Three Paramecium caudatum.
Microscopic animals are animals too small to be seen with the naked eye. Microorganisms such as bacteria are too small to be seen unaided, although they do not qualify as animals. Single-celled eukaryotic organisms (complex cells) with animal-like characteristics are called protists, but are also not considered part of the Kingdom Animalia (also known as metazoans). True animals are multicellular and have differentiated tissues.
Nematodes can live on land or in the sea.
Animals too small to be seen without a microscope are the most numerous of all animals. If aliens were ordered to pull a random animal from Earth, they would probably get some sort of microscopic animal. The most common include planaria (flatworms); many types of mites, including dust mites and spider mites; and aquatic crustaceans such as copepods and cladocerans (water fleas). The most numerous are nematodes (roundworms), rotifers (water filters) and tardigrades (water bears). Nematodes, in particular, are probably the most numerous animals on Earth, accounting for at least 90% of all deep-sea life, and are ubiquitous in all habitable environments on land and sea.
two protists.
Microscopic animals are part of a size continuum 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 be magnified up to 500 times. The smallest object that can be seen with the naked eye ranges from 1/40 to 1 mm, but “microscopic” generally refers to any animal less than 1 mm across, especially less than 1/10 mm across.
Some animals are too small to be visible to the naked eye.
These small animals are extremely important to the global ecosystem, constituting a significant portion of the biomass and representing the base of some food chains. Smaller ones, such as rotifers, live mainly on bacteria, while larger individuals consume smaller animals or suck fluids from trees. Mites are specially adapted 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 almost every human home on the planet, where they survive on dead skin cells that fall off human inhabitants. A common strategy for killing these tiny beasts is to reduce ambient humidity.
Because microscopic animals are so numerous and widely distributed, science has only described a fraction of them. Others will surely be discovered in the future, adding to scientists’ knowledge of the planet’s biodiversity.