Placozoa

(Placozoans)

Phylum Placozoa

Number of families 1

Thumbnail description
A large, flat, amoeba-like creature up to 0.078 in (2 mm) in diameter; the grayish white body of the organism consists of several thousand cells that form two epithelia (thin layers of tissue composed of closely apposed cells) but are not organized into tissues and organs

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 adhaerens 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 mesozoans 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

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 placozoans 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.

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

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