Two eyes are so last year.
Why have two eyes when you can have four. Or eight. Or even two hundred.
No, I’m not envisioning some galactic alien from the far reaches of our universe. Here, on Planet Earth, there are a handful of species that have more than two eyes.
But why do some animals need so many eyes and which species have the most?
Oftentimes, additional eyes help animals to regulate their internal temperature or they facilitate navigation via the sun, such as the 200 eyes of a scallop. However, some species, such as the silent, eight-eyed jumping spider, use the extra eyes to home in on prey whilst hunting.
Whether it’s benthic mollusks or jungle-dwelling arachnids, here are 9 unusual animals that have more than 2 eyes.
1. Scallop
We’re bringing out the big guns for the first contender on the list.
Scallops are oceanic bivalves that can have up to 200 eyes!
Unlike most other species, which use lenses within their eyes to focus light, scallops use concave mirror-like structures to help them see.
Scientists hypothesize that the eyes of a scallop work similar to that of a telescope. The mini mirrors reflect wavelengths of light to the retina, giving them a spatial view of their surrounding environment.
Of course, a scallop can’t just layer itself in reflective metal. Instead, in produces an organic compound, known as guanine, that is naturally reflective.
Astonishingly, every one of a scallop’s 200 eyes has the ability to independently process light, and can form distinct images. These images are processed in a primitive brain-like structure to produce a cohesive image.
Not bad, for a shell.
2. Horseshoe Crab
The prehistoric horseshoe crab has, in total, ten eyes.
Two comparatively large eyes can be found on the side of the horseshoe crab’s body. These side eyes are referred to as compound eyes, and are not typically found in vertebrates. Each compound eye houses roughly 1,000 ommatidia.
Another scientific word to add to the dictionary, ommatidia are pigmented photoreceptor cells, each of which are capable of creating separate imagery.
The ommatidia within these compound eyes are used primarily to find mates during the breeding season. This is because, during the night when horseshoe crabs are most active, the compound eyes are chemically altered to become more sensitive to any wavelengths of light, ultimately allowing potential mates to find one another.
In addition to these compound eyes, a horseshoe crab will have a further seven eyes spread across its prosoma.
The prosoma, also referred to as the cephalothorax, is the fused region of the head and thorax. It is the largest area of the horseshoe crab.
At the front of the prosoma, two median eyes and an endopariental eye can be found. Below the compound eyes, two rudimentary lateral eyes can be found.
These eyes allow the horseshoe crab to detect ultraviolet rays emitted from the sun and the reflected rays from the moon. This is particularly important during the spawning seasons, as the crabs use the lunar cycle of the new and full moon to come ashore and release eggs and sperm.
On the underside, close to the mouth, two further eye-like structures, called the ventral eyes, can be found. Scientists are still investigating the roles these eyes play in the life of the horseshoe crab, but one hypothesis suggests they may help the crustacean to swim.
So, that’s nine eye accounted for. What about the tenth?
For this one, we need to head to the posterior region, or the tail. Here, a series of light sensors across the top and sides of the tail help synchronise the brain with light and dark cycles.
Not only do these ancient crabs have a complex collection of eyes, but they also have some amazing healing abilities too.
3. Giant Clam
Giant clams can be found on tropical reefs across the Indo-Pacific region.
Not only are they one of the biggest mollusc species in the world, giant clams are thought to have more eyes than nearly any other mollusc species.
However, and there seems to be a theme forming already, these eyes are not what you might expect. Hundreds of pinhole-type eyes, no larger than 0.5 mm, can be found dotted around the mantle. Each pinhole contains a spherical cavity, in which a pupil-like structure is housed. At the base of this structure, an accumulation of some 100 or more photoreceptors can be found.
It has been found that the 200 or so pinhole eyes of the giant clam are most sensitive to blue-green, blue and ultraviolet wavelengths.
Although the giant clam cannot generate clear images, they are capable of differentiating between light and dark – a potential anti-predatory response.
In response to sudden environmental dimming, the giant clams were observed suddenly withdrawing their siphon and mantle back inside their bodies.
4. Jumping Spiders
Yes, all spiders have eight eyes. In fact, it is one of the ways you can tell an insect and a spider apart – almost all insects will have between 2 to 5 eyes.
However, I want to focus on one particular family of spiders. A group of spiders that have one of the highest rates of hunting success.
The jumping spiders.
Even within this super successful family, there is one individual genus that comes out on top: Portia.
Portia species have, along with all other known spiders, eight eyes.
Three pairs, or six eyes, can be found on its fused head and thorax, (or the cephalothorax if you want to get sciency). These eyes are responsible for motion detection and give the spider an almost 360 degree field of vision.
A single pair of large, forward-facing eyes can be found in the middle of the spider’s head. These are referred to as the principle eyes. These eyes allow Portia species to see in colour, as well as allowing it to distinguish objects. The end result is an incredibly high resolution. In fact, scientists believe that jumping spiders, Portias in particular, have the best eyesight of any spider, rivalling that of larger vertebrates, such as pigeons and cats.
All eight eyes work independently from one another, however, each eye works cooperatively together to paint a picture of the world in the spider’s brain.
5. Saniwa ensidens
We’re taking a trip back down memory lane for this one.
Saniwa ensidens is an extinct relative of today’s monitor lizards that once roamed modern-day Wyoming, some 49 million years ago.
Scientists working on fossil records made an incredible discovery. We’re talking about a discovery so big, it changed the way we looked at evolution.
