Smallest organisms

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 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 long.

''Pelagibacter ubique''

Pelagibacter ubique is one of the smallest known free-living bacteria, with a length of and an average cell diameter of. 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 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 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.

Single-celled eukaryotes (protists)

e of the genus Ostreococcus are the smallest free-living eukaryote. The single cell of an Ostreococcus measures across.

Heliozoa

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

Diatoms

The smallest diatoms with diameters as small as 1.9 μm can be found in the genera Mediolabrus and Minidiscus. ''Mediolabrus comicus'' is the smallest known marine diatom.

Viruses

Some biologists consider viruses to be non-living because they lack a cellular structure and cannot metabolize or reproduce 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 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 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. As a whole, the viral family geminiviridae is about in length. However, the two capsids making up the virus are fused; divided, the capsids would be 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 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 ; parvoviruses have smaller capsids, at, 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 never grow larger than. One of the smallest species is no more than when fully grown, making it the smallest known animal.

Molluscs (Mollusca)

Bivalvia

The shell of the nut clam Condylonucula maya grows long.

Gastropods (Gastropoda)

The smallest water snail is Ammonicera minortalis in North America, originally described from Cuba. It measures.
The smallest land snail is Acmella nana. Discovered in Borneo, and described in November 2015, it measures. The previous record was that of Angustopila dominikae from China, which was reported in September 2015. This snail measures.

Cephalopods (Cephalopoda)

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

Arthropods (Arthropoda)

The smallest arthropods are mites Cochlodispus minimus of the family Microdispidae. The body length of the smallest measured individual was.

Crustaceans (Crustacea)

The smallest crustaceans belong to the class Tantulocarida. The single smallest species may be Tantulacus dieteri, with a total body length of only. Another candidate is Stygotantulus stocki, with a length of.

Arachnids (Arachnida)

There is a debate about which spider is smallest. According to Guinness World Records, "Two contenders are from the Symphytognathidae genus Patu: males of Patu digua described in Colombia had a body length of, while the Samoan moss spider could be as small as long." Other possible smallest spider species are the Frade cave spider known as Anapistula ataecina, and the dwarf orb weaver, the females of which are and respectively. Males of both species are potentially smaller than the females, but no male A. ataecina or A. caecula have been measured yet.
Cochlodispus minimus is the smallest mite. An adult individual measured with a body length of.
Insects (Insecta)
Adult males of the parasitic wasp Dicopomorpha echmepterygis can be as small as long, smaller than some species of protozoa ; females are 40% larger. Megaphragma caribea from Guadeloupe, measuring long, is another contender for smallest known insect in the world.
Beetles of the tribe Nanosellini are all less than long; the smallest confirmed specimen is of Scydosella musawasensis at long; a few other nanosellines are reportedly smaller, in historical literature, but none of these records have been confirmed using accurate modern tools. These are among the tiniest non-parasitic insects.
The western pygmy blue is one of the smallest butterflies in the world, with a wingspan of about.
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.

Sea urchins

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

Starfish

Patiriella parvivipara is the smallest starfish, at across.

Fish

One of the smallest vertebrates and one of the smallest fish is Paedocypris progenetica from Indonesia, with mature females measuring as little as in standard length. This fish, a member of the carp family, has a translucent body and a head unprotected by a skeleton.
One of the smallest fish based on the minimum size at maturity is Schindleria brevipinguis from Australia, their females reach and males. Males of S. brevipinguis have an average standard length of ; a gravid female was. This fish, a member of the goby family, differs from similar members of the group in having its first anal fin ray further forward, under dorsal fin 4.
Male individuals of the anglerfish species Photocorynus spiniceps have been documented to be at maturity, and thus claimed to be a smaller species. However, these survive only by sexual parasitism and the female individuals reach the significantly larger size of.
Amphibians (Amphibia)

Frogs and toads (Anura)

The smallest vertebrate known is Brachycephalus pulex, a Brazilian flea toad, with a minimum adult snout–vent length of. Brachycephalus dacnis is similarly tiny, with a minimum adult length of. Other very small frogs include:

Paedophryne amauensis from Papua New Guinea, ranging in length from, and on average.
Brachycephalus didactylus from Brazil
several species of Eleutherodactylus such as E. iberia, E. limbatus, and Eleutherodactylus orientalis from Cuba,
Gardiner's Frog Sechellophryne gardineri from the Seychelles,
several species of Stumpffia such as S. tridactyla and S. pygmaea
Wakea madinika from Madagascar.

