Vertebrates () comprise all animal taxa within the subphylum Vertebrata () (

chordates A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These five ...

with backbones), including all

mammal Mammals () are a group of vertebrate animals constituting the class Mammalia (), characterized by the presence of mammary glands which in females produce milk for feeding (nursing) their young, a neocortex (a region of the brain), fur or ...

s, birds,

reptile Reptiles, as most commonly defined are the animals in the class Reptilia ( ), a paraphyletic grouping comprising all sauropsids except birds. Living reptiles comprise turtles, crocodilians, squamates (lizards and snakes) and rhynchocephalians ( ...

amphibian Amphibians are tetrapod, four-limbed and ectothermic vertebrates of the Class (biology), class Amphibia. All living amphibians belong to the group Lissamphibia. They inhabit a wide variety of habitats, with most species living within terres ...

s, and fish. Vertebrates represent the overwhelming majority of the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

Chordata A chordate () is an animal of the phylum Chordata (). All chordates possess, at some point during their larval or adult stages, five synapomorphies, or primary physical characteristics, that distinguish them from all the other taxa. These fiv ...

, with currently about 69,963 species described. Vertebrates comprise such groups as the following: * jawless fish, which include hagfish and lampreys *

jawed vertebrates Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...

, which include: **

cartilaginous fish Chondrichthyes (; ) is a class that contains the cartilaginous fishes that have skeletons primarily composed of cartilage. They can be contrasted with the Osteichthyes or ''bony fishes'', which have skeletons primarily composed of bone tissue ...

( sharks, rays, and ratfish) ** bony vertebrates, which include: *** ray-fins (the majority of living

bony fish Osteichthyes (), popularly referred to as the bony fish, is a diverse superclass of fish that have skeletons primarily composed of bone tissue. They can be contrasted with the Chondrichthyes, which have skeletons primarily composed of cartilag ...

) *** lobe-fins, which include: ****

coelacanths The coelacanths ( ) are fish belonging to the order Actinistia that includes two extant species in the genus ''Latimeria'': the West Indian Ocean coelacanth (''Latimeria chalumnae''), primarily found near the Comoro Islands off the east coast ...

and lungfish **** tetrapods (limbed vertebrates)

Extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

vertebrates range in size from the frog species '' Paedophryne amauensis'', at as little as , to the blue whale, at up to . Vertebrates make up less than five percent of all described

animal species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriate sex ...

; the rest are invertebrates, which lack vertebral columns. The vertebrates traditionally include the hagfish, which do not have proper vertebrae due to their loss in evolution, though their closest living relatives, the lampreys, do. Hagfish do, however, possess a

cranium The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, the ...

. For this reason, the vertebrate subphylum is sometimes referred to as " Craniata" when discussing morphology. Molecular analysis since 1992 has suggested that hagfish are most closely related to lampreys, and so also are vertebrates in a

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

sense. Others consider them a sister group of vertebrates in the common taxon of craniata.

Etymology

The word ''vertebrate'' derives from the Latin word ''vertebratus'' ( Pliny), meaning ''joint of the spine.'' ''Vertebrate'' is derived from the word '' vertebra'', which refers to any of the bones or segments of the

spinal column The vertebral column, also known as the backbone or spine, is part of the axial skeleton. The vertebral column is the defining characteristic of a vertebrate in which the notochord (a flexible rod of uniform composition) found in all chordates ...

Anatomy and morphology

All vertebrates are built along the basic chordate body plan: a stiff rod running through the length of the animal (vertebral column and/or

notochord In anatomy, the notochord is a flexible rod which is similar in structure to the stiffer cartilage. If a species has a notochord at any stage of its life cycle (along with 4 other features), it is, by definition, a chordate. The notochord consis ...

