Placozoans

How Phylum Placozoa Feed

Evolution and systematics

Phylum Placozoa includes only one species, Trichoplax adhaerens. Because this phylum is so small, it has no classes or orders assigned to it. There is one family, the Trichoplacidae. Little is known about this organism because it has never been observed in the wild; it has been studied from samples cultured in laboratory aquaria around the world. Trichoplax ad-haerens was first discovered in the aquarium of the Graz Zoological Institute in Austria in 1883. A second species, Trichoplax reptans, was found in 1896. This species, however, has never been seen since it was first described, causing some researchers to doubt its existence.

The placozoans were formerly assigned to the phylum Mesozoa together with the dicyemids, orthonectids, and Salinella on the basis of their simple body organization. It became evident, however, that placozoans are not like other meso-zoans and do not fit into any other metazoan phylum. As a result, phylum Placozoa was established in 1971. Since that date, specimens of T. adhaerens have been found in aquaria around the world. It is not known whether placozoans are cosmopolitan (widely distributed around the world); they are, however, so cryptic (hidden) that their diversity may be much greater than we realize.

Some zoologists have inferred that placozoans may represent the earliest form of animal life. Others regard them as a modified form of the planula stage of a cnidarian. Recent molecular phylogenetic studies suggest that the placozoans are closely related to cnidarians. If this finding is confirmed, it would imply that the placozoans are a secondary simplification of more complex ancestors that possessed fully differentiated tissues and organs, including muscles and nerves.

The taxonomy for this species is: Trichoplax adhaerens Schulze, 1883, aquarium of the Zoological Institute in Graz, Austria.

Physical characteristics

Placozoans are extremely simple multicellular animals, with no anterior-posterior (front to rear) polarity or bilateral (right to left) symmetry. They have only dorsoventral (upper surface to lower surface) polarity. The bodies of placozoans consist of several thousand cells but only four cell types. The dorsal epithelium is thin and loosely constructed of cover cells that bear a single flagellum (microscopic whip-like appendage), and contain droplets of fatty material that refract light. The ventral epithelium is composed of a thicker, denser layer of nonflagellated gland cells and columnar cylindrical

Trichoplax Adhaerens
Trichoplax adhaerens. (Illustration by Dan Erickson)

cells with a single flagellum. A fluid-filled space lies between the two epithelia. It contains mesenchyme, a network of loosely organized cells known as fiber cells. The fiber cell is a syncytium, or mass of protoplasm containing many nuclei. Fiber cells are star-shaped, and connected to one another and to both epithelia by branched extensions. Microtubules and microfilaments can be seen within the cytoplasm of the fiber cells; they may be associated with the contraction of cells. Each fiber cell has a synapse-like structure at the point of its attachment to neighboring cells. The fiber cells are thought to function as both muscle and nerve cells. An additional feature of the fiber cells is the presence of symbiotic bacteria within the space of the endoplasmic reticulum.

Distribution

Placozoans are distributed in aquaria located in the littoral zones of tropical and subtropical seas.

Habitat

As of 2003, nothing is known about the biology of placo-zoans under natural conditions. They may occur on the surface of underwater rocks and benthic marine organisms with shells.

Behavior

Placozoans move around by waving or beating their cilia (tiny hairlike projections), and their outer shape changes continuously. Feeding behavior depends on the amount of available food. When food concentration is low, the organisms move rapidly with frequent random changes in shape. At high concentrations, however, they flatten themselves and move around less.

Placozoans

Feeding ecology and diet

In aquaria and laboratory cultures, placozoans have been observed to creep over the substrate by beating the cilia on their ventral surface. They often lift themselves off the substrate, forming a digestive bag in which they can digest their food more efficiently. They are able to digest detritus, living protozoans, and algae by extracellar digestion. The ventral epithelium absorbs the digested material by phagocytosis.

Reproductive biology

Placozoans appear to undergo asexual reproduction in three ways: fission, in which the body divides in half; frag-

mentation, in which small parts separate from the body; and budding. Sexual reproduction has occasionally been observed in laboratory vessels containing two different clones of pla-cozoans.

Conservation status

This species is not listed by the IUCN.

