Sinking through the inky ocean, it would seem that there is little light at depth: but you'd be wrong. 'In the mesopelagic realm [200 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, long time collaborators Tamara Frank, Sonke Johnsen, Steven Haddock, Edith Widder and Charles Messing teamed up to find out just how much light is produced by seabed residents. The team discovered that bioluminescent animals are relatively rare but blue-green bioluminescence produced when plankton collide with obstacles is relatively common. They also found that deep-sea predators have incredibly sensitive colour vision and they publish their discoveries in a pair of papers in the Journal of Experimental Biology at http:/jeb.biologists.org.
Descending to the bottom of the ocean near the Bahamas in Harbor Branch Oceanographic's Johnson- Sea-Link submersible, switching off all the lights and adapting to the darkness, the group 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 (Johnsen et al. 2012). 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 495nm. However, a family of soft corals known as the pennatulaceans produced green light, with spectra peaking from 505 to 535nm. 'We were working at the absolute limits of what the equipment can do', remembers Johnsen, recalling the frustration of working in the cramped, pitch-dark conditions on the boat. 'It gives you respect for our vision, we can see the bioluminescence fine, but getting it recorded on an instrument or a camera is much harder', he adds. And as if that wasn't challenging enough, proving that anything living down there could even see the spectacular light display was even trickier.
Devising a strategy for collecting crustaceans ranging from crabs to isopods under dim red light – to protect their sensitive vision – by luring or gently sucking them into light-tight boxes, the submersible's crew then sealed the animals in boxes to protect their vision from harsh daylight when they reached the surface. Back on the RV Seward Johnson, Frank painstakingly measured the weak electrical signals produced by the animals' eyes in response to dim flashes of light ranging from 370nm to over 600nm and found that the majority of the creatures were most sensitive to blue/green wavelengths, ranging from 470nm to 497nm (Frank at al. 2012). Most surprisingly, two of the animals were capable of detecting UV wavelengths. Even though there is no UV left from the sun at this depth, Johnsen explains, 'Colour vision works by having two channels with different spectral sensitivities, and our best ability to discriminate colours is when you have light of wavelengths between the peak sensitivities of the two pigments.' He suspects that combining the inputs from the blue and UV photoreceptors allows the crustaceans to pick out fine gradations in the blue-green spectrum that are beyond our perception, suggesting, 'These animals might be colour-coding their food': they may discard unpleasant-tasting green bioluminescent coral in favour of nutritious blue bioluminescent plankton.
Finally, after recording the crustacean's spectral sensitivity, Frank – from Nova Southeastern University, USA – measured how much light the animals' eyes had to collect before sending a signal to the brain (the flicker rate). She explains that there is a trade-off between the length of time that the eye collects light and the ability to track moving prey. Eyes that are sensitive to dim conditions lower the flicker rate to gather light for longer before sending the signal to the brain. However, objects moving faster than the flicker rate become blurred and their direction of motion may not be clear. The crustaceans' flicker rates ranged from 10 to 24Hz (human vision, which is sensitive to bright light, has a flicker rate of 60Hz) and the team were amazed to find that one crustacean, the isopod Booralana tricarinata, had the slowest flicker rate ever recorded: 4Hz. According to Frank, the isopod would have problems tracking even the slowest-moving prey. She suggests that as it is a scavenger, it is possible that it may be searching for pockets of glowing bacteria on rotting food and it might achieve the sensitivity required to see this dim bioluminescence with extremely slow vision.
IF REPORTING ON THESE STORIES, PLEASE MENTION THE JOURNAL OF EXPERIMENTAL BIOLOGY AS THE SOURCE AND, IF REPORTING ONLINE, PLEASE CARRY A LINK TO: http://jeb.biologists.org/content/215/19/3335.abstract AND http://jeb.biologists.org/content/215/19/3344.abstract
REFERENCES: Johnsen, S., Frank, T. M., Haddock, S. H. D., Widder, E. A. and Messing, C. G. (2012). Light and vision in the deep-sea benthos: I. Bioluminescence at 500m depth in the Bahamian Islands. J. Exp. Biol. 215, 3335-3343.
Frank, T. M., Johnsen, S. and Cronin, T. W. (2012). Light and vision in the deep-sea benthos: II. Vision in deep-sea crustaceans. J. Exp. Biol. 215, 3344-3353.
This article is posted on this site to give advance access to other authorised media who may wish to report on this story. Full attribution is required, and if reporting online a link to jeb.biologists.com is also required. The story posted here is COPYRIGHTED. Therefore advance permission is required before any and every reproduction of each article in full. PLEASE CONTACT firstname.lastname@example.org
Kathryn Knight | EurekAlert!
How molecules teeter in a laser field
18.01.2019 | Forschungsverbund Berlin
Discovery of enhanced bone growth could lead to new treatments for osteoporosis
18.01.2019 | University of California - Los Angeles
The scientific and political community alike stress the importance of German Antarctic research
Joint Press Release from the BMBF and AWI
The Antarctic is a frigid continent south of the Antarctic Circle, where researchers are the only inhabitants. Despite the hostile conditions, here the Alfred...
World first experiments on sensor that may revolutionise everything from medical devices to unmanned vehicles
The new sensor - capable of detecting vibrations of living cells - may revolutionise everything from medical devices to unmanned vehicles.
Dead and alive at the same time? Researchers at the Max Planck Institute of Quantum Optics have implemented Erwin Schrödinger’s paradoxical gedanken experiment employing an entangled atom-light state.
In 1935 Erwin Schrödinger formulated a thought experiment designed to capture the paradoxical nature of quantum physics. The crucial element of this gedanken...
Cellulose obtained from wood has amazing material properties. Empa researchers are now equipping the biodegradable material with additional functionalities to produce implants for cartilage diseases using 3D printing.
It all starts with an ear. Empa researcher Michael Hausmann removes the object shaped like a human ear from the 3D printer and explains:
The phenomenon of so-called superlubricity is known, but so far the explanation at the atomic level has been missing: for example, how does extremely low friction occur in bearings? Researchers from the Fraunhofer Institutes IWM and IWS jointly deciphered a universal mechanism of superlubricity for certain diamond-like carbon layers in combination with organic lubricants. Based on this knowledge, it is now possible to formulate design rules for supra lubricating layer-lubricant combinations. The results are presented in an article in Nature Communications, volume 10.
One of the most important prerequisites for sustainable and environmentally friendly mobility is minimizing friction. Research and industry have been dedicated...
16.01.2019 | Event News
14.01.2019 | Event News
12.12.2018 | Event News
18.01.2019 | Materials Sciences
18.01.2019 | Life Sciences
18.01.2019 | Health and Medicine