Key Takeaways on Squid Eyes
The smallest squid eye is about 15 millimeters, while the largest squid eye can get up to 11 inches in diameter.
Squid eyes are like human eyes since they have two camera-type eyes. However, they have photoreceptors at the front of the eye, instead of at the back, allowing them to see in dimly lit ocean waters.
Squid eyes have evolved and adapted over time as a deep-sea arms race. These keen eyes help squid hunt, stay alive and communicate with one another in some cases.
There are over 300 species of squid inhabiting every ocean on Earth and these creatures boast truly remarkable eyes. Living in the depths of the ocean means their vision is highly adapted to hunting, navigating, and communicating in low-light environment.
Unsurprisingly, given the wide variation in species of squid the size of their eyes differs greatly.
How Big Is a Squid’s Eye?
The smallest squid species – pygmy squid – reaches only a few centimeters in length with its tiny eyes maxing out at around 15 millimeters.
On the other side of the spectrum, Colossal and giant squid are known to have the largest eyes amongst any animal alive today. They can reach up to 11 inches in diameter or about the size of a dinner plate.
Other species, known as cockeyed squid, have developed eyes of different sizes.
“They are remarkable in in appearance, because they have one very large eye and one relatively small eye,” says Bruce Robison, a senior scientist at Monterey Bay Aquarium Research Institute.
Read More: Once Thought Mythical, Colossal Squid Spotted Alive for the First Time
Unique Features and Human Comparison
Squid have two eyes which are known as camera-type eyes, like our own. Squid, however, have photoreceptors at the front of the eye, rather than at the back. That means that light doesn’t need to travel all the way through the eye, so less information is lost. This structure and the large size of squid eyes proportional to their bodies enables them to see remarkably well in dimly lit, deep ocean waters.
Where squid eyes are found on the head sometimes differs by species, giving them a different field of view. Colossal squid, for example, face forward enabling them with binocular vision while other species’ eyes are on the side of the head, allowing the squid to see both in front and behind in the search for prey or predators.
Their eyes also come in different colors. The cockeyed squid, for example, has one large yellow eye while the other smaller eye is often black. Other squid species can have blue or even red eyes, that can change depending on the source of light they are exposed to.
Some squid eyes also contain light organs called photophores that are used to camouflage themselves by mirroring the dim light from the surface, such as the glass squid. In other species these act as “headlights,” illuminating the way ahead as they look forward.
Evolution of the Squid Eye
Squid eyes have evolved and adapted over time as part of what Robison describes as a deep-sea arms race. These keen eyes help squid hunt, stay alive and communicate with one another in some cases.
In the case of the cock-eyed squid, its peculiar one-big eye, one-small eye adaptation enables it to see predators and prey both above and below. Research by Robison and his colleagues, that required many hours of observation in a submersible, revealed their function to be different than was previously assumed.
Contrary to belief before their study, a squid’s big eye usually aims upwards, and the small eye looks downward, he says.
“They use the large eye to look above them to see the silhouettes of potential prey or potential predators silhouetted against the lighted waters,” says Robison.
The cockeyed squid has also developed a pigment in their lenses that causes a shift in light wavelengths and how they're detected, Robison explains. That means that the bioluminescence that other creatures use to camouflage themselves can be distinguished.
“The effort that the potential prey has evolved to erase their shadows is nullified by the fact that the squid have this pigment that changes the spectral signature of the light,” he says.
Meanwhile, the smaller eye peers into the depths. “The combination of the two eyes gives them an extraordinarily broad field of view in their habitat, with the ability to detect predators and potential prey in a fashion that normal set of quid eyes could not provide,” says Robison.
Read More: Why Is the Elusive Colossal Squid So Hard to Come Across?
How Squid Use Their Eyes to Survive
The extraordinarily large eyes of colossal and giant squid, for instance, mean that these enormous creatures can spot a sperm whale from hundreds of meters away, according to research, enabling the squid to avoid predation.
Squid also communicate in the deep and their eyes play a key role. Humboldt squid, for example, commonly feed in frenzies. It’s not uncommon for these to consume a fellow squid, says Robison. During these onslaughts, the squid glow, illuminating their bodies.
“We think that this is signaling to each other,” Robison says. “Those squid have really big eyes, and clearly they're able to see reasonably well in in dim light.”
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article
Britannica. Southern pygmy squid
Smithsonian Museum. Largest Eye in the World, Giant Squid
Glaucoma UK. Why is a squid eye “better evolved” than a human eye?
Museum of New Zealand. The eyes of the colossal squid
Journal of the Royal Society Interface. Open water camouflage via ‘leaky’ light guides in the midwater squid Galiteuthis
Monterey Bay Aquarium Research Institute. Deciphering the visual language of Humboldt squid
Sean Mowbray is a freelance writer based in Scotland. He covers the environment, archaeology, and general science topics. His work has also appeared in outlets such as Mongabay, New Scientist, Hakai Magazine, Ancient History Magazine, and others.