They had discovered a four-eyed lizard – the first, and only, species of jawed vertebrates known to possess four eyes.
But hold your horses. Things get a bit technical. These aren’t any old eyes.
Similarly to modern-day monitor lizards, Saniwa ensidens had two regular, side-facing eyes. However, the also owned pineal and parapineal eyes, which could be found on top of its head.
These “eyes” are thought to have been connected to special organs called the pineal and parapineal organs. Ten out of ten for originality.
Whilst the main, side eyes most likely served as motion detection, prey detection and predator avoidance, the top set of smaller, light-sensitive eyes are thought to aid in navigation – almost like a compass. The pineal and parapineal eyes would have helped Saniwa ensidens orient themselves.
6. Jawless Lampreys
Now, we’ve just said that Saniwa ensidens is the only jawed vertebrate with four eyes.
But what about jawless vertebrates?
Enter, the lamprey along with their many teeth…and eyes.
As with Saniwa ensidens, most species of lampreys have four eyes. They are the only living species alive today with such a unique trait.
Despite being from two very different lineages, the eyes of the extinct Saniwa ensidens and the extant jawless lampreys are remarkably similar.
While the main two eyes of the lamprey are surprisingly complex, the third and forth eyes are more along the lines of eye-like structures. They are photosensory receptors that feed sensory information to the pineal and parapineal organs. It is thought that these additional “eyes” play important roles for orientation and navigation.
Scientists also speculate the third and fourth eyes help the lampreys develop a spatial awareness of the circadian (24 hours) and circannual (seasonal) cycles.
7. Starfish
Starfish are fast healers. They are capable of regrowing lost limbs.
But just how many eyes does a starfish have?
A simple question, with a slightly more complicated answer.
A starfish will have the same number of eyes, as it does legs. So, for a starfish that has 5 legs, it will have 5 eyes.
For a starfish that has 40 legs, well, you guessed it, they have 4o eyes.
Despite their sheer quantity of eyes, starfish have relatively poor vision. For starters, they can’t see the world in colour. But, for species that live in the ocean deep, this is less of an issue.
The compound eyes of a starfish are capable of formulating imagery, albeit with low spatial resolutions.
However, despite being of a poor quality, this is all a starfish needs to navigate its way back to their territory after being displaced.
8. Dragonflies
Everyone loves a bit of controversy.
So, let’s stir things up. Let’s talk about the dragonfly.
Two eyes, or thousands of eyes? The ongoing debate.
Well, the answer is neither. Technically.
A dragonfly has five eyes. Two, large compound eyes, found bulging from the side of its head and three, smaller ocelli. The main function of the ocelli is to detect light, enabling the individual to orientate themselves.
However, to those who thought dragonflies had thousands of eyes, you wouldn’t be far wrong.
Those two big, bulging compound eyes contain many thousands – with some estimates suggesting up to 30,000 – lenses, or ommatidia, in each eye. These large compound eyes wrap around almost the entirety of the dragonflies head, giving it a near 360 degree field of vision.
Within the ommatidia, light-sensitive opsin proteins can be found. Us humans have opsin proteins in our eyes, after all, they are the universal photoreceptor molecule of all visual systems within the Animal Kingdom.
Dragonflies have up to five different opsin proteins, enabling them to see colours far beyond our human capabilities, such as ultraviolet.
So, instead of two complex eyes, a dragonfly has many thousands of ommatidia that can detect a range of different wavelengths and can create a mosaic of different imagery that can be processed by the brain.
It is also thought that dragonflies are one of the animals that can move their eyes independently.
9. Box Jellyfish
Ah, jellyfish.
The gelatinous blobs that aimlessly drift through the ocean.
But, wait.
Not all jellyfish drift aimlessly.
The box jellyfish, distributed throughout the warm Indo-Pacific, can swim. And it is guided by its 24 eyes!
That’s right, I bet you didn’t know the box jellyfish has 24 eyes.
Of these 24 eyes, four are comparable to that of a vertebrate, complete with retina, corneas and lenses.
These complex-ish eyes allow the jellyfish to not only navigate, but also more complex tasks such as avoiding obstacles (oceanic debris or potential predators) and change direction. They have the ability to form images. This is pretty remarkable for an animal that doesn’t even have a brain.
The other 20 eyes are simple ocelli. These simple ocelli are capable of distinguishing between light and dark.
What About Group Living?
Well, this is an interesting point that I feel we need to touch upon.
Many animals, such as herding ungulates of the African plains, live in large groups. Not only is there safety in numbers, but there are plenty of collective eyes to look out for predators.
The term “many-eyes hypothesis” was coined as a potential theory to suggest why animals group together.
So, although we may not be talking about a single individual with many eyes, the group as a whole works together as a many-eyed organism to look out for danger.
But ungulates, such as antelopes or zebras, are not the only animals that deploy the many-eyes hypothesis. Fish, such as shoaling herring and anchovies, and birds, such as starlings and ostriches, all deploy this method.
Essentially, it greatly increases the quantity of eyes for any given individual from two, to many thousands.
Closing Thoughts
Eyes are so much more than just mechanisms of sight.
Yes, they do play an important role in navigation, finding food and avoiding predation.
However, they also play a role in orientation, spawning cycles and thermoregulation.
Of course, many vertebrates with just two eyes cannot carry out these functions with their eyes alone. This is why species, from oceanic to terrestrial, have evolved more than two eyes.
