Last week, I cited a paper from Nature on the search for the Urmetazoan, a paper that briefly mentioned Trichoplax adhaerens as "the simplest known living animal". So what happens is that I get a bunch of e-mail asking, "what the heck is a Trichoplax?" It's easiest if I just tell you all at once, so here you go.

Salt-water aquarists may already know what Trichoplax is—it's a common salt-water scavenger that crops up on the glass walls of aquaria, sliding around and eating algae. It's not a particularly attractive aquarium resident, though.

It's a flattened blob, a few millimeters across and made up of a few thousand cells. It's main claim to fame is its remarkable simplicity: it is a multicellular animal that consists of only four apparent cell types, and the only obvious organization is into an upper and lower surface. The upper surface consists of a sheet of covering cells, while the lower surface contains two cell types: the gland cells that secrete digestive enzymes onto whatever the animal is sitting on, and the cylinder cells that absorb whatever nutrients are released. In between is a loose network of fiber cells that are responsible for the animal's movement.

The four classically recognized cell types of Trichoplax: cover cells of the upper epithelium, fiber cells of the intermediate layer, and cylinder cells and gland cells of the lower layer

Trichoplax completely lacks any kind of nervous system. It's genome is tiny, only 50Mb—significantly smaller than our 3Gb, and not much bigger than that of yeast, at 12Mb. It is considered the simplest multicellular animal in existence.

There are subtle hints that some of that simplicity may only be apparent, though: the latest molecular evidence suggests that there is some regional specificity, and that there may be a few other cell types scattered throughout the organism. They do have one Hox-like gene, Trox-2, that is expressed in scattered cells in the marginal zone. That and a few other molecules suggest that there may be some positional information in the animal to distinguish margin from center. There are also a few cells in the margin that secrete RFamide, a common neurotransmitter used by cnidarians (Hydra, for instance).

(C) Secp1, which encodes a putative small secreted protein, is expressed uniformly in the marginal zone, as shown in the top view (upper panel), and in all three layers, as shown in the transverse section (lower panel). (D) The Gsx-type homeobox gene, Trox-2 is also expressed in the marginal zone, but in discrete cells. (E) Another homeobox gene, Not, is expressed in folds in intact animals, as shown here, as well as in regenerating wounds. (F) Birefringent granules, possibly of calcitic calcium carbonate, are also limited to the marginal zone. (G) The T-box transcription factor Brachyury is expressed in a few cells or groups of cells in the marginal outgrowth zones of large Trichoplax individuals.

One other strange thing: in culture, Trichoplax is consistently asexual and reproduces by fission, but older cultures at high density begin to produce small motile presumptive sperm cells, and as individual animals desintegrate, they spew out ova. The two have never been observed to come together, though, so there is no fertilization, and while the ova may divide a half dozen times, they all eventually die. It is possible that there is another stage in the life cycle that is not viable under laboratory conditions and has never been observed.

Another likely possibility hinted at by these scattered scraps of complexity is that Trichoplax is a degenerate form, a descendant of a more complex ancestor that secondarily abandoned complexity. It's not clear who that ancestor would have been, though—some of the molecular evidence rules out the cnidaria, the obvious candidate. The enigmatic status of this animal has prompted the DOE Joint Genome Institute to sponsor an effort to sequence Trichoplax.

The sole representative species of the phylum Placozoa, Trichoplax adhaerens represents the simplest known animal, with the smallest known animal genome. The DNA sequence of the 50-Mbp Trichoplax genome will have far-reaching scientific importance, providing significant genomic insights into our understanding of how animal life evolved. This genome will have enormous utility to the scientific community, becoming the standard basal group for the comparative analysis of animal genomes, genes, and biological processes. Researchers who study the lower metazoans will clearly benefit, as will the bioinformatics and comparative genomics community. Prospects for future funding of functional annotation, microarrays, and evolutionary genomic studies are excellent, and available biological resources include established laboratory cultures, high-quality genomic DNA, arrayed fosmid and full-length cDNA libraries. A Trichoplax genome will make it possible to determine the gene and proteome content of the simplest known animals and provide the first genomics platform in a simple multicellular system.

You can also get a more detailed summary from the Trichoplax sequencing proposal (pdf).

Ender A, Schierwater B (2003) Placozoa Are Not Derived Cnidarians: Evidence from Molecular Morphology. Mol. Biol. Evol. 20(1):130-134.

Miller DJ, Ball EE (2005) Animal Evolution: The Enigmatic Phylum Placozoa Revisited. Curr. Biol. 15(1):26-28.