Placozoans (PLACK-uh-zoh-uhns) are shape-shifting blobs up to one-sixteenth of an inch (2 millimeters) in diameter. There is only one species, Trichoplax adhaerens. The grayish white body is covered with tiny structures that look like hairs. Placozoans consist of several thousand cells that form two thin layers of tissue but not organs. There is only one known species of placozoan, and it is a tiny, flattened bag of cells that has a top and a bottom but no front or rear. A narrow space between the two layers of cells is filled with a gel-like substance.
Some scientists believe that placozoans may be the earliest form of animal life. Results of molecular studies suggest that placozoans are closely related to the group that includes corals and jellyfish. If confirmed, this finding would imply that placozoans are a simple form of more complex ancestors that had tissues and organs, including muscles and nerves.
Although the bodies of placozoans are made up of several thousand cells, there are only a few cell types. The top layer is thin and loosely constructed of cover cells that bear a single whiplike structure and contain droplets of fatty material. The bottom layer is made up of a thicker, denser layer of gland cells without a whip as well as other cells with a whip. The fluid-filled space between the top and bottom layers contains a network of loosely organized cells known as fiber cells. The fiber cells are connected to one another and to the top and bottom layers by branched extensions. The fiber cells are thought to function as both muscle and nerve cells. The fiber cells contain bacteria that live in placozoans.
Scientists do not know whether placozoans are widely distributed around the world. These animals are so hidden that their diversity may be much greater than scientists realize. The placozoans that have been studied are samples cultured in laboratory aquariums near warm seas.
Scientists do not know where placozoans live under natural conditions. They may live on the surface of underwater rocks and on the shells of bottom-dwelling sea animals.
Placozoans eat waste material, protozoans (proh-tuh-ZOH-uhns), and algae (AL-jee). Protozoans are one-celled living things that resemble animals in that they get their food from their surroundings rather than making it themselves as plants do. Algae are plantlike growths that live in water and have no true roots, stems, or leaves.
BEHAVIOR AND REPRODUCTION
In aquariums and laboratories, placozoans have been observed to creep along by beating the hairlike structures on their bottom surface. They often lift themselves, forming a bag in which they digest their food. The feeding behavior of placozoans depends on the amount of available food. When there is little food, placozoans move rapidly with frequent, random changes in shape. When there is much food, however, placozoans flatten themselves and move around less.
Placozoans appear to use asexual (ay-SEK-shuh-wuhl) reproduction, or reproduction without the uniting of egg and sperm for the transfer of DNA from two parents, in three ways. In the first method, the body divides in half to make two new individuals. In the second method, small parts separate from the body and grow into new individuals. In the third method, buds develop on the parent, grow to full size, and then break off to live as new individuals. A few scientists have observed sexual reproduction in laboratory vessels containing two different clones of placozoans.
PLACOZOANS AND PEOPLE
Placozoans have no known importance to humans.
Placozoans are not threatened or endangered.
FOR MORE INFORMATION
Grel, Karl G., and A. Ruthmann. "Placozoa." In Microscopic Anatomy of Invertebrates: Placozoa, Porifera, Cnidaria, and Ctenophora. Edited by Frederick W. Harrison and J. A. Westfall. New York: Wiley, 1991.
Howey, Richard L. "A Weird Wee Beastie: Trichoplax adhaerens." Microscopy-UK. http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/artoct98/tricho.html (accessed on December 13, 2004).
"Introduction to Placozoa: The Most Simple of All Known Animals." University of California, Berkeley, Museum of Paleontology. http://www.ucmp.berkeley.edu/images/WebLiftComb.gif (accessed on December 13, 2004).