Astyanax mexicanus is a species of fish commonly known as the Mexican tetra. It is native to freshwater rivers and caves in Mexico and parts of the United States. This fish is notable for its two forms: the surface-dwelling form, which has eyes and pigmentation, and the cave-dwelling form, which is blind and lacks color due to living in dark environments. The Mexican tetra is often studied in scientific research, particularly in genetics and evolutionary biology. Its adaptations to different environments provide insights into how species evolve over time. Additionally, Astyanax mexicanus serves as a popular aquarium fish due to its unique appearance and behavior.