The smallest organisms found on Earth can be determined according to various aspects of organism size, including volume, mass, height, length, or genome size. Given the incomplete nature of scientific knowledge, it is possible that the smallest organism is undiscovered. Furthermore, there is some debate over the definition of life, and what entities qualify as organisms; consequently the smallest known organisms (microrganisms) may be nanobes that can be 20 nanometers long.

Microorganisms

Obligate endosymbiotic bacteria

The genome of Nasuia deltocephalinicola, a symbiont of the European pest leafhopper, Macrosteles quadripunctulatus, consists of a circular chromosome of 112,031 base pairs. The genome of Nanoarchaeum equitans is 491 Kbp nucleotides long.

Pelagibacter ubique

Pelagibacter ubique is one of the smallest known free-living bacteria, with a length of 370 to 890 nm and an average cell diameter of 120 to 200 nm. They also have the smallest free-living bacterium genome: 1.3 Mbp, 1354 protein genes, 35 RNA genes. They are one of the most common and smallest organisms in the ocean, with their total weight exceeding that of all fish in the sea.

Mycoplasma genitalium

Mycoplasma genitalium, a parasitic bacterium which lives in the primate bladder, waste disposal organs, genital, and respiratory tracts, is thought to be the smallest known organism capable of independent growth and reproduction. With a size of approximately 200 to 300 nm, M. genitalium is an ultramicrobacterium, smaller than other small bacteria, including rickettsia and chlamydia. However, the vast majority of bacterial strains have not been studied, and the marine ultramicrobacterium Sphingomonas sp. strain RB2256 is reported to have passed through a 220 nm ultrafilter. A complicating factor is nutrient-downsized bacteria, bacteria that become much smaller due to a lack of available nutrients.

Nanoarchaeum

Nanoarchaeum equitans is a species of microbe 200 to 500 nm in diameter. It was discovered in 2002 in a hydrothermal vent off the coast of Iceland by Karl Stetter. A thermophile that grows in near-boiling temperatures, Nanoarchaeum appears to be an obligatory symbiont on the archaeon Ignicoccus; it must be in contact with the host organism to survive. Guinness World Records recognizes Nanoarchaeum equitans as the smallest living organism.

Eukaryotes (Eukaryota)

Prasinophyte algae of the genus Ostreococcus are the smallest free-living eukaryote. The single cell of an Ostreococcus measures 800 nm across.

Heliozoa

The Erebor lineage of Microheliella maris is the smallest known heliozoan with an average cell body diameter of 2.56 μm.

Viruses

Some biologists consider viruses to be non-living because they lack a cellular structure and cannot metabolize by themselves, requiring a host cell to replicate and synthesize new products. Some hold that, because viruses do have genetic material and can employ the metabolism of their host, they can be considered organisms. Also, an emerging concept that is gaining traction among some virologists is that of the virocell, in which the actual phenotype of a virus is the infected cell, and the virus particle (or virion) is merely a reproductive or dispersal stage, much like pollen or a spore. The smallest viruses in terms of genome size are single-stranded DNA (ssDNA) viruses. Perhaps the most famous is the bacteriophage Phi-X174 with a genome size of 5,386 nucleotides. However, some ssDNA viruses can be even smaller. For example, Porcine circovirus type 1 has a genome of 1,759 nucleotides and a capsid diameter of 17 nm. As a whole, the viral family geminiviridae is about 30 nm in length. However, the two capsids making up the virus are fused; divided, the capsids would be 15 nm in length. Other environmentally characterized ssDNA viruses such as CRESS DNA viruses, among others, can have genomes that are considerably less than 2,000 nucleotides. The smallest RNA virus in terms of genome size is phage BZ13 strain T72 at 3,393 nucleotides length. Viruses using both DNA and RNA in their replication (retroviruses) range in size from 7,040 to 12,195 nucleotides. The smallest double-stranded DNA viruses are the hepadnaviruses such as hepatitis B, at 3.2 kb and 42 nm; parvoviruses have smaller capsids, at 18 - 26 nm, but larger genomes, at 5 kb. It is important to consider other self-replicating genetic elements, such as obelisks, ribozymes, satelliviruses and viroids.

Animals (Animalia)

Several species of Myxozoa (obligately parasitic cnidarians) never grow larger than 20 μm. One of the smallest species (Myxobolus shekel) is no more than 8.5 μm when fully grown, making it the smallest known animal.

Molluscs (Molluska)

Bivalvia

The shell of the nut clam Condylonucula maya grows 0.54 mm long.

Gastropods (Gastropoda)

The smallest water snail (of all snails) is Ammonicera minortalis in North America, originally described from Cuba. It measures 0.32 to 0.46 mm. The smallest land snail is Acmella nana. Discovered in Borneo, and described in November 2015, it measures 0.7 mm. The previous record was that of Angustopila dominikae from China, which was reported in September 2015. This snail measures 0.86 mm.

Cephalopods (Cephalopoda)

Maximites was the smallest known ammonoid. Adult specimens reached only 10 mm in shell diameter.

Arthropods (Arthropoda)

The smallest arthropods are crustaceans in the class Tantulocarida. The single smallest species may be Tantulacus dieteri, with a total body length of only 85 um. Another candidate is Stygotantulus stocki, with a length of 94 um.

