The Mexican tetra is a small freshwater fish native to the rivers and lakes of Mexico and Texas. Known scientifically as *Astyanax mexicanus*, it typically measures about 5 to 10 centimeters in length. This fish is notable for its adaptability to different environments, including both surface and cave habitats. One of the most interesting features of the Mexican tetra is its two distinct forms: the surface-dwelling form, which has functional eyes and pigmentation, and the cave-dwelling form, which is blind and lacks color. This variation makes the Mexican tetra a popular subject for studies on evolution and adaptation in isolated environments.