The zebrafish (Danio rerio) is a species of danionin belonging to the genus Danio in the suborder Cyprinoidei. Native to South Asia, it is a popular, hardy aquarium fish, frequently sold under the trade name zebra danio (thus often mislabeled as a "tropical fish", even though they also inhabit subtropical and temperate waters).
The zebrafish is extensively used as a vertebrate model organism in scientific research, particularly developmental biology, but also gene function, oncology, teratology, and drug development, in particular pre-clinical development, due to its scalability from high numbers of offspring and ease of drug delivery through water into the gills. It is also notable for its regenerative abilities, and has been modified by researchers to produce many transgenic strains.
Taxonomy The zebrafish is a derived member of the genus Danio, of the family Cyprinidae. It has a sister-group relationship with Danio aesculapii. Zebrafish are also closely related to the genus Devario, as demonstrated by a phylogenetic tree of close species.
Distribution Range The zebrafish is native to freshwater habitats in South Asia where it is found in India, Pakistan, Bangladesh, Nepal and Bhutan. The northern limit is in the South Himalayas, ranging from the Sutlej river basin in the Pakistan–India border region to the state of Arunachal Pradesh in northeast India. Its range is concentrated in the Ganges and Brahmaputra River basins, and the species was first described from Kosi River (lower Ganges basin) of India. Its range further south is more local, with scattered records from the Western and Eastern Ghats regions. It has frequently been said to occur in Myanmar (Burma), but this is entirely based on pre-1930 records and likely refers to close relatives only described later, notably Danio quagga and Danio kyathit. Likewise, old records from Sri Lanka are highly questionable and remain unconfirmed.
Zebrafish have been introduced to a variety of places outside their natural range, including California, Connecticut, Florida and New Mexico in the United States, presumably by intentional release by aquarists or by escape from fish farms. The New Mexico population had been extirpated by 2003 and it is unclear if the others survive, as the last published records were decades ago. Elsewhere the species has been introduced to Colombia and Malaysia.
Habitats Zebrafish typically inhabit moderately flowing to stagnant clear water of quite shallow depth in streams, canals, ditches, oxbow lakes, ponds and rice paddies. There is usually some vegetation, either submerged or overhanging from the banks, and the bottom is sandy, muddy or silty, often mixed with pebbles or gravel. In surveys of zebrafish locations throughout much of its Bangladeshi and Indian distribution, the water had a near-neutral to somewhat basic pH and mostly ranged from in temperature. In contrast, zebrafish in acidified freshwater were found to have reduced survival rates and impairments of their lateral line system. One unusually cold site was only and another unusually warm site was , but the zebrafish still appeared healthy. The unusually cold temperature was at one of the highest known zebrafish locations at above sea level, although the species has been recorded to .
Description The zebrafish is named for the five uniform, pigmented, horizontal, blue stripes on the side of the body, which are reminiscent of a zebra's stripes, and which extend to the end of the caudal fin. Its shape is fusiform and laterally compressed, with its mouth directed upwards. The male is torpedo-shaped, with gold stripes between the blue stripes; the female has a larger, whitish belly and silver stripes instead of gold. Adult females exhibit a small genital papilla in front of the anal fin origin. The zebrafish can reach up to in length, although they typically are in the wild with some variations depending on location. Its lifespan in captivity is around two to three years, although in ideal conditions, this may be extended to over five years. In the wild it is typically an annual species.
Reproduction
The approximate generation time for Danio rerio is three months. A male must be present for ovulation and spawning to occur. Zebrafish are asynchronous spawners and under optimal conditions (such as food availability and favorable water parameters) can spawn successfully frequently, even on a daily basis. Females are able to spawn at intervals of two to three days, laying hundreds of eggs in each clutch. Upon release, embryonic development begins; in absence of sperm, growth stops after the first few cell divisions. Fertilized eggs almost immediately become transparent, a characteristic that makes D. rerio a convenient research model species. Sex determination of common laboratory strains was shown to be a complex genetic trait, rather than to follow a simple ZW or XY system.
The zebrafish embryo develops rapidly, with precursors to all major organs appearing within 36 hours of fertilization. The embryo begins as a yolk with a single enormous cell on top (see image, 0 h panel), which divides into two (0.75 h panel) and continues dividing until there are thousands of small cells (3.25 h panel). The cells then migrate down the sides of the yolk (8 h panel) and begin forming a head and tail (16 h panel). The tail then grows and separates from the body (24 h panel). The yolk shrinks over time because the fish uses it for food as it matures during the first few days (72 h panel). After a few months, the adult fish reaches reproductive maturity
To encourage the fish to spawn, some researchers use a fish tank with a sliding bottom insert, which reduces the depth of the pool to simulate the shore of a river. Zebrafish spawn best in the morning due to their Circadian rhythms. Researchers have been able to collect 10,000 embryos in 10 minutes using this method. In particular, one pair of adult fish is capable of laying 200–300 eggs in one morning in approximately 5 to 10 at time. Male zebrafish are furthermore known to respond to more pronounced markings on females, i.e., "good stripes", but in a group, males will mate with whichever females they can find. What attracts females is not currently understood. The presence of plants, even plastic plants, also apparently encourages spawning.
