4 examples of microscopic animals (described)

Several examples of the extent to which animals can be the size of single-celled organisms.

When we think of living things, we automatically think of dogs, cats, the odd invertebrate and, if we are lucky, perhaps a plant.

No wonder, because macroscopic organisms surround us from the time we start in the morning until we go to bed: that bird singing while we go to work, ants busy lining up to feed, and many other living beings surround us on a daily basis. However anthropized the environment may be, life makes its way as it can.

If, when reflecting on life, you turn to creatures you can see with your eyes, we can't blame you. There are an estimated 8.7 million species on the planet, virtually all of them observable to the naked eye. What you may not know is that, behind all these "evolutionarily complex" organisms, there is a microscopic charge that holds, as if it were a giant, all the ecosystems of the Earth.

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

The differences between microorganisms and microscopic animals

Microorganisms are essential to understanding 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, second only to plants, which contribute 80% of the total.

Microorganisms are unicellular living beings with an elementary Biological organization.. Their only common characteristic is that they cannot be observed with the naked eye and 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 to say, it includes taxa that do not have a common ancestor. Its only usefulness is informative, since it does not provide relevant information on the taxonomic category and phylogenetic position of these beings.

Thus, "microorganism" is a sort of catch-all for anything consisting of only one cell (or acellular, according to some authors, such as viruses), whereas microscopic animals are governed by a much more complex set of classification criteria. (or acellular, according to some authors, such as viruses), while microscopic animals are governed by a series of much more complex classificatory criteria. For a living being to be considered part of the kingdom Animalia, it must meet a series of parameters:

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

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

Examples of microscopic animals and their characteristics

Once we have differentiated without any margin of error the animals from the rest of the groups of living beings, we are ready to show you some examples of microscopic animals. Don't miss them.

1. Copepods

Copepods are a subclass of very small maxillopod crustaceans.. It is a small group that includes about 8,500 species, mostly marine, generally semitransparent in color. Most of these animals measure between 1 and 5 millimeters, so they fit perfectly into the definition of "microscopic". However, there are parasitic copepods that reach up to 32 centimeters in length, although this is a complete exception.

Because of their microscopic size, copepods are considered part of the zooplankton, the tiny fraction of aquatic fauna that feeds by ingestion of already processed organic matter (as opposed to phytoplankton, mostly composed of algae). They are the main source of nutrients for many macroscopic marine organisms, so they are an essential part of the food supply for many marine organisms.They are the main source of nutrients for many macroscopic marine organisms, and are therefore an essential part of the base of the trophic chain of aquatic ecosystems.

2. Tardigrades

Tardigrades are some of the most curious and interesting living creatures on Earth. They are some of the smallest animals we know of, with the tiniest measuring less than 0.1 millimeters and the the tiniest ones measure less than 0.1 millimeters and the largest sizes range up to 1.5 millimeters.. In addition, they occupy a somewhat delicate phylogenetic position, as they are included in the panarthropoda clade, which contains tardigrades, onychophorans and arthropods themselves. They are not arthropods as such but neither are they microorganisms, so they "float" between two taxonomic waters.

Most tardigrades are phytophagous (plant-eating) or bacteriophagous, 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" mammalian morphology, with various segments with legs that resemble those of a bear and a mouth with multiple stilettoes. They are also famously known for their extreme resistance, as they are capable of entering a state of cryptobiosis when conditions are unfavorable, reducing their water content in the organism by up to 1%.

3. Rotifers

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

These animals have a completely atypical bilateral symmetry within the animal kingdom: they 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, unicellular algae and certain protozoa.

4. Dust mites

Although when we think of the subclass Acari we automatically think of animals of a very small size, this is far from being a general rule. This category, which belongs to the class Arachnida, includes ticks, plant mites and many other macroscopic invertebrates that can be seen with the naked eye, although many other representatives are microscopic.

Therefore, to get to this last example, we have to split hairs a little more finely. We are referring to the genus Dermatophagoides or dust mites, microscopic invertebrate animals measuring 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 are extremely simple at the evolutionary level, as they lack a stomach and have a very simple intestine, which digests small particles of organic matter present in the environment.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 an enormous amount of eggs during their last weeks of life.

Summary

Beyond fish, reptiles, mammals, amphibians and birds, there is a world of microscopic invertebrates that escapes our sight, but which are nonetheless essential to food chains, ecosystems and the world of research. For example, marine ecosystems could not exist without zooplankton: no matter how small the animal, its work is invaluable and unparalleled, wherever it is found.

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

Bibliographical 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.