More than a kilometer below the ocean’s surface, where the sunless water is inky black, scientists have documented one of nature’s most spectacular living light shows. An underwater survey has found that roughly 20% of bottom-dwelling organisms in the Bahamas produce light. Moreover, all of the organisms surveyed by the researchers proved to have visual senses tuned to the wavelengths of light generated by this bioluminescence. The work speaks to the important role self-generated light plays in deep-sea communities, marine biologists say.

Bioluminescence has evolved many times in marine species and may help organisms find mates and food or avoid predators. In the middle depths of the ocean—the mesopelagic zone that is located 200 to 1000 meters below the surface—the vast majority of organisms can bioluminesce. Much less was known about bioluminescence in organisms living close to the sea floor. Such benthic organisms are harder to visit or sample and therefore study, says Sönke Johnsen, a marine biologist at Duke University in Durham, North Carolina.

With Tamara Frank, a marine biologist at Nova Southeastern University in Florida, and colleagues, Johnsen recently explored four sites in the northern Bahamas in a submersible. The researchers collected the benthic organisms by suctioning them gently into a lightproof box with a vacuum hose. Once back in their shipboard labs, they stimulated bioluminescence in the captured organisms by softly prodding the animals. Those that glowed were tested further to determine the exact wavelength of light emitted.

As the survey team reported online on 5 September in The Journal of Experimental Biology,about 20% of the species they gathered were capable of producing bioluminescence when touched, including several species of coral, sea anemones, and an unusual species of shrimp that vomited bioluminescent chemicals into the water surrounding it. Most of the organisms glowed blue, except for a family of corals known as pennatulaceans, which produced green light.

Although fewer benthic species produce light than in the middle depths of the ocean, where approximately three-quarters of the organisms glow when touched, the sea floor was much brighter than upper depths. “It was like glowing rain,” Johnsen says. “We saw big flashes, then little ones, then streaks of larger gelatinous animals being squished against the windshield.”

This paradox—that fewer benthic organisms bioluminesce, but they do so more frequently—may be explained by how the phenomenon is triggered. A sea-dwelling species doesn’t produce light constantly; it typically does so only when touched by another object. Mesopelagic organisms float freely in the ocean and infrequently encounter other plants and animals. Benthic species, on the other hand, are constantly bumping up against corals or being jostled by microscopic plankton.

In a second study, Frank and Johnsen determined the wavelengths of light to which the captured organisms are most sensitive by placing a tiny electrode on the animal’s cornea or light-capturing organ. When they recorded a tiny jolt of electricity, it meant that the light had been detected by the animal. Most of the benthic organisms were most sensitive to blue-green light between 470 to 497 nanometers, Frank and her colleagues reported in a second article in that same issue of the journal.

Frank also discovered that two species of crab (Eumunida picta and Gastroptychus spinifer), were also sensitive to UV light, a surprising find because there is no UV light that deep in the ocean. Johnsen thinks this additional sensitivity may help the crabs avoid toxic corals, which produce a greenish glow, and home in on the edible organisms that emit blue light. Frank and Johnsen say they will conduct behavioral experiments to see whether their hypothesis about the color-coded benthic buffet is correct.

“It’s a splendid piece of work,” says Peter Herring, a retired bioluminescence expert at the National Oceanography Centre, Southampton, in the United Kingdom. “It’s a good collection of data from benthic animals at this location, and it uses the best technology as far as imaging is concerned.”

Source: Science Now