Monday Organism – Mexican Tetra

Blind cave fish, A. mexicanus

Blind cave fish, A. mexicanus

Astyanax mexicanus, or “blind cave fish”, as it is commonly known, is an evolutionary wonder. The tetra lives freshwater rivers in Mexico, particulary in dark caves in which eyesight is redundant.

The most fascinating aspect of the blind cave fish, as the name implies, is its characteristic lack of eyes.

The  Mexican Tetra is in fact a moniker for two varieties of the same species, only one of them blind, that are genetically related, and are, in fact, only isolated reproductively by geographic location.

Stating the obvious, this means that the blind tetra and the surface tetra can mate and reproduce, while the blind tetra grows to full maturity without ever developing eyes.

The evolutionary tale doesn’t end here, though. There is another related species confounded with the Mexican Tetra called Astyanax jordani. This species probably evolved from the surface version of A. mexicanus, and, amazingly, develops eyes which are consequently covered by skin until they degenerate.

A. jordani - notice the skin-covered eyes!

A. jordani - notice the skin-covered eyes!

That is a stunning demonstration of neutral evolution in action: a surface fish that changes its habitat to cave-dwelling loses eyesight without adaptive costs, but  without arresting completely the development of eyes.

There’s also material evidence that the surface and eyeless versions of A. mexicanus are, in fact, the same species or , at the very least, extremely related: lens cells were cross-planted in both varieties of A. mexicanus, causing the surface fish embryos to become eyeless and the blind fish embryos to develop fully formed eyes, lenses, irises, lock, stock and barrel.

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3 Responses to “Monday Organism – Mexican Tetra”

  1. Nessa Says:

    I wonder… is the blindness genetic or epigenetic in the mexican ones?

    And also, if one blind cave fish and one surface fish mate, will the offspring be blind or not?

  2. Freidenker Says:

    I’m betting the blindness is genetic – if only because blindness would be adaptive for the cave-fish variety. Not that I can tell for sure..

    As for the hybridization question : good one. I’m betting on a recessive mutation responsible for the blindness “gene” (probably a “gene complex”) – so I assume there will be a bias towards sighted fish. I’m only betting on this because in my very humble experience, “devolution” adaptation tend to be recessive mutations, a la sickle cell anemia.

  3. No Pressure | Obsessed With Reality Says:

    […] In short – a certain law in evolution states that useless traits tend to devolve and diminish (a la eyes in cave fish). […]

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