In Mexico’s Cueva del Azufre (the Sulphur Caves), a small fish called the shortfin molly (Poecilia mexicana) is on the way to becoming two separate species. Those that live in the dark interior of the caves are very different to their relatives that swim in the bright, surface waters. They have lighter colours and live more solitary lives. Their eyes are smaller, less sensitive and have lower levels of light-sensitive pigment. Instead, they rely instead on a hypersensitive pressure detector – the lateral line – to sense disturbances in the water.

Their differences aren’t just skin deep either. Michael Tobler from the Texas A&M University has been studying the mollies for years, and has shown that the surface