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Placozoaflat animals

By Jeremy Wright

Diversity

Phylum Placozoa contains just two species of very small (2 to 3 mm in diameter and only 15 to 20 µm in width), simply organized, non-bilaterian metazoan organisms that superficially resemble amoebas. Treptoplax reptans has not been seen since it was first described from the waters of Naples, Italy in 1896, but Trichoplax adhaerens, first discovered on the walls of a marine aquarium in Australia in 1883, is found in tropical and subtropical marine waters around the world. These organisms are composed of differentiated dorsal and ventral epithelial cell layers, which enclose a mesenchymal syncytial net. Placozoans move via gliding, aided by the ciliated cells of the basal epithelial layer, and feed by engulfing particles of organic detritus. They are able to reproduce asexually via fission, but are also known to reproduce sexually. (Brusca and Brusca, 2003; Martinelli and Spring, 2003; Minot, 1883; Monticelli, 1893; Pearse and Voigt, 2007; Schierwater, et al., 2011; Schulze, 1883)

Geographic Range

Placozoans are found globally in tropical and subtropical marine waters. (Pearse and Voigt, 2007; Schierwater, et al., 2011)

Habitat

Placozoans were first identified from the walls of a marine aquarium. While they are considered benthic organisms, they are also found in the water column. They are most commonly found near shore, in the littoral zone, in warm waters. (Pearse and Voigt, 2007; Voigt, et al., 2004)

Systematic and Taxonomic History

Trichoplax adhaerens was first described by the German zoologist Franz Eilhard Schulze in 1883, with an English-language report of his findings published in the United States in the same year by Charles S. Minot. Treptoplax reptans, the only other named member of phylum Placozoa, was described by the Italian biologist Francesco Monticelli in 1893. However, this species has not been observed since, and many question whether it ever existed as a valid taxon. (Eitel, et al., 2013; Martinelli and Spring, 2003; Miller and Ball, 2005; Minot, 1883; Monticelli, 1893; Schulze, 1883)

At its first discovery, it was obvious to many scientists that Trichoplax adhaerens presents significant differences from other known groups of organisms. However, a report classified it as the larval form of the cnidarian species Eleutheria krohi, which became the accepted hypothesis for the next several decades. In the 1970s, scientists, particularly the German protozoologist Karl Gottlieb Grell, renewed examinations of this species and demonstrated that the individuals examined were, in fact, adults. Trichoplax adhaerens has since been widely accepted as the sole representative of the phylum Placozoa, a name coined by Grell in 1971. Several recent morphological and molecular phylogenetic analyses have indicated that many additional species likely exist within this phylum, including several higher order taxonomic groups. To date, however, none of these have been formally described or named. (Brusca and Brusca, 2003; Eitel, et al., 2013; Grell, 1971a; Grell, 1971b; Guidi, et al., 2011; Krumbach, 1907; Lecointre and Le Guyader, 2006; Martinelli and Spring, 2003; Miller, 1971; Signorovitch, et al., 2006; Syed and Schierwater, 2002; Voigt, et al., 2004)

Our understanding of the relationships of placozoans to other animal phyla remains in a state of flux. Early molecular phylogenetic analyses generally recovered them as the sister group to cnidarians or ctenophores. Subsequent studies, using much higher numbers of independent genetic markers, indicated that they occupy a basal position within metazoans as the sister group to the Eumetozoa, being placed between sponges (phylum Porifera) and all other multicellular animals. In contrast, results from analyses of mitochondrial genomes and total evidence approaches suggested that animals form two clades based on the organization of their tissues, with Placozoa representing the basal lineage in the dibloblast clade, which also includes sponges, cnidarians, and ctenophores. However, the most recent molecular phylogenetic analyses indicate that there is no strong support for any of these hypotheses, leaving the true phylogenetic position of placozoans as a matter of intense, widespread scientific interest and debate. (DeSalle and Schierwater, 2008; Eitel, et al., 2013; Osigus, et al., 2013; Phillipe, et al., 2011; Schierwater, et al., 2009a; Schierwater, et al., 2009b; Schierwater, et al., 2010; Siddall, 2010; Srivastava, et al., 2008; Srivastava, et al., 2010)

  • Synonyms
    • Eleutheria krohi (Krumbach, 1907)
  • Synapomorphies
    • Placozoans have two cell layers delimiting a space containing nucleated mesenchyme cells in an extracellular matrix.
    • These organisms have external digestion via temporary formation of a digestive chamber on the ventral surface.

