The Mexican Blind Cavefish (Astyanax mexicanus) may look totally different to the Mexican Tetra (bot

The Mexican Blind Cavefish (Astyanax mexicanus) may look totally different to the Mexican Tetra (bottom left), but is in fact the same species. Adaptations to life in pitch dark caves such as heightened sense of smell, as well as use of the lateral line organ, have made vision and pigmentation unimportant. 

The two different morphs made this species an excellent candidate for research into circadian rhythms and metabolic processes, which you can read about here.

Photos:Josh More,Damian Moran