I know this is my second consecutive post featuring a zoological biology topic, and I really do prefer to mix up my topics more. However, I simply couldn't resist, as the cuttlefish is simply too amazing a species and thus cannot be denied. The cuttlefish is a cephalopod that bears a resemblence to, and is sometimes mistaken for it's relative the squid. However, cuttlefish and squid are quite different, both taxonomically and morphologically.
Images of the cuttlefish. Note the striking color variation. Cuttlefish are able to change color rapidly due to an extremely intricate network of specialized cells called chromatophores (and others) that contain sacs of pigment activated by muscle contractions.
As I alluded to above, cuttlefish are cephalopods, a contraction from greek meaning "head-foot." To me this nomenclature is counter intuitive, as there are 8 arms and 2 tentacles projecting from their heads. Nowhere is the term "foot" used in the anatomical description.
The cephalopods are part of the class of invertebrates that include octopuses, squids, and nautiluses, all grouped under the phylum of mollusk. The speciation of cuttlefish and other cephalopods is quite complex, as can be seen in this Encyclopedia of Life link. The common cuttlefish is known as sepia officinalis.
The cephalopods are part of the class of invertebrates that include octopuses, squids, and nautiluses, all grouped under the phylum of mollusk. The speciation of cuttlefish and other cephalopods is quite complex, as can be seen in this Encyclopedia of Life link. The common cuttlefish is known as sepia officinalis.
Squid eye Cuttlefish eye (note the "W" shape)
There are any number of excellent articles on the differences between the cuttlefish and other cephalopods/molluscs. I would like to jump straight into the topic of the post title, that is, camouflage. Cuttlefish exhibit a capacity to blend into their background by changing color within a half second. In addition to changing color, they can also modify the texture of their skin. for example, spiky-like protuberances in the photo below:
A cuttlefish displaying skin protuberances that help it blend into it's surrounding rocky background
The biological mechanism behind this intricate and fast acting camouflage capability includes a network of more than 20 million specialized cells called chromatophores that contain sacs of pigment. As in Figure 1 below, the chromatophore cells are attached to muscle fibers that connect to the cell body radially and are innervated with nerve axons.
The colors of the chromatophore pigmentation are yellow, red, and brown. There are other cells involved in cuttlefish coloration, but the chromatophores are the most kinetic of the set owing to their nexus with the neuromuscular system.
Figure 1: Chromatophore
In the illustration in Figure 2 below, we see on the left a chromatophore cell at rest. On the right we see the cell activated via expansion of the radial muscle fibers. Note how the yellow pigment sac in this instance is stretched out by the strands of muscle, increasing it's surface area sufficiently to render the color visible.
A chromatophore cell at rest Figure 2 An activated chromatophore
As might be expected in nature, this mechanism is more complicated in practice and relies upon other supporting pigmentation cells. Firstly, there are several layers of chromatophore sheets, and below these are oval shaped iridescent cells called iridophores that reflect the colors blue, green, red, and pink. All this structure, finally, undergirded by a network of yet more cells rendering white coloration called leucophores!
This color palette ensures that the cuttlefish can produce virtually any color in the color spectrum. When you're an extant species belonging to a biological class that originated in the Ordovician era, between 485 and 444 million years ago, there's been much time for evolution to solve certain adaptation problems.
This color palette ensures that the cuttlefish can produce virtually any color in the color spectrum. When you're an extant species belonging to a biological class that originated in the Ordovician era, between 485 and 444 million years ago, there's been much time for evolution to solve certain adaptation problems.
Relative positions of chromatophores, iridophores, and leucophores
Although the issue is not definitively settled, the current consensus among marine biologists appears to be that cuttlefish process their background environment visually, via their "W" shaped eyes.
That visual sensory input subsequently undergoes highly sophisticated neural processing, sending commands to the radial muscle fibers attached to the chromatophore cell bodies. Specifically, on how to expand in the correct sequence and combinations in order to produce the desired skin color and body shape. The fact that this process is coordinated at so many levels of organization so quickly staggers the imagination.
Obviously the ability to innervate over 20 million cells in order to produce complex colors and shapes, informed by background color/textures cues no less, requires gargantuan neural processing power. It turns out that cuttlefish, along with other cephalopods, have large brains and a high encephalization quotient. This metric is thought to be a predictor of sorts of the intelligence of an animal.
The PBS NOVA video embedded below from 2007 offers a fascinating glimpse into the behavior and biology of this magnificent animal, and is well worth the time.
If you don't have sufficient time to view the entire video, make sure to see the section on how chromatophores produce color, around 9:30 into this video:
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