Trichoplax adhaerens Schulze 1883
Trichoplax adhaerens, the only described species in the phylum Placozoa, is the structurally simplest metazoan (multicellular animal). It is quite small (just 2 to 3 mm in diameter) and consists of several thousand cells arranged as a double-layered plate. It lacks anterior-posterior polarity and symmetry. However, the cells of the upper and lower layers differ in shape and there is a consistent dorsal-ventral orientation of the body relative to the substrate. Trichoplax moves by ciliary gliding, changing its shape along the edges as it moves, like an amoeba. Very small (presumably young) individuals can swim, but larger individuals crawl. It appears that Trichoplax feeds by phagocytosis of organic detritus. (Brusca and Brusca 2003) Although the discovery and description of Trichoplax adhaerens in 1883 (in saltwater tanks in Austria) sparked controversy among zoologists about what its significance might be with respect to inferring the characteristics of early metazoans, interest faded away as the (incorrect) idea took hold that these enigmatic organisms were simply abnormal larvae of hydrozoan cnidarians. Although strong evidence against this view was published in 1912 and 1914, there was no more discussion of Trichoplax in the zoological literature--nor citation of the key critiques of 1912 and 1914-- for more than half a century (although the misconception that it was a cnidarian was repeated in textbooks). Beginning in the 1960s, several researchers re-focused attention on Trichoplax, demonstrating that it is an adult form of a new phylum dubbed, in 1971, Placozoa. It became clear that placozoans could be found worldwide in the shallow waters of subtropical and tropical regions (Pearse 1989; Eitel and Schierwater 2010). (Syed and Schierwater 2002) Recent data all indicate that the Placozoa represent one of the earliest branching lineages in the metazoan tree, but the exact placement of this branch remains uncertain (Schierwater et al. 2009; Ball and Miller 2010 and references therein).
Dellaporta et al. (2006) report on the complete mitochondrial genome of Trichoplax adhaerens, which they note is the largest known metazoan mtDNA genome--at 43,079 bp, it is more than twice the size of the typical metazoan mtDNA. Srivastava et al. (2008) present a preliminary analysis of the genome of Trichoplax adhaerens, which is among the smallest known metazoan genomes at about 98 million base pairs.
The placozoan Trichoplax adhaerens is a small (1 to 2 mm) ciliated disc-shaped microscopic marine animal. The thin body consists only of an epithelium and an internal mesenchyme. The epithelium has two regions: an upper free, or dorsal, epithelium of cover cells and a lower attached, or ventral, epithelium of cylinder and gland cells. Some differences between the central and marginal areas in the body have also been reported. When the densely ciliated ventral epithelium is in contact with the substrate, the animals display a gliding or creeping locomotion. When the animal is feeding, its shape changes periodically. (Ueda et al. 1999; Maruyama 2004 and references therein; Srivastava et al. 2008)
Placozoans have been found only in shallow, nearshore, tropical and subtropical marine environments between approximately 30 degrees north and south of the Equator. Within these habitats, they tend to be absent from areas of high current and from bare sand bottoms. Because they are very sensitive to reductions in salinity, they will presumably not be found in fresh or brackish water or seasonally in areas that are subjected to heavy monsoon rains. (Ball and Miller 2010 and references therein; Eitel and Schierwater 2010) They are often found on mangrove tree roots, reefs, boat docks in the eulittoral and littoral zone, and around stony beaches (Eitel and Schierwater 2010). They can be abundant on coral reefs. Although placozoans have long been viewed as benthic organisms, Pearse and Voigt (2007) had more success collecting them on glass slides suspended 20 to 60 cm above the bottom than on glass slides situated on the bottom itself.
Pearse and Voigt (2007) summarized all the sites where placozoans were known to have been collected up to the time of their publication. Based on current knowledge, Eitel and Schierwater (2010) predict that most placozoans are found between the equator and 20 degrees North.
When large or stressed, placozoans can undergo binary fission (split into two individuals--sometimes three!) or bud to produce numerous multicellular flagellated "swarmers", which are capable of regenerating the amoeboid stage. However, this is not the complete life cycle, as there is also evidence of sexual reproduction. (Brusca and Brusca 2003; Ball and Miller 2010 and references therein) Sexual reproduction has not been observed in culture, but putative oocyte formation in degenerating animals is seen routinely. Population genetic analyses demonstrate allelic variation and evidence for genetic recombination in animals in the wild that is consistent with sex. (Signorovitch et al. 2005; Srivastava et al. 2008)
Pearse and Voigt (2007) report that in their sampling using glass slides, certain general types of organisms were often found in association with placozoans, including several kinds of sessile ciliates (solitary and colonial vorticellids as well as folliculinids); spirorbid and other serpulid polychaetes; and, in smaller numbers, free-living loxosomatid kamptozoans (entoprocts). Placozoans have been observed capturing and eating ciliates in a culture dish. Wild placozoans are probably opportunistic grazers and scavengers on organic detritus and on algae and bacteria in biofilms covering a diversity of substrates. (Pearse and Voigt 2007 and references therein)
Evolution and Systematics
Systematics and Taxonomy
Trichoplax adhaerens is the only described species in the phylum Placozoa, but recent work (Eitel and Schierwater 2010 and references therein) suggests that there may be many unrecognized placozoan species.
- Trichoplax reptans (author unknown) (synonym)