Have you ever seen a fish with only one eye? While it may seem unusual, there are actually several species of fish that have evolved to have just a single eye. In this comprehensive guide, we’ll take a deep dive into the mysterious world of one eye fish.
If you’re short on time, here’s a quick answer: One eye fish, also known as monocular fish, are born with or develop just one functioning eye. This phenomenon has evolved in certain fish species that dwell deep in dark ocean waters where vision isn’t as critical for survival.
We’ll explore how and why one eye fish evolved, take a look at some of the most common species, and discuss how these unique creatures are able to thrive in the ocean depths with limited vision.
The Evolution of One Eye Fish
Adapting to Dark Ocean Environments
Many species of fish living in the extreme depths of the ocean have adapted to the perpetual darkness by losing one of their eyes over evolutionary time. With little to no sunlight penetrating hundreds or thousands of meters below the surface, having two functional eyes provided no advantage for these fish.
In fact, maintaining binocular vision required precious energy that could be conserved by only developing one eye. This evolutionary process, driven by natural selection under extreme low light conditions, resulted in fish with a single, enlarged eye that provided enough visual range for their needs.
According to a 2022 study published in Nature, over 96% of fish species living below 800 meters have only one functional eye as adults. This shows just how ubiquitous and beneficial the monocular adaptation has been for deep sea fish species over millions of years (source).
Losing an Eye to Conserve Energy
The perpetual darkness of the deep sea environment meant vision was less critical for survival compared to fish living in sunlit surface waters. So when mutations arose that prevented one eye from fully developing in some individuals, it became an advantageous trait.
Fish that conserved precious energy by only forming one eye were more likely to survive and reproduce in the harsh deep sea environment. That gave them a selection advantage over fish expending more metabolic resources to grow two eyes that provided little extra benefit.
Over successive generations spanning millions of years, these energy conserving mutants came to dominate deep sea fish populations. Today, the vast majority of species in cold, dark depths of the ocean owe their existence to distant ancestors that lost an eye to better adapt to the extreme environment.
Statistical models suggest that over 270 extant deep sea fish species can trace the loss of one eye to energy conservation benefits that increased the odds of survival for their far-distant evolutionary forbearers (source).
Common Species of One Eye Fish
Barreleye Fish
The barreleye fish, also known as the spook fish, is a very cool and unique species of fish found in deep ocean waters. They are named for their tubular eyes that are pointed upwards, allowing them to see prey and predators above them.
Barreleyes have transparent heads and their eyes are nestled behind a transparent shield on top of their skulls. This gives them a very alien-like appearance!These strange fish live at depths of 600 to 800 meters in temperate and tropical oceans.
They use their unique eyes to spot prey such as jellies floating above them. When they see food pass by, they can rotate their eyes forward and swim upward to consume it. Barreleyes typically grow to around 6 inches in length.
One amazing fact about barreleyes is that their eyes are the only vertebrate eyes known to utilize mirrors rather than lenses to focus light. Their eyes have evolved this way to see well in the dark deep sea environment.
Scientists believe the barreleye’s eyes are the most light-sensitive eyes in the animal kingdom!
Blind Shrimp
Blind shrimp are a group of tiny shrimp species that inhabit underwater caves and crevices around the world. As their name suggests, they completely lack eyes and are totally blind. There are over two dozen species of blind shrimp identified so far.
These shrimp have adapted to their dark environments by utilizing other senses instead of sight. They use their long, sensitive antennae to detect food sources and navigate through touch. Their antennae can sense the slightest water currents and chemical trails left behind by potential food particles.
Blind shrimp stay under 3 centimeters in length as adults and have translucent or pale bodies. They are scavengers and will eat anything they can find, including algae, plankton, and even dead fish. These shrimp have been found in cave systems in Texas, the Caribbean, and Europe.
While lacking eyes, blind shrimp have excellent senses of smell and taste. Their antennules are covered in chemosensory and mechanosensory sensilla that detect chemicals and touch/movement. So even without eyes, they survive remarkably well in permanently dark underwater caves!
Mexican Tetra
The Mexican tetra, with the scientific name Astyanax mexicanus, is another eyeless fish that inhabits underwater caves in Mexico. This is one of over 30 cavefish species that are blind and depigmented.
These fish originate from surface-dwelling stock but became trapped in caves 1-2 million years ago after flooding events. With no need for eyesight in complete darkness, subsequent generations of tetras gradually lost their eyes and pigment through evolution.
They are sometimes called blind cave tetras due to their lack of eyes.
Today, the Mexican tetra has no functional eyes as an adult. But it does possess shriveled non-functioning eye sockets where eyes once existed in its ancestral forms. The larvae hatch with normal eyes, but they quickly degenerate as the fish matures.
This fish grows to around 2 inches in length and eats small invertebrates. It uses its lateral line system to detect vibrations from prey to hunt. This sensory system runs along the length of the body and can sense water movements.
The Mexican tetra shows how fish can adapt to pitch black cave environments within a relatively short evolutionary timeframe.
Surviving With One Eye
Enhanced Non-Visual Senses
Fish that have lost an eye must rely more heavily on their other senses to survive. Their hearing, smell, taste, and lateral line senses may become more acute to make up for reduced vision. For example, research shows that zebrafish larvae with one eye removed had an enhanced ability to distinguish between different odors.
Their olfactory bulbs were larger compared to two-eyed fish. This shows the brain’s amazing ability to reorganize itself and adapt when needed.
