An adipose eyelid is a transparent

eyelid An eyelid is a thin fold of skin that covers and protects an eye. The levator palpebrae superioris muscle retracts the eyelid, exposing the cornea to the outside, giving vision. This can be either voluntarily or involuntarily. The human eyel ...

found in some species of

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

, that covers some or all of the eye. They are most commonly found on deep sea (

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

) fish, but can also be seen on non-benthic fish. Fish with this feature include

milkfish The milkfish (''Chanos chanos'') is the sole living species in the family Chanidae. However, there are at least five extinct genera from the Cretaceous. The repeating scientific name (tautonym) is from Greek ( ‘mouth’). The species has many ...

, isospondyls (including

herring Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Oceans, i ...

), jacks, mullets, and

mackerel Mackerel is a common name applied to a number of different species of pelagic fish, mostly from the family Scombridae. They are found in both temperate and tropical seas, mostly living along the coast or offshore in the oceanic environment. ...

. The overall anatomy of the eyelids plays an important role in understanding their possible purpose.

Purpose

Though the true purpose of these bodily structures is not known, it is generally accepted that the eyelids play some role in affecting the vision of the fish or otherwise serving to protect the fish. There are four proposed theories. * The eyelid may act as a lens, increasing the fish's ability to focus on specific objects and interpreting its surroundings better. * It may provide the fish the capability of seeing polarized light. * It may block out ultraviolet light. * It may serve as a physical barrier against foreign objects in the waters. Scientists are still unsure of the biological implications for the second theory. The first theory is more practical because of the presence of this feature on deep sea fish. This ability to see better is crucial for their survival as visibility is severely reduced in the deeper parts of the ocean. The opposite however can be said for the non-benthic fish because there is more light near the surface of the ocean, so there would not necessarily be a need for a stronger sense of sight.

Anatomy

Most adipose eyelids are separated into anterior and posterior parts, commonly referred to as layers. These layers also vary in number, anywhere from three to five, depending on the fish. The layers are arranged in a parallel fashion, relative to the dorsal fin to the fish's belly. This arrangement makes for eyelids that are optically positive, meaning that a certain amount and type of light will be able to enter the eye. The eyelids also are able to filter out certain wavelengths of light and have different refractive indices for light. The skin of the eyelid is the thinnest over the eyeballs and begins to thicken as it moves on to the cheek and face of the fish.

Epithelial Epithelium or epithelial tissue is one of the four basic types of animal tissue, along with connective tissue, muscle tissue and nervous tissue. It is a thin, continuous, protective layer of compactly packed cells with a little intercellula ...

tissue and

connective tissue Connective tissue is one of the four primary types of animal tissue, along with epithelial tissue, muscle tissue, and nervous tissue. It develops from the mesenchyme derived from the mesoderm the middle embryonic germ layer. Connective tiss ...

formed from several fibers of

collagen Collagen () is the main structural protein in the extracellular matrix found in the body's various connective tissues. As the main component of connective tissue, it is the most abundant protein in mammals, making up from 25% to 35% of the whole ...

are the main components of these layers. In some fish, there are three layers, with the middle layer being made of connective tissue, which helps to bind the other two epithelial layers together.

Filter specifics

One special detail about adipose eyelids is their ability to filter out specific wavelengths of light. For example, different fish have a different concentration of epithelial tissue in their eyelids. However, there is a range that most of the eyelids will filter. Most adipose eyelids can filter out light that has a wavelength shorter than 305 nanometers. Another fact is that these eyelids can also reflect light and this level of reflection corresponds to polarization of the light and the angle at which it is shone. To test for these ranges, different fish eyelids were exposed to light generated from photometers and polarizing microscopes.

References

{{reflist Fish anatomy

Fishes Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...