The Mexican tetra (Astyanax mexicanus) is a freshwater fish of the characin family (family Characidae) of order Characiformes.^{[1] [2]} The type species of its genus, it is native to the Nearctic ecozone, originating in the lower Rio Grande and the Neueces and Pecos Rivers in Texas as well as the central and eastern parts of Mexico. Scientific classification or biological classification is how biologists group and categorize extinct and living species of organisms (as opposed to folk taxonomy). ... Phyla Subregnum Parazoa Porifera (sponges) Subregnum Agnotozoa Placozoa (trichoplax) Orthonectida (orthonectids) Rhombozoa (dicyemids) Subregnum Eumetazoa Radiata (unranked) (radial symmetry) Ctenophora (comb jellies) Cnidaria (coral, jellyfish, anemones) Bilateria (unranked) (bilateral symmetry) Acoelomorpha (basal) Orthonectida (parasitic to flatworms, echinoderms, etc. ... Typical Classes Subphylum Urochordata - Tunicatas Ascidiacea Thaliacea Larvacea Subphylum Cephalochordata - Lancelets Subphylum Myxini - Hagfishes Subphylum Vertebrata - Vertebrates Petromyzontida - Lampreys Placodermi (extinct) Chondrichthyes - Cartilaginous fishes Acanthodii (extinct) Actinopterygii - Ray-finned fishes Actinistia - Coelacanths Dipnoi - Lungfishes Amphibia - Amphibians Reptilia - Reptiles Aves - Birds Mammalia - Mammals Chordates (phylum Chordata) include the vertebrates, together with... Orders See text The Actinopterygii are the ray-finned fish. ... Families  Acestrorhynchidae  Anostomidae - Headstanders  Characidae - Characins and tetras  Citharinidae  Ctenoluciidae - Pike-characids  Curimatidae  Erythrinidae - Trahiras  Gasteropelecidae - Freshwater hatchetfishes  Hemiodontidae  Hepsetidae  Lebiasinidae The Characiformes are an order of ray-finned fish, comprising the characins and their allies. ... Genera Subfamily Alestiinae - African tetras  Alestes (or Brycinus)  Hydrocynus  Micralestes  Phenacogrammus  Rhabdalestes  and 13 other genera Subfamily Characidiinae  8 genera Subfamily Characinae  Acestrorhynchus - Pike characins  Aphyocharax  Boehlkea  Chalceus  Charax  Cynodo  Exodon  Hasemania  Hydrolycus  Nematobrycon  Petitella  Priocharax  Rhaphiodon  Roeboides  Thayeria  ... Subfamily Crenuchinae  Crenuchus  Poecilocharax Subfamily Glandulocaudinae  16 genera Subfamily Iguanodectinae  Iguanodectes... Species Categories: Animal stubs | Characins ... In biology, binomial nomenclature is the formal method of naming species. ... 1853 was a common year starting on Saturday (see link for calendar). ... For the village on the Isle of Wight, see Freshwater, Isle of Wight. ... The Guppy, also known as guppie (Poecilia reticulata) is one of the most popular freshwater aquarium fish species in the world. ... Families  Acestrorhynchidae  Anostomidae - Headstanders  Characidae - Characins and tetras  Citharinidae  Ctenoluciidae - Pike-characids  Curimatidae  Erythrinidae - Trahiras  Gasteropelecidae - Freshwater hatchetfishes  Hemiodontidae  Hepsetidae  Lebiasinidae The Characiformes are an order of ray-finned fish, comprising the characins and their allies. ... In biological classification, family (Latin: familia, plural familiae) is 1) a rank or 2) a taxon in that rank. ... Genera Subfamily Alestiinae - African tetras  Alestes (or Brycinus)  Hydrocynus  Micralestes  Phenacogrammus  Rhabdalestes  and 13 other genera Subfamily Characidiinae  8 genera Subfamily Characinae  Acestrorhynchus - Pike characins  Aphyocharax  Boehlkea  Chalceus  Charax  Cynodo  Exodon  Hasemania  Hydrolycus  Nematobrycon  Petitella  Priocharax  Rhaphiodon  Roeboides  Thayeria  ... Subfamily Crenuchinae  Crenuchus  Poecilocharax Subfamily Glandulocaudinae  16 genera Subfamily Iguanodectinae  Iguanodectes... Scientific classification or biological classification refers to how biologists group and categorize extinct and living species of organisms. ... Families  Acestrorhynchidae  Anostomidae - Headstanders  Characidae - Characins and tetras  Citharinidae  Ctenoluciidae - Pike-characids  Curimatidae  Erythrinidae - Trahiras  Gasteropelecidae - Freshwater hatchetfishes  Hemiodontidae  Hepsetidae  Lebiasinidae The Characiformes are an order of ray-finned fish, comprising the characins and their allies. ... Type specimens When a new species is discovered, more important than creating a new and unique name for the species is developing a reasonably detailed description. ... In biology, a genus (plural genera) is a taxonomic grouping. ... The Nearctic is one of the eight terrestrial ecozones dividing the Earths land surface. ... Ecozones are global divisions which have their own characteristic interplay of climatic factors, morphodynamics, soil-forming processes, living conditions for plants and animals, and production potentials for agriculture and forestry. ... The Rio Grande flowing in Big Bend National Park The Rio Grande in its lower course, between Matamoros and Brownsville Known as the Rio Grande in the United States and as the Río Bravo (or, more formally, the Río Bravo del Norte) in Mexico, the river, 3,034... The Nueces River is a river in the U.S. state of Texas, approximately 315 mi (507 km) long. ... The Pecos River rises in northern New Mexico, USA, and flows for 926 miles (1,480 km) through the eastern portion of that state and neighboring Texas before it empties into the Rio Grande near Del Rio. ... This article does not cite its references or sources. ...

