Not all eyes are equal—even on the same animal. The upward-looking left eye (visible top) of the squid Histioteuthis heteropsis is twice the size of its right eye, the better to spot prey in light from above. The squid’s smaller eye (not seen) points down into the dark below, detecting bioluminescent prey and predators. Among the tools of aquatic animals with camera-like eyes: a lens with varying tissue density, sharpening vision underwater. - National Geographic