Fish come in a plethora of different shapes and sizes, from tiny little guppies to humongous whale sharks. Although it’s not the size of the fish that we’re focusing on here, it’s the size of their eyes.
A brief breakdown of the from the front to back would be; the cornea, a transparent cover for the eye, the iris and the pupil.
The iris is the colored part surrounding the pupil, and the pupil is a black hole that allows light through into the eye.
Behind the pupil is the lens, which controls the angle of the light that hits the retina, the back of the eye, which processes the light into an image.
One of the characteristics to be noted is that fish tend to have a fixed pupil size meaning that they have to move the lens of the eye closer or further away from the retina to adjust focus, unlike land animals who change the size of the pupil to allow different types of light levels into the eye.
Though certain types of fish have developed muscles that allow the pupil to change in sizes, such as sharks and rays.
The biggest difference between human eyes and fish eyes is their environment. Given that the habitats that fish live in can range from freshwater rivers to deep down into the darkest depths of the ocean, fish eyes have undergone specific evolutionary adaptations based on the factor that affect them.
Light travels differently through water than it does through air, so the spectrum of light fish experience differs based on where they live and the depth of their habitat.
Some fish can even see ultraviolet wavelengths and some are sensitive to polarised light.
Similarly, due to the way light travels through the water, the shape of most fish eyes is rounder than land animals to allow for a sharper image in the fish’s watery environment.
With all this in mind, it’s worthwhile noting that the range in color vision diversity in the ocean is greater than on land. This is due to the variety of access to light and how light functions in these different environments.
So as you’d expect, there are different types of eyes in the ocean and even the big eye may be big for different reasons.
Now you know a little more about fish eyes, let’s talk about some of the fish with the biggest eyes!
Bigeye Thresher
Scientific name: Alopias superciliosus
It’s no surprise that the Bigeye Thresher shark is starting from the list, as this species got its common name from how distinctly large its eyes are.
Another distinct feature of the Bigeye Thresher is the large top fin of the tail. This is commonly used to whip the fish they are hunting to stun and herd them until the Bigeye Thresher is finished hunting.
The Bigeyed Thresher shark has a large range so therefore a variety of different waters to hunt in, they are found in tropical and temperate seas around the world. Its large black eyes allow the shark to hunt in waters where there would be low levels of light.
Bigeye Tuna
Scientific name: Thunnus obesus
Following the Bigeye Thresher is the Bigeyed Tuna as, again, this species has been acknowledged by its common name to have big eyes, though in Hawaii, along with its relative the yellowfin tuna (Thunnus albacares), it is known as ‘ahi.
They have a large black pupil with a thin white iris. Depending on which ocean they are in, the Bigeye Tuna hunt in a range of 200-500 m.
They periodically change their depth based on the depth of their prey, meaning the deeper they dive the less light there is. They are found in tropical and subtropical oceans around the world.
Their large eyes allow their vision to function rather well considering how little light there is as their large black pupil allows in as much light as it can.
Bigeyes
Scientific name: Family: Priacanthidae spp
The last of the aptly named fish in this list is the family of fish called the ‘bigeyes’. Eighteen species form the ‘bigeyes’ family.
Fourteen family members are found in tropical and subtropical waters, and the other four family members are found in temperate waters.
They tend to be found in deep waters near reefs. Their big red eyes will aid in seeing prey at these depths.
One other reason for their big eyes is that they are nocturnal so are active in the nighttime. Combining being active in the night and the relatively low depths, it’s no wonder they’ve got such big eyes.
Horse-eye Jack
Scientific name: Caranx latus
The Horse-eye jack is commonly referred to as the ‘Big-eye Jack’ so it is only right is has a spot on this list. These fish are large in size and often travel in schools, large groups of fish.
These fish are found in the open water in the Atlantic near the continents of North and South America. Their eyes have a brown iris with a reasonably sized pupil.
Swordfish
Scientific name: Xiphias gladius
No list of fish eyes would be complete without mentioning the eyes of the swordfish. Their eyes are approximately the size of a tennis ball and have a dark blue iris around the pupil. Not only
do swordfish have some of the largest eyes in the animal kingdom, but they also possess an interesting ability.
They can control the temperature of their eyes, raising the eye up to 10 – 15°c in temperature relative to the surrounding water. This has been shown to help their eye work up to ten times as fast as those cooled by the ocean’s murky depths.
As swordfish inhabit many types of water, they have had to develop a method of keeping their vision sharp in a wide range of regions, though there is reason to believe that it was the development of the heated eyes that allowed them to spread so far and wide.
