An adipose eyelid is a transparent eyelid found in some species of fish, that covers some or all of the eye. They are most commonly found on deep sea (benthic) fish, but can also be seen on non-benthic fish. Fish with this feature include milkfish, isospondyls (including herring), jacks, mullets, and mackerel.

Source : Wikipedia

The purpose of the eyelid has not yet been agreed amongst biologists. Proposed explanations include :

• As an extra lens for focusing
• As a polarising filter
• As a UV filter
• As a physical protection barrier
• As a 'streamlining' structure.