Does the eye of a fish have a nictitating membrane?

Answer 1

Some do, only they are called nictitating membranes, instead of eyelids.

Here's an answer from a friend of mine on another site:

Just to add to your confusion, but perhaps to resolve some other unclarity: Indeed, there are "fishes" (plural allowed due to the plurality of species) that have eyelids. As someone poited out, some sharks have nictitating membranes, while others do not and roll their eyes backwards to protect them instead. The essential problem of your question, however, is the word "fish". For biologists, fish are not a truly monophyletic group. This is the fancy way of saying the critters usually subsumed under the category "fish" are actually not really closely related to one another. A shark is about as closely related to a gold fish as we are to a frog. What has been referred to as "fish" is usually an aquatic animal with some form of torpedo-shaped body (certain variations permitted), gills and fins. But if you take a closer look at the anatomy and genes of these animals, huge discrepancies and differences are revealed.

Eyelids in land animals largely serve the purpose of keeping the eye moist, a function not required in aquatic animals. But they also protect the eye from mechanical imapcts, and this certainly is a problem for all animals, aquatic or terrestrial, especially when they are predators. Take the shark example again: sharks often prey on large animals that in case of a struggle could injure the shark's eye. To protect itself, the shark with a nictitating membrane will close the membrane to cover the eye. Sharks lacking the membrane will roll back their eyes and thus protect their eyes with their sclera that contains cartelage. (If you ever have the chance to hold an eye ball of a shark, you will notice how stable it is and how you'd have a really, really hard time to bend it.) Elasmobranchs (sharks, skates, and rays) have eye lids (an upper and lower one) which cannot, however, cover the entire eye. The nictitating membrane can be viewed as a third eyelid that does the job.

In other "fishes," such as teleost, eyes are essentially lidless. Instead, here the eyes are covered with a translucent skin called the conunctiva.