Physical Description

Placozoans are very small animals, measuring just 2 to 3 mm in diameter and typically 15 to 20 µm thick. They have traditionally been described as being composed only four types of cells: cover (squamous), columnar, glandular, and fiber. They are asymmetrical (although smaller animals tend to be circular), and their bodies lack anterior or posterior ends. They do, however, have distinct ventral and dorsal sides, and are essentially made up of three layers: dorsal epithelia, mesenchyme, and ventral epithelia. Dorsal epithelial cells are cover, or squamous, cells; they are flattened, contain lipid droplets, and each has a single cilia. Ventral epithelial cells are more columnar, lack lipid droplets, but are also mainly monociliate. The ventral layer also has unciliated glandular cells. Between these two layers, forming the interior of the animal, is a layer of mesenchyme, composed of star-shaped fiber cells. The points of the “stars” are connected, creating a network. There appears to be no basement membrane between the epithelial layers and mesenchyme. (Brusca and Brusca, 2003; Martinelli and Spring, 2003; Miller and Ball, 2005; Pearse and Voigt, 2007)

In situ hybridization studies of gene expression in the cells of Trichoplax adhaerens have indicated that this organism likely possesses more than just four cell types. Though the newly identified cell types are morphologically indistinguishable, differential gene expression patterns between them and previously characterized cell types strongly suggest that they possess unique, albeit currently unknown, functions. (Martinelli and Spring, 2003; Miller and Ball, 2005)

  • Sexual Dimorphism
  • sexes alike

Development

New animals may be produced via binary fission or budding. Budding creates multicellular flagellated “swarmers,” each of which becomes a new individual. Sexual reproduction may occur, in which case holoblastic cell division proceeds following fertilization. At the 64 cell stage, cell division ceases, while nuclear DNA multiplication continues until the nucleus bursts. (Brusca and Brusca, 2003; Collins, 2000; Miller and Ball, 2005)

Reproduction

Reproduction in placozoans is mainly asexual. Sexual reproduction may be observed during times of high population density, high water temperature (23°C and greater), and food depletion. Sexual reproduction occurs only through degeneration of the mother. A single egg/oocyte as well as small, unflagellated cells (assumed to be sperm) develops in the interspace of a degenerating placozoan. (Brusca and Brusca, 2003; Collins, 2000; Eitel, et al., 2011; Miller and Ball, 2005)

Placozoans may reproduce asexually (via transverse fission or budding) or sexually. Sexual reproduction seems to be triggered by environmental factors including water temperature; this implies that, in some regions, animals may have both sexual and asexual phases that may be dependent on the season. (Eitel, et al., 2011; Pearse and Voigt, 2007)

Parental investment is not known to occur in placozoans. (Miller and Ball, 2005)

  • Parental Investment
  • no parental involvement

Lifespan/Longevity

While strains of Trichoplax adhaerens have been maintained in lab settings for many years, little data is available regarding their lifespans. (Eitel, et al., 2011)

Behavior

Placozoans move using ciliary action and changes in body shape; there is evidence that smaller (presumably younger) individuals may swim. (Brusca and Brusca, 2003)

Communication and Perception

Little is known regarding how placozoans may perceive their environments. In laboratory settings, they have been observed to react strongly when exposed to ultraviolet radiation. (Pearse and Voigt, 2007)

  • Perception Channels
  • ultraviolet

Food Habits

Placozoans feed by phagocytosis, using their ventral surfaces (some of the glandular cells located there produce digestive enzymes). In laboratory settings, they are known to feed on flagellated chromists (Cryptomonas sp.) and chlorophytes (Chlorella sp.), other algae, the nauplii of Artemia species, and commercial fish food. It is suspected that they are opportunistic grazers, and they may feed on organic detritus as well. (Collins, 2000; Miller and Ball, 2005; Pearse and Voigt, 2007)

Predation

Potential predators have been observed reacting negatively to placozoans. In one instance, a snail was observed touching a placozoan with its tentacle, then recoiling; in another study, placozoans dropped onto the tentacles of hydroids caused paralysis. Structures known as "shiny spheres," present in the upper epithelium of placozoans may somehow serve as predator deterrents, though the mechanism by which they may do this is completely unknown. The only predators reported for placozoans are snails in genus Rhodope and a small nemertean species. (Pearse and Voigt, 2007)

Ecosystem Roles

Although they are ciliates, nematodes, and other small animals have been observed around or even on placozoans, they do not seem to elicit any response and no parasitic or commensal relationships are known. (Pearse and Voigt, 2007)

Economic Importance for Humans: Positive

Beyond potential scientific interest, there are no positive effects of placozoans on humans.

