Bright life in the benthos: Sinking through the inky ocean, it would seem that there is little light at depth: but that's totally wrong. ‘In the mesopelagic realm [200–1000 m] bioluminescence [light produced by animals] is very common’, says Sönke Johnsen from Duke University, USA, explaining that many creatures are capable of producing light, yet rarely do so. But how much light do the inhabitants of the ocean floor (benthos) generate? Explaining that some bioluminescence is generated when organisms collide, Johnsen says, ‘In the benthos you have a current moving over complicated ground with all the things in the water banging into it, so one idea was that there would be a fair amount of bioluminescence.’ However, few people have visited this remote and inhospitable habitat. Intrigued by the animals that dwell there and the possibility that bioluminescent bacteria coating the ocean floor might glow faintly, Johnsen teamed up with long-time collaborators Tamara Frank, Steven Haddock, Edith Widder and Charles Messing to find out just how much light is produced by seabed residents.
Descending to the bottom of the ocean near the Bahamas, switching off all the lights and adapting to the inpenetrable darkness, Johnsen and his colleagues were amazed to find themselves continually surrounded by tiny flashes of light as bioluminescent plankton collided with coral and boulders strewn across the floor. However, there was no evidence of the all-pervasive glow produced by bioluminescent bacteria that the team had hoped to find. ‘We weren't in regions where the currents were slow enough to allow for collection of detritus,’ says Frank, adding, ‘it's not that this phenomenon doesn't exist…we just weren't able to observe it on these dives.’
Next the submariners began searching for bioluminescent inhabitants, gently tapping coral, crabs and anything else they could reach with the submersible's robotic arm to see whether any of the organisms emitted light. The team found that only 20% of the species that they encountered produced bioluminescence. Collecting specimens and returning to the surface, Johnsen and Haddock then photographed the animals' dim bluish glows – ranging from glowing corals and shrimp that literally vomit light (spewing out the chemicals that generate light where they mix in the surrounding currents) to the first bioluminescent anemone that has been discovered – and carefully measured their spectra. The duo found that most of the species produced blue and blue-green spectra, peaking at wavelengths ranging from 455 to 495 nm. However, a family of soft corals known as the pennatulaceans produced green light, with spectra peaking from 505 to 535 nm.