Gastrotric: Characteristics, Habitat, Reproduction, Feeding

The gastrotricos (Gastrotricha) constitute a phylum of the Animalia kingdom that is located in the group of the so-called asqueminths, together with the nematodes and rotifers, among others.

They were described by the Russian microbiologist Ilia Mechnikov in 1864. This phylum is made up of two orders: Chaetonotida and Macrodasyida. Between them they cover a little more than 500 species.

Exemplary of gastroteric. Source: Nebarnix [CC BY-SA 3.0 (]

These animals inhabit various bodies of water on the planet and due to their similarity with rotifers, sometimes, for inexperienced eyes, they can be confused with them.

Article index

  • one


  • two


    • 2.1

      – External anatomy

    • 2.2

      – Internal anatomy

  • 3


  • 4

    Habitat and distribution

  • 5


    • 5.1

      Asexual reproduction

    • 5.2

      Sexual reproduction

  • 6


  • 7



Gastrotics are considered multicellular organisms, since they are made up of different types of cells that are specialized to perform various functions. Similarly, in your cells, DNA is limited to the cell nucleus, packed together to form chromosomes.

These types of animals are tripoblastic, bilaterally symmetrical and pseudocoelomed. During its embryonic development, the three germ layers known as mesoderm, endoderm and ectoderm are formed. From them the organs of the adult individual will be formed.

Regarding symmetry, it can be established that these animals are made up of two halves that are exactly the same. They are pseudocoelomed because it has an internal cavity, the pseudocoelom, which has certain similarities with the coelom of other animals, but different embryonic origin.

They are hermaphrodites, with internal fertilization, oviparous (some species can be viviparous) and of direct development. They are also heterotrophs.


– External anatomy

The body of gastrotricks is elongated (like a bowling pin), small in size (no more than 4 mm). It is divided into three regions: head, neck and trunk.

On the head they have small extensions that are known as cilia. They have lateral location. In this region they also have an opening: the mouth.

The neck is a very small region that functions more as a transition zone between the head and the trunk.

The trunk is covered by a thin cuticle. Towards the ventral part you can see the cilia and in its dorsal part there are scales that have spines.

Exemplary of gastroteric with enlargement of its body surface. Source: Maria Balsamo [CC BY 4.0 (]

The body wall is made up of the union of several layers or layers, from the inside out: pseudocele, longitudinal muscle layer, circular muscle layer, basement membrane, syncytial epidermis and cuticle.

In the terminal part of the trunk a species of tubercles can be located. In these there are some (adhesive) glands that secrete substances that help them to attach to the substrate.

– Internal anatomy

The gastrotricos have digestive, excretory, nervous and reproductive systems. While they lack a respiratory and circulatory system.

Digestive system

The digestive system has an entrance orifice (mouth) and an exit orifice (anus). It is also quite simple, with the absence of specialized organs such as the stomach or esophagus.

It starts in the mouth, which gives way to the pharynx. This communicates with the outside through channels that allow it to expel excess water so as not to interfere with digestion. After the pharynx is the intestine, whose appearance appears multilobed. This eventually flows into the anal orifice.

Excretory system

It is represented by two protonephridiums, which consist of very long tubes that are contoured and wound on themselves. These flow into an excretory pore that opens to the outside on the ventral surface of the animal.

Reproductive system

Gastrotricks are hermaphrodites, which means that they have both male and female reproductive organs.

The male organs are made up of a pair of testicles, from which some ducts (deferens) start that lead to a structure similar to a sac, from which the copulatory organ emerges.

The female reproductive organs consist of the ovaries, which connect with oviducts that lead to a large organ that some specialists call the copulatory bag.

Nervous system

It is of the ganglionic type, since on both sides of the pharynx there are two ganglia that are joined together by some nerve fibers. From these ganglia nerves emerge that are distributed throughout the body of the animal.


The taxonomic classification of gastrotric is as follows:

-Domain: Eukarya.

-Animalia Kingdom.

-Superphile: Spiralia.


-Filo: Gastrotricha.

Habitat and distribution

Gastrotics are animals typical of aquatic ecosystems. Among these, they do not have any type of preference for salt or fresh water, that is, they can be found both in seas and in rivers or lagoons.

In terms of distribution, they are widely located all over the planet. Factors such as temperature do not seem to be limiting elements for these to develop in any habitat.

Gastrotricks are part of the animals called interstitials. This means that, in aquatic ecosystems, they occupy very small natural spaces such as gaps and fissures. Due to the small size of these animals, the reduced space is not a problem.


These types of animals are hermaphrodites, that is, they have both female and male reproductive structures. In them it is possible that there are two types of reproduction that exist, sexual and asexual.

Asexual reproduction

In asexual reproduction, the fusion of sex cells does not occur, so there is no exchange of genetic material between individuals. Among the wide range of asexual reproduction processes that exist, gastrotricks present parthenogenesis.

Parthenogenesis is a process that consists of a new individual being generated from an unfertilized female sex cell (ovum). In this sense, the ovum, due to the action of various factors, some of which are not yet sufficiently clear, begin to undergo successive divisions until they become an adult individual.

Sexual reproduction

Sexual reproduction involves the union of male sex cells (sperm) with female sex cells (eggs). Despite the fact that they are hermaphrodites, self-fertilization is not frequent, but they mate with other individuals.

In some species, fertilization is direct, that is, the sperm are deposited directly in the gonopore. While in others it is indirect, through spermatophores, in which the sperm are packed.

After fertilization occurs, the eggs are laid. These measure approximately 50 microns. Now, in these organisms there are two types of eggs. In the first place there are the regular ones, known as direct ones, which are characterized by having a thin cover and in which the embryos develop rapidly.

The other type of eggs are known as duration eggs. These have a thick and rough shell that protects the embryo from any adverse environmental conditions.

These eggs are very useful when the temperature or humidity conditions are not the most ideal, since they keep the embryo protected until the conditions are favorable again.

With regard to the development of these animals, it is straightforward. Gastrotricks do not go through larval stages, since the individual that emerges from the egg presents the characteristics similar to those of adult gastrotricks. The time it takes for an egg to hatch ranges from 1 to 4 days.


Gastrotricks are heterotrophic organisms. This means that they are not able to synthesize their own nutrients, so they need to feed on other living beings or substances made by them.

Because these animals are very small, they must feed on almost microscopic particles. This is known as a micro-phage diet.

The gastrotric diet is made up of bacteria, detritus and some unicellular algae, as well as some protozoa.

Food is ingested through the mouth and passes directly into the pharynx. Excess water is eliminated through tubes that come out of the pharynx. Food continues to the intestine, where nutrients are absorbed. Substances that are not used by the body are discharged through the anal opening.


  1. Barnes, Robert D. (1982). Invertebrate Zoology. Holt-Saunders International.
  2. Brusca, RC & Brusca, GJ, (2005). Invertebrates, 2nd edition. McGraw-Hill-Interamericana, Madrid
  3. Curtis, H., Barnes, S., Schneck, A. and Massarini, A. (2008). Biology. Editorial Médica Panamericana. 7th edition
  4. Hejnol, A. (2015) Gastrotricha. Chapter of the book: Evolutionary developmental biology of invertebrates2: Lophotrochozoa (spiralia). Springer vienna
  5. Hickman, CP, Roberts, LS, Larson, A., Ober, WC, & Garrison, C. (2001). Integrated principles of zoology (Vol. 15). McGraw-Hill.
  6. Kanneby, T. and Hochberg, R. (2014). Phylum Gastrotricha. Chapter of the book: Ecology and general biology: Thorp and Covich’s freshwater invertebrates. 4 th Academic Press

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