Ocean Exploration and Drug Discovery in the Twilight Zone

The transition between shallow coral reef communities and aphotic deep-water communities in the tropics (= 50m to 150m) is often called the "twilight zone", and it is characterized by reduced light and consequently photosynthesis. But while reef-building coral diversity decreases with increasing depth, many sponges, soft corals, and even low-light acclimatized primary producers are abundant. Thus, this is a zone of potentially incredible ecological significance, unique biodiversity, and unusual physiological adaptations where one might expect to find important natural products that might be used as pharmaceuticals or other biotechnology products. This is also one of the least studied ecosystems in the marine environment since it exceeds the limits of traditional SCUBA, but is considered too shallow to warrant costly submersible time. We have examined the Twilight Zone off the Exuma Sound Shelf, Bahamas, over the last three years using technical mixed gas diving. Biodiversity data indicate a rich sponge community consisting of 92 species and approximately 15% endemism. Sponge diversity and density increased with depth as picoplankton food abundance increased. Sponges were also larger and faster growing at depth than in shallow-water conspecific populations. This unique biodiversity represents an important biotechnological resource as well; two thirds of the sponges exhibit promising biomedical activity compared to less than one third of the shallow reef sponge species. In addition, 5 of 7 species that co-occur at shallow and deep sites exhibited greater bioactivity due to quantitative and qualitative differences in the natural products produced in each depth range.