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Chromogenic behavior and physiology of squid chromatophores

Gonatus changes the its color by expanding individual chromatophores. credit: P. Daniel
Gonatus changes the its color by expanding individual chromatophores. credit: P. Daniel

Chromatophores in the skin of squid, octopus, and cuttlefish share a common design, each is an elastic pigment body, spherical at rest, surrounded by a halo of muscle fibers with both excitatory (glutamatergic) and inhibitory (serotonergic) nerve supply. The various communication and camouflage displays for which cephalopods are famous result from differential retraction/expansion of these neuromuscular end organs.

While the full chomogenic repertoire of a squid can be caught on camera in the wild, glass-like transparency of its skin allows coordinated activity of individual components to be captured in the laboratory at intermediate magnifications, right down to the single chromatophore or even single muscle fiber level. This gives the system great theoretical potential, difficult or impossible to access in other motor systems; for besides color, the thousands of chromatophores per square cm of skin engaged in the delivery of chromogenic behavior possess a less well-known general feature. They also carry an ontogenetc signature; their pigment bodies (spots) belong in a size/age hierarchy. When the system is at rest, spots of any one generation can be seen to be smaller than those of the previous. Image-analysis is a particularly important element of this work.

Classical comparisons between two ecologically different species provide valuable insights into structure and function. In the open-ocean Humboldt squid (Dosidicus gigas), only red chromatophores are present, used in signalling displays of repetitive flashing, whereas in the coastal market squid (Doryteuthis opalescens), successive layers of brown, red, and yellow chromatophores generate spatial patterning that is used for both signalling and camouflage. The latter species is regularly available in
Monterey Bay, and our lab has ample facilities for collecting and holding live animals and for studies in the laboratory using a wide variety of techniques. Humboldt squid are presently available in the Gulf of California where we are developing a field station for both field and lab studies (See SURMAR).

Rosen H, Gilly W. (2017) Myogenic activity and serotonergic inhibition in the chromatophore network of the squid Dosidicus gigas (family Ommastrephidae) and Doryteuthis opalescens (family Loliginidae). Journal of Experimental Biology 220: 4669-4680.

Rosen H, Gilly W, Bell L, Abernathy K, Marshall G (2015) Chromogenic behaviors of the Humboldt squid (Dosidicus gigas) studied in situ with an animal-borne video package. Journal of Experimental Biology 218: 265–275.