), with a hollow tube of nervous tissue (the spinal cord) above it and the

gastrointestinal tract The gastrointestinal tract (GI tract, digestive tract, alimentary canal) is the tract or passageway of the digestive system that leads from the mouth to the anus. The GI tract contains all the major organ (biology), organs of the digestive syste ...

below. In all vertebrates, the mouth is found at, or right below, the anterior end of the animal, while the

anus The anus (Latin, 'ring' or 'circle') is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, the residual semi-solid waste that remains after food digestion, which, d ...

opens to the exterior before the end of the body. The remaining part of the body continuing after the anus forms a tail with vertebrae and spinal cord, but no gut.Romer, A.S. (1949): ''The Vertebrate Body.'' W.B. Saunders, Philadelphia. (2nd ed. 1955; 3rd ed. 1962; 4th ed. 1970)

Vertebral column

The defining characteristic of a vertebrate is the vertebral column, in which the

(a stiff rod of uniform composition) found in all

has been replaced by a segmented series of stiffer elements (vertebrae) separated by mobile joints (intervertebral discs, derived embryonically and evolutionarily from the notochord). However, a few vertebrates have secondarily lost this anatomy, retaining the notochord into adulthood, such as the

sturgeon Sturgeon is the common name for the 27 species of fish belonging to the family Acipenseridae. The earliest sturgeon fossils date to the Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretace ...

and coelacanth.

Jawed vertebrates Gnathostomata (; from Greek: (') "jaw" + (') "mouth") are the jawed vertebrates. Gnathostome diversity comprises roughly 60,000 species, which accounts for 99% of all living vertebrates, including humans. In addition to opposing jaws, living ...

are typified by paired appendages (fins or legs, which may be secondarily lost), but this trait is not required in order for an animal to be a vertebrate. Naturkundemuseum Berlin - Dinosaurierhalle

Naturkundemuseum Berlin - Dinosaurierhalle

Gills

All

basal Basal or basilar is a term meaning ''base'', ''bottom'', or ''minimum''. Science * Basal (anatomy), an anatomical term of location for features associated with the base of an organism or structure * Basal (medicine), a minimal level that is nec ...

vertebrates breathe with gills. The gills are carried right behind the head, bordering the posterior margins of a series of openings from the pharynx to the exterior. Each gill is supported by a cartilaginous or bony gill arch. The

have three pairs of arches,

have five to seven pairs, while the primitive jawless fish have seven. The vertebrate ancestor no doubt had more arches than this, as some of their chordate relatives have more than 50 pairs of gills. In

s and some primitive bony

fishes Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

, the larvae bear external gills, branching off from the gill arches. These are reduced in adulthood, their function taken over by the gills proper in fishes and by

lung The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of t ...

s in most amphibians. Some amphibians retain the external larval gills in adulthood, the complex internal gill system as seen in fish apparently being irrevocably lost very early in the evolution of tetrapods.Clack, J. A. (2002): Gaining ground: the origin and evolution of tetrapods. ''

Indiana University Press Indiana University Press, also known as IU Press, is an academic publisher founded in 1950 at Indiana University that specializes in the humanities and social sciences. Its headquarters are located in Bloomington, Indiana. IU Press publishes 140 ...

'', Bloomington, Indiana. 369 pp While the more derived vertebrates lack gills, the gill arches form during fetal development, and form the basis of essential structures such as jaws, the

thyroid gland The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobe (anatomy), lobes. The lower two thirds of the lobes are connected by a thin band of Connective tissue, tissue cal ...

, the

larynx The larynx (), commonly called the voice box, is an organ in the top of the neck involved in breathing, producing sound and protecting the trachea against food aspiration. The opening of larynx into pharynx known as the laryngeal inlet is about ...

, the ''columella'' (corresponding to the stapes in

s) and, in mammals, the malleus and incus.

Central nervous system

The central nervous system of vertebrates is based on a hollow nerve cord running along the length of the animal. Of particular importance and unique to vertebrates is the presence of neural crest cells. These are progenitors of

stem cell In multicellular organisms, stem cells are undifferentiated or partially differentiated cells that can differentiate into various types of cells and proliferate indefinitely to produce more of the same stem cell. They are the earliest type o ...

s, and critical to coordinating the functions of cellular components.Teng, L.; Labosky, P. A. (2006)
"Neural crest stem cells"
In: Jean-Pierre Saint-Jeannet, ''Neural Crest Induction and Differentiation'', pp. 206-212, Springer Science & Business Media. . Neural crest cells migrate through the body from the nerve cord during development, and initiate the formation of neural ganglia and structures such as the jaws and skull. The vertebrates are the only chordate group with neural

cephalisation Cephalization is an evolutionary trend in which, over many generations, the mouth, sense organs, and nerve ganglia become concentrated at the front end of an animal, producing a head region. This is associated with movement and bilateral symmetr ...

