Chromatophores are highly specialized cells that allow an organism to change the hues and patterns of its skin. Examples of this trait in species distantly related across the genetic tree of life, living in very different environments, argues in support of natural selection and the theory of evolution.
The chromatophores in the skin of many cephalopods exhibit polychromatism. As an example of the common mechanism, Loligo pealeii color cells are manipulated by a wagon-wheel of muscles (between 15 and 25) that encircle and stretch the central, elastic pigment sac, pulling it from a tiny sphere into a disc; typical relaxed sphere diameters of 10-100 µm contrast with 0.1-1 mm when flattened into discs. When these muscles relax, the sac recovers its original shape1. Various colors of pigment sac exist, each with a specific layer and size of chromatophore: yellow rates smallest, brown the largest. The sac transition from “off” to “on” requires approximately 700 milliseconds, and Japanese researchers propose that Sepioteuthis lessonianas' enjoys direct control over its body color and pattern, as neural signal frequencies correspond to shifts.2
The chromatophores in the skin of many lizards of family Chamaeleonidae accomplish color and pattern change in a very different way. Pigments in these cells are stored in fatty sacs called vesicles, and are not manipulated by muscles but by hormonal domino effects. Chameleon chromatophores release their stockpiled vesicles when triggered by these chemicals, which drift about and soon burst, scattering “dye” molecules throughout the cell to temporarily alter its color. Below a transparent outer skin layer, these pigment cells are stacked vertically with respect to their vesicle “paint palette”. Yellow xanthophores are topmost, followed by red erythrophores, blue iridiphores (which use guanine as their “paint”), then finally melanophores, employing melanin to create browns and blacks. Amazingly, this dying process is additive, with greens and oranges possible from xanthophores reacting in sync with iridiphores and erythrophores.3
Both of these far-removed families of organisms utilize endogenous dyes, but this is where their isomorphism ends. Chameleons alter integument by expending and mixing vibrant chemicals, while octopi, cuttlefish and squid are more economical, having devised stretchy, reusable pigment sacs to unfurl with motor impulses. The “bombs of paint” technique versus the “hoist the flag” approach is a fascinating example of widely divergent evolutionary lines selecting similar color-altering talents, yet employing very different cell designs in the two. The hypothetical direct neural control seen in cephalopods further contrasts with the endocrine and limbic drivers in Chamaeleonidae chromatophores; such structural control differences suggest intentional color shifts in the former, versus emotional and reactive in the latter. Yet regardless of the regulation of these two systems, their similar function supports the claim that natural selection yields maximally efficient, and often convergent, structures.
1. Macroscale and Microscale Structural Characterization of Cephalopod Chromatophores.
Keith M. Kirkwood, Eric D. Wetzel, George Bell, Alan M. Kuzirian, and Roger T. Hanlon
Proceedings of the Army Science Conference, Orlando, FL, 29 November 2010.
2. Suzuki, M., Kimura, T., Ogawa, H., Hotta, K., & Oka, K. (2011). Chromatophore Activity during Natural Pattern Expression by the Squid Sepioteuthis lessoniana: Contributions of Miniature Oscillation. Plos ONE, 6(4), 1-8. doi:10.1371/journal.pone.0018244
3. High Dynamic Range Image rendering of color in chameleons' camouflage using optical thin films. Mark Prusten. Proc. SPIE 7057, The Nature of Light: Light in Nature II, 705709 (August 11, 2008); doi:10.1117/12.802177
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