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)
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.