PLACOZOANS: CHARACTERISTICS, REPRODUCTION, FEEDING - BIOLOGY - 2024

The Placozoa (Placozoa) are a phylum of the Eumetazoa subkingdom found in organisms evolved very little flat and simple appearance. They were described for the first time during the 19th century (year 1883), but it was not until 1971 when they were established as a cutting edge with its own characteristics.

Placozoans are fairly simple animals, of which very little data is available, since they have been observed on very few occasions. There are not enough records to establish their behavior, feeding or reproduction patterns.

Placozoan specimen. Source: Bernd Schierwater

Of the two species that make up this phylum, only Trichoplax adhaerens has been observed relatively frequently naturally. The other species, Treptoplax reptans, has not been found or observed in its natural environment for more than a century.

This means that these animals are almost unknown to specialists in the area, which is why there is no further data on the members of this phylum. Placozoans are the great unknown of the animal kingdom.

Taxonomy

The taxonomic classification of placozoas is as follows:

- Domain: Eukarya.

- Animalia Kingdom.

- Subkingdom: Eumetazoa.

- Phylum: Placozoa.

characteristics

Placozoans are multicellular eukaryotic organisms. This means that they are made up of cells whose genetic material is enclosed and delimited within the cell nucleus. Likewise, the cells that make it up are specialized in specific functions.

Likewise, they do not present any type of symmetry. The specialists who have been in charge of their study have determined that they do not have radial or bilateral symmetry.

They are quite primitive organisms that feed on other living beings or substances produced by them, which is why they are considered heterotrophs. It is not yet well determined if they have predatory habits.

Placozoans apparently have a predilection for marine environments, with a moderate salinity level. They have not been found in freshwater habitats.

Morphology

External anatomy

Placozoans are extremely simple animals. In fact, it is believed that they are the simplest organisms that make up the animal kingdom. There are still many data that are unknown about its morphology.

Regarding the shape that the few specimens that have been observed have presented, it is amoeboid or globose, with average measurements of 1 to 2 mm in diameter. With regard to color, placozoans do not have a specific color. Some transparent specimens have been seen, as well as some with shades from the pink palette.

As their name suggests, placozoans appear to be a simple plate. However, within its simplicity there is a certain degree of complexity.

Internal anatomy

Internally they present a cavity that is filled with liquid, which is subjected to certain levels of pressure. Likewise, it is made up of the apparent union of several layers of cells. Placozoans have a ventral surface and a dorsal surface.

The ventral surface is made up of ciliated cylindrical cells and glandular-like cells that do not have cilia. It should be noted that the cells on this surface have been shown to produce some digestive enzymes.

On the other hand, the dorsal surface is made up of cells that have cilia and are flattened in shape. They also have a last type of cell known as fiber cells, which are intermediate in location; that is, they are located between the ventral and dorsal surfaces.

Taking this into account, it can be affirmed with total security that the members of the phylum placozoa are made up of only 4 types of cells, thus confirming the simple and primitive nature of these animals. Despite the fact that there are only 4 types of cells, of each one, there are thousands of copies carrying out their functions.

Regarding specialized systems, placozoans do not have any type of organs capable of performing complex functions such as breathing or excreting, among others. Similarly, there is no presence of a basement membrane or an extracellular matrix.

What there are in placozoa are microtubules and filaments that pass through the extensions between each fiber cell. This type of system is believed to provide the animal with stability, as well as the ability to move around the substrate on which it sits.

It is important to note that when it comes to genetic material (DNA), placozoans are also characterized by being the living organism that has the least amount of DNA in its genome.

Reproduction

Asexual and sexual reproduction mechanisms have been observed in placozoans.

-Asexual reproduction

It is the most common and frequent in these organisms. It is also the type of reproduction that has been shown to be most successful in placozoa, generating viable offspring, capable of continuing the hereditary lineage.

Placozoans reproduce asexually through two processes: vegetative fission and fragmentation. It is important to mention that asexual reproduction allows the obtaining of a large number of individuals in a short time.

Vegetative fission

It is one of the reproductive methods most used by placozoans. It does not require the union of gametes nor does it involve the exchange of any type of genetic material.

For binary fission to occur in a placozoa, what happens is that in the midline of the member the animal begins to strangle or constrict, in such a way that it ends up dividing into two exactly the same physically and of course, with the same genetic information..

Fragmentation

In this process, as its name indicates, small fragments are shed from the body of the placozoa, from which an adult individual regenerates, thanks to the process of cell multiplication known as mitosis.

-Sexual reproduction

As is well known, sexual reproduction involves the union of male and female sex cells or gametes. In placozoa, it is not fully proven that sexual reproduction is natural in them, since what has been done is to induce it under controlled conditions in the laboratory.

Likewise, the fertilization process has not yet been properly documented, so it is not certain how it occurs in these organisms. What is known is that at a certain point in the life of the placozoa, especially when the population density increases, they begin to degenerate.

In the interspace (between the dorsal plate and the ventral plate) an oocyte develops. Sperm cells come from small, non-flagellated cells that form when the animal begins to degenerate.

After fertilization, for which reliable data are not yet available, the zygote begins to develop. However, at an experimental level, the successful development of an individual has not been achieved by this method, since all of them die when they reach the 64-cell stage.

Sexual reproduction has not been observed in its natural habitat.

Feeding

Like many aspects of placozoans, food also remains, in part, unknown. There are no reliable data on the food preferences of this type of organisms. However, the data collected by specialists seems to indicate that they feed on some microorganisms.

In the cultures that have been carried out at the laboratory level, they have been fed with protozoa such as those of the Cryptomonas genus or chlorophyta algae such as those belonging to the Chlorella genus.

Regardless of the food they eat, it has been established that placozoans form a kind of sac on their ventral surface. There, with the help of the digestive enzymes secreted by the cells in that area, digestion takes place. The process of excretion of wastes from metabolism is not yet clear.

Classification

Placozoans are a relatively new group. This is made up of a single case, Trichoplacoidea, as well as a single family, Trichoplacidae.

However, with regard to genera, two have been described: Trichoplax and Treptoplax. Each of these genera has only one species.

Specimen of Trichoplax adhaerens. Source: Neil W. Blackstone, 2009

In the case of the genus Trichoplax, the species is Trichoplax adhaerens, while the species of the genus Treptoplax is Treptoplax reptans.

However, with regard to Treptoplax reptans, it has only been seen and described once, in the year 1896. After that moment, no specimen of this species has been found again, so there are still those who question its existence.

Trichoplax adhaerens

This species was discovered in 1883 by the German zoologist Franz Schulze. The discovery was made in the aquarium of the Zoological Institute of Graz, in Austria.

This species is the one that has served as a model to describe the placozoa. It is the species that has provided all the information available about this group of organisms.

References

1. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
2. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition

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