Everything about Placozoa totally explained
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 hasn't 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. 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.
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're easily collected in tropical and subtropical latitudes around the world.
Further Information
Get more info on 'Placozoa'.
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