What transparent eyelid protects a penguin’s eye underwater?

A penguin’s eye is protected underwater by a nictitating membrane.

The nictitating membrane is a translucent or transparent third eyelid. It can sweep across the eye while allowing some light to pass through, helping protect the cornea from water and debris without completely blocking vision.

Penguins use this adaptation while diving and swimming. Their eyes also have other specialized features that help them function in different light conditions above and below the surface.

The membrane is not unique to penguins. Many birds possess one, and similar protective structures occur in some reptiles, amphibians, fish, and mammals. It should not be confused with the ordinary upper and lower eyelids.