4 examples of microscopic animals (described)

When we think of living beings, we automatically turn to dogs, cats, the odd invertebrate and, hopefully, perhaps a plant.

It is not for less, since macroscopic organisms surround us from the beginning of the morning until we go to bed: that song of a bird while we go to work, ants busy lining up to feed and many other living beings surround us in the day to day. No matter how anthropized the environment is, life makes its way as best it can.

If when reflecting on life you go to beings that you can see with your eyes, we cannot blame you. It is estimated that there are 8.7 million species on the planet, practically all of them observable with the naked eye. What you may not know is that, behind all those "evolutionarily complex" organisms, there is a microscopic load that holds, as if it were a giant, all the ecosystems of the Land.

Today we sat in front of the microscope to show you some examples of microscopic animals. In addition, we take the opportunity to distinguish between a microorganism and an animal extraordinarily small, because, although it may not seem like it, they are completely different concepts and in no case interchangeable. Get ready to discover a world invisible to the human eye but exciting.

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The differences between microorganisms and microscopic animals

Microorganisms are essential to understand how the Earth works. Without going any further, it is estimated that, of the 550 gigatons (Gt) of carbon (c) present on the planet, bacteria contribute 15%. This makes them the second largest reserve of organic matter in all ecosystems, being only surpassed by plants, which contribute 80% of the total.

Microorganisms are unicellular living beings with an elemental biological organization. Their only characteristic in common is that they cannot be observed with the naked eye and they are "evolutionarily simple", since a virus has little to do with a protozoan, for example. The term "microorganism" refers to a polyphyletic group, that is, it encompasses taxa that do not have a common ancestor. Its only usefulness is informative, since it does not report relevant information on the taxonomic category and phylogenetic position of these beings.

So that, "Microorganism" is a kind of mixed bag where everything that is made up of only one cell fits (ie acellular according to some authors, like viruses), while microscopic animals are governed by a series of much more complex classification criteria. For a living being to be considered part of the Animalia kingdom, it must meet a series of parameters:

• Being eukaryotic: the cells that make up this organism must present a true nucleus that encompasses its genetic information. Bacteria are prokaryotic and animals, plants, and fungi are eukaryotic.
• Being multicellular: the body of the living being must be made up of more than one cell. A protozoan, for example, is unicellular.
• Being heterotrophic: the animal must obtain its energy from organic matter. Based on this parameter, plants are excluded from the Animalia kingdom.
• It must present a tissue organization (except poriferous): the animal must present tissues, which are specialized cellular organizations based on a function.

In addition, animals are characterized by an excellent movement capacity (in most cases), by lacking chloroplasts, because they do not have a cell wall (as plants and fungi do) and because they have an embryonic development with certain common guidelines. Based on all these parameters, we discard plants, fungi and all microorganisms.

Examples of microscopic animals, and their characteristics

Once we have differentiated animals without any margin of error from other groups of living beings, we are ready to show you some examples of microscopic animals. Do not miss them.

1. Copepods

Copepods are a subclass of very small maxilopod crustaceans. It is a small group that includes about 8,500 species, most of them marine, generally semi-transparent in color. Most of these animals measure between 1 and 5 millimeters, so they perfectly fit the definition of "microscopic." In any case, there are parasitic copepods that reach up to 32 centimeters in length, although this is a complete exception.

Due to their microscopic size, copepods are considered part of zooplankton, the fraction of the aquatic fauna in size tiny that feeds by ingestion of already processed organic matter (unlike phytoplankton, mostly composed of algae). They are the main source of nutrients for many macroscopic marine organismsThus, they represent an essential part of the base of the trophic chain of aquatic ecosystems.

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2. Tardigrades

Tardigrades are one of the most curious and interesting living things on Earth. They are some of the smallest animals we know of, since the smallest are less than 0.1 millimeters and the largest sizes are 1.5 millimeters. In addition, they occupy a somewhat delicate phylogenetic position, as they are encompassed within the panarthropoda clade, which contains tardigrades, onychophores and arthropods themselves. They are not arthropods as such but neither are microorganisms, so they "float" between two taxonomic waters.

