Maturing at a total length of about 12cm (4.7in), the Mexican tetra is of typical characin form, albeit with silvery, unremarkable scalation, likely an evolutionary adaptation to its natural environment.[4] By comparison, the species' blind "cave" form has scales which evolved a pale, pinkish-white color, somewhat resembling an albino,[8] as it inhabits pitch-black caverns and subterranean streams and has no need for a colorful appearance (i.e. for attracting mates).
Likewise, the blind cave tetra has fully "devolved" (lost) the use of its eyes by living in an environment completely devoid of natural light, with only empty sockets in their place. The blind tetra instead has sensory organs along its body, as well as a heightened nervous system (and senses of smell and touch), and can immediately detect where objects or other animals are located by slight changes in the surrounding water pressure, a process vaguely similar to echolocation—another adaptation known from cave-dwelling, as well as aquatic, species, such as the bats and cetaceans.
The Mexican tetra's blind variant has experienced a steady surge in popularity among modern aquarists.[9]
A. mexicanus is a peaceful, sociable schooling species, like most tetras, that spends most of its time in midlevel waters above the rocky and sandy bottoms of pools, and backwaters of creeks and streams. Coming from an environment somewhere between subtropical climate, it prefers water with 6.5–8 pH, a hardness of up to 30 dGH, and a temperature range of 20 to 25°C (68 to 77°F). In the winter, some populations migrate to warmer waters. The species' natural diet consists largely of crustaceans, annelids and arthropods and their larvae, including both aquatic insects, such as water beetles, and those that land on or fall in the water, like flies or arachnids. It will also supplement its diet with algae or aquatic vegetation; in captivity, it is largely omnivorous, often doing well on a variety of foods such as frozen/thawed or live cultured blackworms, bloodworms, brine shrimp, daphnia, and mysis shrimp, among other commercially available fish foods.[4][9]
The Mexican tetra has been treated as a subspecies of A. fasciatus, though this is not widely accepted.[4] Additionally, the hypogean blind cave form is sometimes recognized as a separate species, A. jordani, but this directly contradicts the phylogenetic evidence.[8][10][11][12][13][14]
A. mexicanus is famous for its blind cave form, which is known by such names as blind cave tetra, blind tetra (leading to easy confusion with the Brazilian Stygichthys typhlops), blind cave characin and blind cavefish. Depending on the exact population, cave forms can have degenerated sight or have total loss of sight and even their eyes, due to down-regulation of the protein αA-crystallin and consequent lens cell death.[15] Despite losing their eyes, cavefish cells respond to light responsive and show an endogenous circadian rhythm.[16] During the start of development, larvae still exhibit a shadow response which is controlled by the pineal eye.[17] The fish in the Pachón caves have lost their eyes completely whilst the fish from the Micos cave only have limited sight.[18] Cave fish and surface fish are able to produce fertile offspring.[18]
These fish can still, however, find their way around by means of their lateral lines, which are highly sensitive to fluctuating water pressure.[19] Blindness in A. mexicanus induces a disruption of early neuromast patterning, which further causes asymmetries in cranial bone structure. One such asymmetry is a bend in the dorsal region of their skull, which is propounded to increase water flow to the opposite side of the face, functionally enhancing sensory input and spatial mapping in the dark waters of caves.[20] Scientists suggest that gene cystathionine beta synthase-a mutation restricts blood flow to cavefish eyes during a critical stage of growth so the eyes are covered by skin.[21]
Currently, about 30 cave populations are known, dispersed over three geographically distinct areas in a karst region of San Luis Potosí and far southern Tamaulipas, northeastern Mexico.[10][22][23] Among the various cave population are at least three with only full cave forms (blind and without pigment), at least eleven with cave, "normal" and intermediate forms, and at least one with both cave and "normal" forms but no intermediates.[22] Studies suggest at least two distinct genetic lineages occur among the blind populations, and the current distribution of populations arose by at least five independent invasions.[10][24] Furthermore, cave populations have a very recent origin (< 20,000 years) in which blindness or reduced vision evolved convergently after surface ancestors populated several caves independently at different times.[25][26] This recent origin suggests that the phenotypic changes in cavefish populations, namely eye degeneration, arose as a result of the high fixation of genetic variants present in surface fish populations in a short period of time.[27]
