A shark’s eyes turning white is often a sign of threat display or predatory behavior. When sharks are in hunting mode, their pupils dilate and their eyes roll back, exposing the white sclera. This gives the eyes an eerie white appearance and indicates the shark is zeroed in on potential prey.
Anatomy of a Shark’s Eye
To understand why a shark’s eyes turn white, it helps to know a bit about their eye anatomy. A shark’s eye is similar to a human eye in some ways:
- It has a cornea, pupil, lens, retina, and optic nerve.
- The cornea and lens help focus light.
- The retina contains photoreceptor cells that detect light and send signals to the brain.
Here are some key differences in a shark’s eye:
- Sharks lack eyelids – their eyes are always open.
- They have a protective third eyelid (nictitating membrane) that covers the eye while hunting.
- Their pupils dilate widely to let in more light, due to living deep underwater.
- A reflective layer of tissue behind the retina (tapetum lucidum) enhances vision in low light.
It’s this extensive pupil dilation and the tapetum lucidum that causes a shark’s eyes to appear white in certain situations.
When and Why Shark Eyes Turn White
There are several contexts in which a shark’s eyes may turn white:
Preparing to Attack
When sharks enter predatory mode, their pupils dilate to their maximum width to let in more light and focus on prospective prey. This exposes more of the tapetum lucidum, causing the eyes to look white as they reflect ambient light. White eyes indicate the shark is zoned in on a target and ready to strike.
Feeding
While feeding on prey, a shark’s eyes often remain rolled back and white. This likely enhances its binocular vision to precisely bite and immobilize struggling prey. A shark’s eyes may intermittently return to normal while eating, then revert white again as it re-focuses.
Excitement
Sharks can exhibit white eyes when excited by the presence of food, mate competition, or stimulating scents in the water. This white-eye excitement response is seen in some species like great whites, makos, and tiger sharks. The dilated pupils allow them to keenly inspect whatever is causing the excitement.
Threat Display
When confronted by danger, sharks may perform threat displays with arched backs, dropped jaws, and exaggerated swimming movements. Rolling the eyes back to display the white sclera accompanies this behavior. White eyes make the shark appear more menacing to predators, asserting its readiness to defend itself.
At the Surface
Sharks often have white eyes when at or near the water’s surface. The bright environment causes their pupils to dilate fully, so light isn’t overwhelming. This is why sharks sometimes display white eyes when approaching divers or biting watercrafts and surfboards.
Tonic Immobility
When sharks enter a natural state of paralysis called tonic immobility, their eyes typically roll back and appear white. This doesn’t indicate an aggressive response, but rather a fear-induced catatonic state sharks experience when inverted or restrained.
Why Sharks Have Tapetum Lucidum
The tapetum lucidum behind a shark’s retina is key to the white eye effect. This mirror-like tissue does the following:
- Reflects light back through the retina, essentially giving photons a second chance to stimulate rod and cone cells.
- Allows sharks to see well in dark, murky waters where light is minimal.
- Enhances night vision to detect bioluminescent prey.
- Provides superior low light sensitivity compared to humans and other animals.
This enhanced visual capability gives sharks a key predatory advantage to spot prey anywhere, day or night. The tapetum lucidum is the main cause of eyeshine in sharks and other animals like cats and crocodiles.
Shark Species Prone to White Eyes
Some shark species are more likely than others to display pronounced white eyes:
| Species | Description |
|---|---|
| Great white shark | Famous for having dark, vacant black eyes that suddenly roll back and turn white before an attack. |
| Mako shark | Often exhibits white eyes when in hunting mode or excited. Makos can elevate body temperature to give an extra predatory boost. |
| Tiger shark | Displays white eyes when aroused by prey or investigating an interesting scent. Tigers have a varied, opportunistic diet. |
| Bull shark | Its eyes tend to go white as it makes a predatory rush. Bull sharks enter murky, shallow waters that favor a white-eye enhancement. |
| Hammerhead shark | Their strange, wide-set eyes give hammerheads nearly 360-degree vision to better see approaching prey. |
Other sharks like makos, blues, and threshers may show white eyes but not as consistently as the species above.
Interpreting White Shark Eyes
Seeing a shark with white eyes can be unnerving. But it doesn’t necessarily mean the shark will attack. Here are some tips for interpreting this behavior:
- Relaxed vs. stiff posture – A shark swimming normally with white eyes is less concerning than one stiffening its body, arching its back, and making exaggerated swimming movements while displaying white eyes.
- Sudden or gradual – A sudden transition from black to white eyes indicates heightened interest or predatory mode. If white eyes gradually develop, it may just be investigating without aggression.
- Jaw position – An open or dropping jaw accompanying white eyes is a sign of threat display.
- Nictitating membranes – If the protective third eyelid is closed over the eye, it’s less likely the shark will immediately attack.
- Direction of focus – A shark with white eyes zooming in on you is much more dangerous than one looking elsewhere.
Understanding these context clues helps determine if a shark’s white eyes warrant evasive action. Caution is always advised around these powerful predators of the deep.
Conclusion
A shark’s eyes rolling back to reveal the opaque white sclera makes for a dramatic, intimidating sight. This inherent characteristic is due to the shark’s anatomical adaptations for optimized underwater hunting – a dilatable pupil and reflective tapetum lucidum that enhances vision in dim light. White shark eyes signify a state of focused attention on prey, threat response, feeding frenzy, or curiosity. While caution around sharks is always wise, a nuanced interpretation of their white-eyed behavior in context may help differentiate predatory aggression from benign investigation.