What are the World's Smallest Animals?

The world's smallest known animals only consist of a few hundred cells and measure as little as 45 micrometers in length (0.0017 inch or 0.045 mm). They are often measured in microns, where 1 micron is equal to 0.00003937 inch (0.001 mm). These microfauna include the smallest gastrotichs (< 60 microns), nematodes (< 80 microns), rotifers (< 100 microns), and micrognathozoans (< 100 microns).

Other extremely small animals include the smallest mites, such as Aberoptus championus (< 120 microns); parasitic wasps, such as Dicopomorpha echmepterygis (137 microns, the smallest insect); and the smallest beetles, such as Scydosella musawasensis (300 microns). Members of the phylum Cycliophora, such as Symbion (347 microns); jaw worms (500 microns); and Trichoplax adhaerens (500 microns) are also included. Nematodes, rotifers, jaw worms, and micrognathozoans are aquatic, while mites, wasps, and beetles are terrestrial.

Most of the smallest animals are aquatic, partially because the layer of mud on the ocean floor is an excellent habitat for tiny organisms. The small invertebrates that live there are known as the meiofauna, infauna, or the benthos. Aside from bacteria, they are probably among the most numerous animals on the planet (especially nematodes). Found at modest densities in nearly every part of the ocean floor, they consume the endless organic "snow" that falls from above. They coexist with abundant bacteria, which they eat. Bacteria and other unicellular organisms are not considered animals because, by definition, an animal is an organism that is made up of multiple cells.

Sometimes, species named the "smallest animals" are not very small at all, such as fish of the genus Paedocrypis, which can be as short as 0.31 inches (8 mm). Aphids have a size between 0.039 and 0.39 inches (1 and 10 mm), and are clearly visible to the naked eye. All of the truly tiny animals are microscopic, as human vision cannot easily discern objects smaller than about 500 microns, and many animals can pass through a 500 micron filter. Many are somewhat poorly studied due to their tiny size, but may hold the key to crucial debates of biological classification. For instance, it is thought that the tiny gastrotich, which may be a relative of nematodes, holds the key to the Edcysozoa hypothesis, over whether animals that shed a cuticle are all descendants of a common ancestor, or whether the quality arose independently in different lineages.

The title of the smallest mammal is held by the Etruscan shrew (Suncus etruscus), which weighs about 1.8 grams and measures around 1.4 inches in length, not including the tail. Despite its diminutive size, this tiny creature has a voracious appetite, needing to consume twice its body weight daily to survive.

The Bee Hummingbird (Mellisuga helenae) of Cuba and the Isle of Youth claims the spot as the world's smallest bird. Males measure about 2.2 inches in length and weigh approximately 1.6 grams. They are known for their iridescent plumage and incredibly rapid wing flapping, which can reach up to 80 beats per second during normal flight.

Discovered in Madagascar, the Brookesia micra chameleon is the smallest known reptile. Adults can be as small as 0.6 inches in length from snout to tail. These tiny chameleons inhabit leaf litter on the forest floor and are remarkable for their ability to blend into their surroundings despite their minuscule size.

Yes, the smallest fish is the Paedocypris progenetica, a member of the carp family found in the peat swamp forests of Sumatra, Indonesia. Mature females can measure just 7.9 millimeters in length. They are adapted to survive in highly acidic water, which is inhospitable to most other fish species.

The smallest amphibian is the Paedophryne amauensis, a tiny frog species discovered in Papua New Guinea. It averages around 7.7 millimeters in length, making it not only the smallest amphibian but also one of the smallest vertebrates on Earth. These frogs are so minute they can perch comfortably on a dime.

Despite their size, many small animals play crucial roles in their ecosystems. For example, the Bee Hummingbird is a vital pollinator, while the Etruscan shrew helps control insect populations. The Paedocypris progenetica fish contributes to the detritus cycle in its swampy habitat, demonstrating that even the smallest creatures can have a significant environmental impact.