Arachnids (Arachnida)

Insects (Insecta)

Echinoderms (Echinodermata)

The smallest sea cucumber, and also the smallest echinoderm, is Psammothuria ganapati, a synaptid that lives between sand grains on the coast of India. Its maximum length is 4 mm.

Sea urchins

The smallest sea urchin, Echinocyamus scaber, has a test 6 mm across.

Starfish

Patiriella parvivipara is the smallest starfish, at 5 mm across.

Fish

Amphibians (Amphibia)

Frogs and toads (Anura)

The smallest vertebrate (and smallest amphibian) known is Brachycephalus pulex, a Brazilian flea toad, with a minimum adult snout–vent length of 6.45 mm. Brachycephalus dacnis is similarly tiny, with a minimum adult length of 6.95 mm. Other very small frogs include: The two species Microhyla borneensis (males: 10.6 –; females: 16 –) and Arthroleptella rugosa (males: 11.9 –; females: 15.5 mm) were once the smallest known frogs from the Old World. In general these extremely small frogs occur in tropical forest and montane environments. There is relatively little data on size variation among individuals, growth from metamorphosis to adulthood or size variation among populations in these species. Additional studies and the discovery of further minute frog species are likely to change the rank order of this list.

Salamanders, newts and allies (Urodela)

The average snout-to-vent length (SVL) of several specimens of the salamander Thorius arboreus was 17 mm.

Sauropsids (Sauropsida)

Lizards and snakes (Squamata)

Turtles and tortoises (Testudines)

The smallest turtle is the speckled padloper tortoise (Homopus signatus) from South Africa. The males measure 6 - 8 cm, while females measure up to almost 10 cm.

Archosaurs (Archosauria)

Crocodiles and close relatives (Crocodylomorpha)

Pterosaurs (Pterosauria)

Nemicolopterus was the smallest pterosaur, it reached about 25 cm in wingspan.

Non-avian dinosaurs (Dinosauria)

Sizes of non-avian dinosaurs are commonly labelled with a level of uncertainty, as the available material often (or even usually) is incomplete. The smallest known extinct non-avian dinosaur is Anchiornis, a genus of feathered dinosaur that lived in what is now China during the Late Jurassic Period 160 to 155 million years ago. Adult specimens range from 34 cm long, and the weight has been estimated at up to 110 g. Parvicursor was initially seen as one of the smallest non-avian dinosaurs known from an adult specimen, at 39 cm in length, and 162 g in weight. However, in 2022 its holotype was concluded to represent a juvenile individual. Epidexipteryx reached 25 - 30 cm in length and 164 - 220 g in weight.

Birds (Aves)

Non-mammalian synapsids (Synapsida)

The smallest Mesozoic mammaliaform was Hadrocodium with a skull of 1.5 cm in length and a body mass of 2 g.

Mammals (Mammalia)

Marsupials (Marsupialia)

The smallest marsupial is the long-tailed planigale from Australia. It has a body length of 110-130 mm (including tail) and weighs 4.3 g on average. The Pilbara ningaui is considered to be of similar size and weight.

Shrews (Eulipotyphla)

The Etruscan shrew (Suncus etruscus), is the smallest mammal by mass, weighing about 1.8 g on average. The smallest mammal that ever lived, the shrew-like Batodonoides vanhouteni, weighed 1.3 g.

Bats (Chiroptera)

The Kitti's hog-nosed bat (Craseonycteris thonglongyai), also known as the bumblebee bat, from Thailand and Myanmar is the smallest mammal, at 29 - 33 mm in length and 2 g in weight.

Carnivorans (Carnivora)

The smallest member of the order Carnivora is the least weasel (Mustela nivalis), with an average body length of 114 –. It weighs between 29.5 – with females being lighter.

Rodents (Rodentia)

The smallest known member of the rodent order is the Baluchistan pygmy jerboa, with an average body length of 3.8 cm.

Primates (Primates)

The smallest member of the primate order is Madame Berthe's mouse lemur (Microcebus berthae), found in Madagascar, with an average body length of 9.2 cm.

Cetaceans (Cetacea)

The smallest cetacean, which is also (as of 2006) the most endangered, is the vaquita, a species of porpoise. Male vaquitas grow to an average of around 135 cm; the females are slightly longer, averaging about 141 cm in length.

Plants (Plantae)

Gymnosperms (Gymnospermae)

Zamia pygmaea is a cycad found in Cuba, and the smallest known gymnosperm. It grows to a height of 25 cm.

Angiosperms (Angiospermae)

Duckweeds of the genus Wolffia are the smallest angiosperms. Fully grown, they measure 300 by and reach a mass of just 150 μg.

Dicotyledons

The smallest known dicotyledon plant is the Himalayan dwarf mistletoe (Arceuthobium minutissimum). Shoots grow up to 5 mm in height.

Other

Nanobes

Nanobes are thought by some scientists to be the smallest known organisms, about one tenth the size of the smallest known bacteria. Nanobes, tiny filamental structures first found in some rocks and sediments, were first described in 1996 by Philippa Uwins of the University of Queensland, but it is unclear what they are, and if they are alive.