Smallest organisms

Viruses

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.^[1]

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.^[2] However, some ssDNA viruses can be even smaller. For example, Porcine circovirus type 1 has a genome of only 1759 nucleotides^[3] and a capsid diameter of only 17 nm.^[4] 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.^[5][6]

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

The genome of Nasuia deltocephalinicola, a symbiont of the European pest leafhopper, Macrosteles quadripunctulatus consists of a circular chromosome of 112,031 base pairs.^[7]

Nanoarchaeum equitans's genome is 490,885 nucleotides long.

Pelagibacter ubique

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.^[8]

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 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.^[9]

Nanoarchaeum equitans and archaeon host, Ignicoccus

Nanoarchaeum

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.

Eukaryotes

Prasinophyte algae of the genus Ostreococcus is the smallest free-living eukaryote. The single cell of an Ostreococcus measures only 0.8 μm across.

Molluscs

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.^[12][13]

The smallest land snail is Acmella nana. Discovered in Borneo, Malaysia, and described in November 2015, it measures only 0.7 mm.^[14] The previous record was that of Angustopila dominikae from China, which was reported in September 2015. This snail measures 0.86 mm.^[15]

Arthropods

Insects

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% larger.^[18]

Megaphragma caribea from Guadeloupe, measuring 170 μm long, is another contender for smallest known insect in the world.

• Beetles

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.^[19]

• Butterflies

Grand Cayman pygmy blue (Brephidium exilis thompsoni)

The western pygmy blue (Brephidium exilis) is one of the smallest butterflies in the world.

Echinoderms

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 only 4 mm.^{[20] [21]}

Vertebrates

Paratype of Paedophryne amauensis (LSUMZ 95004) on a US dime.

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).^[22][23] Previously, the title of smallest vertebrate was held by members of the fish genus Paedocypris of Indonesia.

Fish

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.^[24] 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).^{[25][26][27][28]}

Amphibians

Reptiles

Lizards

A juvenile Brookesia micra on a finger tip

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).^[29] 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)^[30] and 15–16  millimetres for male B. micra,^[31] 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.^[32][33]

Dinosaurs

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).^[36] Nevertheless, sizes of dinosaurs are commonly labelled with a level of uncertainty, as the available material often (or even usually) is incomplete.

Birds

Bee hummingbird (Mellisuga helenae) immature male

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".^[37] 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.

Mammals

The vulnerable Kitti's hog-nosed bat (Craseonycteris thonglongyai), also known as the bumblebee bat, from Thailand and Myanmar^[38] 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 g (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).

Flowering plants (angiosperms)

Wolffia arrhiza on human fingers. Every speck of less than 1 mm length is an individual plant.

Duckweeds of the genus Wolffia are the world's smallest flowering plants.^[41] Fully grown, they measure only 300 µm by 600 µm and reach a mass of just 150 µg.

Nanobes

Nanobes are thought by some scientists to be the smallest known organisms^[42] 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.