The smallest organism found on Earth can be measured using a variety of different methods, and can be defined as the smallest by volume, mass, height, or length. Because there is some dispute over what the definition of life is and what entities qualify as organisms, the actual "smallest organism" (microorganisms) may be unclear. A further complication is the incomplete nature of scientific knowledge: it is possible that the smallest organism has yet to be discovered.
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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 kb and particle diameters of 80 nm. The smallest 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.[1] It is important to consider other self replicating genetic elements such as satelliviruses, Viroids and Ribozymes.
The Porcine circovirus type 2 is a single stranded DNA virus having a capsid diameter of 17 nm.[2]
Nanobes are thought by some to be the smallest known organism, 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
Nanoarchaeum equitans is a species of tiny microbe 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. Its cells are only 400 nm in diameter, making it the smallest known living organism, with the possible exception of nanobes (if they are considered to be living). Its genome is only 490,885 nucleotides long; as of 2005 it remains the smallest non-viral genome ever sequenced.
Mycoplasma genitalium, a parasitic bacterium which lives in the primate 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 220 nm ultrafilter. A complicating factor is nutrient-downsized bacteria, bacteria that become much smaller due to a lack of available nutrients.[3]
Prasinophyte algae of the genus Ostreococcus are the smallest free-living eukaryote. The single cell of an Ostreococcus measures only 0.8 μm across.
Eriophyid mites are among the smallest of all arthropods, measuring only 125 to 250 μm in length.[4]
Adult males of the parasitic wasp Dicopomorpha echmepterygis can be as small as 139 μm long; females are 40 percent larger.[5]
Beetles of the tribe Nanosellini are all less than 1 mm long; the smallest include Scydosella musawasensis at 300 μm long, Vitusella fijiensis at 310 μm, and Nanosella at 300 to 400 μm. These are among the tiniest non-parasitic insects.[6]
The smallest crustacean, and indeed the smallest arthropod is the tantulocarid Stygotantulus stocki, at a length of only 94 µm (0.0037 in).[7]
The smallest sea cucumber, and also smallest echinoderm is Psammothuria ganapatii, a synaptid which lives between sand grains on the coast of India. Its maximum length is only 4 mm.[8] [9]
The smallest sea urchin, Echinocyamus scaber, has a test only 6 mm across.[9]
Patiriella parvivipara is the smallest starfish, at only 5 mm across.[9]
The smallest vertebrates (and smallest fishes) known are part of the genus Paedocypris of Indonesia, which dwell in peat swamps and can survive extreme drought in part due to their small size (just 7.9 mm long at maturity). Much of their habitat is acid water, with pH as low as 2.9. The members of the genus are thought to be highly endangered.
Male individuals of the species anglerfish Photocorynus spiniceps have been documented to be 6.2-7.3 mm 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 mm.[10][11][12][13]
At 9.6 to 9.8 millimetres (0.38 to 0.39 in) long, the Monte Iberia Eleuth (Eleutherodactylus iberia) and the Brazilian Gold Frog (Brachycephalus didactylus) are tied as the smallest frog, amphibian, or tetrapod.[14]
The average length of several specimens of the salamander Thorius arboreus was only 17 millimetres (0.67 in) .
The Jaragua Sphaero (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).[15] A few Brookesia chameleons from Madagascar may be equally small, with a reported snout-vent length of 15–18 mm. for male B. minima, and 14–19 mm. for male B. tuberculata, though females are larger.[16] Of the aforementioned geckos, S. ariasae was first described in 2001 by 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.[17][18]
The world's smallest turtle is the Speckled Padloper Tortoise (Homopus signatus) from South Africa. The males measure 6–8 cm (2.4–3.1 in), while females measure up to almost 10 cm (3.9 in).[19]
The smallest crocodilian is the Cuvier's Dwarf Caiman (Paleosuchus palpebrosus) from northern and central South America. It reaches up to 1.6 m (5.2 ft) in length.[20]
One of the smallest snakes known is the recently discovered Barbados Threadsnake (Leptotyphlops carlae). Adults average about 10 cm long, which is only about twice as long as the hatchlings.
The smallest known 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 (13 in) long, and the weight has been estimated at up to 110 g (3.9 oz).[21] 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.
M. helenae, called the zunzún in its native habitat on Cuba 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".[22] The Bee Hummingbird is said to eat half its total body mass and drink eight times its total body mass each day. Its nest is 3 cm across.
The vulnerable Kitti's Hog-nosed Bat (bumblebee bat; Craseonycteris thonglongyai) from Thailand and Myanmar[23] is the smallest mammal, at 30–40 mm in length and 1.5 to 2 g in weight.
The Etruscan Shrew is the smallest mammal by mass, though it exceeds the Bumblebee Bat in skull size. The smallest mammal that ever lived, the shrew-like Batodonoides vanhouteni, weighed only 1.3 g.
The smallest member of the primate order is Madame Berthe's Mouse Lemur (Microcebus berthae), found in Madagascar, [24] with an average body length of 92 mm (3.6 in).
Duckweeds of the genus Wolffia are the world's smallest flowering plants.[25] Fully grown, they measure only 300 µm by 600 µm and reach a mass of just 150 µg.