, the concentration of brain functions in the head. A slight swelling of the anterior end of the nerve cord is found in the lancelet, a chordate, though it lacks the eyes and other complex sense organs comparable to those of vertebrates. Other chordates do not show any trends towards cephalisation. A peripheral nervous system branches out from the nerve cord to innervate the various systems. The front end of the nerve tube is expanded by a thickening of the walls and expansion of the

central canal of spinal cord The central canal (also known as spinal foramen or ependymal canal) is the cerebrospinal fluid-filled space that runs through the spinal cord. The central canal lies below and is connected to the ventricular system of the brain, from which it r ...

into three primary brain vesicles: The prosencephalon (forebrain), mesencephalon (midbrain) and rhombencephalon (hindbrain), further differentiated in the various vertebrate groups.Hildebrand, M.; Gonslow, G. (2001): Analysis of Vertebrate Structure. 5th edition. ''John Wiley & Sons, Inc''.

New York New York most commonly refers to: * New York City, the most populous city in the United States, located in the state of New York * New York (state), a state in the northeastern United States New York may also refer to: Film and television * '' ...

Two laterally placed

eye Eyes are organs of the visual system. They provide living organisms with vision, the ability to receive and process visual detail, as well as enabling several photo response functions that are independent of vision. Eyes detect light and conv ...

s form around outgrowths from the midbrain, except in hagfish, though this may be a secondary loss. The forebrain is well-developed and subdivided in most tetrapods, while the midbrain dominates in many fish and some salamanders. Vesicles of the forebrain are usually paired, giving rise to hemispheres like the cerebral hemispheres in

s. The resulting anatomy of the central nervous system, with a single hollow nerve cord topped by a series of (often paired) vesicles, is unique to vertebrates. All invertebrates with well-developed brains, such as insects, spiders and

squid True squid are molluscs with an elongated soft body, large eyes, eight arms, and two tentacles in the superorder Decapodiformes, though many other molluscs within the broader Neocoleoidea are also called squid despite not strictly fitting t ...

s, have a ventral rather than dorsal system of

ganglion A ganglion is a group of neuron cell bodies in the peripheral nervous system. In the somatic nervous system this includes dorsal root ganglia and trigeminal ganglia among a few others. In the autonomic nervous system there are both sympatheti ...

s, with a split brain stem running on each side of the mouth or gut.

Molecular signatures

In addition to the morphological characteristics used to define vertebrates (i.e. the presence of a notochord, the development of a vertebral column from the notochord, a dorsal nerve cord, pharyngeal gills, a post-anal tail, etc.), molecular markers known as conserved signature indels (CSIs) in protein sequences have been identified and provide distinguishing criteria for the subphylum Vertebrata. Specifically, 5 CSIs in the following proteins: protein synthesis

elongation factor-2 Eukaryotic elongation factor 2 is a protein that in humans is encoded by the ''EEF2'' gene. It is the archaeal and eukaryotic counterpart of bacterial EF-G. This gene encodes a member of the Guanosine triphosphate, GTP-binding translation elongat ...

(EF-2),

eukaryotic translation initiation factor 3 Eukaryotic initiation factor 3 (eIF3) is a multiprotein complex that functions during the initiation phase of eukaryotic translation. It is essential for most forms of cap-dependent and cap-independent translation initiation. In humans, eIF3 con ...

(eIF3), adenosine kinase (AdK) and a protein related to

ubiquitin carboxyl-terminal hydrolase Ubiquitin C-terminal hydrolase may refer to: * Ubiquitin carboxy-terminal hydrolase L1, an enzyme * Ubiquitinyl hydrolase 1, an enzyme {{Short pages monitor A dagger (†) denotes an

extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and ...

