More recent work has attempted to classify them based on the DNA sequences in their genome; this has placed the phylum between the sponges and the eumetazoa.[51] In such a feature-poor phylum, molecular data are considered to provide the most reliable approximation of the placozoans' phylogeny.

Their exact position on 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

The Placozoa descending side by side with the sponges, cnidarians and ctenophores from a gallertoid by processes of differentiation
amoebae they superficially resemble, they continually change their external shape. In addition, spherical phases occasionally form which may facilitate movement. Trichoplax lacks tissues and organs. There is no manifest body symmetry, so it is not possible to distinguish anterior from posterior or left from right. It is made up of a few thousand cells of six types in three distinct layers.[52]

On the basis of their simple structure, the Placozoa were frequently viewed as a model organism for the transition from unicellular organisms to the multicellular animals (

Metazoa
) and are thus considered a sister taxon to all other metazoans:

Metazoa

Placozoa

Sponges (Porifera)

Animals with tissues (Eumetazoa)

According to a functional-morphology model, all or most animals are descended from a

pelagic) sphere in seawater, consisting of a single ciliated layer of cells supported by a thin, noncellular separating layer, the basal lamina. The interior of the sphere is filled with contractile fibrous cells and a gelatinous extracellular matrix. Both the modern Placozoa and all other animals then descended from this multicellular beginning stage via two different processes:[53]
Trichoplax adhaerens

While the probability of encountering food, potential sexual partners, or predators is the same in all directions for animals floating freely in the water, there is a clear difference on the seafloor between the functions useful on body sides facing toward and away from the substrate, leading their sensory, defensive, and food-gathering cells to differentiate and orient according to the vertical – the direction perpendicular to the substrate. In the proposed functional-morphology model, the Placozoa, and possibly several similar organisms only known from the fossils, are descended from such a life form, which is now termed placuloid.

Three different life strategies have accordingly led to three different possible lines of development:

  1. Animals that live interstitially in the sand of the ocean floor were responsible for the fossil crawling traces that are considered the earliest evidence of animals; and are detectable even prior to the dawn of the
    bilaterally symmetrical
    worms, but the hypothesis presented here views animals derived from placuloids, and thus close relatives of Trichoplax adhaerens, to be the producers of the traces.
  2. Animals that incorporated
    Mistaken Point) were in deep water, below the photic zone, and hence those individuals could not dependent on endosymbiotic photosynthesisers
    .
  3. Animals that grazed on
    dorsal) accordingly led to the physiologically distinct cell layers of Trichoplax adhaerens that can still be seen today. Consequently, these are analogous, but not homologous, to ectoderm and endoderm
    – the "external" and "internal" cell layers in eumetazoans – i.e. the structures corresponding functionally to one another have, according to the proposed hypothesis, no common evolutionary origin.

Should any of the analyses presented above turn out to be correct, Trichoplax adhaerens would be the oldest branch of the multicellular animals, and a relic of the

Ediacaran fauna, or even the pre-Ediacara fauna. Although very successful in their ecological niche, due to the absence of extracellular matrix and basal lamina, the development potential of these animals was of course limited, which would explain the low rate of evolution of their phenotype (their outward form as adults) – referred to as bradytely.[citation needed
]

This hypothesis was supported by a recent analysis of the Trichoplax adhaerens

mitochondrial genome in comparison to those of other animals.[54] The hypothesis was, however, rejected in a statistical analysis of the Trichoplax adhaerens whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species, but only at the p = 0.07 level, which indicates a marginal level of statistical significance.[51]

Epitheliozoa hypothesis

A concept based on purely morphological characteristics pictures the Placozoa as the nearest relative of the animals with true tissues (

Porifera
):

  
Metazoa
  

 

Porifera

  
Epitheliozoa
  
     

