Adipose eyelid
An adipose eyelid is a transparent extra eyelid that covers all or part of a fish’s eye. It’s most common in deep-sea fish but can be found in other species as well, including milkfish, herring, jacks, mullets, and mackerel.
The exact purpose of this eyelid isn’t known, but scientists think it may help the eye see better or protect it. There are several ideas about why it exists; the most practical is that it aids vision in the dark depths of the ocean. Surface-dwelling fish have more light, so they might not need this benefit as much.
An adipose eyelid usually has anterior and posterior parts and can have three to five layers. The layers run parallel to the body from the back toward the belly, making the lid optically positive—letting some light through while also filtering certain wavelengths and changing how light bends.
The skin over the eye is the thinnest, and it thickens toward the cheek. The layers are made mainly of epithelial tissue with collagen-rich connective tissue. In some species, the middle layer is connective tissue that helps hold the outer layers together.
A notable feature is its ability to filter specific light wavelengths. Many adipose eyelids block wavelengths shorter than about 305 nanometers. They can also reflect light and affect polarized light, depending on the angle of the light. Scientists test these properties using light sources, photometers, and polarizing microscopes.
This page was last edited on 2 February 2026, at 08:14 (CET).