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