  Placozoa

  Eumetazoa

The above view could be correct, although there is some evidence that the

ctenophores, traditionally seen as Eumetazoa, may be the sister to all other animals.[55]
This is now a disputed classification.[56] Placozoans are estimated to have emerged 750–800 million years ago, and the first modern neuron to have originated in the common ancestor of cnidarians and bilaterians about 650 million years ago (many of the genes expressed in modern neurons are absent in ctenopheres, although some of these missing genes are present in placozoans).[57][58]

The principal support for such a relationship comes from special cell to cell junctions – belt

apomorphies), and thus form the basis of a common taxon for all animals that possess them.[citation needed
]

One possible scenario inspired by the proposed hypothesis starts with the idea that the monociliated cells of the epitheloid in

Trichoplax adhaerens evolved by reduction of the collars in the collar cells (choanocytes) of sponges as the hypothesized ancestors of the Placozoa abandoned a filtering mode of life. The epitheloid would then have served as the precursor to the true epithelial tissue of the eumetazoans.[citation needed
]

In contrast to the model based on functional morphology described earlier, in the Epitheliozoa hypothesis, the ventral and dorsal cell layers of the Placozoa are homologs of endoderm and ectoderm — the two basic embryonic cell layers of the eumetazoans. The digestive gastrodermis in the Cnidaria or the gut epithelium in the bilaterally symmetrical animals (Bilateria) may have developed from endoderm, whereas ectoderm is the precursor to the external skin layer (epidermis), among other things. The interior space pervaded by a fiber syncytium in the Placozoa would then correspond to connective tissue in the other animals. It is unclear whether the calcium ions stored in the syncytium would be related to the lime skeletons of many cnidarians.[citation needed]

As noted above, this hypothesis was supported in a statistical analysis of the Trichoplax adhaerens whole genome sequence, as compared to the whole-genome sequences of six other animals and two related non-animal species.[51]

Eumetazoa hypothesis

A third hypothesis, based primarily on molecular genetics, views the Placozoa as highly simplified

eumetazoans. According to this, Trichoplax adhaerens is descended from considerably more complex animals that already had muscles and nerve tissues. Both tissue types, as well as the basal lamina of the epithelium, were accordingly lost more recently by radical secondary simplification.[59]

Various studies in this regard so far yield differing results for identifying the exact sister group: in one case the Placozoa would qualify as the nearest relatives of the Cnidaria, while in another they would be a sister group to the Ctenophora, and occasionally they are placed directly next to the Bilateria. Currently, they are typically placed according to the cladogram below:[60]

  
Metazoa
  

 

Porifera

  Eumetazoa /  
Epitheliozoa
  
  
  

In this cladogram the

Diploblasts, and the Ctenophora
are basal to them.

An argument raised against the proposed scenario is that it leaves morphological features of the animals completely out of consideration. The extreme degree of simplification that would have to be postulated for the Placozoa in this model, moreover, is known only for parasitic organisms but would be difficult to explain functionally in a free-living species like Trichoplax adhaerens.[citation needed]

This version is supported by statistical analysis of the Trichoplax adhaerens whole genome sequence in comparison to the whole genome sequences of six other animals and two related non-animal species. However, ctenophora was not included in the analyses, placing the placozoas outside of the sampled Eumetazoans.[51][61]

Cnidaria-sister hypothesis

DNA comparisons suggest that placozoans are related to Cnidaria, derived from planula larva (as seen in some Cnidaria).[62] The Bilateria also are thought to be derived from planuloids.[63][64][65][66][67][68][69][70] The Cnidaria and Placozoa body axis are overtly similar, and Placozoan and Cnidarian cells are responsive to the same neuropeptide antibodies despite extant Placozoa's not developing any neurons.[71][72]

  Choanozoa  

 Choanoflagellata

  
Animalia
  

 

Porifera

  Eumetazoa  

 Ctenophora

  
  ParaHoxozoa  
     
     

 Placozoa

 Cnidaria

 

Triploblasts

680 mya
760 mya
  
950 mya
  
  

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External links

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