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What is the nictitating membrane?

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Photo of a bird blinking shows why the nictitating membrane is referred to as the third eyelid.
Photo of a bird blinking shows why the nictitating membrane is referred to as the third eyelid.

By Blaise Jones

Though sharks are some of the most ferocious predators in saltwater or freshwater, they are not invincible. Sharks can be injured and damaged. With little to protect their face during an attack, the eyes of a shark are particularly vulnerable.

Being even partially blind reduces their hunting efficiency significantly. A shark that cannot hunt also cannot eat, making starvation a possibility or, at the very least, becoming sluggish enough that other predators are emboldened to attack. However, some sharks have developed a specialized adaptation to protect their eyes: the nictitating membrane.

Third Eyelid
The nictitating membrane, commonly known as the third eyelid, is exactly that: a third eyelid. Many different types of animal have nictitating membranes and there are just as many different uses for it. Camels, lizards, and frogs use the nictitating membrane to remove dirt and other obstructions from their eyes. Birds and crocodiles use nictitating membranes as goggles, allowing them to see while flying into harsh winds or while swimming through murky water, respectively. Sharks, however, use their nictitating membrane solely to protect their eyes during an attack.

Keeping it in the Family

The nictitating membrane of a blue shark.
The nictitating membrane of a blue shark.

Only one order of shark possesses nictitating membranes: Carcharhiniformes. With more than 270 species, Carcharhiniformes are the largest order of shark. Often referred to as ground sharks, Carcharhiniformes can vary greatly in body shape.

The major families of shark that fall under the order Carcharhiniformes are the Carcharhinidae, Galeocerididae, Galeorhinidae, Proscylliidae, Pseudotriakidae, Scyliorhinidae, Sphyrnidae, and Triakidae. Or, more colloquially, the tiger sharks, hammerhead sharks, cat sharks, finback catsharks, false catsharks, smoothhound sharks, and requiem sharks.

Ground sharks use the nictitating membrane to protect their eyes during predation. The last thing a shark wants is for their intended prey to lash out against them and bite or jab a spine into their eye. The nictitating membrane slides over the eye upon the moment of a bite, creating a protective shield over the shark’s eye.

Whites of Their Eyes
Other sharks rely on a different strategy to protect their eyes during hunts. Sharks without nictitating membranes roll their eyes upward underneath their top eyelid, exposing only the white, fibrous underside of their eyes. This gives sharks their infamous, dead-eyed appearance that many find terrifying.

New Kids on the Block
Why do only the ground sharks possess the nictitating membrane?
Scientists divide sharks into one of two categories based on their evolution. Two-thirds of all sharks are considered to be less advanced on the evolutionary scale and are placed in the group Squalae. The remaining third of shark species are in the Galae group, which began developing additional traits approximately 150 million years ago.

Ground sharks are in the Galae group because one of the key evolutionary changes they developed was the nictitating membrane, which their relatives do not possess. The eyes of ground sharks like the hammerhead were similar to their cousins the great white and other mackerel sharks (Lamnidae) until approximately 150 million years ago. Developing a third eyelid to protect their sight during predation, instead of rolling the eyes back into the skull, was a major evolutionary change for sharks, which date back more than 400 million years.

SOURCES:

http://animals.mom.me/animal-three-eyelids-1300.html

“The Encyclopedia of Sharks” by Steve Parker

http://eol.org/pages/1901/overview

http://elasmo-research.org/education/classification/classification.htm#galea_squalea

“Sharks: The Animal Answer Guide” by Gene Helfman and George H. Burgess

http://www.elasmo-research.org/education/shark_profiles/carcharhiniformes.htm

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