Photo credit: Marx Perfecto C. Garcia (Philippines) Review: Abdel Rahman El Gamal (Founder the website)
The inserted photo shows the eyelid in a mullet specimen (Mugil sp.).
There are several theories related to the possible roles of eyelids in fishes. It is generally accepted that the eyelids may play a role in affecting the vision and enhance the ability of fish to focus on specific objects. This is supported by the presence of eyelids in deep sea fish whereas better vision and focusing would be of top importance for the survival and well-being of fish in the deeper part of ocean as the visibility is severely reduced. It is also believed that eye lids serve as a physical barrier and protect fish eyes against foreign objects in the waters.
It is questionable whether the adipose eyelid could block out ultraviolet light and so prevent the UV damage to the fish’s retina in some species. Apart from above, the eyelids served as a taxonomy tool in some fish groups such as mullets. In flathead grey mullet, Mugil cephalus, the adipose eyelid is well developed, covering most of pupil with only a narrow slit over the pupil, while in thin-lip grey mullet (Chelon ramada), the adipose eyelid is poorly developed.