  • Positive Impacts
  • research and education

Economic Importance for Humans: Negative

There are no known adverse affects of placozoans on humans.

Conservation Status

There is no concern of either known placozoan species becoming threatened or endangered at this time. (IUCN, 2013)

  • IUCN Red List [Link]
    Not Evaluated

Contributors

Jeremy Wright (author), University of Michigan-Ann Arbor, Leila Siciliano Martina (editor), Animal Diversity Web Staff.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

World Map

Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

World Map

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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asexual

reproduction that is not sexual; that is, reproduction that does not include recombining the genotypes of two parents

benthic

Referring to an animal that lives on or near the bottom of a body of water. Also an aquatic biome consisting of the ocean bottom below the pelagic and coastal zones. Bottom habitats in the very deepest oceans (below 9000 m) are sometimes referred to as the abyssal zone. see also oceanic vent.

carnivore

an animal that mainly eats meat

coastal

the nearshore aquatic habitats near a coast, or shoreline.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

herbivore

An animal that eats mainly plants or parts of plants.

heterothermic

having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

holarctic

a distribution that more or less circles the Arctic, so occurring in both the Nearctic and Palearctic biogeographic regions.

World Map

Found in northern North America and northern Europe or Asia.

intertidal or littoral

the area of shoreline influenced mainly by the tides, between the highest and lowest reaches of the tide. An aquatic habitat.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

World Map

pelagic

An aquatic biome consisting of the open ocean, far from land, does not include sea bottom (benthic zone).

reef

structure produced by the calcium carbonate skeletons of coral polyps (Class Anthozoa). Coral reefs are found in warm, shallow oceans with low nutrient availability. They form the basis for rich communities of other invertebrates, plants, fish, and protists. The polyps live only on the reef surface. Because they depend on symbiotic photosynthetic algae, zooxanthellae, they cannot live where light does not penetrate.

saltwater or marine

mainly lives in oceans, seas, or other bodies of salt water.

seasonal breeding

breeding is confined to a particular season

sedentary

remains in the same area

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

solitary

lives alone

temperate

that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

year-round breeding

breeding takes place throughout the year

References

Brusca, R., G. Brusca. 2003. Invertebrates, 2nd Edition. Sunderland, MA: Sinauer Associates.

Collins, A. 2000. "Introduction to Placozoa" (On-line). University of California Museum of Paleontology. Accessed April 09, 2013 at http://www.ucmp.berkeley.edu/phyla/placozoa/placozoa.html.

DeSalle, R., B. Schierwater. 2008. An even “newer" animal phylogeny. Bioessays, 30: 1043-1047.

Eitel, M., L. Guidi, H. Hadrys, M. Balsamo, B. Schierwater. 2011. New insights into Placozoan sexual reproduction. PLOS One, 6/5: e19639. Accessed April 09, 2013 at http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0019639.

Eitel, M., H. Osigus, R. DeSalle, B. Schierwater. 2013. Global Diversity of the Placozoa. PLoS ONE, 8/4: e57131.

Grell, K. 1971. Embryonalenwicklung bei Trichoplax adhaerens F. E. Schulze. Naturwissenschaften, 58: 570.

Grell, K. 1971. Trichoplax adhaerens F. E. Schulze und die Entstehung der Metazoen. Naturwissenschaftliche Rundschau, 24: 160-161.

Guidi, L., M. Eitel, E. Cesarini, B. Schierwater, M. Balsamo. 2011. Ultrastructural analyses support different morphological lineages in the Placozoa, Grell 1971. Journal of Morphology, 272: 371-378.

IUCN, 2013. "The IUCN Red List of Threatened Species. Version 2013.2" (On-line). Accessed December 31, 2013 at http://www.iucnredlist.org/.

Krumbach, T. 1907. Trichoplax, die umgewandelte Planula einer Hydromeduse. Zoologischer Anzeiger, 31: 450-454.

Lecointre, G., H. Le Guyader. 2006. The Tree of Life: A Phylogenetic Classification. Cambridge, MA: Harvard University Press.

Martinelli, C., J. Spring. 2003. Distinct expression patterns of the two T-box homologues Brachyury and Tbx2/3 in the placozoan Trichoplax adhaerens. Development Genes and Evolution, 213: 492-499.

Miller, D., E. Ball. 2005. Animal evolution: The enigmatic phylum Placozoa revisited. Current Biology, 15/1: R26-R28. Accessed April 09, 2013 at http://www.sciencedirect.com/science/article/pii/S0960982204009790.