Exposure to environmentally relevant concentrations of diisononyl phthalate (DINP), commonly used in a large variety of plastic items, disrupt the endocannabinoid system, disrupt cranio-facial cartilage of offsprings, and thereby affect reproduction in a sex-specific and developmental manner.
In the aquarium Zebrafish are hardy fish and considered good for beginner aquarists. Their enduring popularity can be attributed to their playful disposition, as well as their rapid breeding, aesthetics, cheap price and broad availability. They also do well in schools or shoals of six or more, and interact well with other fish species in the aquarium. However, they are susceptible to Oodinium or velvet disease, microsporidia (Pseudoloma neurophilia), and Mycobacterium species. Given the opportunity, adults eat hatchlings, which may be protected by separating the two groups with a net, breeding box or separate tank. In captivity, zebrafish live approximately forty-two months. Some captive zebrafish can develop a curved spine.
The zebra danio was also used to make genetically modified fish and were the first species to be sold as GloFish (fluorescent colored fish).
Strains In late 2003, transgenic zebrafish that express green, red, and yellow fluorescent proteins became commercially available in the United States. The fluorescent strains are trade-named GloFish; other cultivated varieties include "golden", "sandy", "longfin" and "leopard".
The leopard danio, previously known as Danio frankei, is a spotted colour morph of the zebrafish which arose due to a pigment mutation. Xanthistic forms of both the zebra and leopard pattern, along with long-finned strains, have been obtained via selective breeding programs for the aquarium trade.
Various transgenic and mutant strains of zebrafish were stored at the China Zebrafish Resource Center (CZRC), a non-profit organization, which was jointly supported by the Ministry of Science and Technology of China and the Chinese Academy of Sciences.
Wild-type strains The Zebrafish Information Network (ZFIN) provides up-to-date information about current known wild-type (WT) strains of D. rerio, some of which are listed below.
AB (AB) AB/C32 (AB/C32) AB/TL (AB/TL) AB/Tuebingen (AB/TU) C32 (C32) Cologne (KOLN) Darjeeling (DAR) Ekkwill (EKW) HK/AB (HK/AB) HK/Sing (HK/SING) Hong Kong (HK) India (IND) Indonesia (INDO) Nadia (NA) RIKEN WT (RW) Singapore (SING) SJA (SJA) SJD (SJD) SJD/C32 (SJD/C32) Tuebingen (TU) Tupfel long fin (TL) Tupfel long fin nacre (TLN) WIK (WIK) WIK/AB (WIK/AB)
Hybrids Hybrids between different Danio species may be fertile: for example, between D. rerio and D. nigrofasciatus.
Scientific research
D. rerio is a common and useful scientific model organism for studies of vertebrate development and gene function. Its use as a laboratory animal was pioneered by the American molecular biologist George Streisinger and his colleagues at the University of Oregon in the 1970s and 1980s; Streisinger's zebrafish clones were among the earliest successful vertebrate clones created. Its importance has been consolidated by successful large-scale forward genetic screens (commonly referred to as the Tübingen/Boston screens). The fish has a dedicated online database of genetic, genomic, and developmental information, the Zebrafish Information Network (ZFIN). The Zebrafish International Resource Center (ZIRC) is a genetic resource repository with 29,250 alleles available for distribution to the research community. D. rerio is also one of the few fish species to have been sent into space.
Research with D. rerio has yielded advances in the fields of developmental biology, oncology, toxicology, reproductive studies, teratology, genetics, neurobiology, environmental sciences, stem cell research, regenerative medicine, muscular dystrophies and evolutionary theory.
Zebrafish are used as a model to study pancreatic development, beta-cell regeneration, and diabetes, as their pancreas is functionally and structurally conserved with that of mammals and they can regenerate insulin-producing beta cells after injury.
Model characteristics As a model biological system, the zebrafish has many advantages for scientists. Its genome has been fully sequenced at ~1.4 billion base pairs, and it has well-understood, easy to see and test developmental behaviors. Its embryonic development is rapid, meaning that it grows up fast, and its embryos are large enough for scientific study without special equipment, robust, and transparent, and are able to develop outside of their mother. Furthermore, well-characterized mutant strains are readily available, which means that zebrafish can be used to study other organisms.