Structural Coloration in Cephalopods
Cephalopods are one of my favorite class of invertebrates, rivaled only by their molluscan cousins, the sea slugs. This article is derived from a presentation I gave in college.
Before getting into how cephalopods change color, I’d like to review what a cephalopod actually is. Then we’ll talk about the structures used in color change as well as how color change is implemented and why. Most people know that the octopus and squid are related, but did you know these guys are also related to the cuttlefish and nautilus? These fab four are the only living cephalopods.
Cephalopods belong to phylum Mollusca and class Cephalopoda. Cephs have been awesome-saucing the world for roughly 500 million years, and there are about 800 species alive today.
All cephalopods are characterized by a merged head and foot with a ring of arms around the head. They have a beak made of chitin and eyes that can form images. They are intelligent and carnivorous. As mollusks, all cephalopods have an internal or external shell.
Interestingly, the nautilus is the only extant ceph with an external shell.
Cephalopods are the masters of disguise thanks to their ability to change the color and texture of their skin. All members of the cephalopod family can change color, except for our nautilus friend. Color change is made possible by a combination of various cell types operating at once. These cells include chromatophores, iridophores, and leucophores.
A chromatophore is a group of cells located just beneath the cephalopod’s skin. This group of cells includes a pigment within an elastic sac (saccule). Several muscles (15-25) are attached to the saccule, and when they contract, they stretch the sac, and that pigment inside is also stretched, covering more surface area. The more surface area covered, the more pigment is seen. Au contraire, when the muscles relax, the saccule shrinks and hides the pigment.
Because chromatophores are controlled by the nervous system, a cephalopod can vary the sizes of its chromatophores independently from one another. This allows the animal to produce complex patterns, such as zebra stripes in aggressive male cuttlefish. Because a cuttlefish in a zebra-striped onesy is terrifying.
A magnified image of cuttlefish chromatophores:
Deep water cephs typically have fewer chromatophores because when your home is the cold pits of darkness, no one can hear you scream. Or see your pretty colors.
The pigments in chromatophores come only in a variety of black, brown, red, orange, and yellow. They are not responsible for producing the blue and green colors seen in some species. These colors are formed by iridophores, aka iridocytes.
Iridophores are found in the next layer under the chromatophores. Iridophores are layered stacks of platelets that are either chitin- or protein-based. Iridophores work by reflecting light. They are responsible for producing the metallic looking greens, blues, and golds seen in some cephalopod species. Iridophores can be found in cuttlefish, some squid, and some species of octopus.
Leucophores are the last layer in the ceph rainbow.
These cells are responsible for the white spots occurring on some species of cephalopods. Leucophores are flattened, branched cells that are thought to scatter and reflect incoming light. In this way, the color of the leucophores will reflect the predominant wavelength of light in the environment. Basically, in white light they will be white, while in blue light they will be blue.
The photo on the left shows leucophores in the skin. You can see these white spots in the octopus to the right.
Cephalopods have one final ability to change color and pattern: the photophores. Duh duh dunnnn. These cells produce light by bioluminescence. Photophores are found in most midwater and deep sea cephalopods, but not so much in shallow-water species.
Bioluminescence is produced by a chemical reaction. Photophores may produce light constantly or flash light intermittently. The mechanism for this is not yet known. Some species also have sacs which contain bacteria that can bioluminesce.
Cephalopods will also use their body position to disguise themselves, either by moving or holding themselves in a particular way, or by changing the texture of their skin. They are even known to act like other organisms or objects. (Mimic octopuses FTW!)
Functions of Color Change
There are essentially two reasons that cephs change color: for communication and for camouflage.
The first is for communication, both within species (intraspecific), as well as with other species (interspecific). The second reason is for camouflage. The ability of the cephalopods to change color is a trait that has evolved over time due to the need to avoid predators and become competitive in an environment shared with vertebrates, like the common dog fish seen below.
Cephs use color change as well as body postures to communicate. Many cephs use chromatic display (color changes) in courting rituals. Often during courtship males will not only have to try to attract females, but also to fend off other males. They can actually do this at the same time, with one side of the body displayed to attract the female and pattern on the other to fend off other males. Fighting between males is not so much physical as an exhibition of color and body posture, essentially arguing over who is bigger and prettier.
Camouflage is a ceph’s primary defense. Since most modern cephalopods lack an external shell, they are an easy-to-digest meal for predators. They are underwater flying spaghetti monsters. It’s easier for them to just hide. Camouflage is also effective in lie-in-wait prey capture.
Here are some other points of interest:
I mean. If you’re like me and obsessed with cephs.
BACKGROUND RESEMBLANCE: This is when the animal changes its color and texture to match as closely as possible that of its background. Many also use different body postures to help with this. Can you spot the cuttlefish in the picture below?
DECEPTIVE RESEMBLANCE: As well as trying to blend into the background, some cephs will attempt to look like specific objects in their environment. Some try to look like rocks or sea weed, or even other organisms. The Mimic Octopus in the top photo is imitating the flounder in the bottom photo.
DISRUPTIVE PATTERNING: This type of colouration is used to disrupt the outline of the animal and confuse predators. Chromatophores will create sharply contrasting patterns on the body. This is usually seen in cuttlefish. Can you spot the cuttlefish again?
COUNTER-SHADING: Fish are actually much better examples of the use of counter-shading. Counter-shading is used when there is nothing around for the cephalopod to blend in with, such as in squid that spend more time in midwater than on the sea floor. Photophores match the light coming in through the water column so it is almost invisible to animals below it.
Counter-shading also makes round surfaces appear flat, so our squid will be harder to spot when viewed from the side.
DIEMATIC BEHAVIOUR: When all else fails, instead of trying to blend in a ceph will begin rapidly changing bold constrasting colours, such as black and white. Cephs will also change their posture to appear bigger when they do this.
To sum it up, cephalopods are intelligent carnivorous invertebrates with an amazing potential for colour change due to the chromatophores, iridiphores, and leucophores found beneath their skin. Colour change is important in communication and camouflage, and mixed with cryptic behaviour cephalopods have evolved into effective predators and elusive prey.
The short version: Cuttlefish, octopuses, and squid are badasses. Nautilus, you’re cool, too.
“Invertebrates: Cephalopods.” Smithsonian National Zoological
Park. n.d. n. page. Web. 8 Sep. 2011.
Izumi, Michi, et al. “Changes in reflectin protein phosphorylation
are associated with dynamic iridescence in squid.” Journal of
the Royal Society Interface. 7.44 (2009): 549-560.
Mäthger, Lydia M., et al. “Mechanisms and behavioural functions
of structural coloration in cephalopods.” Journal of the Royal
Society Interface. 6. (2008): 149-163. Web. 19 Sep. 2011.
Rima, Chaddha. “Kings of Camouflage.” NOVA. n.d. n. page. Web.
12 Sep. 2011. nationalzoo.si.edu>.
*Original image adapted from: Journal of Royal Society Interface; Mechanisms and behavioural functions of structural coloration in cephalopods