Double cone
Encyclopedia
Double cones also known as twin cones in some literature, are two cone cell
s (colour detecting photoreceptor
s) joined together that may also be coupled optically/electrically. They are the most common type of cone cells in fish
, reptile
s and bird
s, and are present in most vertebrates, though they have been noted as absent in placental mammals, elasmobranches and catfish
. There are many gap junction
s between the cells
of fish double cones. Their function
, if they have any unique function compared to single cones, is largely unknown; proposed uses include achromatic (non-colour vision) tasks such as detecting luminance, motion and polarization vision.
Some double cones have members with identical visual pigments, while others have members with different cone types (members have a different peak absorption spectra). Behavioural research on the reef dwelling triggerfish Rhinecanthus aculeatus has provided evidence that individual members of double cones can act as independent channels of color information.
In a book about vision in fishes
, James Bowmaker writes that double cones tend to be sensitive to longer wavelength
s of light than single cones. He also states that the single cones are usually smaller than the individual members of the double cones.
Cone cell
Cone cells, or cones, are photoreceptor cells in the retina of the eye that are responsible for color vision; they function best in relatively bright light, as opposed to rod cells that work better in dim light. If the retina is exposed to an intense visual stimulus, a negative afterimage will be...
s (colour detecting photoreceptor
Photoreceptor
A photoreceptor cell is a specialized type of neuron found in the eye's retina that is capable of phototransduction. The great biological importance of photoreceptors is that they convert light into signals that can stimulate biological processes...
s) joined together that may also be coupled optically/electrically. They are the most common type of cone cells in fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate 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...
, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s and bird
Bird
Birds are feathered, winged, bipedal, endothermic , egg-laying, vertebrate animals. Around 10,000 living species and 188 families makes them the most speciose class of tetrapod vertebrates. They inhabit ecosystems across the globe, from the Arctic to the Antarctic. Extant birds range in size from...
s, and are present in most vertebrates, though they have been noted as absent in placental mammals, elasmobranches and catfish
Catfish
Catfishes are a diverse group of ray-finned fish. Named for their prominent barbels, which resemble a cat's whiskers, catfish range in size and behavior from the heaviest and longest, the Mekong giant catfish from Southeast Asia and the second longest, the wels catfish of Eurasia, to detritivores...
. There are many gap junction
Gap junction
A gap junction or nexus is a specialized intercellular connection between a multitude of animal cell-types. It directly connects the cytoplasm of two cells, which allows various molecules and ions to pass freely between cells....
s between the cells
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
of fish double cones. Their function
Function (biology)
A function is part of an answer to a question about why some object or process occurred in a system that evolved through a process of selection. Thus, function refers forward from the object or process, along some chain of causation, to the goal or success...
, if they have any unique function compared to single cones, is largely unknown; proposed uses include achromatic (non-colour vision) tasks such as detecting luminance, motion and polarization vision.
Some double cones have members with identical visual pigments, while others have members with different cone types (members have a different peak absorption spectra). Behavioural research on the reef dwelling triggerfish Rhinecanthus aculeatus has provided evidence that individual members of double cones can act as independent channels of color information.
In a book about vision in fishes
Vision in fishes
Vision is an important sensory system for most species of fish. Fish eyes are similar to terrestrial vertebrates like birds and mammals, but have a more spherical lens. Their retinas generally have both rod cells and cone cells , and most species have colour vision. Some fish can see ultraviolet...
, James Bowmaker writes that double cones tend to be sensitive to longer wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
s of light than single cones. He also states that the single cones are usually smaller than the individual members of the double cones.