[ATJ]

Luke,

First of all, the fact that you can "observe" anything under the blacklight suggests that it also includes visible light as human eyes are not sensitive to UVR. How do you know whether the reaction of the corals, zoanthids and clams are a result of the UVR or the visible light?

Second, assuming it was the UVR from the blacklight that was causing the reaction, how do you know whether the "day mode" is a positive or negative reaction? The observed "day mode" could be the organisms protecting themselves from the UVR rather than obtaining some kind of benefit.

With the exception of some work on deep water corals (which I will cover below) not one of the many papers on fluorescent pigments in corals has suggested that the purpose of the fluorescent pigments in shallow water corals are for enhancing photosynthesis. Most actually suggest the opposite, that the pigments are photoprotective. They use as evidence the fact that a) the fluorescent pigments are found above the zooxanthellae rather than below and b) expression of the fluorescent pigments tend to decrease with increasing depth (i.e. decreasing light).

If the fluorescent pigments were enhancing photosynthesis they would most likely be found under the zooxanthellae where the "reflection" of the shifted light would reach the zooxanthellae. With the pigments being above the zooxanthellae, most of the shifted light will end up in the water (which is also why we see it).

Corals in shallow water are exposed to much more light and UVR than those in deeper water. Corals in shallow water have much less of a need for enhancement of photosynthesis. In fact, many have been shown to actually receive too much light and have various adaptations to either reduce the light they receive or to shut photosynthesis down (photoinhibition) for periods when the light is at its highest. In these cases photoprotection seems a more obvious role than photoenhancment.

The exception to the above is the work done by Schlichter and Fricke (and others) on Leptoseris fragilis from very deep water in the Red Sea. These corals are found at depths between 100 and 160 m where visible light is extremely low. They were found to have fluorescent pigments below the zooxanthellae instead above as in most other corals and it is very likely that the pigments are enhancing photosynthesis under very tough lighting conditions for the coral.

There is no doubt that a lot of UVR reaches the Earth's surface, including the ocean and some of it penetrates the ocean (UVA more so than UVB). Note that less radiation (visible and UV) gets into the water the lower the Sun is because of simple reflection on the water surface. As already mentioned, corals in shallow water already get more than enough visible light so the need to enhance this is does not really exist. Just because something is there (UVR) doesn't mean it is used or needed.

Can you be certain that the blue you are seeing is actually fluorescence not simple reflection of the small amount of violet light emitted with the UVR from the blacklight? What species of coral appear blue? What happens if you place something white under the light? I'm not saying that you don't have blue fluorescence but it is very uncommon. I also know at least one researcher who denies the existence of a fluorescent pigment in corals that emits in the blue although Salih et al. (2000) show a pigment that has an excitation peak of 384 nm and an emission of 486 nm which should appear blue-green.

Finally, while you haven't observed any obvious harm to your organisms from the blacklight (and I wouldn't expect any) how can you be sure that they wouldn't have done better without it? I'm not saying they would have done better or worse with or without the blacklight but merely pointing out you have only only one alternative so it is difficult to make any meaningful conclusions.