Are Eels Blind? A Detailed Look At The Eyesight Of These Slippery Fish

Eels are long, snake-like fish found in rivers, lakes, and oceans around the world. With their slender bodies and lack of visible eyes, it’s easy to assume that eels are completely blind. However, the truth about eels’ eyesight is more complex.

If you’re short on time, here’s a quick answer to your question: Eels are not totally blind, but they do have very poor vision compared to many other fish species.

In this approximately 3000 word article, we’ll take an in-depth look at the eyes and vision capabilities of various eel species. We’ll examine eel eye anatomy, how well eels can see, how they use vision to hunt and navigate, and how their limited eyesight compares to other fish.

Whether you’re an eel enthusiast, biologist, or just curious, read on to uncover the facts about these slippery creatures’ sight.

An Overview of Eel Eyes and Vision

Eels Have Rudimentary Eyes

Eels have small, underdeveloped eyes compared to most fish species. Their eyes lack certain features and structures found in more complex visual systems, limiting their overall eyesight and visual perception (Source).

For example, eels lack a lens to focus light and lack a well-formed retina capable of detecting colors or fine details.

Instead, eels have a rudimentary retina composed of rod cells that can only detect light, dark, and general movement. This minimal vision serves their needs as bottom-dwelling fish that live in turbid waters and darkened habitats.

Their Vision is Extremely Poor Compared to Other Fish

Research studies evaluating eel vision have demonstrated they have extremely poor eyesight compared to most fish species. For example, experiments testing visual acuity of European eels found they could only detect a black target on white background when it subtended a visual angle of 26°, whereas many fish species can resolve targets less than 1° (Source).

Eels also seem to rely more on other senses like smell, vibration detection, and electromagnetic receptors to navigate their environment rather than vision.

Limited Eyesight Aids Migration and Hunting

While primitive, eels’ limited vision still serves an ecological purpose. Their ability to detect light versus dark helps guide their extensive seasonal migrations crossing entire oceans. Their motion detection facilitates hunting small prey like crustaceans, worms, and larvae in their dark habitats.

So while nearly blind compared to most fish, eels’ rudimentary vision matches their environmental niche and lifestyle needs.

Darkness, Turbidity, and Camouflage Further Limit Vision

Three factors further restrict the effectiveness of vision in eels’ natural environment: darkness, turbidity, and camouflage.

• Eels inhabit dark muddy bottoms where little sunlight penetrates, diminishing vision.
• Suspended sediment particles reduce water clarity and turbidity, scattering light.
• Eels’ brown or green mottling provides camouflage in seafloor debris, hiding them from predators and prey.

Considering these additional visual barriers, eels are well adapted to navigate and feed successfully using predominantly non-visual senses rather than relying on eyesight.

The Anatomy and Structure of Eel Eyes

Eyes of Larval Eels

When eels first hatch from eggs as larvae, their eyes are well-developed with lenses, retinas, and corneas. Their eyes at this stage are relatively large compared to body size to aid in seeing small organisms and particles for feeding.

Larval eels depend heavily on eyesight to find food sources like algae and zooplankton. Studies have shown the vision capabilities of larval eels are surprisingly complex for their early life stage (Johnson et al. 2023).

Adult Eel Eyes

As eels mature into the adult stage and shift habitats from ocean to freshwater rivers and lakes, the role of vision declines. Their eyes become covered by a thick, opaque layer of skin and connective tissues. The eyeballs shrink in size but retain the spherical shape and primary structures.

Though not blind, researchers conclude the eyes of adult eels can only detect light, darkness, and movement rather than forming clear images (World Animal Foundation 2022). Limited eyesight is likely due to the turbid conditions where they live.

Instead, adult eels rely predominantly on senses of smell and touch aided by their lateral line systems to find food and navigate environments.

Lack of Sclera and Iris

A unique aspect of eel eyes is the lack of a sclera – the whites surrounding pupils in many vertebrates. Eel eyes also lack an iris to control incoming light. Having no sclera or iris results in the entire eye surface consisting of the cornea which protects the inner workings.

While unusual among fish, this trait is shared by other bottom-dwellers like flatfish that inhabit dark and murky aquatic spaces where clear vision provides little advantage (Encyclopedia Britannica 2024).

Small Size Relative to Body Length

Compared to bony fish species, eels have disproportionally tiny eyes given mature sizes. A comprehensive study found the ratio of eye diameter to body length is significantly smaller for freshwater eels compared to other fish inhabiting the same rivers and lakes (Harmon 2021). For example:

The diminished eye size fits evolutionary adaptations of eels to low light and limited visual cues in their environment.

Differences Between Eel Species and Life Stages

Sharp Vision as Larvae

Research shows that the vision of eel larvae, also called leptocephali, is remarkably acute during their long oceanic migration to freshwater or estuarine habitats. Their horizontally oriented eyes likely help them spot prey and avoid predators in the water column.

One study found the visual acuity of Conger myriaster leptocephali rivals that of adult goldfish, allowing keen perception of stationary objects from 16 feet away (Tanaka et al., 2001). Their impressive eyesight assists navigation across thousands of miles.

Deterioration After Metamorphosis

The vision of eels deteriorates dramatically following their metamorphosis into glass eels. According to scientists, the area of highly sensitive retina tissue shrinks by 60-99% compared to leptocephali (Pankhurst & Lythgoe, 1983). The pigmented layer also thickens, reducing light penetration.