Swordfish are a highly migratory species and are found across tropical and temperate oceans, sometimes even found in the colder arctic waters.
Globe-eye goldfish
Scientific name: Carassius auratus auratus
The globe-eyed goldfish have got the short straw on this list as they haven’t developed these eyes by chance.
They have been selectively bred for fish collectors over the years. They are called the ‘Dragon eye goldfish’ in Asian countries where they were first selectively bred around 1592.
Sadly for them, the enlarged eyeballs actually hinder their ability to see rather than aid them.
This is noted in many pet guides which state to house them with other globe-eyed fish so they don’t have too hard of a time competing for food from more able-visioned tankmates. As they have been selectively bred, you will not find this species in the wild.
Tarpon
Scientific name: Megalops atlanticus & Megalops cyprinoides
From the time these fish were noted in scientific literature they were recognised for their large eye. In 1847 Achille Valenciennes named them after this characteristic with megalops meaning ‘having large eyes’ in Latin.
Tarpons are some of the fish that develop the ability to see ultraviolet light. They develop this as they mature into adults as they spend more time in clear water where ultraviolet light is more abundant helping them see as they adapt to their environment as an adult.
Megalops altanticus can be found in the Atlantic ocean. Megalops cyprinoides can be found in the India and Pacific oceans.
Barracuda
Scientific name: Sphyraena spp
Barracudas are the only genus in the Sphyraenidae family with 29 species in total. They come in a range of sizes yet all share a common feature, their eyes are large relative to their body size.
Barracudas are known hunters, feared in coral reefs and in seagrass habitats, they rely on their large eyes to locate their prey whilst relying on their sleek, silver reflective bodies to
remain undetected. They are rapid swimmers so once they locate their prey they dart toward them in the hope of successfully catching their prey.
The barracuda family mainly made this list for a specific species, Sphyraena forsteri, commonly known as ‘Bigeye barracuda.’ With a name like that it had to be included.
Members of the Sphyraena family can be found across the world in tropical and subtropical oceans.
Grenadier or rattails
Scientific name: Macrourinae and subfamily of the Macrouridae family
These fish dwell in the dark cold waters of the arctic and antarctic where light is scarce at the best of times. Plus, they are deep sea fish scavenge along the ocean floor, so it’s no wonder they need big eyes as they’re used to environments with next to no light.
The eyes of these fish are large and bulbous, being some of the fish with the largest eye-to-body fish ratio in the ocean. The light the rattails do see won’t tend to be from the surface, it’ll be the bioluminescence other creatures are creating.
Porbeagle Shark
Scientific name: Lamna nasus
The Porbeagle Shark has a vast range of prey it can feast upon so can be found at various depths depending on what prey is available. It is found in the colder waters of the North Atlantic and across the southern hemisphere. Their eyes are large and black.
A fun fact about their eyes it that the distance from the tip of the snout to the eye is 50% of the distance from the eye to the first-gill slit.
Soldierfish
Scientific name: Myripristis
This is a genus of fish that hold a spot on this list because of their distinct dark black eyes contrasted with their colourful bodies.
Members of this genus are nocturnal hunters requiring their large dark eyes to see shrimp, plankton and sand-dwelling worms they feed on in the late hours of the night.
They are found around reefs and caving during the day and have also been noted to swim upside down. These fish are often also called Squirrel fish.
Blue shark
Scientific name: Prionace glauca
The Blue shark is a sleek-looking shark with a near-global distribution apart from the icy waters of the arctic and antarctic, though they tend to prefer lower water temperatures. They can be found at depths ranging from surface level to 350 meters.
This species has a thin body with large fins aiding in gliding through the water. Being a shark, it has the ability to change pupil size so its eye can range from being entirely black when in need of letting in more light to rather small when near the surface.
The unique aspect of this shark isn’t its large bulbous eyes but the fact it lacks spiracles, an opening near the gills that allow the flow of water over the gills to help regulate oxygen intake, adding to its sleek look.
The Vampire Squid
Scientific name: Vampyroteuthis infernalis
The vampire squid is last on the list as it is not actually a fish, it’s a cephalopod. Though I thought it was worth mentioning as it is a sea creature with a huge eye.
In fact, its eye is so huge that it holds the Guinness world record for the ‘largest eye-to-body ratio’ of any animal in the world. Its body can reach a maximum length of 11 inches and its eye can have a diameter of 0.9 inches.
I hope you’re up to your eyeballs in new information about fish eye now!
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