Significance to humans

There is no known significance to humans.

Resources Books

Grell, Karl G., and A. Ruthmann. "Placozoa." In Microscopic Anatomy of Invertebrates, vol. 2, edited by Frederick W. Harrison and J. A. Westfall. New York: John Wiley and Sons, 1991.

Periodicals

Grell, Karl G. "Einbildung und Forschung von Trichoplax adhaerens F. E. Schulze (Placozoa)." Zeitschrift für Morphologie der Tiere 73 (1972): 297-314.

Grell, Karl G., and G. Benwitz. "Die Ultrastruktur von Trichoplax adhaerens F. E. Schulze." Cytobiologie 4 (1971): 216-240.

Kim, Jihee, W. Kim, and C. W. Cunningham. "A New Perspective on Lower Metazoan Relationships from 18S rDNA Sequences." Molecular Biology and Evolution 16, no. 3 (1999): 423-427.

Hidetaka Furuya, PhD

Salinella Argentine

Evolution and systematics

Phylum Monoblastozoa includes only one species, Salinella salve. Because this phylum is so small, it has no classes or orders assigned to it. Salinella salve was found in 1892 in a culture of 2% saline solution made from material taken from the salt beds of Cordova, Argentina. According to Frenzel, the author of the only published record of the organism, the body of Salinella consists of a single cell layer and lacks tissues and organs. Salinella, however, has never been observed since its discovery. As a result, some zoologists doubt that it really exists.

Salinella has been classified within the phylum Mesozoa, together with the dicyemids, orthonectids, and Trichoplax. The body of Salinella, however, differs from those of any other metazoans in lacking internal cells. Its body organization is clearly different from the usual pattern of relegating some cells to the interior of the organism to form endoderm. In this regard, Salinella appears to be more closely related to unicellular (one-celled) organisms than to living multicellular animals. If contemporary researchers are able to find new specimens of Salinella and study them in detail, they might find that the organism represents an intermediate stage between unicellular and multicellular animals.

The taxonomy for this species is: Salinella salve Frenzel, 1892, in a culture made from materials taken from the salt beds of Cordoba, Argentina.

Physical characteristics

The body of Salinella consists of about a hundred cells and a single cell layer enclosing a digestive cavity. The cavity is open at both ends, with the openings functioning as a mouth and anus respectively. Distinct bristles can be seen around the mouth and anus. The organism's dorsal surface carries a sparse collection of bristles. The ventral surface is somewhat flattened but is heavily ciliated. The cell walls facing the inner cavity are also heavily ciliated.

Distribution

Salinella salve has been identified only in Argentina.

Habitat

As of 2003, nothing is known about the biology of Salinella under natural conditions.

Behavior

Salinella is reported to move by ciliary gliding in the manner of ciliates and small flatworms.

Feeding ecology and diet

Salinella appears to feed by ingesting organic detritus in its internal cavity. Undigested materials are carried to the anus by the movement of the cilia.

Placozoans

Reproductive biology

Asexual reproduction occurs by transverse fission; however, another mode of reproduction was frequently seen in culture. Salinella appears to form a cyst by the conjugation or coupling of two individuals. Although the details of the process are unknown, a unicellular individual that possibly came from the cyst was found in the culture. It is not known whether sexual reproduction takes place within the cyst.

Conservation status

The species is not listed by the IUCN.

Significance to humans

There is no known significance to humans.

Resources

Periodicals

Frenzel, Johannes. "Untersuchungen über die mikroskopische Fauna Argentiniens." Archiv für Naturgeschichte 58 (1892): 66-96.

Hidetaka Furuya, PhD

Phylum Rhomobozoa Number of families 3

Thumbnail description

Characteristic parasites of the kidney of benthic cephalopod mollusks; the body consists of only 8-40 cells, which are fewer in number than in any other metazoans and are organized very simply

Photo: Dicyemodeca deca within its host Octopus dofleini. (Photo by Hidetaka Furuya. Reproduced by permission.)

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Responses

  • niklas viren
    What do placozoans feed on?
    6 years ago
  • Tegan Dickson
    How do phylum: placozoa feed?
    6 years ago

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