Trichoplax
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| iTrichoplax adhaerens | ||||||||||||||
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| Trichoplax adhaerens F.E. von Schultze, 1883 |
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; the only other species, 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 metazoan phylum.[1]
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. 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.
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 project is currently underway.[2]
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.[3] 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.[4] The full extent of their natural range is unknown, but they are easily collected in tropical and subtropical latitudes around the world.
If several Trichoplax are placed in the same dish they interact with each other. Movies of such interactions can be viewed here [1]
[edit] References
- ^ Dellaporta et al (2006). 'Mitochondrial genome of Trichoplax adhaerens supports Placozoa as the basal lower metazoan phylum'. Proceedings of the National Academy of Sciences 103 (23): 8751-6. PMID 16731622.
- ^ Trichoplax genome project.
- ^ Signorovitch AY, Dellaporta SL, Buss LW (2005). Molecular signatures for sex in the Placozoa. Proceedings of the National Academy of Sciences 102 (43): 15518-22. PMID 16230622.
- ^ Maruyama YK (2004). Occurrence in the field of a long-term, year-round, stable population of placozoans. Biol Bull 206 (1): 55-60. PMID 14977730.
[edit] External links
- The Trichoplax Genome Project at the Yale Peabody Museum
- A Weird Wee Beastie: Trichoplax adhaerens
- Information page from the University of California at Berkeley
- Mitochondrial DNA and 16S rRNA analysis and phylogeny of Trichoplax adhaerens: full text online article Entrez PubMed 12519915.
