Chameleons, Cuttlefish, and Color: How Shape-Shifters Do Their Thing

Part of what makes being a biologist so fun is the huge variety of animals. They come in every color of the rainbow! 

    

A few of the especially exceptional examples, the mandarin dragonet, Synchiropus splendidus, a poison dart frog, Ranitomeya amazonica, and the leaf viper, Atheris squamigera.  

Sometimes, animals even change color. Most of these color changes occur seasonally and happen slowly, as you might see when a fox sheds its winter coat.

Fluffy and gorgeous to sleek and chic, the arctic fox, Vulpes lagopus, changes its coat color with season. 

However, some animals can instantaneously and dramatically change their appearance. How do they do that? And why do they do it? Today we’re going to look at how rapid color changes work, and examine some of the most famous color-changing animals to find out what’s going on.

As we investigate this, you may wonder why animals change their color at all. There are actually a lot of reasons! For example, they may want to hide. Many animals make use of cryptic coloring to blend in all the time. 

A peacock flounder, Bothus mancus, demonstrates some strategic cryptic coloring. 

However, if you live in a variable environment, matching your background can be tricky, so being able to change it up is useful. Second of all, just like you might get dressed up for an important meeting or a night on the town, in some animals color change can be an important visual signal. They might want to tell other animals that they think they’re cute, or maybe tell them to back off their turf.

Now we can take a look at how color works in animals. Many animals, such as reptiles, amphibians, fish, crustaceans, and cephalopods, owe their color to skin cells called chromatophores. What makes color changing animals so special is the ability to alter their chromatophores so that their skin becomes a different color. However, even chromatophores can only hold a limited number of pigments, so along with them, some of these animals have iridiphores. Iridiphores are cells that reflect light, and these allow animals to create some beautiful iridescent patterns like those seen in the blue ring octopus. 

A blue-ringed octopus, Hapalochlaena lunulata, shows off its iridescent rings. 

Chromatophores and iridphores working in concert present the possibility for a huge variety of colors and patterns. Let’s take a closer look at some of the most famous color changing animals and see what we can learn about their color-changing secrets.

When many people try to think of an animal that can change color, a chameleon will be the first thing to come to mind. 

Looking at this Indian chameleon, Chamaeleo zeylanicus, it’s not hard to see why! 

Color changes in chameleons are well known, and our understanding of them is old and has changed a lot over time. The color-changing capabilities of chameleons were first noted by Aristotle in about 350BC. He claimed that the cause of chameleon color changes was the psychic character of fear (Best and Litt, 1968). We now know that how chameleons actually change their color is by using multiple layers of chromatophores and two layers of iridiphores in their skin. The upper layer of iridiphores is just underneath a layer of red and yellow pigmented chromatophores. By adjusting a lattice of guanine crystals in the upper layer of iridiphores, chameleons can change how light reflects off their iridiphores and through their chromatophores and change what color it appears to the eye (Teyssier et al., 2015).

Figure 1. This figure shows the color changes that can be seen in these panther chameleons, Furcifer pardalis. (a) The difference in colors between excited and relaxed chameleons. (b) Using a chromaticity chart to quantify color changes over time. (c) A cross-section of chameleon skin, showing both layers of iridiphores. (d) The structure of the upper and (e) lower layer of iridiphores (Teyssier, 2009).

What behaviors underlie these crystalline color changes in chameleons? Scientists originally thought, and many people still think, that chameleons changed their color to camouflage into their environment. Current researchers think that the story is actually more complex than that. While chameleons can change to avoid predators (Stuart-Fox et al., 2006), they also change based on the temperature of their environment in order to absorb more heat while basking (Walton and Bennett, 1993). The Namaqua chameleon, Chamaeleo namaquensis, which lives in the desert, will change its color throughout the day to adapt to high temperatures. 

Chameleons also change color to signal to other chameleons. In fact, one study suggests that the evolution of these striking color changes was driven by social signaling (Stuart-Fox and Moussalli, 2008). By making themselves strikingly different from the background, chameleons can make themselves highly visible to other chameleons. This allows them to convey a variety of signals. Males may use colored signals to intimidate competitors, and females can use color changes to aggressively reject males they aren’t interested in.

Chameleons might be the mascot for color change, but a close second are the cephalopods: squids, octopi, and cuttlefish. 

Some colorful cousins demonstrating their versatility. Here we have a flamboyant cuttlefish, Metasepia pfefferi, a coconut octopus, Amphioctopus marginatus, and a bigfin reef squid, Sepioteuthis lessoniana. 

The mechanism that cephalopods use is completely different from chameleons, but equally fascinating. The chromatophores in cephalopods are considered neuromuscular organs (Messenger, 2001). Each chromatophore contains an elastic sac full of pigment, and is surrounded by a ring of muscle fibers, which expand and contract to alter the amount of pigment visible in the cell (Florey, 1969). These cells can be altered by direct neural control, each muscle is surrounded by neurons that originate in the brain and travel directly to the chromatophores (Hanlon, 2007). This mechanism allows cephalopods to react rapidly to their environment and enact direct and delicate color changes that allow for their incredibly precise and diverse patterns. Cephalopods, like chameleons, also have iridiphores, but their iridiphores consist of sack-like cells containing stacks of reflective plates (Mäthger et al., 2008). 