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 article is being considered for deletion in accordance with Wikipedias deletion policy. ... Mid-19th century tool for converting between different standards of the inch An inch is an Imperial and U.S. customary unit of length. ... Albinism is a genetic condition resulting in a lack of pigmentation in the eyes, skin and hair. ...

This fish is reasonably popular among aquarists. This is especially true of the blind cave form. For other uses, see Aquarium (disambiguation). ...

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 crustaceans, insects, and annelids, although in captivity it is omnivorous. Subtropical (or semitropical) areas are those adjacent to the tropics, usually roughly defined as the ranges 23. ... pH is a measure of the acidity of a solution, in terms of activity of hydrogen ions (H+). For dilute solutions, however, it is convenient to substitute the activity of the hydrogen ions with the molarity (mol/L) of the hydrogen ions (however, this is not necessarily accurate at higher... The title given to this article is incorrect due to technical limitations. ... The degree Celsius (symbol: °C) is an SI derived unit of temperature. ... Fahrenheit is a temperature scale named after the German physicist Gabriel Fahrenheit (1686–1736), who proposed it in 1724. ... Classes & Subclasses Branchiopoda Phyllopoda Sarsostraca Remipedia Cephalocarida Maxillopoda Thecostraca Tantulocarida Branchiura Pentastomida Mystacocarida Copepoda Ostracoda Myodocopa Podocopa Malacostraca Phyllocarida Hoplocarida Eumalacostraca The crustaceans (Crustacea) are a large group of arthropods (55,000 species), usually treated as a subphylum [1]. They include organisms such as lobsters, crabs, shrimp and barnacles. ... Classes & Orders See taxonomy Insects are invertebrate animals of the Class Insecta, the largest and (on land) most widely-distributed taxon within the phylum Arthropoda. ... Classes and subclasses Class Polychaeta      (paraphyletic?) Class Clitellata    Oligochaeta -      (Earthworms, etc. ... Omnivores are organisms that consume both plants and animals. ...

The Mexican tetra has been treated as a subspecies of A. fasciatus, the banded tetra, but this is not widely accepted.^[1]

Blind cave form

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 caves and have lost the power of sight and even their eyes. These fish can still, however, find their way around by means of their lateral lines, which are highly sensitive to fluctuating water pressure. Alternate meanings: Cave (disambiguation) This article is about natural caves; for artificial caves used as dwellings, such as those in north China, see yaodong. ... Visual perception is one of the senses, consisting of the ability to detect light and interpret (see) it as the perception known as sight or naked eye vision. ... Closeup of a blue-green human eye. ... In fish, the lateral line is a sense organ used to detect movement in the surrounding water. ... Water pressure is the pressure in any system for supplying water, usually a domestic water system, although the term is used in other contexts as well, such as a municipal water system. ...

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.^[3]

Evolution research

The surface and cave forms of the Mexican tetra have proven popular subjects for scientists studying evolution. A hypothetical phylogenetic tree of all extant organisms, based on 16S rRNA gene sequence data, showing the evolutionary history of the three domains of life, bacteria, archaea and eukaryotes. ...

