Mexican tetra
Encyclopedia
The Mexican tetra or Blind Cave Fish (Astyanax mexicanus) is a freshwater
fish
of the characin family
(family Characidae
) of
order
Characiformes
.
The type species
of its genus
, it is native to the Nearctic
ecozone
, originating in the lower Rio Grande
and the Neueces
and Pecos River
s in Texas
as well as the central and eastern parts of Mexico.
Growing to a maximum overall length of 12 cm (4.7 in), the Mexican tetra is of typical characin shape, with unremarkable, drab coloration. Its blind cave form, however, is notable for having no eyes and being albino, that is, completely devoid of pigmentation; it has a pinkish-white color to its body.
This fish, especially the blind variant, is reasonably popular among .
A. mexicanus is a peaceful species that spends most of its time in the mid-level of the water above the rocky and sandy bottoms of pools and backwaters of creeks and rivers of its native environment. Coming from a subtropical climate, it prefers water with 6.0–7.8 pH
, a hardness
of up to 30 dGH
, and a temperature range of 20 to 25 °C (68 to 77 °F
). In the winter it migrates to warmer waters. Its natural diet consists of crustacean
s, insect
s, and annelid
s, although in captivity it is omnivorous.
The Mexican tetra has been treated as a subspecies of A. fasciatus, the banded tetra, but this is not widely accepted.
A. mexicanus is famous for its blind cave form, which is known by such names as blind cave tetra, blind tetra, and blind cavefish. Some thirty distinct populations of Mexican tetras live in deep cave
s and have lost the power of sight
and even their eye
s. These fish can still, however, find their way around by means of their lateral line
s, which are highly sensitive to fluctuating water pressure.
The eyed and eyeless forms of A. mexicanus, being members of the same species, are closely related and can interbreed. Astyanax jordani
, however, is another blind cave fish, independently and recently evolved from the sighted surface form, which is sometimes confused with the cave form of A. mexicanus. However, when born, the cave dwelling form of A. mexicanus has eyes. As they grow older, skin just grows over them and the eyes degenerate completely, because there is no need for sight in the pitch-black world of a cave.
They prefer subdued lighting with a rocky substrate, like gravel. It is recommended that the tank mimic their natural environment, so artificial or natural rock is highly appreciated. They become semi-aggressive as they age, but are by nature schooling fish, and a group of at least three is a good idea. However, it isn't imperative, as they can be kept as a single specimen. As stated earlier, they can get to their food rather swiftly, even more so than fish with the ability to see, so they aren't really good for a community, but one is OK for a semi-aggressive community. They are also incredibly fast, so catching them with the net is a chore in itself while conducting maintenance.
.
A recent study suggests that there are at least two distinct genetic lineages among the blind populations, arguing that these represent a case of convergent evolution
.
One theory to the cave fish's evolution says that because of its dark habitat, the fish embryo saves energy it would normally use to develop eyes to develop other body parts, and this developmental choice would eventually dominate the population. This is called economical adaptation.
However, studies have shown that blind cave fish embryos begin to grow eyes during development but then something actively stops this process and flesh grows over the partially grown eyes.
Another theory is that some Mexican tetra randomly don't develop eyes (which is represented by broken genes in the fish's genome), and this lack of eyes spreads to the rest of the population despite having no advantage or disadvantage. This is called the neutral theory.
In one experiment studying eye development, University of Maryland
scientists transplanted lens
es from the eyes of sighted surface-form embryo
s into blind cave-form embryos, and vice versa. In the cave form, lens development begins within the first 24 hours of embryonic development, but quickly aborts, the lens cells dying; most of the rest of the eye structures never develop. Researchers found that the lens seemed to control the development of the rest of the eye, as the surface-form tetras which received cave-form lenses failed to develop eyes, while cave-form tetras which received surface-form lenses grew eyes with pupil
s, cornea
s, and iris
es. (It is not clear whether they possessed sight, however.)
sense by having taste buds all over its head, and by being able to store four times more energy as fat allowing it to deal with irregular food supplies more effectively. However, it is the lack of eyes that has been at the centre of discussion of the Mexican cave tetras among creationists
.
