PSEUDOCOELOMATES: CHARACTERISTICS, REPRODUCTION AND NUTRITION - BIOLOGY - 2025

The pseudocelomados are animal group comprised of organisms with body cavity it is of mesodermal origin and is called pseudoceloma. This works like a true coelom. From an evolutionary perspective, the appearance of a body cavity represented a series of advantages for its owners, related to the movement and development of organs.

According to the evidence, this cavity could have evolved on multiple occasions in the different branches of the animal lineages. Therefore, the term describes generalities of a body plan and not a taxonomic unit.

Source: Joel Mills

Representatives of this group are of little popular knowledge, and include nematodes, nematomorphs, acantocephalus, rotifers, and marine kinorhkids.

Historically, these vermiform animals were grouped in a phylum called Asquelmintos. Today, each of the classes that constituted them has been raised to edges. The taxonomy of the group is still subject to controversy, supporting the idea that they do not constitute a monophyletic group.

What is the coelom?

The coelom (a term derived from the Greek koilos, meaning cavity) is defined as a body cavity totally surrounded by mesoderm, one of the three embryonic layers of triblastic animals.

It is formed during the gastrulation event, where the blastocele is filled (partially or completely) by the mesoderm. The way it is formed varies depending on the group studied.

Animals that possess a true coelom are known as coelomats. In the same way, there are animals that lack said body cavity and are known as acellomates.

It is necessary to mention that the cellophane animals are triblastic. Those that lack mesoderm do not have a coelom but are not considered acellomate.

One group of animals exhibits a different body plan, which appears to be in between: a body cavity that is not completely lined with mesoderm called a pseudocoelom. Embryologically, this cavity is a persistent blastocele.

Some authors consider that the use of the coelom as a feature of high taxonomic value has been exaggerated, as have all the discussions related to its origin and possible homologies between the groups.

The pseudocoelom: a body cavity

In addition to its particular embryonic origin, the pseudocoelom is characterized by being filled with fluid or with a substance of gelatinous consistency with some mesenchymal cells.

Functionally it shares some aspects with true coeloms, namely: (i) freedom of movement; (ii) add extra space for the development and differentiation of the digestive system and other organs, since they will not be compressed within a solid matrix; (iii) participate in the circulation of materials with the body and (iv) in the storage of waste substances.

In addition, the cavity can function as a hydrostatic skeleton, capable of providing support to the animal, as occurs with the coelom in earthworms.

Despite these adaptive potentials, the pseudocoelom does not fulfill all these functions in all species that possess it.

Taxonomy and phylogeny

Organisms with a pseudocoelomed plan of organization are characterized by being small, most of them shaped like a worm; and they can have a free life or be parasites of some animal.

The term pseudocoelomate groups together a series of highly heterogeneous organisms and describes a body plan, not a taxonomic unit. In other words, they are not a monophyletic group.

The absence of synapomorphies or shared derived characters suggests that each phylum evolved independently and the similar structure in terms of the body cavity is the result of convergent evolution.

As the body of these animals is soft, their fossil record is extremely scarce, a fact that does not help to clarify the phylogenetic relationships between groups of pseudocoelomates.

Holley classification

Although the classification of this group is surrounded by controversy and discrepancies, we will use a simple classification used by Holley (2016), consisting of six edges, grouped into two superphiles:

- Superphylum Lophotrochozoa.

- Phylum Rotifera.

- Phylum Acanthocephala.

- Phylum Gastrotricha.

- Superphylum Ecdysozoa.

- Phylum Nematoda.

- Phylum Nematomorpha.

- Phylum Kinorhyncha.

Characteristics: reproduction, nutrition and habitat

We will now describe the characteristics of the most prominent groups of pseudocoelomates.

Phylum Rotifera

Rotifers are a group that inhabits bodies of fresh and salty waters and semi-terrestrial environments, with a cosmopolitan distribution that includes more than 1800 species described so far. Most species have a solitary lifestyle, and a few can form colonies.

They are small animals, between 1 and 3 mm long, widely varied in their shape and coloration: we find from transparent shapes to very colorful variants.

The digestive system is complete. Some species feed on small organic particles and algae that they manage to capture by suspension, while others have the ability to trap small prey, such as protozoa.

They are dioecious, sexually dimorphic, where males are usually smaller. Reproduction can be sexual or through parthenogenesis.

Phylum Acanthocephala

It is made up of about 1000 species of animals that are strict parasites of fish, birds and mammals. They do not usually infect humans.

They are larger than rotifers. They are generally about 20 cm long, although some species are over a meter long.

Their lifestyle as parasites is associated with the total loss of the digestive system, while other systems have been considerably reduced.

They are dioecious, and the females are larger. Reproduction is sexual in nature, and development leads to the formation of an acanthor larva.

Phylum Gastrotricha

It includes about 450 species of aquatic animals, inhabitants of oceans and fresh waters. They are small, in a range of 1 to 3 mm, with a body divided into head and trunk. They resemble rotifers.

They are predators of protozoa, bacteria, algae, and detritus, which they can consume through the mouth, and digest them through the entire digestive system.

They are hermaphroditic and generally undergo cross fertilization, which leads to the development of eggs with hard shells.

Phylum Nematoda

Among the pseudocoelomates, there is no doubt that nematodes are the most relevant organisms in diversity, abundance, and impact on humans. More than 25,000 species of nematodes have been reported, although more than 500,000 are presumed to exist.

To get an idea of ​​its abundance, consider the following fact: in a single rotten apple more than 90,000 nematodes have been found.

They have a worldwide distribution and inhabit a wide range of ecosystems, such as soil, ocean and bodies of fresh water, or being parasites of animals or plants.

Most are dioecious, with males smaller than females. Fertilization is internal and the eggs are usually stored in the uterus until the moment of deposition.

References

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