The giant adorable eyes of axolotls are one of their most distinctive features. If you’ve ever seen a photo of these aquatic salamanders, their shiny black eyes definitely catch your attention. But besides being cute, what’s really going on with axolotl eyes?
What do axolotls actually see and how does their vision work?
Axolotls have excellent vision and can see in color, detect motion, and even see above water when their eyes are still submerged. Their eyes are specially adapted to see clearly underwater as well as focus on prey within striking distance.
Read on for an in-depth guide to the anatomy, abilities, and unique features of axolotl vision.
Anatomy of Axolotl Eyes
Large, Lidless Eyes
One of the most striking features of axolotls are their large, lidless eyes. Their eyes are quite large in proportion to their head size and protrude out from the top of their heads. Unlike most amphibians and vertebrates, axolotls do not have eyelids or a nictitating membrane to protect their eyes.
Their eyes are left exposed under the water. This gives them excellent vision from multiple angles to spot food and potential predators. Their lidless eyes provide nearly 360 degrees of vision, but they cannot see directly in front or behind themselves.
Nictitating Membrane
While axolotls lack eyelids, they do possess a transparent nictitating membrane. This membrane is located underneath the eyeball and helps lubricate and protect the eye. It sweeps across the eyeball from the inside, clearing away debris.
The nictitating membrane gives axolotls the ability to see clearly even when under water. In some species of salamander, the nictitating membrane also aids in underwater vision by acting as a corrective lens. It is not fully known if the axolotl nictitating membrane serves this purpose as well.
Lens Structure
The lens of the axolotl eye is large, spherical, and rigid. A 2014 study published in Investigative Ophthalmology & Visual Science found that the lenses of axolotls contain tightly packed fiber cells that limit flexibility (source). This rigid lens structure results in axolotls being far-sighted.
They have excellent vision for detecting food or predators at a distance, but they cannot focus well on objects close to their faces. Their eyes also lack the musculature to accommodate for near vision. While axolotls have great acuity underwater, they are considered practically blind on land.
Axolotl Vision Abilities
Can See In Color
Axolotls have excellent color vision and can see in the red, green, blue, and ultraviolet spectrums! Their eyes contain cone cells with photopigments that are sensitive to specific wavelengths of light, allowing them to distinguish between colors.
This gives axolotls a rich visual experience of their surroundings.
Researchers have found that axolotls have four types of cone cells, compared to three in humans. The additional cone type allows them to see into the ultraviolet range, which is invisible to human eyes.
Ultraviolet light creates visual patterns that stand out sharply to axolotls but are imperceptible to us. This enhanced color vision likely helps axolotls spot food, avoid predators, and interact with potential mates in their aquatic environments.
Motion Detection
Axolotls have excellent motion detection and can perceive rapid movements in their surroundings. Their eyes have a high density of rod cells, which are highly sensitive to light and motion. This allows axolotls to spot fast-moving prey like insects, crustaceans, and small fish with ease.
Experiments have shown that axolotls can detect movements as subtle as 0.04mm, around the diameter of a single human hair! They can even detect prey in lighting conditions too dim for humans to see. This impressive motion detection provides a critical hunting advantage to these predator species.
Aerial Vision
One amazing axolotl vision ability is that they can see clearly both under water and above water. Axolotls spend their lives fully submerged, but their eyes are adapted to focus both in and out of the water.
Specializing in aquatic vision often comes at the cost of aerial vision, but axolotls have excellent sight in both environments.
This adaptation allows axolotls to accurately strike at prey above the water’s surface. Their eyes have the unique ability to change focus when transitioning from underwater to aerial viewing. Axolotls can visually monitor their surroundings at the water’s edge, keeping watch for food, predators, or competitors.
Adaptations for Underwater Vision
Focus on Nearby Prey
Axolotls have excellent vision that allows them to spot prey right in front of their faces. Their eyes are positioned on the top of the head to look upward for food. This gives them a high-resolution view of prey swimming above them, which they can quickly snatch with their wide mouths.
Studies show that axolotls have remarkable focusing ability that lets their eyes accommodate within just 1-2 millimeters. This allows sharp vision of food drifting extremely close by. Their eyes can swiftly adjust focus between different distances to trap nutrition coming from all directions in their aquatic world.
Research also reveals their eyes have up to 700 cones per millimeter which is hundreds more than humans. This dense concentration produces very detailed images of nearby meals. Special retinal neurons help filter background light so prey is not camouflaged and easily spotted.
