It's hard for ONR's oceanographer Steve Ackleson to believe he hadn't thought of it before: imaging underwater in fluorescent light.* But the first time he did so on a Floridian coral reef, he couldn't believe his eyes. Corals in brilliant colors not visible under sunlight illumination were suddenly all around him.
Corals are tiny living sea creatures which secrete a calcium carbonate material during their lifetimes, which, in turn, slowly builds up and creates what we call the coral reef. Living corals fluoresce in several colors when illuminated with blue light, and the ways these fluorescent colors mix produces a wide range of coral appearances. But here's the rub…scientists still aren't sure why the animals do it.
"This is not bioluminescence," says Ackleson, "which is the biological production of light. Sea creatures of all kinds use bioluminescence to search for mates, for example, and as camouflage. Fluorescence on the other hand is the absorption of light at one wavelength and its re-emission at another wavelength, and it ceases when the source of light is removed. What that boils down to is that some things will glow when you shine the ‘right light’ on them, and the 'right light' can be different for different marine organisms. Why the corals do this is a complete mystery at this point."
Fluorescence in corals is related to species (and so it can be used to map specific coral groups within a reef) as well as coral health. When a fluorescent animal dies, the fluorescence at some point stops.
"Fluorescence in the ocean tends to come from living things. Dead things, and man-made objects like discarded refrigerators and spent ordinance, generally do not fluoresce," says Ackleson. "The Navy finds this very interesting. We believe that eventually we can use fluorescent underwater survey techniques for managing reefs in sensitive military areas, for reclamation efforts, and in searching for mines and other man-made objects within a reef environment."
More about Ackleson's research can be found at the following websites: http://www.psicorp.com/cobop/cobop.html and images can be found here: http://www.nightsea.com/steveackleson.htm
*Underwater fluorescent photography has only been around since the late 50's. The first documented record of an observation of marine fluorescence only dates to 1927.