Definitions

monociliated

Trichoplax

Trichoplax adhaerens is a simple balloon-like marine animal with a body cavity filled with pressurized fluid. It is given its own phylum, called Placozoa, sometimes referred to commonly as the tablet animals, which is a direct translation from the Greek scientific name; the only other species assigned to this taxon, Treptoplax reptans, was described in 1896 and has not been seen since, leading to doubts about its existence.

Individual Trichoplax are soft-bodied, about 0.5 mm across, and somewhat resemble a large amoeba. The name T. adhaerens was given because it tends to stick to its substrate, including glass pipettes and microscope slides. Its evolutionary relationships are still being investigated, but it may be allied with the cnidarians and ctenophores. Dellaporta et al have reported the complete mitochondrial genome sequence of Trichoplax adhaerens and showed that Placozoa are the most basal (that is, they branched off earliest) living eumetazoan phylum.

Trichoplax lacks organs and most tissues, including nerve cells and a nervous system, although evidence suggests that they evolved from species with nerve cells. It is made up of a few thousand cells of four types in three distinct layers: monociliated dorsal and ventral epithelia cells, ventral gland cells and the syncytial fiber cells. But sensory cells and muscle cells are apparently absent. The outermost layer (the monociliated cells) have a single cilium, which allow the adult to move. The epithelia of Trichoplax lack a basal membrane and the cells are connected by belt desmosomes. Lipid inclusions, called 'shiny spheres', are regularly distributed over the dorsal epithelia.

It feeds by absorption and has been observed to form temporary bulges to trap food. It climbs atop its food and uses the ventral surface as a temporary extraorganismal gastric cavity. Digestion is both extracellular and by phagocytosis.

When not feeding Trichoplax is actively motile with movement effected by ventral ciliation and by the fiber cell layer and lacks any polarity in its movement.

Mansi Srivastava and her colleagues drew the first genome draft for Trichoplax in 2008. They estimated that there are about 11,514 protein coding genes in Trichoplax genome. The genome size for Trichoplax is about 98 million base pair.

The haploid number of chromosomes is six. It has the smallest amount of DNA yet measured for any animal with only 50 megabases (80 femtograms per cell). A Trichoplax genome sequencing project has recently been completed.

Putative eggs have been observed, but they degrade at the 32-64 cell stage. Neither embryonic development nor sperm have been observed, however Trichoplax genomes show evidence of sexual reproduction. Asexual reproduction by binary fission is the primary mode of reproduction observed in the lab.

Trichoplax were discovered on the walls of a marine aquarium in the 1880s, and have rarely been observed in their natural habitat. The full extent of their natural range is unknown, but they are easily collected in tropical and subtropical latitudes around the world.

Of the 11,514 genes identified in the six chromosomes of Trichoplax, 80% are shared with cnidarians and bilaterians. Trichoplax also shares over 80% of its introns—the regions within genes that are not translated into proteins—with humans. This junk DNA forms the basis of regulatory gene networks. The arrangement of genes is conserved between the Trichoplax and human genomes. This contrasts to other model systems such as fruit flies and soil nematodes that have experienced a paring down of non-coding regions and a loss of the ancestral genome organizations.

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