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.^[4] It has been suggested that Morphological convergence be merged into this article or section. ...

In one experiment studying eye development, University of Maryland scientists transplanted lenses from the eyes of sighted surface-form embryos 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 pupils, corneas, and irises. (It is not clear whether they possessed sight, however.)^{[5] [6]} The University of Maryland, College Park (also known as UM, UMD, or UMCP) is a public university located in College Park, Maryland, just outside Washington, D.C., USA. As the flagship institution of the University System of Maryland, the university is most often referred to as the University of Maryland... Light from a single point of a distant object and light from a single point of a near object being brought to a focus by changing the curvature of the lens. ... Embryos (and one tadpole) of the wrinkled frog (Rana rugosa). ... The human eye The pupil is the central transparent area (showing as black). ... The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber, providing most of an eyes optical power [1]. Together with the lens, the cornea refracts light and, as a result, helps the eye to focus. ... Iris has three main meanings, unrelated except for their derivation from the Greek word for rainbow: Iris (mythology), a messenger of the gods in Greek mythology, identified with the rainbow Iris (anatomy), the sphincter around the pupil of the eye, named after the colors in human and animal eyes Iris...

Evolution controversy

The blind form of the Mexican tetra has been used by opponents of evolution as a criticism of natural selection. The basic argument is that because in this instance the evolutionary trend is from complex (having eyes) to less complex (not having eyes), this widely-quoted example of adaptation to a new environment (in this case, to completely dark caves) tells us nothing about how evolution can be responsible for trends of increasing complexity, i.e., from simple organisms to more complex ones. Furthermore, since this is an example of the "deterioration of creation" after the mythical Fall of Man, it appears better to correspond with the beliefs of creationists. The creation-evolution controversy (also termed the creation vs. ... Natural selection is the process by which individual organisms with favorable traits are more likely to survive and reproduce. ... Essentially, original sin is the doctrine, shared in one form or another by most Christian churches, that the sin of Adam and Eve in the Garden of Eden changed or damaged human nature, such that all human beings since then are innately predisposed to sin, and are powerless to overcome... The Creation of Light by Gustave Doré. In the Abrahamic religions, creationism is the belief that humans, life, the Earth, and the universe have a miraculous origin in a deity or supreme beings supernatural intervention. ...

Biologists would simply view this argument as stemming from a misunderstanding of "complexity" and "simplicity" in the context of evolution and adaptation. Having eyes, for example, may seem better or more advanced to us, but to an animal living in a cave, the best and most advanced morphology is having a body form that is best suited to its environment. If not having eyes means that the animal has more energy for growth and reproduction without suffering any loss in terms of being able to find food or swim away from predators and obstacles, then being blind is the optimal state for a species living inside caves. Simply put, evolution isn't about making things more or less complex, but about making them better suited to their environment and ecological niche. Often, as is the case with the blind cave tetra, as well as organisms like parasites, the perceived trend from our point of view may indeed seem to be from a more complex body form to a simpler one. A parasite is an organism that lives in or on the living tissue of a host organism at the expense of it. ...

References

1. ^ ^a b Astyanax mexicanus. FishBase. Ed. Ranier Froese and Daniel Pauly. March 2006 version. N.p.: FishBase, 2006.
2. ^ Astyanax mexicanus (TSN 162850). Integrated Taxonomic Information System. Accessed on 1 July 2006.
3. ^ Astyanax jordani. FishBase. Ed. Ranier Froese and Daniel Pauly. March 2006 version. N.p.: FishBase, 2006.
4. ^ Dowling, T. E., D. P. Martasian, and W. R. Jeffrey (2002). "Evidence for Multiple Genetic Forms with Similar Eyeless Phenotypes in the Blind Cavefish, Astyanax mexicanus". Molecular Biology and Evolution 19: 446–55.
5. ^ Yamamoto, Yoshiyuki, and William R. Jeffrey (2000). "Central Role for the Lens in Cave Fish Eye Degeneration". Science 289 (28 July): 631–3.
6. ^ Pennisi, Elizabeth (2000). "Embryonic Lens Prompts Eye Development". Science 289 (28 July): 522–3.

• Sharpe, Shirlie. "Blind Cave Fish". Your Guide to Freshwater Aquariums. Accessed on 3 November 2004.