Darwin said of sightless fish -
Modern genetics has made clear that the lack of use does not, in itself, necessitate a feature's disappearance. http://www.findarticles.com/p/articles/mi_m1134/is_5_114/ai_n13811128 In this context, the positive genetic benefits have to be considered, i.e., what advantages are obtained by cave-dwelling tetras by losing their eyes? Possible explanations include:
Another likely explanation for the loss of its eyes is that of selective neutrality and genetic drift: in the dark environment of the cave, the eyes are neither advantageous nor disadvantageous and thus any genetic factors that might impair the eyes (or their development) can take hold with no consequence on the individual or species. Because there is no selection pressure for sight in this environment, any number of genetic abnormalities that give rise to the damage or loss of eyes could proliferate among the population with no effect on the fitness of the population.
Among some creationists
the cave tetra is seen as evidence against evolution. One argument claims that this is an instance of "devolution" -- showing an evolutionary trend of decreasing complexity. But evolution is a non-directional process, and while increased complexity is a common effect, there is no reason why evolution cannot tend towards simplicity if that makes an organism better suited to its environment.
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate 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...
of the characin family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
(family Characidae
Characidae
The Characidae, characids or characins are a family of freshwater subtropical and tropical fish, belonging to the order Characiformes. The name "characins" is the historical one, but scientists today tend to prefer "characids" to reflect their status as a by and large monophyletic group at family...
) of
order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...
Characiformes
Characiformes
The Characiformes are an order of ray-finned fish, comprising the characins and their allies. Grouped in 18 recognized families, there are a few thousand different species, including the well-known piranha and tetras.-Taxonomy:...
.
The type species
Type species
In biological nomenclature, a type species is both a concept and a practical system which is used in the classification and nomenclature of animals and plants. The value of a "type species" lies in the fact that it makes clear what is meant by a particular genus name. A type species is the species...
of its genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
, it is native to the Nearctic
Nearctic
The Nearctic is one of the eight terrestrial ecozones dividing the Earth's land surface.The Nearctic ecozone covers most of North America, including Greenland and the highlands of Mexico...
ecozone
Ecozone
An ecozone is the broadest biogeographic division of the Earth's land surface, based on distributional patterns of terrestrial organisms.Ecozones delineate large areas of the Earth's surface within which organisms have been evolving in relative isolation over long periods of time, separated from...
, originating in the lower Rio Grande
Rio Grande
The Rio Grande is a river that flows from southwestern Colorado in the United States to the Gulf of Mexico. Along the way it forms part of the Mexico – United States border. Its length varies as its course changes...
and the Neueces
Nueces River
The Nueces River is a river in the U.S. state of Texas, approximately long. It drains a region in central and southern Texas southeastward into the Gulf of Mexico. It is the southernmost major river in Texas northeast of the Rio Grande...
and Pecos River
Pecos River
The headwaters of the Pecos River are located north of Pecos, New Mexico, United States, at an elevation of over 12,000 feet on the western slope of the Sangre de Cristo mountain range in Mora County. The river flows for through the eastern portion of that state and neighboring Texas before it...
s in Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...
as well as the central and eastern parts of Mexico.
Growing to a maximum overall length of 12 cm (4.7 in), the Mexican tetra is of typical characin shape, with unremarkable, drab coloration. Its blind cave form, however, is notable for having no eyes and being albino, that is, completely devoid of pigmentation; it has a pinkish-white color to its body.
This fish, especially the blind variant, is reasonably popular among .
A. mexicanus is a peaceful species that spends most of its time in the mid-level of the water above the rocky and sandy bottoms of pools and backwaters of creeks and rivers of its native environment. Coming from a subtropical climate, it prefers water with 6.0–7.8 pH
PH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
, a hardness
Hard water
Hard water is water that has high mineral content . Hard water has high concentrations of Ca2+ and Mg2+ ions. Hard water is generally not harmful to one's health but can pose serious problems in industrial settings, where water hardness is monitored to avoid costly breakdowns in boilers, cooling...
of up to 30 dGH
DGH
Degrees of general hardness is a unit of water hardness, specifically of general hardness.General hardness is a measure of the concentration of metal divalent ions such as calcium and magnesium per volume of water...