Figure 2. Some of the astonishing color capabilities of cephalopods, specifically the longfin inshore squid, Doryteuthis pealeii, and the giant cuttlefish, Sepia apama. (a-b) Examples of iridescence. (c-d) The transformative capability of a single species of cuttlefish. (e) The location of the chromatophores (ch) in a cross-section of the skin and iridiphores (ir). (f-h) Close-up images of cuttlefish and squid skin showing chromatophores. (i) Electron micrograph showing stacks of iridiphore plates (Mäthger et al. 2008).

As in chameleons, some color changes in cephalopods are for communication. For example, squids use iridiphores on their sides to flash signals at each other (Mäthger et al., 2008). However, the real benefit of these complex chromatophore systems is that allows cephalopods to be masters of disguise. They can mimic almost any background (Hanlon, 2007), and can thus blend in almost anywhere.

Figure 3. This assay probes perception in the common cuttlefish, Sepia officinalis, and shows their subsequent dynamic patterns. They drastically alter their coloration in order to match their background (Hanlon 2007).

These animals are largely predatory, so this remarkable camouflage ability gives them a huge advantage when waiting for prey. Furthermore, they have been shown to use their complex pigment systems to mimic other sea creatures! The mimic octopus, Thaumoctopus mimicus, can change its shape to look like a sea snake or a ray, and pharaoh cuttlefish, Sepia pharaonis, will imitate hermit crabs. 

Despite its long history, this area of research doesn’t lack for fresh mysteries. The biochemical pathways that underlie chameleon chromatophore change are still unknown. We know that other vertebrates that use chromatophores, like fish and amphibians, rely on hormonal regulation (Kindermann et al., 2014; Sköld et al., 2008), but that hasn’t been studied yet in chameleons. Cephelopod chromatophores are still not completely understood either. A recent study suggests that their pigment cells may actually be photosensitive, like our eyes are (Kingston et al., 2015). 

Figure 4. This shows rhodopsin in the skin of a longfin inshore squid, Doryteuthis pealeii , stained green. Rhodopsin is a photosensitive pigment found in the cones of human eyes (Kingston et al. 2015). 

The crystalline color changing system of the chameleon and the neuromuscular adaptability of the cephalopod allow for some intense diversity of colors, but this blog is only an introduction to the types and uses of color-changing skin cells in the animal kingdom. There is still a lot to learn about the how and why of biological color changes, and who knows what abilities these animals have that are still yet unknown.

References:

Best, A.E., and Litt, B. 1968. The discovery of the mechanism of colour-changes in the chameleon. Annals of Science, 24(2): 147-167.

Florey, E. 1969. Ultrastructure and function of cephalopod chromatophores. Integrative and Comparative Biology, 9(2) 429-442.

Hanlon, R. 2007. Cephalopod dynamic camouflage. Current Biology, 17(11) 400-404.

Kindermann, C., Narayan, E.J., and Hero, J.M. 2014. The neuro-hormonal control of rapid dynamic skin colour change in an amphibian during amplexus. PLoS One, doi: 10.1371/journal.pone.0114120.

 

Kingston, A.C.N., Kuzirian, A.M., Hanlon, R., and Cronin, T.W. 2015. Visual phototransduction component in cephalopod chromatophores suggest dermal photoreception. Journal of Experimental Biology, 218: 1596-1602.

Mäthger, L.M., Denton, E.J., Marshall, N.J., and Hanlon, R. 2008. Mechanisms and behavioral functions of structural coloration in cephalopods. Journal of the Royal Society Interface, 6: 149-163.

Messenger, J.B. 2001. Cephalopod chromatophores: neurobiology and natural history. Biological Reviews, 76(4)4 473-528.

Sköld, H.N., Amundsen, T., Svensson, P.A., Mayer, I, Bjelvenmark, J, and Forsgren, E. 2008. Hormonal regulation of female nuptial coloration in a fish. Hormones and Behavior, 54(4): 549-556.

Stuart-Fox, D., and Moussalli, A. 2008. Selection for social signaling drives the evolution of chameleon colour change. PLOS Biology, doi:10.1371/journal.pbio.0060025.

Stuart-Fox, D., Whiting, M.J., and Moussalli, A. 2006. Camouflage and colour change: antipredator responses to bird and snake predators across multiple populations in a dwarf chameleon. Biological Journal of the Linnean Society, 88(3): 437-446.

Teyssier, J., Saenko, S.V., van der Marel, D., and Milinkovitch, M.C. 2015. Photonic crystals cause active colour change in chameleons. Nature Communications, doi:10.1038/ncomms7368.

Walton, B.M., and Bennett, A.F. 1993. Temperature-dependent color change in Kenyan chameleons. Physiological Zoology, 66(2): 270-287.

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