NOAA Ocean Explorer seems to be making The Life Aquatic with Steve Zissou come true: they've found a fluorescent spotted shark. Actually, it's a chain dogfish, a very pretty animal, that also happens to glow when illuminated with a blue light.

Fluorescing fish is most likely not a useful adaptation, but only an accidental byproduct of a common property of many molecules. Fluorescence is not a process that generates light. Instead, molecules absorb a photon—this is the tricky part, where the molecule must be resilient enough to absorb the energy of light and distribute it within its bonds in such a way that the molecule is not destroyed by the event—and then re-emit it a minute fraction of a second later. A little energy is lost in the process so the re-emitted light has a lower energy, or longer wavelength. The chain dogfish was illuminated with blue light, and then glowed at a longer, lower energy blue-green wavelength.

Black-light posters work the same way. Shine a purplish (short wavelength) illuminator on them, and the pigments absorb the short wavelength light and re-emit it in the longer yellow-green spectrum. It looks particularly vivid to us because our eyes are relatively poor at detecting violets and blues, but have a peak of sensitivity in the yellow green.

Some dishwashing detergents pull a similar trick. When they say they get clothes "whiter than white", they actually do: they load up the clothes with fluorescent compounds that reflect the shorter wavelengths present in the broad spectrum lighting we use in our homes, and translate it into wavelengths our eyes see more easily, adding a subtle glow.

But back to the dogfish—fluorescing is probably not at all useful to a deep-dwelling shark. Conceivably, it could make colors more vivid if the shark were illuminated, but if it's living where it is normally dark, it's going to have no effect at all. Similarly, my zebrafish embryos are loaded with yolk proteins that can fluoresce spectacularly, but it's not for visual effect—it's because they contain complex carbon compounds with lots of double bonds that make them capable of absorbing photons.

It is a useful property that the divers can use to scan for interesting organisms, and there's a good reason for outfitting a submersible with filtered illuminators. People who study scorpions use the same trick—walk around the desert at night with a UV light source, and the scorpions all light up like glow-in-the-dark halloween decorations.