Accommodate Refraction
Another clever visual adaption of axolotls is their ability to see clearly underwater despite the refractive effects. Light bends significantly more when passing from water to an air-filled eyeball. But the axolotl eye components are evolutionarily designed to handle this.
The lens shape and position adeptly focus light rays entering the eye from water. The cornea has adapted a flatter and more convex form compared to land animals. This effectively corrects the amount of refraction for superior aquatic vision.
The muscles controlling lens shape also contract differently to fine tune according to environmental conditions.
Additionally, axial length of the eye is just right to establish an ideal retinal focus of underwater images. The components essentially miniaturize an air-based design to function smoothly in water instead.
So despite living in different optical worlds, the anatomy brings light to a sharp point on retinal photoreceptors.
Protect Sensitive Eyes
Living underwater also constantly exposes eyes to contaminants and pathogens, so axolotls have special protective features. Transparent movable eyelids act like goggles, safeguarding while maintaining visibility. If irritated, the eyes can be covered by a thin filmy membrane for emergency defense.
Axolotls have exceptional regeneration abilities and can recreate damaged body parts like limbs or tail. But interestingly, once injured their eyes cannot heal fully. So they must keep vision safe against scraped corneas that could cause permanent blindness.
Chemical secretions from specialized glands coating the eyes ward off external risks.
Oddly, axolotls lack tear ducts and cannot produce tears. But studies indicate their eyes stay moistened through a high blood circulation rate combined with water absorbed through the skin or cloacal region. This keeps eyes hydrated without tears so they can open safely while swimming.
Regeneration of Damaged Eyes
Axolotls have an incredible ability to regenerate damaged body parts, including their eyes. If an axolotl’s eye becomes wounded or infected, it can fully regrow the eye and restore vision within a matter of weeks.
This astounding regenerative power makes axolotls an important model organism in research on tissue regeneration and wound healing.
When an axolotl’s eye is injured, the cells around the damaged eye begin rapidly multiplying to heal the wound. Stem cells are activated that can differentiate into any needed type of eye tissue. Within 2-3 weeks, an entirely new eye will form in place of the damaged one.
The new eye is fully functional, complete with a lens, iris, retina, and optic nerve connected to the brain to restore sight.
Researchers have found that a protein called PAX6 plays a key role in guiding eye regeneration in axolotls. When PAX6 production is disrupted, axolotls cannot properly rebuild wounded eyes. Studies of the axolotl regeneration process aim to uncover more helpful genetic pathways that could aid regenerative medicine therapies in humans one day.
One amazing aspect of axolotl eye regeneration is that they can recreate fully patterned eyes, rather than only simple tissue repair. The ability of axolotls to integrate regenerated parts with the existing neural networks and lymphatic system demonstrates an incredible healing capability worth further scientific inquiry.
Eye regeneration studies in amphibian models like axolotls will pave an enlightening path toward relieving human vision impairment or eye injuries.
Comparisons with Other Species
Very few other species share the axolotl’s robust regenerative abilities. Salamanders are the most adept vertebrate regeneration models besides axolotls. In contrast, mammals like humans have very limited natural regenerative properties.
While mice can regrow heart tissue after injury, they cannot regenerate damaged body parts like salamanders can.
| Species | Eye Regeneration Ability |
|---|---|
| Axolotls | Full eye regeneration within 2-3 weeks |
| Salamanders | Can regenerate eye tissues, lenses |
| Humans | Extremely limited eye regeneration potential |
When studied in detail, the extraordinary capability of axolotls to regrow complete anatomical structures offers tantalizing clues into improving mammalian regeneration. The axolotl genome was fully sequenced in 2018 (Bryant et al.
2018
), giving more insight into the genetic mechanisms behind their unusual regenerative skills. Decoding how axolotls accomplish scarless tissue and organ regeneration could be a boon for mammalian regenerative medicine in areas like eyesight restoration.Conclusion
The eyes of axolotls reveal the amazing visual adaptations that allow these aquatic salamanders to thrive in their underwater environments. From lidless eyes to color vision to regeneration abilities, axolotl vision is fine-tuned to suit their habitat and hunting needs.
Their huge sensitive eyes not only capture our attention, but give axolotls the visual acuity to capture prey. The next time you gaze into the eyes of an axolotl, remember the complex anatomy and physiology behind their stare!
