A larger eye is a better photon catcher
In the deep ocean, vision is often less about forming a bright, detailed picture than about detecting a faint change against near-darkness. A large eye can help by admitting more light: its broad pupil acts like a wider opening in a camera, allowing more photons—the tiny packets of light—to reach the light-sensitive retina. That extra input can make a dim signal easier to detect.
The advantage is especially useful when the light is not coming from the animal being watched. Many deep-sea organisms produce brief flashes of bioluminescence, or light made by living things. A passing animal can disturb them and set off scattered points of light, creating a moving pattern in the darkness.
The whale may show itself indirectly
One proposed explanation for the giant squid’s huge eyes is that they help it detect the disturbance made by an approaching sperm whale. Rather than clearly seeing the whale’s body, the squid might notice the tiny flashes around its movement—a bit like recognizing a dark shape by the glow it triggers nearby.
That would make the eyes useful as an early-warning system, not simply as equipment for seeing distant prey. Even a rough indication that something large is moving closer could matter in a place where there is little time or light to react.
A compelling idea, hard to watch
The explanation remains difficult to verify because the encounters happen far below the surface, where observing both animals and the surrounding flashes is exceptionally challenging. The proposed mechanism fits a basic advantage of large eyes—collecting scarce light—but confirming what a squid actually sees during a whale’s approach requires evidence from an environment that is almost entirely hidden from direct observation.