The two species Microhyla borneensis and Arthroleptella rugosa 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 of several specimens of the salamander Thorius arboreus was.

Sauropsids (Sauropsida)

Lizards and snakes (Squamata)

The miniature chameleon Brookesia nana, with a snout-vent length of, may represent the smallest known lizard and smallest reptile.
The dwarf gecko is also one of the smallest known reptile species, with a snout-vent length of. S. ariasae was first described in 2001 by the biologists Blair Hedges and Richard Thomas. This dwarf gecko lives in Jaragua National Park in the Dominican Republic and on Beata Island, off the southern coast of the Dominican Republic. A few Brookesia chameleons from Madagascar are equally small, with a reported snout-vent length of for male dwarf chameleons, for male Mount d'Ambre leaf chameleons and for male B. micra, though females are larger.
One of the smallest known snakes is the recently discovered Barbados threadsnake. Adults average about long, which is only about twice as long as the hatchlings. The common blind snake measures long, occasionally up to long.
Turtles and tortoises (Testudines)

The smallest turtle is the speckled padloper tortoise from South Africa. The males measure, while females measure up to almost.

Archosaurs (Archosauria)

Crocodiles and close relatives (Crocodylomorpha)

The smallest extant crocodilian is the Cuvier's dwarf caiman from northern and central South America. It reaches up to in length.
Pterosaurs (Pterosauria)

Nemicolopterus was one of the smallest pterosaurs, it reached about in wingspan.

Non-avian dinosaurs (Dinosauria)

Sizes of non-avian dinosaurs are commonly labelled with a level of uncertainty, as the available material often 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 long, and the weight has been estimated at up to. Parvicursor was initially seen as one of the smallest non-avian dinosaurs known from an adult specimen, at in length, and in weight. However, in 2022 its holotype was concluded to represent a juvenile individual. Epidexipteryx reached in length and in weight.

Birds (Aves)

With a mass of approximately and a length of, the bee hummingbird is the smallest known dinosaur as well as the smallest bird species, and the smallest warm-blooded vertebrate. Called the zunzuncito in its native habitat on Cuba, it is lighter than a Canadian or U.S. penny. It is said that it is "more apt to be mistaken for a bee than a bird". The bee hummingbird eats half its total body mass and drinks eight times its total body mass each day. Its nest is across.
The smallest waterfowl is the pygmy goose. African species reaches the average weight of about for males and for females and length of single wing between and. The second smallest waterfowl is the extinct Mioquerquedula from the Miocene.
The smallest penguin species is the little blue penguin, which stands around tall and weighs.
The smallest bird of prey is the black-thighed falconet, with a wingspan of, roughly the size of a sparrow.
Non-mammalian synapsids (Synapsida)

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

Mammals (Mammalia)

Marsupials (Marsupialia)

The smallest marsupial is the long-tailed planigale from Australia. It has a body length of and weighs on average. The Pilbara ningaui is considered to be of similar size and weight.

Shrews (Eulipotyphla)

The Etruscan shrew, is the smallest mammal by mass, weighing about on average. The smallest mammal that ever lived, the shrew-like Batodonoides vanhouteni, weighed.

Bats (Chiroptera)

The Kitti's hog-nosed bat, also known as the bumblebee bat, from Thailand and Myanmar is the smallest mammal, at in length and in weight.

Carnivorans (Carnivora)

The smallest member of the order Carnivora is the least weasel, with an average body length of. It weighs between 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.

Primates (Primates)

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

Cetaceans (Cetacea)

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

Embryophytes (Embryophyta)

Gymnosperms (Gymnospermae)

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

Angiosperms (Angiospermae)

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

Dicotyledons

The smallest known dicotyledon plant is the Himalayan dwarf mistletoe. Shoots grow up to in height.

Other

Nanobes

s 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.