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

, whereas all other clades have living descendants. NochelaspisMaeandrine-PaleozoologicalMuseumOfChina-May23-08

Note that, as shown in the cladogram above, the †"

Ostracodermi Ostracoderms () are the Armour (zoology), armored Agnatha, jawless fish of the Paleozoic Era. The term does not often appear in classifications today because it is paraphyletic (excluding jawed fishes) (may also be polyphyletic if anaspids are cl ...

" (armoured jawless fishes) and †" Placodermi" (armoured jawed fishes) are shown to be paraphylectic groups, separated from Gnathostomes and Eugnathostomes respectively. Also note that

Teleostei Teleostei (; Ancient Greek, Greek ''teleios'' "complete" + ''osteon'' "bone"), members of which are known as teleosts ), is, by far, the largest class (biology), infraclass in the class Actinopterygii, the ray-finned fishes, containing 96% of a ...

( Neopterygii) and Tetrapoda (

s, birds) each make up about 50% of today's vertebrate diversity, while all other groups are either

or rare. The next cladogram shows the extant clades of tetrapods (the four-limbed vertebrates), and a selection of

(†) groups: Note that reptile-like amphibians, mammal-like reptiles, and non-avian dinosaurs are all

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

. The placement of hagfish on the vertebrate tree of life has been controversial. Their lack of proper vertebrae (among with other characteristics found in lampreys and jawed vertebrates) led phylogenetic analyses based on morphology to place them outside Vertebrata.

Molecular data In genetics and biochemistry, sequencing means to determine the primary structure (sometimes incorrectly called the primary sequence) of an unbranched biopolymer. Sequencing results in a symbolic linear depiction known as a sequence which succi ...

, however, indicates they are vertebrates closely related to lampreys. A study by Miyashita ''et al''. (2019), 'reconciliated' the two types of analysis as it supports the Cyclostomata hypothesis using only morphological data.

Number of extant species

The number of described vertebrate species are split between tetrapods and fish. The following table lists the number of described

extant Extant is the opposite of the word extinct. It may refer to: * Extant hereditary titles * Extant literature, surviving literature, such as ''Beowulf'', the oldest extant manuscript written in English * Extant taxon, a taxon which is not extinct, ...

species for each vertebrate

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

as estimated in the

IUCN Red List of Threatened Species The International Union for Conservation of Nature (IUCN) Red List of Threatened Species, also known as the IUCN Red List or Red Data Book, founded in 1964, is the world's most comprehensive inventory of the global conservation status of biologi ...

, 2014.3.The World Conservation Union. 2014. ''

'', 2014.3. Summary Statistics for Globally Threatened Species
Table 1: Numbers of threatened species by major groups of organisms (1996–2014)
The IUCN estimates that 1,305,075 extant invertebrate species have been described, which means that less than 5% of the described animal species in the world are vertebrates.

Vertebrate species databases

The following databases maintain (more or less) up-to-date lists of vertebrate species: * Fish:

Fishbase FishBase is a global species database of fish species (specifically finfish). It is the largest and most extensively accessed online database on adult finfish on the web.

* Amphibians
Amphibiaweb
* Reptiles: Reptile Database * Birds
Avibase
* Mammals
Mammal species of the World

Reproductive systems

Nearly all vertebrates undergo sexual reproduction. They produce

haploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

gametes by meiosis. The smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. These fuse by the process of

fertilisation Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

to form

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Sets of chromosomes refer to the number of maternal and paternal chromosome copies, respectively ...

zygotes, which develop into new individuals.

Inbreeding

During sexual reproduction, mating with a close relative ( inbreeding) often leads to inbreeding depression. Inbreeding depression is considered to be largely due to expression of deleterious

recessive In genetics, dominance is the phenomenon of one variant (allele) of a gene on a chromosome masking or overriding the effect of a different variant of the same gene on the other copy of the chromosome. The first variant is termed dominant and t ...

mutations. The effects of inbreeding have been studied in many vertebrate species. In several species of fish, inbreeding was found to decrease reproductive success. Inbreeding was observed to increase juvenile mortality in 11 small animal species. A common breeding practice for pet dogs is mating between close relatives (e.g. between half- and full siblings). This practice generally has a negative effect on measures of reproductive success, including decreased litter size and puppy survival. Incestuous matings in birds result in severe fitness costs due to inbreeding depression (e.g. reduction in hatchability of eggs and reduced progeny survival).