The eyed and eyeless forms of A. mexicanus, being members of the same species, are closely related and can interbreed[28] making this species an excellent model organism for examining convergent and parallel evolution, regressive evolution in cave animals, and the genetic basis of regressive traits.[29] This, combined with the ease of maintaining the species in captivity, has made it the most studied cavefish and likely also the most studied cave organism overall.[22]
The blind and colorless cave form of A. mexicanus is sometimes recognized as a separate species, A. jordani, but this leaves the remaining A. mexicanus as a paraphyletic species and A. jordani as polyphyletic.[8][10][11][12][13][14] The Cueva Chica Cave in the southern part of the Sierra del Abra system is the type locality for A. jordani.[8] Other blind populations were initially also recognized as separate species, including antrobius described in 1946 from the Pachón Cave and hubbsi described in 1947 from the Los Sabinos Cave (both subsequently merged into jordani/mexicanus).[8] The most divergent cave population is the one in Los Sabinos.[8][30]
Another cave-adapted population of Astyanax, varying from blind and depigmented to individuals showing intermediate features, is known from the Granadas Cave, part of the Balsas River drainage in Guerrero, southern Mexico, but it is a part of A. aeneus (itself sometimes included in A. mexicanus).[8][23][31]
Evolution research
The surface and cave forms of the Mexican tetra have proven powerful subjects for scientists studying evolution.[28] When the surface-dwelling ancestors of current cave populations entered the subterranean environment, the change in ecological conditions rendered their phenotype—which included many biological functions dependent on the presence of light—subject to natural selection and genetic drift.[29][32] One of the most striking changes to evolve was the loss of eyes. This is referred to as a "regressive trait" because the surface fish that originally colonized caves possessed eyes.[28] In addition to regressive traits, cave forms evolved "constructive traits". In contrast to regressive traits, the purpose or benefit of constructive traits is generally accepted.[29] Active research focuses on the mechanisms driving the evolution of regressive traits, such as the loss of eyes, in A. mexicanus. Recent studies have produced evidence that the mechanism may be direct selection,[33][34] or indirect selection through antagonistic pleiotropy,[35] rather than genetic drift and neutral mutation, the traditionally favored hypothesis for regressive evolution.[32]
Pleiotropy is hypothesized to be important in cave fish because there are genes that might be selected for one trait and automatically cause another trait to be selected for it if it is governed by the same gene.[36] As selective pressure on one trait can coordinate change in others, pleiotropy could explain why independent adaptation to the cave environment has been observed in multiple populations of the species.[37] One example is the relationship between taste bud amplification and eye loss controlled by sonic hedgehog expression (Shh) in cave fish.[38] It has been shown that with an over expression of Shh there is an increased number of taste buds and reduced eye development.[38] It is hypothesized that since caves are food and nutrient limited, having an increased amount of taste buds is important and may be under strong selection while at the same time causing evolution of eye loss.[38]
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 olfactory 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.[39]
By the time that an animal had reached, after numberless generations, the deepest recesses, disuse will on this view have more or less perfectly obliterated its eyes, and natural selection will often have effected other changes, such as an increase in the length of antennae or palpi, as compensation for blindness.
Modern genetics has made clear that the lack of use does not, in itself, necessitate a feature's disappearance.[41][42] 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 but not egg production.[15] However the species does use other methods to locate food and detect danger, which also consume energy that would be conserved if it had eyes or transparent eyelids.
There remains less chance of accidental damage and infection, since the previously useless and exposed organ is sealed with a flap of protective skin. It is unknown why this species did not develop transparent skin or eyelids instead, as some species of reptiles did.
