Editing Cephalopod (section)

=== Chromatophores ===
Coleoids, a shell-less subclass of cephalopods (squid, cuttlefish, and octopuses), have complex pigment containing cells called chromatophores which are capable of producing rapidly changing color patterns. These cells store pigment within an elastic sac which produces the color seen from these cells. Coleoids can change the shape of this sac, called the cytoelastic sacculus, which then causes changes in the translucency and opacity of the cell. By rapidly changing multiple chromatophores of different colors, cephalopods are able to change the color of their skin at astonishing speeds, an adaptation that is especially notable in an organism that sees in black and white. Chromatophores are known to only contain three pigments, red, yellow, and brown, which cannot create the full color spectrum.<ref>{{Cite book |last=Hanlon |first=R. |title=Animal Camouflage: Mechanisms and Function |publisher=Cambridge University Press |year=2011 |location=Cambridge, UK |pages=145–161 |chapter=Rapid adaptive camouflage in cephalopods |display-authors=et al.}}</ref> However, cephalopods also have cells called iridophores, thin, layered protein cells that reflect light in ways that can produce colors chromatophores cannot.<ref>{{cite journal| url=https://doi.org/10.1093/icb/23.3.581 |doi=10.1093/icb/23.3.581| title=Chromatophore Organs, Reflector Cells, Iridocytes and Leucophores in Cephalopods | date=1983 | last1=Cloney | first1=Richard A. | last2=Brocco | first2=Steven L. |journal=American Zoologist|volume=23|issue=3| pages=581–592 }}</ref> The mechanism of iridophore control is unknown, but chromatophores are under the control of neural pathways, allowing the cephalopod to coordinate elaborate displays. Together, chromatophores and iridophores are able to produce a large range of colors and pattern displays.

==== Adaptive value ====
Cephalopods utilize chromatophores' color changing ability in order to camouflage themselves. Chromatophores allow coleoids to blend into many different environments, from coral reefs to the sandy sea floor. The color change of chromatophores works in concert with papillae, epithelial tissue which grows and deforms through hydrostatic motion to change skin texture. Chromatophores are able to perform two types of camouflage, mimicry and color matching. Mimicry is when an organism changes its appearance to appear like a different organism. The squid ''[[Sepioteuthis sepioidea]]'' has been documented changing its appearance to appear as the non threatening herbivorous parrotfish to approach unaware prey. The octopus ''[[Thaumoctopus mimicus]]'' is known to mimic a number of different venomous organisms it cohabitates with to deter predators.<ref>{{cite journal|url=https://doi.org/10.1098/rspb.2001.1708| doi=10.1098/rspb.2001.1708 | title=Dynamic mimicry in an Indo–Malayan octopus | date=2001 | last1=Norman | first1=Mark D. | last2=Finn | first2=Julian | last3=Tregenza |first3=Tom| journal=Proceedings of the Royal Society of London. Series B: Biological Sciences|volume=268| issue=1478 | pages=1755–1758 | pmid=11522192 | pmc=1088805 }}</ref> While background matching, a cephalopod changes its appearance to resemble its surroundings, hiding from its predators or concealing itself from prey. The ability to both mimic other organisms and match the appearance of their surroundings is notable given that cephalopods' vision is monochromatic.

Cephalopods also use their fine control of body coloration and patterning to perform complex signaling displays for both conspecific and intraspecific communication. Coloration is used in concert with locomotion and texture to send signals to other organisms. Intraspecifically this can serve as a warning display to potential predators. For example, when the octopus ''[[Callistoctopus macropus]]'' is threatened, it will turn a bright red brown color speckled with white dots as a high contrast display to startle predators. Conspecifically, color change is used for both mating displays and social communication. Cuttlefish have intricate mating displays from males to females. There is also male to male signaling that occurs during competition over mates, all of which are the product of chromatophore coloration displays.

==== Origin ====
There are two hypotheses about the evolution of color change in cephalopods. One hypothesis is that the ability to change color may have evolved for social, sexual, and signaling functions. Another explanation is that it first evolved because of selective pressures encouraging predator avoidance and stealth hunting.

For color change to have evolved as the result of social selection the environment of cephalopods' ancestors would have to fit a number of criteria. One, there would need to be some kind of mating ritual that involved signaling. Two, they would have to experience demonstrably high levels of sexual selection. And three, the ancestor would need to communicate using sexual signals that are visible to a conspecific receiver. For color change to have evolved as the result of natural selection different parameters would have to be met. For one, one would need some phenotypic diversity in body patterning among the population. The species would also need to cohabitate with predators which rely on vision for prey identification. These predators should have a high range of visual sensitivity, detecting not just motion or contrast but also colors. The habitats they occupy would also need to display a diversity of backgrounds.<ref>{{Cite book |title=Animal camouflage: Mechanisms and function |publisher=Cambridge University Press |year=2011 |editor-last=Stevens |editor-first=M. |editor-last2=Merilaita|editor-first2=S.}}</ref> Experiments done in dwarf chameleons testing these hypotheses showed that chameleon taxa with greater capacity for color change had more visually conspicuous social signals but did not come from more visually diverse habitats, suggesting that color change ability likely evolved to facilitate social signaling, while camouflage is a useful byproduct.<ref>{{cite journal|doi=10.1371/journal.pbio.0060025| doi-access=free | title=Selection for Social Signalling Drives the Evolution of Chameleon Colour Change | date=2008 | last1=Stuart-Fox | first1=Devi | last2=Moussalli | first2=Adnan |journal=PLOS Biology|volume=6| issue=1 | pages=e25 | pmid=18232740 | pmc=2214820 }}</ref> Because camouflage is used for multiple adaptive purposes in cephalopods, color change could have evolved for one use and the other developed later, or it evolved to regulate trade offs within both.

==== Convergent evolution ====
Color change is widespread in ectotherms including anoles, frogs, mollusks, many fish, insects, and spiders.<ref>{{Cite book |last1=Bagnara |first1=J. T. |title=Chromatophores and color change: The comparative physiology of animal pigmentation |last2=Hadley |first2=M. E. |publisher=Prentice-Hall |year=1973}}</ref> The mechanism behind this color change can be either morphological or physiological. Morphological change is the result of a change in the density of pigment containing cells and tends to change over longer periods of time. Physiological change, the kind observed in cephalopod lineages, is typically the result of the movement of pigment within the chromatophore, changing where different pigments are localized within the cell. This physiological change typically occurs on much shorter timescales compared to morphological change. Cephalopods have a rare form of physiological color change which utilizes neural control of muscles to change the morphology of their chromatophores. This neural control of chromatophores has evolved convergently in both cephalopods and teleosts fishes.<ref>{{cite journal| url=https://doi.org/10.1159/000113909 | doi=10.1159/000113909 | title=Chromatophore Systems in Teleosts and Cephalopods: A Levels Oriented Analysis of Convergent Systems | date=1992| last1=Demski | first1=Leo S. |journal=Brain, Behavior and Evolution|volume=40| issue=2–3 | pages=141–156 | pmid=1422807}}</ref>