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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 organism (microorganism) is debatable.
- 1 Microorganisms
- 2 Animals
- 2.1 Molluscs
- 2.2 Arthropods
- 2.3 Echinoderms
- 2.4 Vertebrates
- 2.5 Fish
- 2.6 Amphibians
- 2.7 Reptiles
- 2.8 Dinosaurs
- 2.9 Mammals
- 3 Plants
- 4 Other
- 5 See also
- 6 References
- 7 External links
Many 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. A minority of scientists hold that, because viruses do have genetic material and can employ the metabolism of their host, they can be considered organisms. As well, an emerging idea that is gaining traction among some virologists is the concept of the virocell, in which the actual phenotype of a virus is the infected cell, and the virus particle is simply 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 5386 nucleotides. However, some ssDNA viruses can be even smaller. For example, Porcine circovirus type 1 has a genome of only 1759 nucleotides and a capsid diameter of only 17 nm. As a whole, the viral family geminiviridae is only 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 as well as others can have genomes that are considerably less that 2,000 nucleotides.  
The smallest RNA viruses in terms of genome size are small retroviruses such as rous sarcoma virus with genomes of 3.5 kilo base pairs (kb) and particle diameters of 80 nanometres (nm). 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 satelliviruses, viroids and ribozymes.
Obligate endosymbiotic bacteria
Nanoarchaeum equitans's genome is 490,885 nucleotides long.
Pelagibacter ubique is one of the smallest known free-living bacterium with a length of 0.37-0.89 μm and an average cell diameter of 0.12-0.20 μm. They also have the smallest free-living bacterium genome; 1.8Mbp, 1354 protein genes, 35 RNA genes. They are one of the most common and smallest organisms in the ocean, with their total weight equaling more than all fish presently in the sea.
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 ultramicrobacteria 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 equitans is a species of tiny microbe 400 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.
The smallest land snail is Acmella nana. Discovered in Borneo, Malaysia, and described in November 2015, it measures only 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.
Beetles of the tribe Nanosellini are all less than 1 mm long; the smallest confirmed specimen is of Scydosella musawasensis at 325 μm 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 (Brephidium exilis) is one of the smallest butterflies in the world.
The smallest vertebrates (and smallest amphibians) known are Paedophryne amauensis frogs from Papua New Guinea, which range in length from 7.0–8.0 millimetres (0.28–0.31 in), and average 7.7 millimetres (0.30 in). Previously, the title of smallest vertebrate was held by members of the fish genus Paedocypris of Indonesia.
The world's smallest fish based on the minimum size at maturity is Paedocypris progenetica from Indonesia, with mature females measuring as little as 7.9 mm (0.31 in) in standard length. This fish, a member of the carp family, has a translucent body and a head unprotected by a skeleton.
Male individuals of the anglerfish species Photocorynus spiniceps have been documented to be 6.2–7.3 millimetres (0.24–0.29 in) 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 50.5 millimetres (1.99 in).
Frogs include the smallest vertebrates known. The smallest known frog species is Paedophryne amauensis, with a snout-vent length reported as 7.7 mm, which occurs among leaf-litter in the tropical montane forests of New Guinea. Other very small frogs include Brachycephalus didactylus from Brazil (reported as 9.6-9.8 mm), several species of Eleutherodactylus such as Eleutherodactylus iberia (around 10mm) from Cuba, Gardiner's Frog Sechellophryne gardineri from the Seychelles (up to 11 mm), several species of Stumpffia such as Stumpffia tridactyla and Stumpffia pygmaea from Madagascar, and the Rough Moss-frog Arthroleptella rugosa of South Africa (11.9 - 14.1 mm). 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.
The dwarf gecko (Sphaerodactylus ariasae) and the Virgin Islands dwarf sphaero (S. parthenopion), two geckos in the genus Sphaerodactylus, are the world's smallest known reptile species and smallest lizard, with a snout-vent length of 16 millimetres (0.63 in). A few Brookesia chameleons from Madagascar are equally small, with a reported snout-vent length of 15–18 millimetres for male dwarf chameleons (B. minima), 14–19 millimetres for male Mount d'Ambre leaf chameleons (B. tuberculata) and 15–16 millimetres for male B. micra, though females are larger. Of the aforementioned geckos, S. ariasae was first described in 2001 by the biologists Blair Hedges and Richard Thomas. This dwarf gecko is endangered and lives in Jaragua National Park in the Dominican Republic and on Beata Island (Isla Beata), off the southern coast of Hispaniola in the Dominican Republic.
The smallest known dinosaur (excluding modern birds) 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 (13 in) long, and the weight has been estimated at up to 110 g (3.9 oz). Nevertheless, sizes of dinosaurs are commonly labelled with a level of uncertainty, as the available material often (or even usually) is incomplete.
With a mass of approximately 1.8 grams (0.063 oz) and a length of 5 centimetres (2.0 in), the bee hummingbird (Mellisuga helenae) is the world's smallest bird species and the smallest warm-blooded vertebrate. Called the zunzún 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 3 cm across.
The vulnerable Kitti's hog-nosed bat (Craseonycteris thonglongyai), also known as the bumblebee bat, from Thailand and Myanmar is the smallest mammal, at 3–4 centimetres (1.2–1.6 in) in length and 1.5–2 grams (0.053–0.071 oz) in weight.
The Etruscan shrew (Suncus etruscus), is the smallest mammal by mass, weighing only about 1.8 grams (0.063 oz) on average. The bumblebee bat has a smaller skull size. The smallest mammal that ever lived, the shrew-like Batodonoides vanhouteni, weighed only 1.3 grams (0.046 oz).
The smallest member of the order Carnivora is the least weasel (Mustela nivalis), with an average body length of 114–260 mm (4.5-10.2 in). It weighs between 29.5 – 250 grams with females being lighter.
The smallest marsupial is the Long-tailed planigale from Australia. It has a body length of 110–130 millimetres (4.3–5.1 in) (including tail) and weigh 4.3 grams (0.15 oz) on average.
The smallest cetacean, which is also (as of 2006) the most endangered, is the vaquita. Male vaquitas grow to an average of around 135 cm (53 in); the females are slightly longer, averaging about 141 cm (55 in) in length.
Flowering plants (angiosperms)
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.
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