Brett Smith for redOrbit.com – Your Universe Online
A team of American scientists has literally shed new light on the activities of creatures living half-a-mile below the ocean surface.
The oceanographers led by Duke University found that some crustaceans living on the sea floor can see ultraviolet light even though thousands of feet of water prevent them from ever being exposed to it. They also found a wide range of creatures, from plankton to fish, living deep in the ocean that are capable of producing a chemically-generated bioluminescence when coming in contact with an object.
Unfortunately, the swift ocean currents of the Bahamas, where the teams conducted their research dives in a submarine, did not allow for the ideal observation of this glowing phenomenon.
“We weren’t in regions where the currents were slow enough to allow for collection of detritus,´ said Tamara Frank of Southeastern University, adding, “it’s not that this phenomenon doesn’t exist“¦we just weren’t able to observe it on these dives.”
According to their report published this week in the Journal of Experimental Biology, the team was able to spot bioluminescence in about 20 percent of the species they encountered by tapping and touching them with the submersible´s robotic arm. The crew then proceeded to collect samples of the glowing specimens, from glowing corals to luminescence-vomiting shrimp, by luring them into light-proof boxes that they took back up to the surface.
Once onboard their research ship, the RV Seward Johnson, Frank and her colleagues began measuring how the creature perceive light by exposing them to dim flashes and recording their optical impulse response. They found the majority of the creatures were sensitive to different subtleties around blue/green wavelengths of light.
Perhaps the most interesting observation was sea creatures receiving certain wavelength of light found in the UV spectrum, despite the fact that natural UV light from the sun never reaches the depths where they live.
“Color vision works by having two channels with different spectral sensitivities, and our best ability to discriminate colors is when you have light of wavelengths between the peak sensitivities of the two pigments,” Duke biologist Sönke Johnsen explained
By combining sensory inputs from both the blue-green and UV photo receptors, the creatures are able to refine their observations of the environment around them, the researchers said.
“Call it color-coding your food,” Johnsen said. He theorized that these animals “sort out the likely toxic corals they’re sitting on, which glow, or bioluminesce, blue-green and green, from the plankton they eat, which glow blue.”
In addition to testing the sea creatures´ range of vision, they also checked the animals’ flicker rate. Similar to the shutter speed setting on a camera, the flicker rate measures how much light an animals’ eyes collect before sending a signal to the brain. Like lower shutter speeds, a low flicker rate will cause an animal to perceive fast moving objects as blurry and be unable to accurately detect a moving object´s direction and pace.
Researchers found the deep sea crustaceans’ flicker rates to range from 10 to 24Hz. Human vision, by comparison, has a flicker rate of 60Hz. One crustacean, the isopod Booralana tricarinata, was found to have a flicker rate of 4HZ, the slowest rate ever recorded. This means that the isopod would have difficulties tracking even the slowest-moving prey and suggests that as it is a scavenger that simply ℠vacuums´ for bacteria along the ocean floor.
The scientists said they would like to collect more specimens and test the animals’ sensitivity to an even wider range of light; however the Johnson-Sea-Link submersible they used in the study is no longer available.
“We would love to go back, get more basic data. We’ve only scratched the surface”, Johnsen said. “When you are down there you are cramped and cold and stiff, but at the end of a dive I never want to come back up.”
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