Double cones (DCs), known as twin cones when the two members are the same, are two
cone cell
Cone cells or cones are photoreceptor cells in the retina of the vertebrate eye. Cones are active in daylight conditions and enable photopic vision, as opposed to rod cells, which are active in dim light and enable scotopic vision. Most v ...
s (
colour
Color (or colour in Commonwealth English; see spelling differences) is the visual perception based on the electromagnetic spectrum. Though color is not an inherent property of matter, color perception is related to an object's light absorp ...
detecting
photoreceptors) joined together that may also be coupled optically/electrically.
They are the most common type of cone cells in
fish
A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...
,
reptile
Reptiles, as commonly defined, are a group of tetrapods with an ectothermic metabolism and Amniotic egg, amniotic development. Living traditional reptiles comprise four Order (biology), orders: Testudines, Crocodilia, Squamata, and Rhynchocepha ...
s,
bird
Birds are a group of warm-blooded vertebrates constituting the class (biology), class Aves (), characterised by feathers, toothless beaked jaws, the Oviparity, laying of Eggshell, hard-shelled eggs, a high Metabolism, metabolic rate, a fou ...
s, and
monotremes such as the
platypus and are present in most
vertebrate
Vertebrates () are animals with a vertebral column (backbone or spine), and a cranium, or skull. The vertebral column surrounds and protects the spinal cord, while the cranium protects the brain.
The vertebrates make up the subphylum Vertebra ...
s, though they have been noted as absent in most
placental mammals (including humans),
elasmobranch
Elasmobranchii () is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks ( division Selachii), and batomorphs (division Batomorphi, including rays, skates, and sawfish). Members of this subclass are characterised by h ...
s and
catfish
Catfish (or catfishes; order (biology), order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Catfish are common name, named for their prominent barbel (anatomy), barbels, which resemble a cat's whiskers, though not ...
.
There are many
gap junction
Gap junctions are membrane channels between adjacent cells that allow the direct exchange of cytoplasmic substances, such small molecules, substrates, and metabolites.
Gap junctions were first described as ''close appositions'' alongside tight ...
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
Color vision, a feature of visual perception, is an ability to perceive differences between light composed of different frequencies independently of light intensity.
Color perception is a part of the larger visual system and is mediated by a co ...
) tasks such as detecting
luminance
Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls wit ...
,
motion
In physics, motion is when an object changes its position with respect to a reference point in a given time. Motion is mathematically described in terms of displacement, distance, velocity, acceleration, speed, and frame of reference to an o ...
and
polarization vision.
Some double cones have members with same
opsin
Animal opsins are G-protein-coupled receptors and a group of proteins made light-sensitive via a chromophore, typically retinal. When bound to retinal, opsins become retinylidene proteins, but are usually still called opsins regardless. Most pro ...
(twin cones), while others have members with different cone types (members have a different
spectral sensitivity
Spectral sensitivity is the relative efficiency of detection, of light or other signal, as a function of the frequency or wavelength of the signal.
In visual neuroscience, spectral sensitivity is used to describe the different characteristics ...
).
Behavioural research on the reef dwelling triggerfish ''
Rhinecanthus aculeatus'' has provided evidence that individual members of double cones can act as independent channels of colour information.
In a book about
vision in fishes,
James Bowmaker writes that double cones tend to be sensitive to longer
wavelength
In physics and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s of light than single cones. He also states that the single cones are usually smaller than the individual members of the double cones.
Further reading
*Walls, G. L. (1942). The vertebrate eye and its adaptive radiation: Bloomfield Hills, Mich. : Cranbrook Institute of Science. esp. pp 58-63.
References
{{reflist
External links
University of Queensland research page
(last updated 2007 but has some interesting information)
Photoreceptor cells