popular article | Reef Encounter | open access
In nature, competition can be fierce. In the absence of major disturbance, the species with superior physiology and capabilities eventually dominate in terms of abundance. For example, on Hawaii’s coral reefs five species of reef-building corals(Porites lobata, Porites compressa, Montipora capitata, Montipora patula and Pocillopora meandrina) account for over 90% of the coral cover in shallow waters. All reef-building corals secrete a calcium carbonate skeleton and share a common strategy for life. They harbor photosynthetic microalgae (zooxanthellae) in their tissue which supply them with food to supplement what their polyp tentacles catch in the clear, nutrient-poor waters surrounding Hawaii. Therefore, all reef-building corals are capable of both autotrophy (synthesizing their own food) and heterotrophy (eating other organisms). Reef-building corals (also called zooxanthellate corals) depend on sunlight to fuel their growth and cannot survive indefinitely in darkness. With increasing depth in the ocean, light becomes weaker and eventually limits the depth at which obligate photosynthetic organisms can survive. In the dark depths, “azooxanthellate” corals (and other suspension feeding animals) which lack photosynthetic symbionts live by catching/extracting plankton from the water. In Hawaii, these obligate heterotrophs grow more slowly than photosynthetic organisms and often cannot compete with them for space in the shallower, sunlit habitats.