ONYCHOPHORES: CHARACTERISTICS, NUTRITION, REPRODUCTION, SPECIES - BIOLOGY - 2025

The onychophores are a phylum of animals that are characterized by presenting an elongated body with a specific number of extensions on the sides, which allow it to move adequately on the substrate.

They are really ancient animals, since the first fossils that have been recovered from them come from the Cambrian period, in the Paleozoic era. However, they were first described by the British naturalist Landsdown Guilding in the 19th century.

Specimen of onychophor. Source: Bruno Vellutini from São Paulo / São Sebastião, Brazil

These animals are mainly found in environments in which humidity predominates. This is because they have very thin skin and cannot counteract the desiccation caused by harsh environmental conditions. Likewise, they live in places far from sunlight and only come out of them at night to hunt their prey.

characteristics

Onychophores are animals that are part of the Eukarya domain and, as such, are made up of eukaryotic cells, in whose nucleus is the genetic material (DNA) that makes up the chromosomes.

They are multicellular, being made up of several types of cells, each specialized in a specific function.

In addition to this, onychophores are coelomed. This means that they have an internal cavity called coelom, which is of mesodermal origin. The coelom is important because it contains the internal organs of the animal, although in these it only surrounds the gonads.

If an imaginary line is drawn along the longitudinal axis of the animal, two exactly equal halves are obtained, which then allows us to affirm that these animals present bilateral symmetry.

They are dioecious, since the sexes are separated, also presenting sexual dimorphism. The females are usually larger than the males.

They reproduce mainly sexually, with internal and external fertilization (depending on the species). They can be oviparous, viviparous, and ovoviviparous.

Morphology

Onychophores have an elongated body, giving the impression of being flattened dorsally ventral. Although most measure only up to 10 cm, specimens have been found that have exceeded this size and have reached more than 20 cm.

In general, its coloration is dark, showing colors that go from black, to dark brown, passing through green. There are also a few that exhibit slightly more vivid colors like orange.

They do not have articulated legs themselves, but they do have a kind of appendage that they can use for movement and locomotion. The number of these varies by species.

The body is divided into two regions: head (anterior) and trunk (posterior). There is no very clear delimitation between the two, so only someone very expert in these animals is able to point out the limits between them.

Morphology of an onychophore. Source: Lansdown Guilding

Head

The most striking feature of the onychophores' head is a pair of forward-facing antennae. At the base of each antenna there is a sensory organ that acts as an eye.

Below the antennae are other appendages known as oral papillae. These are of great importance in the feeding process of the animal, since they are in charge of expelling the liquid that paralyzes the prey.

On the head is also the mouth, from which the jaws emerge, which are another pair of appendages here.

The posterior surface of the head is plagued with chemo receptor papillae, which have a sensory function. They are particularly abundant on the antennae.

Trunk

The most striking elements of the body are the extensions that come out of it and that many insist on loving legs, but which are not such. The correct name is Lobopods. The number of these varies with the species.

Each one has trailing pads, in number from 3 to 6 and they are those that are in constant contact with the ground.

The body wall of onychophores is made up of three layers. From the outermost to the innermost they are: the cuticle, which is made of chitin, thin and very flexible; the epidermis; and lastly, several layers of smooth muscle tissue.

Taxonomy

The taxonomic classification of onychophores is as follows:

-Domain: Eukarya

-Animalia Kingdom

-Superphile: Ecdysozoa

-Filo: Onychophora

Nutrition

Heterotrophs - carnivores

Onychophores are heterotrophic organisms, which means that they are not capable of synthesizing their own nutrients. Because of this, they must feed on other living beings or on substances made by others.

In this sense, it has been established that these animals are predatory carnivores, their diet being represented mainly by a wide variety of animals such as arthropods.

The size of the prey does not seem to be a limiting element in this diet, since they eat both small animals and animals a little larger than themselves.

Prey capture

Thanks to the receivers that are located on their antennas, they can perceive possible prey, even at a distance.

Once the prey is identified, the animal throws a kind of silk whose function is to immobilize it. It is important to note that initially the silk is in a liquid state, but when it comes into contact with the environment it undergoes a solidification process, becoming a network that captures and immobilizes the prey.

