Mexican tetra
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| Astyanax mexicanus (De Filippi, 1853) |
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.
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 is reasonably popular among aquarists. This is especially true of the blind cave form.
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.
The Mexican tetra has been treated as a subspecies of A. fasciatus, the banded tetra, but this is not widely accepted.[1]
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[edit] 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.
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]
[edit] Evolution research
The surface and cave forms of the Mexican tetra have proven popular subjects for scientists studying evolution.
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]
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]
[edit] Blind cave tetras and creationism
The blind form of the Mexican tetra is different to 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.[7] 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 -
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 affected other changes, such as an increase in the length of antennae or palpi, as compensation for blindness.
—Charles Darwin, Origin of Species (1859)
Modern genetics has made clear that the lack of use does not, in itself, necessitate a feature's disappearance[1] - as seen in humans with useless evolutionary retentions such as wisdom teeth and the appendix. In this context, the positive genetic benefits for the 'degeneration' have to be considered, i.e., what advantages are obtained by cave-dwelling tetras by losing their eyes? Possible explanations include[2]:
- not developing eyes allows the individual more energy for growth and reproduction[3]
- there remains less chance of accidental damage and infection, since the previously useless and exposed organ is sealed with a flap of protective skin
Among 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 -- and that this is inconsistent with the main tenet of evolutionary theory, as it implies a loss of 'information'. Another argument, though representing a minor and controversial subset within creationists, maintains that such 'degeneration' is a product of a "fallen, cursed creation"[4]. Others see such 'degeneration' as an abnormal disability.[5]
From the scientific perspective, such arguments are based on a common misunderstanding of the theory. Mainstream evolution defines a non-directional process, and increased complexity is a common, but not necessarily inherent effect of it. Evolution does not dictate that an organism becomes more or less complex, but rather better suited to their environment.[8]
[edit] References
- ^ a b "Astyanax mexicanus". FishBase. Ed. Ranier Froese and Daniel Pauly. March 2006 version. N.p.: FishBase, 2006.
- ^ Astyanax mexicanus (TSN 162850). Integrated Taxonomic Information System. Accessed on 1 July 2006.
- ^ "Astyanax jordani". FishBase. Ed. Ranier Froese and Daniel Pauly. March 2006 version. N.p.: FishBase, 2006.
- ^ 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.
- ^ Yamamoto, Yoshiyuki, and William R. Jeffrey (2000). "Central Role for the Lens in Cave Fish Eye Degeneration". Science 289 (28 July): 631–3.
- ^ Pennisi, Elizabeth (2000). "Embryonic Lens Prompts Eye Development". Science 289 (28 July): 522–3.
- ^ Helfman G., Collette B., & Facey D.: The Diversity of Fishes, Blackwell Publishing, p 315, 1997, ISBN 0-86542-256-7
- ^ Dawkins, R.: Climbing Mount Improbable, W. W. Norton & Co, 1997, ISBN 0393316823
