Fluorescence is pretty common underwater. It is when a subject absorbs light of one colour and emits light of another. It is usually pretty hard to capture photographically.
I am sure you have all seen examples of fluorescence - such as an Anemone fluorescing red, or a mushroom coral fluorescing orange or green, or in the Caribbean a coral, often the great star coral (M. cavernosa) fluorescing orange. These are just examples I can think of now.
Generally the way it works is blue light (i.e. underwater ambient light - and usually reasonably deep) shines on the subject and it emits fluorescent light of a different colour. And it is only certain individuals that fluoresce - it never seems to be all the population. This is a completely different process to phosphorescence or bioluminesence.
There is a good explanation here: http://www.nightsea.com/aboutfluor.htm
Anyway it has always been difficult to capture. When you see your bright red anemone or bright orange coral you take a picture (with flash) and it comes out brown and much more muted than it looked underwater. In fact usually they look like all the other corals do.
Curiously I have found that available light images (taken with a filter - presumably of any brand) will capture this fluorescence as it appears underwater.
Here is an example of Great Star Coral in Cayman - that was clearly fluorescing underwater:
and
You can see from the first picture that I was able to capture the fluorescence while rendering the rest of the reef (and my fin in the bottom left corner!) in normal colours.
Alex
p.s. The next day I photographed some other fluorescent coral with flash for comparrison that I will post once I have found them.
