Have you ever seen a fish swimming upside down or sideways? If so, you may have noticed their eyes don’t seem to move – they stay fixed in place, even as the fish changes orientation. This unique eye placement gives these fish a 360-degree view of their surroundings, but how exactly does it work?
And what are the advantages and disadvantages of eyes on top of the head?
If you’re short on time, here’s a quick answer to your question: Many fish species like catfish, hatchetfish, and anomalops have eyes positioned on top of their heads. This gives them a nearly panoramic field of vision to spot food and predators.
The tradeoff is they have a small blind spot below their body.
In this comprehensive guide, we’ll cover everything you need to know about fish with eyes on top of their heads. You’ll learn about the anatomy behind this eye placement, how it aids their vision, what types of fish have dorsal eyes, and how this adaptation helps them survive in their environments.
Anatomy Behind Dorsal Eyes in Fish
The unique dorsal eyes of many fish species allow them to see above and around themselves, providing a wide field of view to spot predators and prey. Let’s dive deep into the anatomical features behind these incredible eyes!
Lack of Eye Muscles
Most dorsal eyes lack the eye muscles that allow movement found in frontal eyes. As a result, dorsal eyes are fixed in place, providing unmoving panoramic vision above the fish. The lack of eye muscles simplifies the anatomy and allows more room in the skull for the eyeball.
Wide Field of View
The fixed dorsal eyes have a wide field of view, often 180 degrees or more! This allows the fish to continuously monitor overhead without having to move their eyes. Some species like the foureye butterflyfish have fields of view up to 192 degrees – talk about keeping an eye on things above you!
Minimal Binocular Vision
While frontal eyes provide a region of binocular vision where the fields of view overlap, dorsal eyes have little to none. This is due to their fixed wide-angle positioning. The lack of binocular vision means judging distance and depth for overhead objects is more difficult.
Small Blind Spot Below
Interestingly, the dorsal eyes have a small blind spot below the fish. This is because their field of view is directed upwards. However, since many predators come from above, the dorsal eyes have excellent vision where it counts most!
The specialized anatomy of dorsal eyes allows fish to continuously monitor the waters above them for food and threats. Vision is directed upwards where they need it most. While sacrificing mobility, dorsal eyes provide panoramic overhead views unmatched by frontal eyes. Vision is amazing! 👀
How Dorsal Eyes Aid Vision
Fish like sharks, manta rays, hatchetfish, and paddlefish have unique eyes positioned on the tops of their heads called dorsal eyes. These distinctive eyes give them enhanced visual capabilities to spot prey and predators more easily, see clearly while swimming in any direction, and adapt to dim light ocean depths.
Spot Prey and Predators Easily
Dorsal eyes allow fish to spot food and threats swimming above them with a greater field of view. Rather than just seeing what’s directly in front or to the sides, dorsal eyes let them also look upward without contorting their bodies.
This offers a 360-degree view of their surroundings so they can constantly monitor all directions.
See Clearly While Swimming Any Direction
Unlike human eyes that point straight ahead, dorsal eyes enable fish to visualize their environment no matter which way they are swimming. With eyes atop their heads, their vision isn’t disrupted based on movements of their body.
This unobstructed perspective while mobile likely aids their reaction times to avoid predators or catch prey.
Adapt to Dim Light Environments
Many fish with dorsal eyes like hatchetfish and paddlefish live deep down where little sunlight penetrates. Their distinctive eye placement and enlarged pupil size allow more light to enter. This helps them see even with minimal illumination.
According to the American Museum of Natural History, some species’ dorsal eyes are so sensitive, they can detect prey in light conditions equivalent to a moonless night.
| Fish Species | Depth Range |
|---|---|
| Hatchetfish | 650-3,300 feet |
| Paddlefish | 130-820 feet |
As the above data shows, dorsal-eyed fish like hatchetfish and paddlefish live at substantial depths where light is very limited. Their distinctive overhead eye placement allows them to utilize even minimal illumination to spot food and avoid predators in their dim habitats.
Fish Species With Eyes on Top of Head
Hatchetfish
Hatchetfish are a group of small, silver freshwater fish that get their name from their uniquely shaped bodies that resemble a hatchet. They have eyes that are located on the very top of their heads, allowing them to see above them for spotting food and predators (1).
This upward-facing vision helps hatchetfish survive in the wild by giving them a nearly 360-degree view of their surroundings. There are around 50 species of hatchetfish found in South America and Africa.
One of the most popular aquarium hatchetfish is the marbled hatchetfish, which grows to about 2 inches long. In the wild, these fish swim in large schools near the surface of the water. Their upward-facing eyes allow them to simultaneously search for insects to eat above while watching for predators below.
Their silvery bodies blend into the sunlight, making it difficult for predators to spot them from below (2).
Catfish
Catfish are known for having whisker-like barbels around their mouths to help detect food. But some catfish species also have eyes sitting high on their heads (3). For example, the upside-down catfish of Asia and Africa got its name from swimming belly up, with its eyes gazing skyward.
This allows it to search for insects and larvae from the water’s surface. But the positioning of its eyes also lets it keep watch for predators approaching from below.
Another catfish with upward sights is the forktail catfish of South America. Its eyes are set high and far forward on its head, while its downturned mouth is adapted for bottom feeding. This species spends its time scavenging along muddy river bottoms but its elevated eyes give it a clear upward view to stay vigilant against aerial predators.
Anomalops
Anomalops are a genus of ray-finned fish found in tropical Pacific and Indian Ocean waters near coral reefs. They are also known as flashlight fish because of light-producing organs under their eyes. They have eyes positioned on the top of their large, flat heads (4).
This allows them to search for prey swimming above while also watching for predators lurking below.
