Color Change in Cephalopods

For millions of years, cephalopods; squid, cuttlefish and octopus, have managed to survive in various ecosystems with the presence of lots of natural predators. How did they achieve this with their soft, squishy bodies and lack of defensive material, teeth, and claws?

The answer to this might not be simple but it’s quite extraordinary.

Cephalopods have different methods of defence and camouflage. While the defence is mostly with their ink sac (although some species do not possess one), they usually depend on their camouflage skills.

How do They Blend in?

Cephalopods rely on their environment in order to change colour. While the ones living in the reef systems have the need to change their colour and texture regularly, deep-sea species do not need this feature since there’s little to no light in their habitat [1,2].

Cephalopods possess three main cells and specialized light organs in order to colour-change 
  1. Chromatophores
  2. Iridophores
  3. Leucophores

   And the light organs: Photophores [3,4,5].

Image 1.: Skin layers of a cephalopod [1].

Chromatophores are groups of cells located right under the cephalopod’s skin. These cells produce colour through the pigments they contain. These pigments can be black, brown, red, orange or yellow. Chromatophores are controlled by the neuromuscular system. If the muscle surrounding the chromatophore is relaxed, it contracts, allowing only a little amount of colour to appear; leaving the animal looking white.

When these muscles tighten, the chromatophore gets pulled open; allowing colours like black, brown, orange or yellow to appear. Many deep-water species possess fewer chromatophores since there’s only little or no light [3].

Image 2: Chromatophores of a cuttlefish [2].

Iridophores are layered stacks of platelets, chitinous in some species and protein-based in others. These cells also contain a protein called reflectin, which reflects light at different wavelengths. This protein is also used to conceal organs. (e.g: silver colouration around the eyes and the ink sac) Iridophores produce metallic greens and blues; also gold and silver in squid, cuttlefish and some species of octopus [6,7].

Iridophores are controlled by hormones, such as acetylcholine, thus the changes observed are much slower compared to the speed of change in chromatophores [7].

Leucophores form the third layer of the cephalopod’s skin and are responsible for the white spots of some species of cuttlefish, squid and octopus (e.g: Octopus cyanea). These cells have a flattened, branched structure and they scatter and reflect the incoming light. The amount of light that reaches leucophores is controlled by the top two layers: chromatophores and iridophores. These cells will reflect the predominant wavelength of light in the environment; which gains the cephalopods the ability to blend in their environment [8].

Image 3: Watasenia scintillans using bioluminescence to produce light [3].

Photophores are specialised organs that consist of light-producing cells through bioluminescence [9]. Photophores can often be found in mid-water and deep-water species [10,11].

References:

  1.  R. T. Hanlon, C. C. Chiao, L. M. Mäthger, A. Barbosa, K. C. Buresch, and C. Chubb, “Cephalopod dynamic camouflage: Bridging the continuum between background matching and disruptive coloration,” Philosophical Transactions of the Royal Society B: Biological Sciences, vol. 364, no. 1516, pp. 429–437, Feb. 2009, DOI: 10.1098/RSTB.2008.0270.
  2. W. Chung, N. M.-P. of the Royal, and undefined 2016, “Comparative visual ecology of cephalopods from different habitats,” royalsocietypublishing.org, vol. 283, no. 1838, Sep. 2016, DOI: 10.1098/rspb.2016.1346.
  3. T. Williams, S. Senft, J. Yeo, … F. M.-M.-N., and undefined 2019, “Dynamic pigmentary and structural coloration within cephalopod chromatophore organs,” nature.com, Accessed: Dec. 03, 2022. [Online]. Available: https://www.nature.com/articles/s41467-019-08891-x
  4. M. J. How, M. D. Norman, J. Finn, W. S. Chung, and N. J. Marshall, “Dynamic skin patterns in cephalopods,” Front Physiol, vol. 8, no. JUN, 2017, doi: 10.3389/FPHYS.2017.00393/FULL.
  5. J. Wood and K. Jackson, “How cephalopods change color,” 2012, Accessed: Dec. 03, 2022. [Online]. Available: http://www.thecephalopodpage.org/cephschool/HowCephalopodsChangeColor.pdf
  6. A. Andouche, Y. Bassaglia, … S. B.-D., and undefined 2013, “Reflectin genes and development of iridophore patterns in Sepia officinalis embryos (Mollusca, Cephalopoda),” Wiley Online Library, vol. 242, no. 5, pp. 560–571, May 2013, doi: 10.1002/dvdy.23938.
  7. T. Wardill, … P. G.-B.-P. of the, and undefined 2012, “Neural control of tuneable skin iridescence in squid,” royalsocietypublishing.org, Accessed: Dec. 03, 2022. [Online]. Available: https://royalsocietypublishing.org/doi/abs/10.1098/rspb.2012.1374
  8. R. Hanlon, L. Mäthger, … G. B.-B. &, and undefined 2018, “White reflection from cuttlefish skin leucophores,” iopscience.iop.org, Accessed: Dec. 03, 2022. [Online]. Available: https://iopscience.iop.org/article/10.1088/1748-3190/aaa3a9/meta?casa_token=Gx7zPyWTT3cAAAAA:WQavGKjHKbzCJ7Hs0fZNX85SpC5CgSXcO-V2IkEgKuVr0iqey4T4Kjvnwzmi4HmlthWgmTROm19HNg
  9. S. H. D. Haddock, M. A. Moline, and J. F. Case, “Bioluminescence in the sea,” 2010, Accessed: Dec. 03, 2022. [Online]. Available: https://digitalcommons.calpoly.edu/bio_fac/184/
  10. M. Cavallaro et al., “New data on morphology and ultrastructure of skin photophores in the deep‐sea squid Histioteuthis bonnellii (Férussac, 1834), Cephalopoda: Histioteuthidae,” Wiley Online Library, vol. 98, no. 3, pp. 271–277, Jul. 2016, DOI: 10.1111/azo.12174.
  11. M. S. Pankey, V. N. Minin, G. C. Imholte, M. A. Suchard, and T. H. Oakley, “Predictable transcriptome evolution in the convergent and complex bioluminescent organs of squid,” Proc Natl Acad Sci U S A, vol. 111, no. 44, pp. E4736–E4742, Nov. 2014, doi: 10.1073/PNAS.1416574111.

Image References:

  1. “Cephalopod Camouflage: A Beauty That’s Skin Deep.” https://www.sciencefriday.com/educational-resources/cephalopod-camouflage-beauty-thats-skin-deep/ (accessed Dec. 03, 2022).
  2. “Cuttlefish chromatophore research – Stock Image – C029/7040 – Science Photo Library.” https://www.sciencephoto.com/media/733941/view/cuttlefish-chromatophore-research (accessed Dec. 03, 2022).
  3. “Now Is the Time to See Squid That Glow Like Fireflies – The New York Times.” https://www.nytimes.com/2016/04/27/science/firefly-squid-toyama-japan.html (accessed Dec. 03, 2022).

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