The lack of eyes disables the "body clock", which is controlled by periods of light and dark, conserving energy. However sunlight does have minimal impact on the "body clock" in caves.[citation needed]
It is important to note that even if natural selection is positively acting to reduce eye growth drift is still present.[36]
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 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.[43]
Inhibition of the HSP90 protein has a dramatic effect in the development of the blind tetra.[44]
In the aquarium
The blind cave tetras seen in the aquarium trade are all based on stock collected in the Cueva Chica Cave in the southern part of the Sierra del Abra system in 1936.[8] These were sent to an aquarium company in Texas, who soon started to distribute them to aquarists. Since then, these have been selectively bred for their troglomorphic traits.[8] Today large numbers are bred at commercial facilities, especially in Asia.[9]
The blind cave tetra is a hardy species.[8] Their lack of sight does not hinder their ability to get food. They prefer subdued lighting with a rocky substrate, like gravel, mimicking their natural environment. They become semi-aggressive as they age, and are by nature schooling fish.[45] Experiments have shown that keeping these fish in bright aquarium set-ups has no effect on the development of the skin flap that forms over their eyes as they grow.
The Amblyopsidae are a fish family commonly referred to as cavefish, blindfish, or swampfish. They are small freshwater fish found in the dark environments of caves, springs and swamps in the eastern half of the United States. Like other troglobites, most amblyopsids exhibit adaptations to these dark environments, including the lack of functional eyes and the absence of pigmentation. More than 200 species of cavefishes are known, but only six of these are in the family Amblyopsidae. One of these, Forbesichthys agassizii, spends time both underground and aboveground. A seventh species in this family, Chologaster cornuta, is not a cave-dweller but lives in aboveground swamps.
Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last common ancestor of those groups. The cladistic term for the same phenomenon is homoplasy. The recurrent evolution of flight is a classic example, as flying insects, birds, pterosaurs, and bats have independently evolved the useful capacity of flight. Functionally similar features that have arisen through convergent evolution are analogous, whereas homologous structures or traits have a common origin but can have dissimilar functions. Bird, bat, and pterosaur wings are analogous structures, but their forelimbs are homologous, sharing an ancestral state despite serving different functions.
Population genetics is a subfield of genetics that deals with genetic differences within and among populations, and is a part of evolutionary biology. Studies in this branch of biology examine such phenomena as adaptation, speciation, and population structure.
The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial cells, known as hair cells, which respond to displacement caused by motion and transduce these signals into electrical impulses via excitatory synapses. Lateral lines play an important role in schooling behavior, predation, and orientation.
Cave tetra may refer to different species of fish:
Astyanax is a genus of freshwater fish in the family Characidae of the order Characiformes. Some of these fish, like many of their relatives, are kept as aquarium pets and known collectively as tetras. With around 150 described species and new ones being described yearly, this genus is among the largest of the entire order; Hyphessobrycon also has more than 145 species and which one is larger at any one time depends on whether more species have been recently described in one or the other. The blind and colorless cave tetra of Mexico is a famous member of the genus, but its taxonomic position is disputed: Some recognize it as part of the Mexican tetra and this is supported by phylogenetic evidence, but others recognize the cave form as a separate species, A. jordani.
Canalisation is a measure of the ability of a population to produce the same phenotype regardless of variability of its environment or genotype. It is a form of evolutionary robustness. The term was coined in 1942 by C. H. Waddington to capture the fact that "developmental reactions, as they occur in organisms submitted to natural selection...are adjusted so as to bring about one definite end-result regardless of minor variations in conditions during the course of the reaction". He used this word rather than robustness to consider that biological systems are not robust in quite the same way as, for example, engineered systems.
