Giant squid eyes are the largest on Earth, some measuring up to 28 centimeters (11 inches) across. Since these creatures live somewhere between 300 and 1,000 meters (between 984 and 3,280 feet) below the ocean's surface where little light penetrates, scientists have long puzzled over why the squid invests in basketball-sized eyes while other deep-sea denizens do not.

A giant squid specimen at the National Institute of Water and Atmospheric Research in Wellington, New Zealand. Credit: NASA

 A giant squid and a swordfish are similar in size but the squid's eyes are proportionally much larger -- three times the diameter and 27 times bigger by volume, Duke University biologist Sonke Johnsen said in a statement. The question is why. Why do squid need such large eyes?

Johnsen and his team say they may have found the reason: whale watching. For big squids, whose primary predator is the sperm whale, watching whales is more about survival than recreation.

Johnsen found that the squid's large eyes collect more light, which allows it to sense small contrast differences in the dim light of its habitat. When sperm whales dive down for a meal, they scatter tiny bioluminescent organisms in their wake. That light allows squids to spot danger coming from as far away as the length of a football field, according to the study published online Thursday in the journal Current Biology.

Squid, however, hardly have a monopoly on extraordinary eyesight in the animal kingdom.

The eagle's eye deserves its proverbial reputation. The entire structure is finely tuned to see precise detail from far away. An eagle retina has many more cone and rod cells to capture light and transmit information to the brain, and the back of an eagle's eye is flatter and larger than a human's, meaning that more of what the eagle sees in its field of view is in focus.

The eagle's retina also has two fovea -- areas where rods and cones are particularly numerous and vision is sharpest -- whereas humans only have one.

A scallop displays its ring of simple eyes. Credit: National Marine Fisheries Service

The scallop is hardly in the eagle's league, eyesight-wise, but it does have an unusually beautiful method of seeing. This mollusk has anywhere from 50 to 200 eyes dotting the edges of its mantle, each with two kinds of retinas: one that responds to light, and another to sudden darkness.

Though its primitive eyesight probably does not allow it to see actual shapes, the scallop can sense quick changes in light, possibly caused by the movement of a predator.

The simple freshwater creature Hydra vulgaris. Credit: Przemyslaw Malkowski

Some creatures don't even need eyes to sense light. Scientists recently discovered that the hydra, a small, simple creature with family ties to jellyfish, has light-sensitive proteins clustered at the ends of its tentacles and around its mouth. Hydra stimulated with LED lights decreased the firing of the stinging cells on their tentacles, a UC Santa Barbara team found in research published in BMC Biology earlier in March.

The researchers proposed that hydras may use light-sensing to detect the shadows of prey or possibly so the tube-like creatures can tell what time of day it is and conserve hunting energy for preferred feeding times at dusk or dawn.

Though giant squid have the largest eyes, these leviathans don't hold the record for largest relative eye size. That honor goes to another cephalopod: the vampire squid that has a body-to-eye ratio of 11:1. In comparison, a 5-foot six-inch person (1.68 meters) with a similar eye-to-body ratio would sport eyes six inches (15 cm) across.