by periods of light and dark, conserving energy. However sunlight does have minimal impact on the "body clock" in caves. Another likely explanation for the loss of its eyes is that of selective neutrality and genetic drift; in the dark environment of the cave, the eyes are neither advantageous nor disadvantageous and thus any genetic factors that might impair the eyes (or their development) can take hold with no consequence on the individual or species. Because there is no selection pressure for sight in this environment, any number of genetic abnormalities that give rise to the damage or loss of eyes could proliferate among the population with no effect on the fitness of the population. Among some creationists, the cave tetra is seen as evidence 'against' evolution. One argument claims this is an instance of "devolution"—showing an evolutionary trend of decreasing complexity. But evolution is a non-directional process, and while increased complexity is a common effect, there is no reason why evolution cannot tend towards simplicity if that makes an organism better suited to its environment. Inhibition of the HSP90 protein has a dramatic effect in the development of the blind tetra. In the aquarium The blind cave tetras seen in the aquarium trade are all based on stock collected in the Cueva Chica Cave in the southern part of the Sierra del Abra system in 1936. These were sent to an aquarium company in Texas, who soon started to distribute them to aquarists. Since then, these have been selectively bred for their troglomorphic traits. Today large numbers are bred at commercial facilities, especially in Asia. The blind cave tetra is a hardy species. Their lack of sight does not hinder their ability to get food. They prefer subdued lighting with a rocky substrate, like gravel, mimicking their natural environment. They become semi-aggressive as they age, and are by nature schooling fish. Experiments have shown that keeping these fish in bright aquarium set-ups has no effect on the development of the skin flap that forms over their eyes as they grow. See also List of freshwater aquarium fish species References Tetras Astyanax (fish) Freshwater fish of Mexico Freshwater fish of the United States Cave fish Blind animals Least concern biota of the United States Fish described in 1853 Taxa named by Filippo De Filippi