Pictures: Squid Iridescence Explained
Nerve cells—dyed red in the above image—are responsible for squids’ shimmering displays of iridescence, new research shows.
A squid’s shifting metallic sheen comes from clusters of tiny platelike structures inside their skin cells. (See squid pictures.)
Known as iridophores, the microscopic ensemble interferes with the way certain wavelengths of light are reflected.
“It’s the same effect you get with shiny colors on a soap bubble or a thin layer of oil on the water surface in a harbor,” said study co-author Paloma Gonzalez-Bellido, a neuroethologist at the Marine Biological Laboratory in Woods Hole, Massachusetts.
Exactly how squid turn these iridophores on and off has, until now, remained a mystery.
A new technique allowed the team to hook up electrodes to individual nerves in squid skin. When they sent electrical impulses into the nerves, the iridophores changed color and brightness.
Longfin inshore squid (pictured) use nerves to control iridescence in their skin—and it’s likely that other squid species do the same thing, said study co-authorTrevor Wardill, also of the Marine Biological Laboratory.
However, it may take time to discover whether close relatives of squid—including cuttlefish and octopus—shimmer via the same process because of the complex neurons and muscles in their skin.
“We can stimulate the skin of cuttlefish, but they are very difficult to work with,” said Wardill, whose study appeared recently in the journal Proceedings of the Royal Society B. “Squid are a simple version.”
A close-up photograph of squid skin shows two varieties of colored spots—darkly pigmented chromatophores overlaying brightly shining iridophores.
Squid can control the color of their skin despite being color-blind. They see predominantly blue light using a single visual pigment in their eyes.
“In the ocean environment once you go down to 60 feet (20 meters), most colors are all washed out,” Wardill said. “So for squid, maybe color is not really important.”
A patch of iridescent squid skin, known as an iridophore, shines brightly after being stimulated by electric pulses.
The role that color-shifting iridescence plays in the lives of squid is still not fully understood.
“As far as we can tell the iridescence is used on a daily basis, potentially for aggression, and it may be involved in camouflage as well,” said Wardill.
“Certainly when the animals are very aggravated, you can see their iridescence gets quite bright, but what they’re actually telling their fellow squid is hard to know.”
Squid adjust their metalic gleam using iridophores in their skin (indicated with an arrow) that are formed from clusters of individual cells called iridocytes, as seen here under a microscope.
The large white objects in this image with pink centers are chromatophores, the pigmented bodies that give squid their darker, changeable colors.
“We not only saw the chromatophores expand, but also saw the iridophores change reflectance, so they change brightness and color,” explained study co-author Wardill.
“The amount of change varies a little bit from one iridophore to the next,” he added.
Scientists use a glass electrode to send pulses along a squid nerve cell, causing the skin to change color.
“The color change takes around 15 seconds, during which time it shifts through the rainbow of colors,” Wardill said.
“It starts at reds and goes through the yellows and then greens, and through to the blues,” he said.
To avoid damaging delicate tissues, the squid skin was dissected from the underside. The two black patches visible in the image are small holes cut through the skin to access nerve bundles.
Squid nerve cells responsible for switching on iridescence glow red in a microscope image.
Iridescence is seen in other animals, but is often fixed, as in butterflies.
“There are fish that have variable iridescence as well, they can have stripes or different patterns on their body surface,” Wardill said.
“But cephalopods [squid, cuttlefish, and octopus] are much better at it than fish,” he added.
Highlighted in green, a nerve divides into intricate filaments deep within a patch of iridescent squid skin.
The research team is now searching for synapses at the end of these nerve fibers, which release neurotransmitter chemicals that turn on the glittering display.
“We can see down to half a micron, but haven’t found the terminals,” Wardill said.
Instead, he thinks the squid could potentially release chemicals at specific sites along the length of the nerves.
“We’re now working on trying to figure out where are the specific sites that neurotransmitter is released,” he added.
Inside the Iridescence
The fine structure inside iridescent squid-skin cells is seen in blue at very high magnification.
In continuing research, the team has discovered that there are nerve cells dedicated exclusively to switching on the reflecting structures that give squid their changing shine.
“It seems that the brain does have the ability to individually control iridophores,” said study co-author Gonzalez-Bellido.
Added Wardill, the “next big question is how neurons individually control the color and brightness.”