Introduction to the Placozoans
Placozoa – Etymology: From the Greek Plakos for flat and Zoon for animal – hence Placozoa is a ‘flat animal’.
Characteristics of Placozoa:
- Have no symmetry or constant shape.
- Have no distinct tissues or organs.
- Have no nervous system.
- Have no body cavity or digestive cavity.
- Body is shaped like a flat disc.
- Body consists of a two layers of single cells.
- Live in marine environments.
Trichoplax adhaerens was first discovered in 1883 in a marine aquarium in Austria. At the time of discovery, and until recently it was considered to be planuloid larvae of some kind. Then in the 1960s it was discovered that they could reproduce sexually and they were given their own phylum. In 2017 two more species, in two new genera, were added to the phylum. Scientists consider that many more species may exist.
All three described species are morphologically identical. Species identification therefore is only possible through DNA analysis.
- Hoilungia hongkongensis
- Polyplacotoma mediterranea
- Trichoplax adhaerens
Basic Biology of the Placozoa
Placozoans are a small flat grayish animal growing to a maximum of about 3mm in diametre.
The placozoan body is made up of a few thousand cells at most. Each body, though basically disc shaped, changes shape like an amoeba as it moves around. It has no front or back and moves in any direction, gliding along on its cilliated under-surface.
Polyplacotoma mediterranea came to scientists attention because it has the unusual ability to be able to adopt a highly ramified body shape. Meaning they can stretch their body out thinly, something other placozoans have never been observed doing. This can result in P. mediterranea achieving a temporary body length of 10 mm.
The polyplacophoran body has only two layers of cells and 4 different types of cells, making it the simplest of all multicellular organisms. The two cell layers, its top and bottom surface, enclose a fluid filled space containing a network of stellate fibre cells. It has a surprisingly small amount of DNA in its cells, similar to species of bacteria, rather than other metazoans.
Ecology of the Placozoa
All known Placozoans are a marine animals, but they are very seldom recognised or identified. Their known distribution reflects the distribution of marine biology research stations, rather than its true geographic distribution.
They feed on protozoans, which they cover with their body and then secretes digestive enzymes onto. The resulting nutrients are absorbed by the same cells that secrete the enzymes.
Reproduction in the Placozoa
Reproduction can be asexual by either binary fission or budding and accidental cutting of the body into several parts, which results in each part becoming a new smaller placozoan. Eggs and sperm are known to be produced but nothing else is currently known about its sexual reproduction.
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