Most tardigrades are phytophages (they eat plants) or bacteriophages, but there are some carnivorous species that feed on other tardigrades. These very curious animals are also known as "water bears", as they have an "almost" morphology. mammal, with various segments with legs reminiscent of a bear and a mouth with multiple stilettos. They are also famously known for their extreme endurance, as they are capable of entering a state of cryptobiosis when conditions are unfavorable, reducing its water content in the body up to a 1%.

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3. Rotifers

Rotifers are a perfect example of microscopic animals, as most of them range in measurements from 0.1 to 0.5 millimeters. They are common in fresh waters around the world, although some marine species have also been exceptionally recorded.

These animals present a completely atypical bilateral symmetry within the animalia kingdom: have a mouth in the ventral area of ​​the cephalic region and this may be surrounded by ciliated bands of the rotator apparatus, which create small currents that attract food particles from the environment. They feed on microscopic organic particles, bacteria, single-celled algae, and certain protozoa.

4. Dust mites

Although when going to the Acari subclass we automatically think of animals of a very small size, this is not a general rule, far from it. This category, encompassed within the Arachnida class, includes ticks, plant mites and many others macroscopic invertebrates that can be seen with the naked eye, although many other representatives are microscopic.

Therefore, to arrive at this last example, we must spin a little finer. We are referring to gender Dermatophagoides or dust mites, microscopic invertebrate animals that measure between 0.2 and 0.5 millimeters. The most common species included in this taxon and spread throughout most of the world are Dermatophagoides farinae, Dermatophagoides pteronyssinus Y Euroglyphus maynei.

These animals they are extremely simple at the evolutionary level, since they lack a stomach and have a very simple intestine, which digests small particles of organic matter present in the environment. Males live from 10 to 19 days, while females last up to 70 days, laying a huge number of eggs during their last weeks of life.

Resume

Beyond fish, reptiles, mammals, amphibians and birds, there is a world of microscopic invertebrates that escapes our sight, but they are still essential for food chains, ecosystems and the world of investigation. Without going any further, marine ecosystems could not exist without zooplankton: no matter how small the animal is, its work is invaluable and unmatched, wherever it is.

Finally, we emphasize the following idea: a microorganism is not the same as a microscopic animal. Remember that bacteria are unicellular and prokaryotic, whereas animals are composed by two or more cells and have a nuclear envelope that delimits their genome from the rest of the body mobile. Based on this simple premise, it is possible to differentiate animals from all other existing taxa.

Bibliographic references:

• Ban, S., Burns, C., Castel, J., Chaudron, Y., Christou, E., Escribano, R.,... & Wang, Y. (1997). The paradox of diatom-copepod interactions. Marine Ecology Progress Series, 157, 287-293.
• Boxshall, G. A., & Halsey, S. H. (2004). An introduction to copepod diversity. Ray Society.
• Dumont, H. J. (1983). Biogeography of rotifers. In Biology of Rotifers (pp. 19-30). Springer, Dordrecht.
• Guidetti, R., & Bertolani, R. (2005). Tardigrade taxonomy: an updated check list of the taxa and a list of characters for their identification. Zootaxa, 845 (1), 1-46.
• Hashimoto, T., Horikawa, D. D., Saito, Y., Kuwahara, H., Kozuka-Hata, H., Shin, T.,... & Kunieda, T. (2016). Extremotolerant tardigrade genome and improved radiotolerance of human cultured cells by tardigrade-unique protein. Nature communications, 7 (1), 1-14.
• Sládeček, V. (1983). Rotifers as indicators of water quality. Hydrobiologia, 100 (1), 169-201.
• Westh, P., & Ramløv, H. (1991). Trehalose accumulation in the tardigrade Adorybiotus coronifer during anhydrobiosis. Journal of Experimental Zoology, 258 (3), 303-311.