Squid have a huge size range, with most being 8 to 12 inches long. But this family also includes the largest invertebrate, the colossal squid, which can be over 40 feet long. Regardless of their size, squid have many characteristics in common, including their proportionately large eyes.
Who Needs Light?
Although some squid swim closer to the surface to feed at night, most hang around the depths of the ocean, where not much light penetrates. Giant squid, for example, tend to stay below 2,000 feet. A squid's large eye lets him spot the movement of prey with just the tiniest amount of light, and he's adept at spying the bioluminescence of prey often found in the deeper ocean.
Bigger is Better
A squid eye takes up a considerable portion of his head area. It's about 100 times larger than a human eye, relative to body size. This, along with strategic placement of the optic nerve, helps eliminate blind spots so the squid can see all around his body. It also allows for a larger pupil to help him focus quickly and accurately, even when light is lacking.
A Black-and-White World
People have rods and cones in their eyes, allowing those eyes to see light and color. Most squid, designed for deep-water hunting, lack the cones that bring color to the world. This means they likely see in black and white. However, the color contrast between prey and a dark ocean, such as a silvery reflection off the scales of a nearby fish, is strong enough that the squid doesn't need color vision to spot it effectively.
Squid might be efficient predators, but they're also prey. Even the giant squid isn't free from predators, often eaten by whales such as sperm whales. The large eyes are especially helpful at spotting big creatures such as whales as they swim toward the squid. Squid eyes also have a reflective surface that turns some light back around toward predators, so the predators don't see the eyes against the black water.