Anomalops use their bioluminescence for communication and camouflage. But scientists believe their upward-facing eyes also help them blend in by hiding the shadows cast by their eyes from predators swimming beneath them. There are around 7 species of anomalops identified so far.
Pufferfish
Many pufferfish species have eyes set high on their heads and can even move each eye independently. This gives them excellent panoramic vision with the ability to see forward, sideways, and upward all at the same time.
For example, the green spotted puffer has eyes that sit atop its head and can rotate almost 90 degrees (5). This allows it to scan in all directions and be aware of predators approaching from any angle.
Another pufferfish with flexible elevated eyes is Arothron meleagris, also known as the guineafowl puffer. Its eyes are set high on its head and can swivel forward and backward on their own. Scientists believe this expanded visual range allows guineafowl puffers to thrive in open water environments where predators can strike from many directions.
Angelfish
Angelfish are a popular aquarium fish known for their unique shape. In the wild, they are found in tropical waters worldwide. Most angelfish have eyes set high on their forehead, giving them an excellent upward view (6).
This helps them watch for prey and predators while swimming in and around coral reefs.
Some species like the grey angelfish have tubular eyes that not only face up but also swivel independently. This allows grey angelfish to look straight ahead and upward at the same time. Other angelfish like the French angelfish have eyes set so high on their foreheads that they can see nearly 90 degrees behind them.
This expanded field of vision is essential for monitoring their surroundings and avoiding ambush from all angles.
Evolutionary Advantages of Dorsal Eyes
Fish that have eyes positioned on the tops of their heads have evolved this adaptation for several key reasons. These dorsal eyes give them a broader field of view to better detect predators and prey, help them camouflage amongst aquatic plants, and allow them to see in dim lighting conditions.
360-Degree Field of View
Eyes on the top of the head, such as in the common sole or flounder, allow for nearly 360 degrees of vision. This gives dorsal-eyed fish the unique ability to see above and below them at the same time with little overlap or blind spots in vision.
This panoramic view of their surroundings offers a considerable evolutionary advantage to spot both threats approaching and prey passing by.
Detect Prey and Threats
With a wide visual field spanning both up and down, fish with dorsal eyes can more easily detect predators swimming above them and prey swimming below. This early visual detection better allows fish to evade predators and capture nutrient-rich meals to improve survival and reproduction.
Camouflage in Aquatic Vegetation
Another advantage conferred by dorsal eyes relates to camouflage and habitat. Species of fish like the sole spend much of their time camouflaged on the seafloor amongst vegetation and sediment. Their overhead position obscures and blends the eyes to match the surroundings below for optimal hiding and surprise ambushing of prey.
Dim Light Vision
Certain dorsal-eyed fish also have specialized visual adaptations to allow better vision in low light conditions. Their eyes are equipped with greater densities of rod photoreceptor cells and light-amplifying pigments compared to lateral-eyed fish.
This biological adjustment enables significantly better nocturnal sight to capitalize on vulnerable nighttime prey.
| Dorsal-Eyed Fish Species | Habitat Depth Range | Countries Found |
|---|---|---|
| Flounders | 0-1,000 m | Worldwide temperate coastal oceans |
| Soles | 0-550 m | Worldwide tropical/subtropical oceans |
To learn more about dorsal-eyed fish, visit the following online resources:
Disadvantages and Tradeoffs of Dorsal Eyes
Limited Binocular Vision
Fish with eyes positioned on top of their heads have a limited range of binocular vision below them (Lane, 2022). This means they have a smaller area where both eyes can see the same object. For example, fish such as the archerfish can only see objects in front of their mouths in binocular vision (Schuster et al., 2004).
Having eyes on top of their heads allows archerfish to accurately judge distances and aim at insects above the water, but reduces their ability to spot predators and food approaching from below or the sides in stereo vision.
Small Blind Spot Underneath
The tradeoff for dorsal eyes in fish like hatchetfish is they have a blind spot underneath their bodies. As their eyes point upwards, they cannot see dangers right below them very well (Moyle & Cech, 2004).
This leaves them vulnerable to attacks from below by predators like crocodiles, snakes, and larger fish. Research on the blind cave fish Astyanax mexicanus found two mutations expanded its blind spot as its eyes moved higher on its head over generations (Yoshizawa et al., 2010).
So while dorsal eyes allow fish to spot food and threats above, it comes at the cost of a larger blind area underneath.
Vulnerability to Birds and Surface Predators
Having eyes on top of their heads also makes some fish more vulnerable to predators attacking from above the water. Species like hatchetfish often stay close to the water’s surface to spot insects and other food items (Moyle & Cech, 2004).
However, this exposes them more to predators like herons, kingfishers, and even monkeys who can snatch them up in their beaks or hands.
Researchers found that freshwater hatchetfish in areas with more bird predators tend to have eyes positioned more towards the sides of their head rather than on top (Temple et al., 2010). This gives them a wider, panoramic field of view to spot hunting birds.
So while dorsal eyes suit many habitats, populations with aerial predators develop less extreme eye positions over time. This highlights the tradeoffs fish make between improving views upwards versus vulnerability to attacks from above.
Conclusion
Fish with eyes on top of their heads demonstrate an ingenious evolutionary adaptation. By positioning their eyes dorsally, these fish gain an expansive field of view to detect food, mates, and threats in their aquatic environment.
While there are some tradeoffs, the advantages of 360-degree vision far outweigh the downsides for species that have evolved this placement of their eyes.
The next time you see a fish like a hatchetfish or catfish swimming sideways or upside down, take a closer look at how their dorsal eyes allow them to perceive the world around them. Their specialized anatomy and vision capabilities are key to their survival in the complex and dynamic underwater domain they inhabit.
