It’s not easy being green–or blue, or purple, or orange–at least not if you’re an octopus. Cephalopods are well-known for their incredible ability to camouflage and communicate via their rapidly color-changing skin. But the process of shifting hues or holding a color takes a significant amount of effort, according to a study published November 18 in the journal Proceedings of the National Academy of Sciences. Octopuses expend the same amount of energy activating their color-changing system as they do maintaining all other aspects of their resting metabolic rate, including digestion, respiration, organ function, and circulation, per the new research.
The findings are the first to quantify how much work goes into switching on chromatophores, the specialized color-changing organs connected to cephalopods’ muscle and nervous systems, which dot the marine invertebrates’ skin like pixels. When at rest, chromatophores are spherical, appearing as small points of pigment–but when expanded, they become flattened disks of color that visually merge together in remarkable displays.
Video: Octopus skin chromatophore expansion under white light. Credit: Sophie Sonner.
Different types of cephalopods have different concentrations of chromatophores. Shallow water octopuses like the species used in the study (Octopus rubescens) have about 230 chromatophores per square millimeter of skin–lending them high resolution hues.
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“Octopuses have the HD or the 4K skin,” says Matthew Birk, a marine biologist studying cephalopods at Saint Francis University in Pennsylvania. Birk wasn’t involved in the new research, but notes, “It’s a really neat study.” The authors “were able to isolate how much of the energy these octopuses consume every day is going towards using their camouflage system…I’m not aware of anybody else that has this cleanly separated out that portion of [metabolism],” Birk explains. “I am quite surprised at how expensive it is to operate.”
When fully at rest, octopuses are very pale–close to white with tiny speckles, notes Sofie Sonner, lead study author who completed the work as part of her Master’s Thesis at Walla Walla University in Washington. Which means that, just about any time you’ve seen an image or video of a shallow water octopus out and about, and it’s appeared purpley brown, reddish, or patterned, it’s been tensed up. “They have to flex muscles in their skin to make this happen,” explains Kirt Onthank, study co-author and a professor of biology at Walla Walla University.
To quantify the exertion necessary for that color flex, Onthank and Sonner took small skin samples from 17 live ruby octopuses. The scientists temporarily anesthetized the cephalopods so that they could undergo their minor biopsies with minimal discomfort. “We put them in ethanol and then raised the ethanol level until we essentially got them so drunk they passed out,” says Onthank. “It didn’t seem to bother them too much after we woke them up,” he adds.
Within 15 minutes of collection, each skin sample was mounted on a specially designed microscope slide set-up, complete with oxygen sensor and 3D-printed chamber. In repeat trials, the scientists exposed each skin bit to dark and light conditions while it was still fresh and the cells remained alive and responsive, which alternately de-activated and expanded the chromatophores. They measured oxygen consumption as a proxy for cellular metabolism and energy use in each phase of the experiment.
Video: Ruby octopuses in the wild near Whidbey Island, Washington, change color. Credit: Kirt L. Onthank.
Previous studies of other color-shifting organisms like fish and newts have attempted to quantify the cost of color across a whole individual, by putting organisms in different environments to spur a change and then assessing oxygen or food consumption. However these past studies are inexact because it’s difficult to separate the impacts of induced color change from the stress of manipulating an animal in a lab environment.
The new method eliminates this challenge, and reveals that the skin samples’ oxygen use shoots up (literally from 0 to 60) when chromatophores are expanded. Using a complex model of octopus surface area, obtained by putting an octopus in a 3D scanner, Onthank and Sonner estimated that the total body cost of activating chromatophores would be about equivalent to the energy used in maintaining all other physiological processes of the animals’ resting states. “It’s as much as their nervous system, their guts–their everything else,” says Onthank. “Though octopuses make color change look effortless, it isn’t for them.”
Even this estimate is likely conservative, says Sonner–given that their octopus surface area assessment oversimplifies and smooths the cephalopods, not accounting for every dip, divot, and bit of texture. Plus, assessing the energy use of a single piece of skin doesn’t measure for the cognitive effort that cephalopods are likely using to coordinate their full-body camouflage. In future work, the researchers would like to compare the chromatophore energy consumption of different species, look at the relationship between mass and chromatophore demand, and develop a better method for measuring real-time, full-body energy use related to color change, says Onthank.
But for now, the apparent extreme expense of chromatophores adds to the evolutionary mystery of how such a costly system developed, says Birk. The ancestors of contemporary cephalopods had shells for protection, but somewhere along the way the lineage lost them–presumably because it became advantageous to be soft-bodied and spry as opposed to hard and heavier. “There’s different possibilities for why they did that, but clearly they did and it’s worked for them– which is impressive when it’s this expensive,” he says.
Video: An octopus at Rosario Beach Marine Laboratory catching a crab, and changing color. Credit: Kirt L. Onthank.
While it prompts new questions, it also provides possible answers for others. The effort needed to activate the chromatophore network may explain aspects of cephalopod behavior and ecology. For instance the study authors hypothesize the energy cost of color change could account for why octopuses spend so much time holed up in their dens and tend towards nocturnality, even with their expert camouflage ability. “It if took you incredible amounts of energy to get up and go out, because you had to get dressed up every time, maybe you would just wait until everybody’s gone and you can go out in your PJ’s,” says Onthank–making octopuses akin to dedicated but reclusive make-up and style influencers.
The new findings might also illuminate why deep-water cephalopods have a reduced chromatophore system. “As you go deeper, it gets darker, which means that octopuses don’t really need to worry as much about predators seeing them,” says Kirt Onthank, study co-author and a professor of biology at Walla Walla University in Washington. “Nature tends to take the easier path,” he adds. “If you don’t need a very expensive system, then you’re probably going to get rid of it.”
