Giant squids possess the largest eyes of any living animal, reaching up to 10 inches in diameter.

The giant squid (Architeuthis dux) lives in the bathypelagic zone, roughly 1,000 to 3,000 feet below the surface. In a 2012 study published in Current Biology, researchers led by Dan-Eric Nilsson of Lund University used mathematical modeling to explain why these eyes are so large. They found that while most fish eyes stop providing benefits after reaching the size of an orange, the giant squid's 10-inch eyes are specifically evolved to detect large moving objects.These eyes do not see fine detail but are exceptionally sensitive to bioluminescence. When a massive predator like a sperm whale swims through the deep ocean, it disturbs tiny organisms that emit light, creating a faint glow. The giant squid's eyes act like wide-aperture lenses to pick up this 'wake' of light from over 400 feet away. This gives the squid enough time to execute an escape maneuver before the whale can strike.The pupil of a giant squid eye can be up to 3.5 inches wide, allowing it to gather a massive amount of photons. For comparison, the largest land animal, the African elephant, has eyes only about 1.5 inches wide. This evolutionary adaptation is a direct response to the extreme pressure and darkness of the deep sea. Without these biological telescopes, the giant squid would be virtually defenseless against its primary predator.