There are some obvious differences between the rivers and caves inhabited by the different populations of A. mexicanus. Perhaps the most obvious is the lack of light in the caves, which are hidden from sunlight. The constant temperature in caves and the lack of sunlight prevents plant growth, which would normally form the bottom of the food chain. The two major sources of nutrition within the caves are probably bat droppings and nutrients that are swept into the caves during seasonal flooding.
The lack of light has another significant effect on cave inhabitants: eyes are not particularly useful. We believe that, when fish were first trapped in caves, those with smaller eyes were more likely to survive and produce offspring because smaller eyes helped them to save energy. After many generations, survival of fish with smaller eyes eventually resulted in fish that completely lack functional eyes. Mexican cavefish and other organisms that lost their eyes this way rely on their other senses instead, like smell, taste, or sensing waterflow. Scientists wonder whether eye loss, and the changes in other senses that accompany eye loss, influence the amount of sleep cave-dwelling fish need.

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