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Placozoans are tiny amazing animals.
Verylittle is known about them because they have never been observed in their natural habitat. No one knows what substratethey live on or what they eat in nature. It is even unknown whether or not they reproduce sexually like most animals. They were discovered in thelate 1880's living on the glass walls of an aquarium in a Europeanlaboratory. Since then, most of what has been learned about their biologyhas come from studying cultures of them kept alive in various laboratories around the world. Not surprisingly, given their small sizeand squishy nature, fossil placozoans have yet to be discovered.

Placozoan from Guam, roughly 400 microns across

Placozoan Morphology
Placozoansare extremely simple animals. Perhaps not coincidentally, they also have the smallest amount of DNA ever measured for any type of animal. Their bodies are made up of a few thousand cells of just four types. You can compare this tosponges, which have anywhere from 10to 20 different kinds of cells, toflies, whichhave roughly 90 different cell types, and to you and othermammals, whichhave over 200 different types of cells. Placozoans are transparent, flat, round (up to 3 millimeters across), and have two distinct sides.A tissue layer composed of two types of cells, column-shapedcylinder cells with cilia andgland cells without cilia, make up the ventral (or bottom) surface.The upper dorsal surface consists of a layer of justcover cells, which are ciliated and flattened toward the outside of the animal. The image above shows the dorsal surface of a small specimen (just over 4/10ths of a millimeter in diameter) seen from abovethrough a microscope. The dorsal and ventral tissues appearto correspond to ectoderm and endoderm, the outer and inner tissuelayers of most animals, but it is not yet known which is endodermand which is ectoderm. The fourth type of placozoan cells are calledfiber cells. These cells are star-shaped and reside inthe space between the two tissue layers. The star shape results fromthin extensions of the cells which connect to each other in a network. Cellularmaterial such as microtubules and microfilaments traverse the extensions fromfiber cell to fiber cell. It is hypothesized that this system of connectedcells in important in coordinating the movement of placozoans. Placozoans can move in two ways, by gliding on their cilia and by changing their shape like an amoeba.
Placozoan Feeding and Reproduction
In the laboratory, placozoans have been kept alive by feeding them the flagellatedchromistCryptomonas orthechlorophyteChlorella. It is unknown what placozoans feed on in nature; they may feedon a number of different organisms. A placozoan feeds with its ventral surface,which produces digestive enzymes. Often, individuals contract part of the ventral surface into a sac where digestion may take place more efficiently.Placozoans can reproduce asexually by either binary fission or, less often,by budding. Some laboratory observations suggest that sexual reproductionmay occur. When the population density becomes high, placozoans start to degenerate. Usually a single egg or oocyte develops in the interspace of a degenerating placozoan. Small cells (without flagella) that also form whenplacozoans degenerate are inferred to be sperm cells. After fertilization, whichdoes not appear to have been documented, cleavage begins. Development has only beenobserved to the 64 cell stage, at which point the cells cease to separate while thenuclear DNA continues to multiply until the nucleus bursts. Placozoan reproductionand development will probably not be fully understood until they are observedin their natural habitat.
The Phylogenetic Position of Placozoa
Some scientists have inferred that placozoans might be the earliest branch of animals (as shown in box A below) because they are so simple. However, the discovery that placozoan epithelial cells are connected by junctions ofextracellular proteins (belt desmosomes), a condition present in allanimals other thansponges, suggested that placozoans may have diverged later in the history of animals(box B below).
More recently, data from molecular sequences (18S)have indicated that placozoans might have diverged even later in the historyof animals (box C above). If this latter view were true, it would imply that placozoansare secondarily simplified from more complex ancestors that had a nervous system. Interestingly enough, placozoans contain cells, which are dispersed around their outer edge, that react withantibodies against a neuropeptide that is present in the nervous systemof cnidarians. In any event, the alternative hypotheses for the phylogenetic position of Placozoa within the animals need further testing with additional data.
Placozoan Diversity
Just two species of placozoans have ever been described,Trichoplaxadhaerens andTreptoplax reptans. The latter of these has never been seen since its description in 1896, causing some to doubt itsexistence. The former, however, has been reported from many tropical andsubtropical locations around the world, including: the Bermudas, the Caribbean Sea, Eastern Australia, the Great Barrier Reef, Guam, Hawaii, Japan, the Mediterranean Sea, Palau, Papua New Guinea, the Red Sea, Vietnam,and Western Samoa. This prompts the question of whetherTricoplaxadhaerens is really a single species. Furthermore, placozoans are so cryptic that their diversity might be much greater than we realize.

Learn more aboutTrichoplax from Richard L. Howey, whose page "A Weird Wee Beastie", is available atMicroscopy-UK.

Sources:

Buchholz, K. and Ruthmann, A., 1995, The mesenchyme-like layer of the fiber cells ofTrichoplax adhaerens (Placozoa), a syncytium,Zeitschrift fuer Naturforschung Section C Biosciences, 50, 282-285.
Collins, A. G., 1998, Evaluating multiple alternative hypotheses for the origin of Bilateria: An analysis of 18S molecular evidence,Proceedings of the National Academy of Sciences, USA, 95, 15458-15463.
Grell, K. C. and Lopez-Ochoterena, E., 1987, A new record ofTrichoplax adhaerens F. E. Schulze (Phylum Placozoa) in the Mexican Caribbean Sea,Anales del Instituto de Ciencias del Mar y Limnologia Universidad Nacional Autonoma de Mexico, 14, 255-256.
Grell, K. G. and Ruthmann, A., 1991, Placozoa, in F. W. Harrison and W. J. A (ed.),Microscopic Anatomy of Invertebrates, Vol. 2. Placozoa, Porifera, Cnidaria, and Ctenophora (New York: Wiley-Liss), pp. 13-28.
Pearse, V. B., 1989, Growth and behavior ofTrichoplax adhaerens: First record of the phylum Placozoa in Hawaii [USA],Pacific Science, 43, 117-121.
Schuchert, P., 1993,Trichoplax adhaerens (Phylum Placozoa) Has Cells That React with Against the Neuropeptide Rf Amide,Acta Zoologica (Copenhagen), 74, 115-117.

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