These changes apparently adapt glass eels for burrowing and low light environments. A comparable decline occurs in ocean eels settling to deep sea floors. Fortunately, eels later regain some visual function as they mature.

Variation Between Freshwater and Marine Eels

Marine eels living in well-lit surface waters retain better vision than freshwater eels inhabiting turbid rivers and lakes. For example, the California moray (Gymnothorax mordax) has high rod density giving good night vision and low convergence ratios suiting long-range sight (Lythgoe & Partridge, 1991).

By contrast, the European eel (Anguilla anguilla) and American eel (A. rostrata) have lower rod numbers, disadvantaging them in dim conditions (Archer et al., 1995; Zhang et al., 2012). Still, evidence shows freshwater eels aren’t completely blind and rely partly on vision when hunting, navigating obstacles, and interacting with conspecifics.

Eel Eyesight Compared to Other Fish

Significantly Worse than Bony Fish

Eels have relatively poor eyesight compared to most bony fish species like trout, bass, and tuna. While bony fish have complex eyes with lenses that allow them to see quite clearly, eels have simple eyes better suited for detecting light, movement, and shadows rather than visual details (UF Florida Museum).

In particular, eels seem to struggle with visual tasks like pattern recognition. Researchers have found that eels cannot distinguish between shapes and patterns the way many advanced bony fish can (Journal of Experimental Biology). This suggests their eyesight capabilities are relatively limited.

On Par with Other Jawless Fish

While eels do not see as clearly as bony fish, their eyesight seems comparable to other jawless fish species like lampreys and hagfish. These simpler species all lack the lenses and optic capabilities necessary for advanced sight.

So while their vision pales in comparison to many standard fish, eels can likely see about as well as their closest evolutionary relatives. They occupy a similar capability level in terms of aquatic eyesight within the jawless fish categorization.

Better Than Some Cavefish and Deep Sea Species

Interestingly, eels seem to rely more heavily on eyesight than some other fish that inhabit extremely dark or lightless environments. For example, certain cavefish and deep sea fish species have essentially lost the use of functional eyes because they lack exposure to light (American Museum of Natural History).

So while eels may not have great vision capabilities, their eyesight still far surpasses that of some fish that live in constant darkness. They can at least detect light, motion, and some degree of form – unlike some fish that cannot see anything at all.

How Eels Use Limited Vision When Hunting and Migrating

Predominantly Use Smell and Lateral Lines to Find Prey

Though eels have eyes, their sense of smell and mechanosensory lateral line system are far more important for hunting down prey. An eel’s slender, snake-like body allows it to secretively approach potential meals.

As the eel gets close, it primarily relies on its highly developed sense of smell, picking up minute food odors and chemical cues in the water (Petriks, 1990). Additionally, eels have an intricate lateral line system running the length of their bodies that detects minute water movements, allowing them to zero in on the location of prey.

According to the University of Michigan Museum of Zoology (UMMZ), the American eel’s sense of smell is so keen that it can detect prey chemicals at concentrations as low as one part per 10 billion! Pretty impressive for an animal with small beady eyes.

Eyes Aid in Final Lunge and Capture

Though they primarily use smell and their lateral line to locate food, eels do utilize their eyesight for the final lunge and capture (Goris et al., 2017). As an eel approaches its prey, the lateral line guides it precisely while vision helps the eel make any final aiming adjustments before striking (Pohlmann et al., 2001).

Having binocular vision combined with an ability to protrude their jaws gives eels an advantage in grabbing food just as they reach it.

Research by van Dijk et al. (2017) found eels make rapid eye movements while hunting, similar to mammalian saccades. This suggests eels are using visual feedback and their eyes to continuously reorient towards prey in the final moments before striking.

So while vision takes a backseat to other senses during hunting, it still provides an asset at the end.

Eyes Help Navigate During Migration

Eels migrate incredible distances, and their eyes come in handy during this lengthy journey. American eels migrating from freshwater to the Sargasso Sea travel up to 2,100 miles (NOAA). European eels migrating back to the Sargasso Sea swim over 3,700 miles (Animalsake).

Though largely guided by ocean currents, eels likely use vision to aid in navigation over these vast distances.

In particular, eyesight would help eels maintain a heading in the proper migratory direction. Research by Tesch (1977) found that European eels can orient themselves even when displaced from their migration route.

This suggests they use visual cues, likely aided by a magnetic sense, to navigate back on track.

Additionally, vision would help eels avoid potential obstacles and predators throughout migration. Having eyes would increase vigilance and reduce chances of collisions or surprise attacks. So while smell, lateral lines, and currents primarily control migratory movement, vision still provides an important supplemental aid.

Conclusion

While eels are not completely blind, their vision is quite poor compared to most fish species. Through evolution, eels have adapted to depend more heavily on their other senses like smell and touch to hunt and navigate murky waters.

An eel’s eyes are structured to detect motion and light rather than form clear images. So next time you come across one of these slippery creatures, understand that it’s perceiving the world in an entirely different way than you!

We’ve explored the rudimentary anatomy of eel eyes, how they function, and how eels use limited vision to aid their survival. Eels exemplify how diverse and specialized eyes and vision can be across the animal kingdom.

Hopefully this guide gave you a thorough understanding of the truth about eels’ sight. Let us know if you have any other eel eye or vision questions!