Written by: Alex Neufeld

If you have ever participated in one of our working dives and have handled our corals, you know that these corals can be quite “slimey”. Why is this? Well, hard corals secrete a mucus layer that serves many beneficial purposes. The coral secretes this slime for its benefit, but the slime has also shown to benefit the surrounding community as well.  For the coral, the mucus layer helps to protect its polyps from sedimentation that may smother it. Sand, particulate matter, and other debris are trapped by the mucus web that overlays the coral. When a strong wave or current comes by, the mucus is blown away, along with all of the smothering particulate matter. The coral can shed and re-secrete the slime once it is sloughed off.

The mucus layer also protects the coral from disease and predation. In the last decade, the microbial communities that exist on the surface of coral colonies have generated a lot of interest from the scientific community. Several studies have shown that a coral’s mucus is packed with microbes and bacteria. In fact, it can contain up to 100 times more bacteria than the surrounding seawater. These microbial communities appear to help prevent the transmission of bacterial disease, either by secreting antibiotics or simply by taking up space on the coral colony. And interestingly, a significant number of these microbes are classified as Archaea, one of the three domains of life (along with eukaryotes and bacteria). Typically, Archaea are found in extreme environments like hydrothermal vents and other anoxic habitats, so their presence in warm, shallow oceans is somewhat puzzling.

The mucus coating is crucial not only for the coral’s survival, but also for maintaining the balance of nutrients elsewhere on the reef. The bacteria found in the mucus are some of the few organisms capable of fixing nitrogen, a process where inorganic nitrogen compounds- unusable to any plant or animal- are converted into usable organic forms. In addition to the high microbe and organic nitrogen levels, coral mucus also contains large amounts of phosphorus and organic carbon, critical nutrients for all reef life. Respiration and decomposition by the microbes breaks down these nutrients and recycles them into the surrounding water.