Nerves in red can be easily traced among the distinctive chromatophores and iridophores that they innervate. Credit: Wardill, Gonzalez-Bellido, Crook & Hanlon, Proceedings of the Royal Society B: Biological Sciences

Squid’s
colorful, changeable skin enables the animal—and their close relatives,
cuttlefish and octopus—to display extraordinary camouflage, the speed
and diversity of which is unmatched in the animal kingdom.

But
how squid control their skin’s iridescence, or light-reflecting
property, which is responsible for the animal’s sparkly rainbow of
color, has been unknown.

In
a new study, MBL (Marine Biological Laboratory) researchers Paloma
Gonzalez Bellido and Trevor Wardill and their colleagues report that
nerves in squid skin control the animal’s spectrum of shimmering
hues—from red to blue—as well as their speed of change. The work marks
the first time neural control of iridescence in an invertebrate species
has been demonstrated.

Squid
skin is extraordinary because it has two ways to produce color and
pattern. Pigmented organs called chromatophores create patterns with
yellow, red, and brown colors.  Underneath the pigments, iridophores,
aggregations of iridescent cells in the skin, reflect light and add
blue, green, and pink colors to the overall appearance of the skin.
Collectively these two groups of skin elements can create spectacular
optical illusions with patterns of color, brightness, and contrast
change.

“For
20 years we have been wondering how the dynamically changeable
iridescence is controlled by the squid,” says study co-author Roger
Hanlon.  “At long last we have clean evidence that there are dedicated
nerve fibers that turn on and tune the color and brightness of
iridophores. It is not an exaggeration to call this “electric skin.” The
complex nerve network distributed throughout the squid’s skin instantly
coordinates tens of thousands of chromatophores with iridescent
reflectors for rapidly changing behaviors ranging from camouflage to
signaling.”

Neurally stimulated squid iridophore. Credit: Wardill, Gonzalez-Bellido, Crook & Hanlon, Proceedings of the Royal Society B: Biological Sciences

Working
with longfin inshore squid (Doryteuthis pealeii), the researchers took a
new approach to investigating the mystery behind the iridophore control
mechanism.  By tracing a highly branched network of nerves and
stimulating them electrically, they found that they could activate
progressive color shifts from red and orange to yellow, green, and blue
in just 15 seconds. The findings suggest that the specific color of each
iridophore, as well as speed of change, is controlled by the nervous
system, as is spatial chromatophore patterning that occurs in the skin
layer just above.

How
squid choose and hold particular skin colors to help camouflage
themselves remains unknown and is particularly interesting because the
animals are completely colorblind.

iridophore

“One
possibility is the animals do not care about the color of the
iridophores, but shifting the color from red to blue will dramatically
increase the relative brightness of iridophores,” says Wardill. “This is
because squid see predominantly blue light. Blue light is especially
important in the ocean as it penetrates best into deeper water.”

The
study, co-authored by Gonzalez Bellido, Wardill, Hanlon of the MBL and
Robyn Crook of University of Texas Medical School at Houston appeared in
the August 15th issue of the journal Biological Sciences.  The work was
funded by grants from the Office of Naval Research (ONR), Defense
Advanced Research Projects Agency (DARPA), and Air Force Office of
Scientific Research.

Source: Marine Biological Laboratory