, and a temperature range of 20 to 25 °C (68 to 77 °F
Fahrenheit
Fahrenheit is the temperature scale proposed in 1724 by, and named after, the German physicist Daniel Gabriel Fahrenheit . Within this scale, the freezing of water into ice is defined at 32 degrees, while the boiling point of water is defined to be 212 degrees...
). In the winter it migrates to warmer waters. Its natural diet consists of crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s, insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...
s, and annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s, although in captivity it is omnivorous.
The Mexican tetra has been treated as a subspecies of A. fasciatus, the banded tetra, but this is not widely accepted.
Blind cave form
Cave
A cave or cavern is a natural underground space large enough for a human to enter. The term applies to natural cavities some part of which is in total darkness. The word cave also includes smaller spaces like rock shelters, sea caves, and grottos.Speleology is the science of exploration and study...
s and have lost the power of sight
Visual perception
Visual perception is the ability to interpret information and surroundings from the effects of visible light reaching the eye. The resulting perception is also known as eyesight, sight, or vision...
and even their eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
s. These fish can still, however, find their way around by means of their lateral line
Lateral line
The lateral line is a sense organ in aquatic organisms , used to detect movement and vibration in the surrounding water. Lateral lines are usually visible as faint lines running lengthwise down each side, from the vicinity of the gill covers to the base of the tail...
s, which are highly sensitive to fluctuating water pressure.
The eyed and eyeless forms of A. mexicanus, being members of the same species, are closely related and can interbreed. Astyanax jordani
Astyanax jordani
Astyanax jordani is a freshwater fishof the characin family oforder Characiformes, native to Mexico.. It is called Cave Tetra , or by its local Spanish name Sardina Ciega.A blind cave fish, A. jordani is recently evolved from the Mexican tetra...
, however, is another blind cave fish, independently and recently evolved from the sighted surface form, which is sometimes confused with the cave form of A. mexicanus. However, when born, the cave dwelling form of A. mexicanus has eyes. As they grow older, skin just grows over them and the eyes degenerate completely, because there is no need for sight in the pitch-black world of a cave.
In the aquarium
A. mexicanus only grows to a length of 12 cm (4.7 in) in the recommended minimum of 30 US gallons. The blind cave tetra is a fairly hardy species. They are not picky on food, as they will eat anything from standard flakes to sinking carnivore pellets. Their lack of sight does not hinder their ability to get any and all food before it hits the bottom.They prefer subdued lighting with a rocky substrate, like gravel. It is recommended that the tank mimic their natural environment, so artificial or natural rock is highly appreciated. They become semi-aggressive as they age, but are by nature schooling fish, and a group of at least three is a good idea. However, it isn't imperative, as they can be kept as a single specimen. As stated earlier, they can get to their food rather swiftly, even more so than fish with the ability to see, so they aren't really good for a community, but one is OK for a semi-aggressive community. They are also incredibly fast, so catching them with the net is a chore in itself while conducting maintenance.
Evolution research
The surface and cave forms of the Mexican tetra have proven popular subjects for scientists studying evolutionEvolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
.
A recent study suggests that there are at least two distinct genetic lineages among the blind populations, arguing that these represent a case of convergent evolution
Convergent evolution
Convergent evolution describes the acquisition of the same biological trait in unrelated lineages.The wing is a classic example of convergent evolution in action. Although their last common ancestor did not have wings, both birds and bats do, and are capable of powered flight. The wings are...
.
One theory to the cave fish's evolution says that because of its dark habitat, the fish embryo saves energy it would normally use to develop eyes to develop other body parts, and this developmental choice would eventually dominate the population. This is called economical adaptation.
However, studies have shown that blind cave fish embryos begin to grow eyes during development but then something actively stops this process and flesh grows over the partially grown eyes.
