Mysterious Phenotypes in Mexican Tetra Fish

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Discussion Overview

The discussion centers on the phenomenon of eye development in Mexican tetra fish, particularly the cave-dwelling form of Astyanax mexicanus, which are born with eyes that degenerate as they mature. Participants explore potential explanations for this trait, its evolutionary implications, and whether similar examples exist in other species.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants question why cave-dwelling Mexican tetras are born with eyes if they lose them later in life, suggesting there may be no advantage in a dark environment.
  • Others reference the concept of vestigiality as a potential explanation for the presence of eyes at birth.
  • One participant challenges the claim that these fish have eyes early in life, asking for sources to support this assertion.
  • A later reply confirms that cave-dwelling A. mexicanus are indeed born with eyes, which later degenerate as skin grows over them, positing that this may be an evolutionary adaptation to protect against damage in a dark environment.
  • Another participant suggests that the evolution of protective tissue over the eyes may not be the most logical path but could be influenced by various barriers or chance events in development.
  • There is speculation about whether cave fish placed in a well-lit environment would retain their eyes and develop similarly to surface fish, with differing opinions on the likelihood of this occurring.
  • A participant draws an analogy to human anatomy, noting that some traits persist despite lacking a clear purpose, such as male nipples.

Areas of Agreement / Disagreement

Participants express differing views on the reasons behind the eye degeneration in cave-dwelling Mexican tetras, with no consensus reached on the evolutionary mechanisms or implications involved.

Contextual Notes

Some claims rely on interpretations of evolutionary biology and developmental processes, which may not be universally accepted or fully understood. The discussion includes references to external sources, but the accuracy and interpretation of these sources are debated.

thorium1010
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So, i have a question, why would these fish continue to have eyes when they are born and loose it when they are older?, eventhough there is no particular advantage for them in the cave. Also these species have been separated from the surface fish (same species ) by about a million years according to the wikipedia article.

Are there other examples ? .
what can be the possible explanations for this kind of phenotypic expression that disappears later in life of the species.

http://en.wikipedia.org/wiki/Mexican_tetra
 
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thorium1010 said:
So, i have a question, why would these fish continue to have eyes when they are born and loose it when they are older?
Now that's a good question!
 
thorium1010 said:
So, i have a question, why would these fish continue to have eyes when they are born and loose it when they are older?, eventhough there is no particular advantage for them in the cave. Also these species have been separated from the surface fish (same species ) by about a million years according to the wikipedia article.

Are there other examples ? .
what can be the possible explanations for this kind of phenotypic expression that disappears later in life of the species.

http://en.wikipedia.org/wiki/Mexican_tetra

I don't see anywhere that says the blind fish have eyes early in life. Do you have a source for this?
 
bobze said:
I don't see anywhere that says the blind fish have eyes early in life. Do you have a source for this?

In this paragraph from wikipedia -

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. However, when born, the cave dwelling form of A. mexicanus has eyes. As they grow older, skin just grows over them and the eyes degenerate completely, because there is no need for sight in the pitch-black world of a cave.
 
bobze said:
I don't see anywhere that says the blind fish have eyes early in life. Do you have a source for this?
From the wiki link the OP gave;
Wikipedia said:
However, when born, the cave dwelling form of A. mexicanus has eyes. As they grow older, skin just grows over them and the eyes degenerate completely, because there is no need for sight in the pitch-black world of a cave.
The passage supplies "Froese, Rainer, and Daniel Pauly, eds. (2006). "Astyanax jordani"" as a citation.

Regarding the growth of skin over the eyes; as far as I'm aware most cave dwelling species become blind not because they evolve to not have eyes but because the evolve to grow protective tissue. Eyes are vulnerable to damage either by something physical or infection/parasitism, a creature with a protective covering would have an advantage over one without especially in an environment that is too dark to see. As to why the tissue grows in later life it could be that this is just how the evolution has gone; creatures that grow tissue at some point in their lives (especially whilst becoming sexually mature) are more likely to proliferate than those that don't. Remember that evolution does not follow the most "logical" or best designed course, it may make more sense to have tissue over the eyes from birth but that hasn't happened either because there are barriers preventing it or making it less likely (barriers such as changes to embryological development decreasing fitness) or purely because of chance.
 
Ryan_m_b said:
From the wiki link the OP gave;

The passage supplies "Froese, Rainer, and Daniel Pauly, eds. (2006). "Astyanax jordani"" as a citation.

Regarding the growth of skin over the eyes; as far as I'm aware most cave dwelling species become blind not because they evolve to not have eyes but because the evolve to grow protective tissue. Eyes are vulnerable to damage either by something physical or infection/parasitism, a creature with a protective covering would have an advantage over one without especially in an environment that is too dark to see. As to why the tissue grows in later life it could be that this is just how the evolution has gone; creatures that grow tissue at some point in their lives (especially whilst becoming sexually mature) are more likely to proliferate than those that don't.

Not that i disagree with this, as the article states it is closely related to the surface fish and can interbreed with them. Is there a possibility, if the cave fish are put in a environment where light is abundant, would not develop a protective covering and function similar to the surface water fish.
 
thorium1010 said:
Not that i disagree with this, as the article states it is closely related to the surface fish and can interbreed with them. Is there a possibility, if the cave fish are put in a environment where light is abundant, would not develop a protective covering and function similar to the surface water fish.
I doubt it as the tissue growth is most likely developmental rather than a response to the environment.
 
For a similar reason that men have nipples, despite that they don't really serve any purpose.
 

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