Did that Rock Just Ink on Me? (A Guest Post)

By Sam Brunner and Ian Straus

Image from NOAA.

Cephalopods, like octopuses, squid, and cuttlefish, are well known for their ability to alter the color and patterns on their bodies for better camouflage, mimicry, and even communication. By developing a unique set of camouflage tools, cephalopods excel at not being seen or being seen but not detected as a cephalopod. There are videos all over the internet showcasing how squid can terrify divers with their flashing red displays, or how some octopuses avoid their predators by mimicking the local venomous snakes. This video provides the perfect example of an octopus using its incredible camouflage to become invisible while convincing you it is merely a clump of algae.

You see, where many animals have lowly organelles in their skin cells responsible for pigments, cephalopods are unique in having a whole organ dedicated to this task. They’re called chromatophores. Each chromatophore is made up of colored pigment granules held in the ever so eloquently named cytoelastic sacculus, which is surrounded by 15 to 25 radially arranged muscle cells (like spokes on a wheel). Each muscle cell is also associated with a neural axon and its supportive glial cells, which puts it under the control of the nervous system.

Image created by Ian Straus.

So, when an octopus wants to change color, a signal travels from the brain and down the neural axon to the chromatophore, telling the muscles to contract. The muscle contraction pulls on the pigment-filled sac, stretching it to change its translucence and thereby changing the amount of color showing through. The chromatophores can produce yellow, orange, red, brown, and occasionally black pigments. The intensity of the color depends on how many muscle fibers are contracted, and therefore how much the sac expands and the pigment is spread out. Once a chromatophore develops, it will stay put for the rest of the animal’s life. As the animal grows, new, smaller chromatophores develop in the spaces between the old ones. These new organs are only able to produce yellow pigment at first, but darken as they get older.

Dieter Froesch of the Zoological Station of Naples conducted an experiment using the common octopus (Octopus vulgaris) to determine which of their nerves control the chromatophore organs in each part of the body. Each octopus examined was anaesthetized, had a nerve cut and was then checked a few days later for the results.

Froesch found that of the thirty nerves leaving the brain of O. vulgaris, ten have control over chromatophores, with each nerve controlling a different region of the body. These regions have well defined borders with no overlap. The head region alone is controlled by five different nerves, especially around the eyes. This suggests that fine control over color patterns around the eye may play an important role in effective camouflage. Furthermore, the coloration and chromatophores in one area of the body, the funnel, didn’t appear to be controlled by any of the nerves cut in this experiment.



This image shows the different chromatophore regions that each nerve controls. The funnel, which does not have nerve-controlled chromatophores, is the tube near the eye. Image is from Froesch’s Marine Biology paper (1973).

In most cephalopods, vision is the most important sense. Information about their surroundings is processed in vision regions of the brain, which then send along information to chromatophore regions of the brain. The chromatophore brain regions, which contain motor neurons, send signals to the chromatophores throughout the body telling them to contract. So, if an octopus sees a bright orange coral structure, the chromatophores will contract in a way that results in bright orange skin being displayed.

The vision-chromatophore pathway may be the most important part of cephalopod camouflage, but it isn’t the only set of structures that play a role. Leucophores allow for white pigment and reflective iridophores are responsible for blues and greens. Cuttlefish and many octopuses also have muscles throughout the skin arranged into papillae, which can form bumps or spikes that transform the texture of the animal into that of seaweed or an inconspicuous rock. In Octopus vulgaris, all these components are arranged into 1 mm wide units distributed across the skin, with the leucophores and iridophores in the central region, papillae at the exact center, and chromatophores distributed throughout. This complex physiological system grants cephalopods the greatest array of possible camouflages and firmly positions them as the coolest of the invertebrates.

Want to know more?  Check these out:

1. Froesch, D. (1973). Projection of chromatophore nerves on the body surface of Octopus vulgaris Marine Biology, 19 (2), 153-155 DOI: 10.1007/BF00353586

2. Messenger JB (2001). Cephalopod chromatophores: neurobiology and natural history. Biological reviews of the Cambridge Philosophical Society, 76 (4), 473-528 PMID: 11762491