why does gellyfish fluoresce uv light?
Amy Z has the right idea - except that fluorescence and reflection are different phenomena. Reflection is where light bounces off a substance. Fluorescence is more complex: light of specific wavelengths is absorbed by the substance, which gives the electrons in it more energy. These more-energetic electrons then lose their energy, decaying back to their original (resting) energy level: they do this by emitting the absorbed energy back out as light. This process is not 100% efficient: some of the energy is lost as heat, so the emitted light has a longer wavelength than the original absorbed light (longer wavelength = less energetic photons), and so is a different colour (it will be shifted towards the red end of the spectrum). Different chemicals have electrons with specific resting energies, and particular absorbance characteristics. So wavelengths of light that makes some chemicals fluoresce will not make other chemicals fluoresce.
They fluoresce because they produce proteins which reflect UV light. So when we see the UV light we are seeing the reflection off of these proteins. One such protein, used to stain many things in cells in GFP (Green fluorescent protein), which comes from the jellyfish Aequorea victoria/Aequorea aequorea/Aequorea forskalea.
