An adipose eyelid is a transparent eyelid found in some species of fish, that covers some or all of the eye.[1][2][3] 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.[4][5] The overall anatomy of the eyelids plays an important role in understanding their possible purpose.