Inbreeding avoidance

As a result of the negative fitness consequences of inbreeding, vertebrate species have evolved mechanisms to avoid inbreeding. Numerous inbreeding avoidance mechanisms operating prior to mating have been described. Toads and many other amphibians display breeding site fidelity. Individuals that return to natal ponds to breed will likely encounter siblings as potential mates. Although incest is possible, ''Bufo americanus'' siblings rarely mate. These toads likely recognize and actively avoid close kin as mates.

Advertisement Advertising is the practice and techniques employed to bring attention to a product or service. Advertising aims to put a product or service in the spotlight in hopes of drawing it attention from consumers. It is typically used to promote a ...

vocalizations by males appear to serve as cues by which females recognize their kin. Inbreeding avoidance mechanisms can also operate subsequent to

copulation Sexual intercourse (or coitus or copulation) is a sexual activity typically involving the insertion and thrusting of the penis into the vagina for sexual pleasure or reproduction.Sexual intercourse most commonly means penile–vaginal penetrat ...

. In guppies, a post-copulatory mechanism of inbreeding avoidance occurs based on competition between

sperm Sperm is the male reproductive cell, or gamete, in anisogamous forms of sexual reproduction (forms in which there is a larger, female reproductive cell and a smaller, male one). Animals produce motile sperm with a tail known as a flagellum, whi ...

of rival males for achieving fertilization. In competitions between sperm from an unrelated male and from a full sibling male, a significant bias in paternity towards the unrelated male was observed. When female sand lizards mate with two or more males, sperm competition within the female's reproductive tract may occur. Active selection of sperm by females appears to occur in a manner that enhances female fitness. On the basis of this selective process, the sperm of males that are more distantly related to the female are preferentially used for fertilization, rather than the

of close relatives. This preference may enhance the fitness of progeny by reducing inbreeding depression.

Outcrossing

Mating with unrelated or distantly related members of the same species is generally thought to provide the advantage of masking deleterious recessive mutations in progeny (see heterosis). Vertebrates have evolved numerous diverse mechanisms for avoiding close inbreeding and promoting outcrossing (see inbreeding avoidance). Outcrossing as a way of avoiding inbreeding depression has been especially well studied in birds. For instance, inbreeding depression occurs in the great tit (''Parus major'') when the offspring are produced as a result of a mating between close relatives. In natural populations of the great tit, inbreeding is avoided by dispersal of individuals from their birthplace, which reduces the chance of mating with a close relative. Purple-crowned fairywren females paired with related males may undertake extra-pair matings that can reduce the negative effects of inbreeding, despite ecological and demographic constraints.

Southern pied babbler The southern pied babbler (''Turdoides bicolor'') is a species of bird in the family Leiothrichidae, found in dry savannah of Botswana, Namibia, South Africa, and Zimbabwe. Description The southern pied babbler is a medium-sized cooperatively b ...

s (''Turdoides bicolor'') appear to avoid inbreeding in two ways: through dispersal and by avoiding familiar group members as mates. Although both males and females disperse locally, they move outside the range where genetically related individuals are likely to be encountered. Within their group, individuals only acquire breeding positions when the opposite-sex breeder is unrelated. Cooperative breeding in birds typically occurs when offspring, usually males, delay dispersal from their natal group in order to remain with the family to help rear younger kin. Female offspring rarely stay at home, dispersing over distances that allow them to breed independently or to join unrelated groups.

Parthenogenesis

Parthenogenesis is a natural form of reproduction in which growth and development of embryos occur without fertilization. Reproduction in squamate reptiles is ordinarily sexual, with males having a ZZ pair of sex determining chromosomes, and females a ZW pair. However, various species, including the Colombian Rainbow boa (''Epicrates maurus''), ''

Agkistrodon contortrix The eastern copperhead (''Agkistrodon contortrix''), also known as the copperhead, is a species of venomous snake, a pit viper, endemic to eastern North America; it is a member of the subfamily Crotalinae in the family Viperidae. The eastern ...