Likewise, it is worth mentioning that the animal can throw this substance at distances as wide as up to 50 cm.

Once the prey has been immobilized, the onychophore approaches it and proceeds to inject it with a substance in which certain digestive enzymes are dissolved. These have the function of beginning to process and digest the tissues of the prey, in order to facilitate the digestion process.

Food tour

After the prey's tissues have been processed and transformed into mush, the onychophore proceeds to ingest them. Inside the body, this nutritional juice travels from the oral cavity to the pharynx and later to the esophagus.

Then it passes into the intestine, which is where the nutrient absorption process takes place, passing these into the circulatory system to be distributed to the various cells.

The substances that are not used by the animal, either because they are not necessary or because it has an inability to digest and absorb them, follow the path of the digestive tract towards the terminal portion of the intestine. Finally they are released in the form of feces to the external environment, through the anal orifice.

Reproduction

Onychophores are dioecious animals, which means that the sexes are separate. There are female individuals and male individuals. In them you can see the two types of reproduction that exist: asexual and sexual.

- Asexual reproduction

This type of reproduction does not involve the union of male and female germ cells (gametes). Because of this, the individuals that are generated through this process are exactly the same, both genetically and physically, as their parents.

There are various asexual reproduction processes. In the case of one of the species of onychophores that only has female individuals, the type of asexual reproduction is parthenogenesis.

Parthenogenesis consists of unfertilized eggs initiating a division and segmentation process, which is believed to be mediated by chemical or environmental factors, although it is not yet very well defined.

This process results in the development of an adult female individual. Of course, all the copies that are obtained through this type of reproduction are exactly the same.

- Sexual reproduction

Sexual reproduction necessarily requires the interaction and fusion of male and female sex cells (gametes). Those cells are represented by eggs and sperm.

Fertilization

The mechanism of reproduction in these animals is really varied and depends on each species. Despite the fact that fertilization is internal in all species of onychophores that reproduce sexually, the mechanism through which it occurs is different.

There are species in which a copulation occurs and the male deposits the sperm directly inside the female's body.

Likewise, there are also species in which reproduction occurs through a structure known as a spermatophore. This consists of a large mass of tissue in which the sperm are contained. The male deposits the spermatophore on the female's body surface, which is the one that subsequently introduces it into her genital pore.

Embryonic development

Embryonic development in onychophores is also varied, as there are some species that are oviparous, others viviparous, and a few ovoviviparous.

In the oviparous case, development occurs in an egg outside the mother's body. The segmentation in these eggs is superficial.

Most species are ovoviviparous, which means that they develop in eggs, but these remain inside the female's body until the individual is fully developed.

Lastly, there are also species of onychophores that are viviparous. In them, the embryo remains inside the mother's body, feeding on it. The new individual is born already fully formed.

Representative species

Eoperipatus totoro

It is one of the new species of onychophores recently discovered. Its first formal description dates from 2013. It can measure up to 6 cm in length and has a series of characteristic hairs on its body surface.

They have scales on the ventral part of the body that have a specific arrangement, which allows them to be differentiated from other onychophores. They are generally hidden in humid places, coming to the surface only during the rainy season.

Eoperipatus totoro specimen

Peripatus juliformis

It has the great honor of having been the first onychophore described, which occurred in 1826. It belongs to the Peripatidae family and is characterized by having a rather dark, almost black color. It is found mainly on the island of San Vicente, in the Caribbean Sea.

Eoperipatus horsti

It belongs to the Peripatidae family and is found mainly in the Asian continent, specifically in the western part of Malaysia. It has the same elongated body as onychophores, with its pair of front antennae that allow it to detect possible prey or dangers.

Austroperipatus aequabilis

It belongs to the Peripatopsidae family and is endemic to the northeast of Australia. Like all members of this family, it has the most primitive characteristics of the onychophores.

References

1. Barnes, R. (1977). Invertebrate zoology. New Interamerican Publishing House.
2. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
3. Curtis, H., Barnes,, Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition.
4. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
5. Morera, B. (2012). The onychophores, walking fossils. National University of Costa Rica
6. Ríos, P. Onychophora. Extracted from: