Fish Camo Trumps Fish Hypervision
September 21, 2016
The open ocean is like the Wild West. Predators and prey stand ten paces from each other in the middle of the main street waiting to hear the word “draw!”
In all that clear, blue emptiness there is nowhere to run to and nowhere to hide so each side has to resort to tricks to get the upper hand, and new research from Duke University is showing that some tricks are better than others, while some may not be tricks at all.
Duke biology professor Sonke Johnsen and colleagues found that a special type of vision, called polarization vision does not work at nearly as long a range as scientists previously thought. They published their findings in the journal, Current Biology.
This is significant because scientists believed for decades that polarization vision was a way to see predators or prey from far away and to see beyond fish camouflage, giving the fish with polarization vision an opportunity to strike or to run before the other fish knew they were even there.
“It’s always funny to be surprised,” Johnsen said. “Everybody went through the sort of five stages of grief when we got the results. We went from total disbelief to anger and took five to six months before we came around to acceptance.”
Polarization vision detects a feature of light that humans and many other animals cannot. Picture light waves as a wavy line. In the diffuse light that we see every day, the waves of those wavy lines crest out at every angle. In polarized light, all of those waves crest and trough in the same direction.
The only way we can tell the difference is if we wear glasses or look through lenses or windows with polarizing glass that filter out light coming from every direction except one. The best we can do is take the glare off a surface of water or make a cool black stripe go across the sky at dawn and dusk.
Animals with polarization vision can do better. Johnsen says scientists can’t be sure exactly how they see polarization, but he says they combine it with other information like color, shape and brightness to make a different picture from what we see.
Scientists thought this could be an advantage because of a trick that other fish use. Many fish have silvery, mirrored scales that reflect the blue water and their nearby surroundings, making a sort of camouflage at long distances. Imagine in the old Western standoff, one cowboy is wearing a mirrored belt buckle and as it shines in the face of the other cowboy, he can’t see exactly where to shoot.
Those mirrored scales also have the unfortunate side effect of polarizing the light, so when scientists discovered the squids, crustaceans and some fish that could detect that polarization, scientists jumped to the logical conclusion that they had that extra sense to cut through the camouflage, like a cowboy wearing polarized sunglasses to cut the glare from the belt buckle.
Johnsen and his colleagues tested exactly how well polarization vision was able to cut the camouflage. They dove near the Great Barrier Reef and took pictures of mirrored fish with a special camera that detects the brightness and polarization of the light that enters the camera.
Using a mathematical model of how light is processed in vision, they determined how far away the mirrored fish could be seen by their polarization and by their brightness.
They found that even though there was a detectable difference in polarization between light bouncing off the mirrored fish and light just travelling through the water, the camera was able to pick up the brightness signal from this fish at a greater distance.
So a fish with polarization vision would see a dark or bright spot in the water and be able to realize that it was a fish before it could recognize an abnormal polarization: not much use in recognizing food or threats from far away.
“Our results overturned things we all assumed were true,” Johnsen said.
Just because the polarization vision cannot aid in detecting fish from far away, does not mean it’s useless. Johnsen says it might be used when fish need to pick their own species or even males or females of their own species out of a crowd.
“These animals are smarter than we think they are, but they’re still not that smart,” Johnsen said. “Close up, polarization vision is just like color. It's added information.”
Future research will examine if there are any angles in the water where polarization vision makes a noticeable difference in the ability to see other fish. For this study, however, Johnson says he was happy to disprove conventional wisdom while working in the wild.
“I think that’s one of the great things about science,” Johnsen said. "The time when science is the most wondrous and magical is when it shows you something you didn’t expect.”
Daniel Lane covers science, medicine, engineering and the environment in North Carolina.