Microscopic animals, such as nematodes, rotifers, and tardigrades, are too small to be seen without a microscope and are not part of the Kingdom Animalia. They are important to the global ecosystem and make up a significant part of the biomass. Only a fraction of them have been described by science.
Microscopic animals are animals that are too small to be seen with the naked eye. Microorganisms such as bacteria are almost all too small to see without assistance, although they do not qualify as animals. Single-celled eukaryotic (complex-celled) organisms with animal characteristics are called protists, but these too are not considered part of the Kingdom Animalia (also known as metazoa). Real animals are multicellular and have differentiated tissues.
Animals too small to be seen without a microscope are the most numerous of all animals. If aliens were instructed to take a random animal from Earth, they would probably take some kind of microscopic animal. Common ones include planarians (flatworms); many types of mites, including dust mites and spider mites; and aquatic crustaceans, such as copepods and cladocera (water fleas). The most numerous are nematodes (roundworms), rotifers (aquatic filter feeders) and tardigrades (water bears). Nematodes, in particular, are probably the most numerous animal on Earth, accounting for at least 90% of all seabed life, and are ubiquitous in all habitable land and sea environments.
Microscopic animals are part of a continuum of size that extends from viruses to the largest living organisms. They were first discovered by Dutch scientist Antonie van Leeuwenhoek, the ‘Father of Microbiology’, in 1675, using microscopes of his own design, some of which could magnify up to 500 times. The smallest object that can be seen with the unaided human eye ranges from about 1/40th to 1mm, but “microscopic” often refers to any animal less than 1mm wide, especially less than 1mm wide. /10mm.
These tiny animals are extremely important to the global ecosystem, making up a significant part of the biomass and forming the basis of some food webs. Smaller ones, such as rotifers, live mainly on bacteria, while larger specimens consume smaller animals or suck fluids from trees. Mites are particularly suited to the latter and are found in large numbers under the leaves of many plants. Dust mites, the most common cause of allergies, are found in nearly every human home on the planet, where they survive on dead skin cells that shed off human inhabitants. A common strategy for killing these tiny beasts is to reduce ambient humidity.
Because microscopic animals are so numerous and distributed, only a fraction of them have been described by science. More will surely be discovered in the future, adding to scientists’ knowledge of the planet’s biodiversity.
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