Introgression, also known as introgressive hybridization, in genetics is the transfer of genetic material from one species into the gene pool of another by the repeated backcrossing of an interspecific hybrid with one of its parent species. Introgression is a long-term process, even when artificial; it may take many hybrid generations before significant backcrossing occurs. This process is distinct from most forms of gene flow in that it occurs between two populations of different species, rather than two populations of the same species.
Astyanax jordani is a freshwater fish of the characin family of order Characiformes, native to Mexico. It is sometimes called the cave tetra, or by its local Spanish name sardina ciega.
Stygichthys typhlops, the blind tetra or Brazilian blind characid, is a species of fish in the family Characidae and the only member of the genus Stygichthys. It is endemic to caves in northern Minas Gerais, Brazil. Like other cave-adapted fish, the Brazilian blind characid is blind and lacks pigmentation. It reaches up to about 4.6 cm (1.8 in) in standard length. It is solitary and when kept together in an aquarium, individuals are indifferent to each other.
P protein, also known as melanocyte-specific transporter protein or pink-eyed dilution protein homolog, is a protein that in humans is encoded by the oculocutaneous albinism II (OCA2) gene. The P protein is believed to be an integral membrane protein involved in small molecule transport, specifically of tyrosine—a precursor of melanin. Certain mutations in OCA2 result in type 2 oculocutaneous albinism. OCA2 encodes the human homologue of the mouse p gene.
Cavefish or cave fish is a generic term for fresh and brackish water fish adapted to life in caves and other underground habitats. Related terms are subterranean fish, troglomorphic fish, troglobitic fish, stygobitic fish, phreatic fish, and hypogean fish.
Visual perception in animals plays an important role in the animal kingdom, most importantly for the identification of food sources and avoidance of predators. For this reason, blindness in animals is a unique topic of study.
Evolutionary biology, in particular the understanding of how organisms evolve through natural selection, is an area of science with many practical applications. Creationists often claim that the theory of evolution lacks any practical applications; however, this claim has been refuted by scientists.
Troglomorphism is the morphological adaptation of an animal to living in the constant darkness of caves, characterised by features such as loss of pigment, reduced eyesight or blindness, and frequently with attenuated bodies or appendages. The terms troglobitic, stygobitic, stygofauna, troglofauna, and hypogean or hypogeic, are often used for cave-dwelling organisms.
Albinism is the congenital absence of melanin in an animal or plant resulting in white hair, feathers, scales and skin and reddish pink or blue eyes. Individuals with the condition are referred to as albinos.
William R. Jeffery is an American professor of evolutionary developmental biology whose studies focus on the evolution of development, especially blind cavefish and tunicates. He is a fellow of the American Association for the Advancement of Science and the Linnean Society of London.
Astyanax aeneus, the banded tetra, is a small species of fish native to southern Central America and northern South America. It can be found in a variety of environments, including lakes, rivers, ponds, and slightly brackish locales like lagoons. As well as a varied habitat, it has a varied omnivorous diet: algae, seeds, leaves, insects, and fish fry appear to be the most common.
Astyanax baileyi is a small freshwater fish native to northern Guatemala. Based on several visual aspects, it was once considered a member of the genus Bramocharax, which is now obsolete, and has since been synonymized with Astyanax. As such, former members of Bramocharax are now a part of Astyanax, like Astyanax bransfordii and Astyanax caballeroi.
Astyanax caballeroi is a small species of freshwater fish endemic to a single lake system in Mexico. It has a longer snout and more slender body than most other species in the genus Astyanax, thought to be the result of predatory behavior; while A. caballeroi eats invertebrates and smaller fish, other Astyanax species are more broadly omnivorous, and have deeper bodies with shorter snouts. This difference in body shape once placed A. caballeroi, along with several other species of Astyanax, into the former genus Bramocharax.
↑ Helfman, G. S.; Collete, B. B.; Facey, D. E. (1997). The Diversity of Fishes. Malden, Massachusetts, USA: Blackwell Science. p.315. ISBN0-86542-256-7.
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