|Image from NOAA.|
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.|
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).|
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