Another theory is that some Mexican tetra randomly don't develop eyes (which is represented by broken genes in the fish's genome), and this lack of eyes spreads to the rest of the population despite having no advantage or disadvantage. This is called the neutral theory.
In one experiment studying eye development, University of Maryland
University of Maryland, College Park
The University of Maryland, College Park is a top-ranked public research university located in the city of College Park in Prince George's County, Maryland, just outside Washington, D.C...
scientists transplanted lens
Lens (anatomy)
The crystalline lens is a transparent, biconvex structure in the eye that, along with the cornea, helps to refract light to be focused on the retina. The lens, by changing shape, functions to change the focal distance of the eye so that it can focus on objects at various distances, thus allowing a...
es from the eyes of sighted surface-form embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
s into blind cave-form embryos, and vice versa. In the cave form, lens development begins within the first 24 hours of embryonic development, but quickly aborts, the lens cells dying; most of the rest of the eye structures never develop. Researchers found that the lens seemed to control the development of the rest of the eye, as the surface-form tetras which received cave-form lenses failed to develop eyes, while cave-form tetras which received surface-form lenses grew eyes with pupil
Pupil
The pupil is a hole located in the center of the iris of the eye that allows light to enter the retina. It appears black because most of the light entering the pupil is absorbed by the tissues inside the eye. In humans the pupil is round, but other species, such as some cats, have slit pupils. In...
s, cornea
Cornea
The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, the cornea refracts light, with the cornea accounting for approximately two-thirds of the eye's total optical power. In humans, the refractive power of the cornea is...
s, and iris
Iris (anatomy)
The iris is a thin, circular structure in the eye, responsible for controlling the diameter and size of the pupils and thus the amount of light reaching the retina. "Eye color" is the color of the iris, which can be green, blue, or brown. In some cases it can be hazel , grey, violet, or even pink...
es. (It is not clear whether they possessed sight, however.)
Blind cave tetras and creationism
The blind form of the Mexican tetra is different from the surface-dwelling form in a number of ways, including having unpigmented skin, having a better olfactoryOlfaction
Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates...
sense by having taste buds all over its head, and by being able to store four times more energy as fat allowing it to deal with irregular food supplies more effectively. However, it is the lack of eyes that has been at the centre of discussion of the Mexican cave tetras among creationists
Creationism
Creationism is the religious beliefthat humanity, life, the Earth, and the universe are the creation of a supernatural being, most often referring to the Abrahamic god. As science developed from the 18th century onwards, various views developed which aimed to reconcile science with the Genesis...
.
Darwin said of sightless fish -
Modern genetics has made clear that the lack of use does not, in itself, necessitate a feature's disappearance. http://www.findarticles.com/p/articles/mi_m1134/is_5_114/ai_n13811128 In this context, the positive genetic benefits have to be considered, i.e., what advantages are obtained by cave-dwelling tetras by losing their eyes? Possible explanations include:
- not developing eyes allows the individual more energy for growth and reproduction http://www.daylightatheism.org/2006/02/evolved-minds.html
- there remains less chance of accidental damage and infection, since the previously useless and exposed organ is sealed with a flap of protective skin
Another likely explanation for the loss of its eyes is that of selective neutrality and genetic drift: in the dark environment of the cave, the eyes are neither advantageous nor disadvantageous and thus any genetic factors that might impair the eyes (or their development) can take hold with no consequence on the individual or species. Because there is no selection pressure for sight in this environment, any number of genetic abnormalities that give rise to the damage or loss of eyes could proliferate among the population with no effect on the fitness of the population.
Among some creationists
Creation-evolution controversy
The creation–evolution controversy is a recurring cultural, political, and theological dispute about the origins of the Earth, humanity, life, and the universe....
the cave tetra is seen as evidence against evolution. One argument claims that this is an instance of "devolution" -- showing an evolutionary trend of decreasing complexity. But evolution is a non-directional process, and while increased complexity is a common effect, there is no reason why evolution cannot tend towards simplicity if that makes an organism better suited to its environment.