Meet the colossal squid with the largest eyes in the animal kingdom

Picture a pair of eyes the size of soccer balls peering out at you from the dark depths of the ocean. For the colossal squid, this is life. The eye size of the colossal squid has been measured to be nearly 11 inches (27-30 cm) in diameter in adult specimens, making them the largest eyes of any living creature on the planet. The massive eyes of the colossal squid are not just a remarkable feature but also a crucial tool for survival in the dark depths of the Southern Ocean, where this deep-sea creature resides. In this regard, the colossal squid’s remarkable vision is as impressive as its massive size. Most animals don’t need eyes this large because there’s enough light near the surface to see by. But the colossal squid’s world is very different.When scientists studied samples of Mesonychoteuthis hamiltoni, they noticed that the eyes were capable of reaching a size of up to 30 cm (12 inches) in diameter, the equivalent of a soccer ball. This is much larger than the eyes of giant squid or any large fish, and even larger than the eyes of whales and other vertebrates of similar size. For example, the largest fish eyes, such as those of a swordfish, only reach a size of 9 cm (3.5 inches) in diameter. The colossal squid exists thousands of feet below the ocean's surface, in a realm where light is scarce to nonexistent. In this extreme habitat, the benefit of having very large eyes is considerable. As a study published in the journal Current Biology explains, the giant eyes of deep-sea squids, both giant and colossal, are not necessarily adapted for grazing or reproduction but are particularly sensitive to the detection of large predators such as sperm whales, which are among their most formidable threats. These predators disrupt the tiny bioluminescent organisms in their path as they swim, creating very faint flashes of light in the dark depths of the ocean. Large eyes with large pupils are better able to collect this faint light, allowing it to be detected from a distance.The study indicates that such giant eyes allow squids to detect these bioluminescent flashes from more than 120 m away, giving them vital seconds to avoid a pursuing whale. In the deep ocean, vision is a matter of life and death, and the larger the eye, the better it can detect faint flashes of light.In the deep water, there is practically no surface light, and the only visible light comes from bioluminescent organisms, which are little tiny creatures that emit light when disturbed. A large eye with a large pupil can collect more photons than a tiny eye. The more photons the animal collects, the better it can discern forms, movement, and contrast, especially in low-light circumstances. The huge squid's enormous eyes decrease light scattering and maximise the possibilities of picking up tiny luminous signals, which are crucial for warning of the approach of whales or other large prey.This is because most other animals do not require eyes of this size. After all, there is enough light near the surface to see by. However, the world of the colossal squid is a very different place. Below sea level by hundreds of meters, life itself depends on the ability to detect even the slightest light signals. Over millions of years, natural selection caused the eyes of the squid to grow larger and larger as they were able to interpret these light signals more effectively. Larger eyes allow the squid to spot predators and prey from greater distances in near-total darkness. This adaptation gives it a crucial survival advantage in the deep, light-starved ocean.