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.
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 synthesise 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. 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. The smallest DNA bacteriophage is the Phi-X174 phage, thought to be larger than Hepatitis B, at about 4 kb. It is important to consider other self replicating genetic elements, such as satelliviruses, viroids and ribozymes.
The smallest virus known to humanity, however, is the single-stranded DNA virus Porcine circovirus type 1. It 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.
Shortest non-viral germs
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.
Prasinophyte algae of the genus Ostreococcus is the smallest free-living eukaryote. The single cell of an Ostreococcus measures only 0.8 μm across. The smallest genome of any Eukaryote is Guillardia theta, with a genome size of only 551 Kilobases.
Several species of Myxozoa never grow larger than 20 µm. One of the smallest species (Myxobolus shekel) is no more than 8.5 µm when fully grown.
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.
Eriophyid mites measure 125 to 250 μm in length.
Adult males of the parasitic wasp Dicopomorpha echmepterygis can be as small as 139 μm long, smaller than some species of protozoa (single-cell creatures); females are 40 percent larger.
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 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 is Paedocypris progenetica from Indonesia, with mature females measuring 7.9 mm (0.31 in). 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).
List of smallest fishes in the world
|Image||Common Name||Species||Family||Length mm (in)|
|Paedocypris||Paedocypris progenetica||Cyprinidae||7.9 mm (0.31 in)|
|Dwarf pygmy goby||Pandaka pygmaea||Gobiidae||9 mm (0.35 in)|
|Stout infantfish||Schindleria brevipinguis||Schindleriidae||8.4 mm (0.33 in)|
|Dwarf goby||Trimmatom nanus||Gobiidae||10 mm (0.39 in)|
The average length of several specimens of the salamander Thorius arboreus was only 17 millimetres (0.67 in).
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 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 g.
Smallest member of rodent order is Baluchistan pygmy jerboa, with an average body length of only 4.4 cm (1.7 in). It is less in weight and dimensions than its nearest competitor.
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 to 130 mm (including tail) and weigh 4.3 grams on average.
Pilbara ningaui is considered to be of similar size and weight.
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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