'' (copperhead snake) and ''

Agkistrodon piscivorus ''Agkistrodon piscivorus'' is a species of pit viper in the subfamily Crotalinae of the family Viperidae. It is one of the world's few semiaquatic vipers (along with the Florida cottonmouth), and is native to the southeastern United States. A ...

'' (cotton mouth snake) can also reproduce by facultative parthenogenesis—that is, they are capable of switching from a sexual mode of reproduction to an asexual mode—resulting in production of WW female progeny. The WW females are likely produced by terminal automixis. Mole salamanders are an ancient (2.4–3.8 million year-old) unisexual vertebrate lineage. In the polyploid unisexual mole salamander females, a premeiotic endomitotic event doubles the number of chromosomes. As a result, the mature eggs produced subsequent to the two meiotic divisions have the same ploidy as the somatic cells of the female salamander.

Synapsis Synapsis is the pairing of two chromosomes that occurs during meiosis. It allows matching-up of homologous pairs prior to their segregation, and possible chromosomal crossover between them. Synapsis takes place during prophase I of meiosis. Whe ...

and recombination during meiotic prophase I in these unisexual females is thought to ordinarily occur between identical sister chromosomes and occasionally between homologous chromosomes. Thus little, if any, genetic variation is produced. Recombination between homeologous chromosomes occurs only rarely, if at all. Since production of genetic variation is weak, at best, it is unlikely to provide a benefit sufficient to account for the long-term maintenance of meiosis in these organisms.

Self-fertilization

Two killifish species, the mangrove killifish (''Kryptolebias marmoratus'') and ''

Kryptolebias hermaphroditus ''Kryptolebias'' is a genus of killifish in the family Rivulidae mostly native to warm parts of South America but with one species (''K. marmoratus'') found north through the Caribbean region to the Gulf Coast states of the United States. They a ...

'', are the only known vertebrates to self-fertilize. They produce both eggs and sperm by meiosis and routinely reproduce by

self-fertilisation Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual. Autogamy is predominantly observed in the form of self-pollination, a reproductive mechanism employed by many flowering plants. However, species of ...

. This capacity has apparently persisted for at least several hundred thousand years. Each individual

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many Taxonomy (biology), taxonomic groups of animals (mostly invertebrate ...

normally fertilizes itself through uniting inside the fish's body of an egg and a sperm that it has produced by an internal organ. In nature, this mode of reproduction can yield highly homozygous lines composed of individuals so genetically uniform as to be, in effect, identical to one another. Although inbreeding, especially in the extreme form of self-fertilization, is ordinarily regarded as detrimental because it leads to expression of deleterious recessive alleles, self-fertilization does provide the benefit of ''fertilization assurance'' (

reproductive assurance Reproductive assurance (fertility assurance) occurs as plants have mechanisms to assure full seed set through selfing when outcross pollen is limiting. It is assumed that self-pollination is beneficial, in spite of potential fitness costs, when the ...

) at each generation.

Population trends

The Living Planet Index, following 16,704 populations of 4,005 species of vertebrates, shows a decline of 60% between 1970 and 2014. Since 1970, freshwater species declined 83%, and tropical populations in South and Central America declined 89%. The authors note that, "An average trend in population change is not an average of total numbers of animals lost." According to WWF, this could lead to a sixth major extinction event. The five main causes of biodiversity loss are

land-use change Land use, land-use change, and forestry (LULUCF), also referred to as Forestry and other land use (FOLU), is defined by the United Nations Climate Change Secretariat as a "greenhouse gas inventory sector that covers emissions and removals of gree ...

, overexploitation of natural resources, climate change, pollution and

invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species ad ...

References

Bibliography

* *

External links

Tree of Life

* ttp://entomology.ifas.ufl.edu/fasulo/vector/chapter_07.htm Vertebrate Pestschapter in United States Environmental Protection Agency and University of Florida/

Institute of Food and Agricultural Sciences The University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) is a teaching, research and Extension scientific organization focused on agriculture and natural resources. It is a partnership of federal, state, and county governmen ...

National Public Health Pesticide Applicator Training Manual
The Vertebrates

Marc W. Kirschner, ''iBioSeminars'', 2008. {{Authority control * Terreneuvian first appearances Extant Cambrian first appearances