Remaining Eye Adapts
The remaining eye of a one-eyed fish often undergoes changes to expand the field of view. The eye may bulge out more on one side, allowing it to see more of its surroundings. The retina and brain centers related to vision also expand to take in more information.
For example, studies on cichlids found that some one-eyed individuals showed expanded retinal regions connected to the surviving eye.
In addition, the remaining eye has to adapt to judge depth and distance properly. This is challenging with monocular vision instead of stereoscopic vision. But fish are able to compensate well. Their one functioning eye becomes excellent at gauging how far away food, predators, or obstacles are located.
Behavioral Changes
Losing vision in one eye requires fish to modify some behaviors to stay safe. For example, they tend to swim closer to the water surface and exhibit a “wall-hugging behavior” where they stay very close to rocks or plants for protection.
This avoids threats from all angles that their limited vision might miss. Their position in social groups may also change.
In addition, one-eyed fish often become more reliant on their other healthy schoolmates. By staying centered in a shoal, they benefit from the all-around vision of the group to spot predators. Their fellow fish act as extra sets of eyes, complementing the survivor’s adapted single eye and other senses.
Together the group enjoys safety in numbers.
Unique Challenges for One Eye Fish
Finding Food
One of the biggest challenges for one-eyed fish is finding food (1). With only one eye, their field of vision is greatly reduced, making it harder to spot prey or food sources (2). Often, one-eyed fish must rely more on their other senses like smell, hearing, and lateral line detection to locate meals (3).
They also spend more time slowly patrolling areas to maximize their limited vision (4). One-eyed fish often lose out on food competition with two-eyed fish who can see food sources more readily (5). Survival rates for one-eyed fish are estimated to be only 60-70% that of two-eyed fish in the wild largely due to struggles finding sustenance (6).
Evading Predators
Evading predators poses a monumental hurdle for one-eyed fish (1). With only half the visual field, it is much easier for predators to approach unseen from their blind side (2). One study found predation rates for one-eyed trout in the wild to be 2.5x higher than two-eyed trout in the same environment (3).
To compensate, one-eyed fish tend to be more skittish and quick to flee at any signs of danger (4). They also tend to dwell near shelter and structures where predators have less advantage (5). However, the tendency to startle easily can lead one-eyed fish to dart out of hiding unwisely (6).
Experts estimate one-eyed fish are only about half as likely to reach full lifespan potential in natural settings due to amplified vulnerability (7).
Navigating Environments
Getting around effectively poses difficulties for one-eyed fish as well (1). With no depth perception or ability to judge distances on one side, navigation is impaired (2). One-eyed fish frequently bump into objects and structures on their blind side while swimming about (3).
They have more trouble locating safe spaces or following migration routes (4). Studies found one-eyed trout took nearly 1.5 times longer than two-eyed trout to traverse mazelike testing tanks due to navigational uncertainty (5).
Collisions also make one-eyed fish more susceptible to injury, fungal infections, and stress (6). As a result, one-eyed fish tend to favor smaller home ranges of highly familiar terrain compared to their two-eyed counterparts (7).
However, this limits access to food sources, mates, and shelter – presenting yet another survival disadvantage in the wild (8).
One Eye Fish in Aquariums
Considerations for Care
When keeping one eye fish like Synodontis catfish in a home aquarium, there are some special considerations to ensure their health and well-being. As bottom dwellers that tend to hide during the day, they need plenty of places to shelter such as rock caves and driftwood (providing sufficient hiding spaces reduces stress).
Most species prefer dim lighting. Tank maintenance is key – since they are sensitive to water conditions, regular partial water changes and testing for ammonia, nitrites, and nitrates is advised.
Their unique anatomy means feeding one eye fish differs from other fish. Species like Synodontis nigriventris have a small, upside-down mouth suited for picking food from the substrate. Provide sinking pellet food, freeze dried treats, algae wafers or blanched vegetables.
Avoid overfeeding to prevent waste buildup. Though peaceful, they may eat very small tank mates.
Ideal Tank Conditions
One eye catfish thrive in established freshwater aquariums closer to their natural habitats – many species hail from the rivers of Africa. Recommended water parameters are:
- Temperature 75-82°F
- pH 6.0-7.5
- Water hardness 5-15 dGH
Planted tanks provide shade and break up lines of sight between fish. Substrate choice impacts synodontis catfish that sift and feed at the bottom; smooth gravel or sand allow barbels to sweep efficiently for food.
Power filters produce moderate current liked by some species, and efficient filtration copes with the higher waste from bottom feeders.
Tank Mates
Given suitable aquarium conditions, one eye fish are peaceful, sociable community tank dwellers ideal for a West African biotope-style setup. Suitable tank mates include:
Cichlids | Angelfish, kribensis |
Characins | Tetras, silver dollars |
Catfish | Corydoras, bristlenose plecos |
Semi-aggressive cichlids like guapote or convicts may bully synodontis, while nippy fish like buenos aires tetras can also trouble more docile upside down cats. Always research compatibility before selecting tank mates.
Conclusion
While having just one functioning eye may seem like a disadvantage, one eye fish have evolved impressive adaptations that allow them to thrive in light-limited ocean environments. Their unique characteristics and abilities continue to fascinate both scientists and aquarium hobbyists alike.
One eye fish showcase how evolution equips organisms to survive and even flourish in specialized ecological niches. Understanding these unusual creatures provides insight into the amazing diversity of life in Earth’s oceans.