When swimming near the ocean’s well-lit surface, some octopus and squid become transparent in order to hide from hungry predators. In deeper waters, found a new study, they do something completely different: They turn deep red.
The study, which offers a rare look into the behavior of deep-sea creatures, documents the first known example of an animal using completely different tricks to achieve camouflage in different situations. And it gives new insight into the secrets of invisibility, which has a range of applications, from sustainable fishing to medicine.
The findings also suggest that a decline in water quality from pollution and other sources may harm undersea animals more than researchers have previously realized.
"A lot of ecosystems are balanced on who can see who and who can hide from who," said Sönke Johnsen, a visual ecologist at Duke University in Durham, N.C. "If you change the optics of the water, all of a sudden you shift the ecosystem for things that relied on sight for the ability to catch their prey. When the water becomes murkier, species diversity drops."
Cephalopods, the group that includes squid and octopuses, have long impressed divers with their rapid color displays. They can turn from clear to green to nearly black in an instant.
To better explain these rainbow-like behaviors, Johnsen and colleague Sarah Zylinski caught members of one species of octopus and one species of squid, caught while on a ship off the coasts of Peru, Chile and Mexico, they put the animals in shipboard tanks and performed a series of experiments.
Under the room's ambient light, the researchers reported today in the journal Current Biology, both the squid and the octopus were transparent. But when the researchers shone blue light on them, they turned deep red. They did not respond the same way to beams of red light, moving light, or other kinds of light disturbances.
Those findings suggested that the animals might use different strategies for camouflage, depending on how deep they were because in shallow waters, being mostly clear makes an animal basically invisible.
But like a windowpane, a transparent animal reflects light at depths below about 800 meters (2,600 feet), where hiding places are scarce and predators lurk with bioluminescent flashlights that shine in the blue wavelengths. In that environment, it is wisest to be red or black, as those colors absorb blue light rather than reflect it – creating a different kind of invisibility.
To confirm that camouflage was the motivation for switching from clear to red and back again, the researchers measured how much light the squid and octopuses reflected in their alternate color schemes. The results showed that their chosen hues did the best job of hiding the animals under various conditions.
"This is the first time that anyone has ever shown that these animals are actively switching between completely different modes of camouflage," Johnsen said. "Being clear or deeply red are completely different tricks that work under completely different principles."
Among other applications, understanding how cephalopods achieve invisibility could help fishermen design nets that are invisible only to the species of fish they want to catch. Studying transparency in animals has already helped scientists understand how human cataracts work.
The new findings also show how cool and underappreciated the animals are that live at extreme depths, said Stephanie Bush, a marine biologist at the University of Rhode Island in Kingston. Revealing the complexities of these creatures, she said, should help inspire motivation to protect them.
"Scientists thought that deep-sea animals were pretty simplistic, and that because they were living in the dark, they didn’t have to do too much to hide," Bush said. "This adds to our knowledge of how many more complex behaviors there are in the deep sea than what we had imagined."