July 26, 2013
The Science Behind Color-Changing Cephalopods
redOrbit Staff & Wire Reports - Your Universe Online
Researchers from the University of California, Santa Barbara (UCSB) have discovered the mechanisms responsible for the dramatic color changes in underwater creatures such as the squid and the octopus.
According to UCSB scientists, color in living organisms can be formed in one of two ways - pigmentation or anatomical structure. Structural colors are the result of the physical interaction between light and biological nanostructures, and while numerous different types of organisms possess this ability, the precise biological mechanisms underlying the process have remained something of a mystery.
Back in 2011, a UCSB team managed to locate the mechanism through which the neurotransmitter acetylcholine drastically changed color in the Doryteuthis opalescens, also known as the opalescent inshore squid or the market squid. Acetylcholine begins a series of events which culminate in the addition of phosphate groups to a family of proteins known as reflectins. The process causes the reflecting surface to condense, serving as a catalyst for the color-changing process.
In the latest study, published in the Proceedings of the National Academy of Science (PNAS), lead author and molecular biology graduate student Daniel DeMartini and his colleagues build upon the previous work by uncovering the mechanisms responsible for the color shifts in squids and octopi.
"Structural colors rely exclusively on the density and shape of the material rather than its chemical properties," the university explained, adding the new study "shows that specialized cells in the squid skin called iridocytes contain deep pleats or invaginations of the cell membrane extending deep into the body of the cell. This creates layers or lamellae that operate as a tunable Bragg reflector."
Bragg reflectors are structures formed from multiple layers of alternating materials with varying refractive index, and are named in honor of the British father-son team that discovered them over a century ago, the researchers explained. The Braggs found how periodic structures reflect light in a regular and predicable manner - a discovery which won the duo a Nobel Prize for Physics in 1915 and factors heavily in the new UCSB study.
"We know cephalopods use their tunable iridescence for camouflage so that they can control their transparency or in some cases match the background," said Daniel E. Morse, co-author of the study and a biotechnology professor in the UCSB Department of Molecular, Cellular and Developmental Biology.
"They also use it to create confusing patterns that disrupt visual recognition by a predator and to coordinate interactions, especially mating, where they change from one appearance to another," added Morse, who is also the director of the university's Marine Biotechnology Center/Marine Science Institute at UCSB. "Some of the cuttlefish, for example, can go from bright red, which means stay away, to zebra-striped, which is an invitation for mating."
Shrinking, Swelling Changes Wavelength Of Reflected Light
DeMartini, Morse and their associates developed antibodies designed specifically to bind to the reflecting proteins. They revealed those proteins are located exclusively inside the thin plate-like structures known as lamellae formed by folds in the cell membrane. They also showed the events that ultimately culminate in the condensation of the reflectins cause the osmotic pressure inside the lamellae to change drastically due to water expulsion, which in turn causes the structures to shrink, become dehydrated, and have their thickness and spacing reduced.
"The movement of water was demonstrated directly using deuterium-labeled heavy water," the university said. "When the acetylcholine neurotransmitter is washed away and the cell can recover, the lamellae imbibe water, rehydrating and allowing them to swell to their original thickness. This reversible dehydration and rehydration, shrinking and swelling, changes the thickness and spacing, which, in turn, changes the wavelength of the light that's reflected, thus 'tuning' the color change over the entire visible spectrum."
"This effect of the condensation on the reflectins simultaneously increases the refractive index inside the lamellae," noted Morse. "Initially, before the proteins are consolidated, the refractive index - you can think of it as the density - inside the lamellae and outside, which is really the outside water environment, is the same. There's no optical difference so there's no reflection. But when the proteins consolidate, this increases the refractive index so the contrast between the inside and outside suddenly increases, causing the stack of lamellae to become reflective, while at the same time they dehydrate and shrink, which causes color changes."
The creatures can control the extent to which this phenomenon happens, allowing it to essentially pick its color, he added. Furthermore, the process is reversible, and Morse called the precision of the tuning by the regulation of the lamellae's nanoscale dimensions "amazing."
Exploring Real-World Applications Of This Color-Changing Phenomenon
A second paper from the UCSB team that was published in the Journal of the Royal Society Interface used a mathematical analysis of the color change to confirm that the changes in refractive index perfectly correspond to the measurements made with live cells.
In addition, a third study, set to appear in an upcoming edition of the Journal of Experimental Biology, discovered the female market squid can display a set of stripes, and a pair of those stripes can switch from being completely transparent to bright white.
"This is the first time that switchable white cells based on the reflectin proteins have been discovered," said Morse. "The facts that these cells are switchable by the neurotransmitter acetylcholine, that they contain some of the same reflectin proteins, and that the reflectins are induced to condense to increase the refractive index and trigger the change in reflectance all suggest that they operate by a molecular mechanism fundamentally related to that controlling the tunable color."
He also suggested the findings could have practical applications in a variety of areas, including telecommunications. "We're moving to more rapid communication carried by light," he explained.
"We already use optical cables and photonic switches in some of our telecommunications devices. The question is - and it's a question at this point - can we learn from these novel biophotonic mechanisms that have evolved over millions of years of natural selection new approaches to making tunable and switchable photonic materials to more efficiently encode, transmit, and decode information via light?"