Miller, R. 1971. Trichoplax adhaerens Schulze 1883: return of an enigma. Biological Bulletin, 141: 374.

Minot, C. 1883. An apparently new animal type. Science, 1: 305.

Monticelli, F. 1893. Treptoplax reptans n.g., n.sp. Atti della Reale Accademia dei Lincei. Rendiconti, 5(II): 39-40.

Osigus, H., M. Eitel, B. Schierwater. 2013. Chasing the urmetazoon: Striking a blow for quality data. Molecular Phylogenetics and Evolution, 66/2: 551-557.

Pearse, V., O. Voigt. 2007. Field biology of placozoans (Trichoplax): distribution, diversity, biotic interactions. Integrative and Comparative Biology, 47/5: 677-692. Accessed April 09, 2013 at http://icb.oxfordjournals.org/content/47/5/677.full.

Phillipe, H., H. Brinkmann, D. Larov, D. Littlewood, M. Manuel, G. Wörheide, D. Baurain. 2011. Resolving difficult phylogenetic questions: why more sequences are not enough. PLoS Biology, 9/3: e1000602.

Schierwater, B., M. Eitel, W. Jakob, H. Osigus, H. Hadrys, S. Dellaporta, S. Kolokotronis, R. DeSalle. 2009. Concatenated analysis sheds light on early metazoan evolution and fuels a modern “Urmetazoon" hypothesis. PLoS Biology, 7/1: e1000020.

Schierwater, B., M. Eitel, H. Osigus, K. von der Chevallerie, T. Bergmann, H. Hadrys, M. Cramm, L. Heck, M. Lang, R. DeSalle. 2010. Trichoplax and Placozoa: one of the crucial keys to understanding metazoan evolution. Pp. 289-326 in R DeSalle, B Schierwater, eds. Key Transitions in Animal Evolution. Boca Raton, FL: CRC Press.

Schierwater, B., S. Kolokotronis, M. Eitel, R. DeSalle. 2009. The Diploblast-Bilateria sister hypothesis: Parallel evolution of a nervous systems may have been a simple step. Communicative and Integrative Biology, 2: 1-3.

Schierwater, B., M. Eitel, R. DeSalle. 2011. "World Placozoa Database" (On-line). Accessed April 09, 2013 at http://www.marinespecies.org/placozoa/index.php.

Schulze, F. 1883. Trichoplax adhaerens, nov. gen., nov. spec. Zoologischer Anzeiger, 6: 92-97.

Siddall, M. 2010. Unringing a bell: metazoan phylogenomics and the partition bootstrap. Cladistics, 26: 444-452.

Signorovitch, A., S. Dellaporta, L. Buss. 2006. Caribbean placozoan phylogeography. Biological Bulletin, 211/2: 149-156.

Srivastava, M., E. Begovic, J. Chapman, N. Putnam, U. Hellsten, T. Kawashima, A. Kuo, T. Mitros, A. Salamav, M. Carpenter, A. Signurovitch, M. Moreno, K. Kamm, J. Grimwood, J. Schmutz, H. Shapiro, I. Grigoriev, L. Buss, B. Schierwater, S. Dellaporta, D. Rokhsar. 2008. The Trichoplax genome and the nature of placozoans. Nature, 454: 955-960.

Srivastava, M., O. Simakov, J. Chapman, B. Fahey, M. Gauthier, T. Mitros, G. Richards, C. Conaco, M. Dacre, U. Hellsten, C. Larroux, N. Putnam, M. Stanke, M. Adamska, A. Darling, S. Degnan, T. Oakley, D. Plachetzki, Y. Zhai, M. Adamski, A. Calcino, S. Cummins, D. Goodstein, C. Harris, D. Jackson, S. Leys, S. Shu, B. Woodcroft, M. Vervoort, K. Kosik, G. Manning, B. Degnan, D. Rokhsar. 2010. The Amphimedon queenslandica genome and the evolution of animal complexity. Nature, 466: 720-726.

Syed, T., B. Schierwater. 2002. Trichoplax adhaerens: Discovered as a missing link, forgotten as a hydrozoan, re-discovered as a key to metazoan evolution. Vie Millieu, 52/4: 177-187.

Voigt, O., A. Collins, V. Pearse, J. Pearse, A. Ender, H. Hadrys, B. Schierwater. 2004. Placozoa – no longer a phylum of one. Current Biology, 14/22: R944-R945. Accessed April 09, 2013 at http://www.sciencedirect.com/science/article/pii/S0960982204008413.

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