Symbiosis, the interdependence of different organisms for the benefit of one or both participants, is much more prevalent in the oceans than on land. Within the spectrum of symbioses, zooxanthellae clearly have a special place. They are not just found in Scleractinia but occur in other cnidarians (soft corals, anemones and their allies) as well as in an assortment of other animals encompassing single-celled ciliated protists, sponges, flatworms and molluscs (including giant clams). Once thought to be a single species, zooxanthellae have been found to be genetically diverse (consisting of many genetic types or ‘clades’), although they all look similar. They can all live independently, although not in such concentrated numbers nor with such long-term security as they can in the tissues of hosts. In corals (but not clams) they live inside the cells of the host organisms – in the innermost (gastrodermal) layer of the two cell layer body wall. All zooxanthellae are tiny, (approximately one hundredth of a millimetre in diameter), and only one to four occur within each coral cell.
Zooxanthellae photosynthesise as do other green plants, but up to 95% of the nutrients they produce are leaked directly to the host organism. Nevertheless corals are voracious feeders on zooplankton and therefore have two very different food sources. Many if not most corals that are kept in darkness (so that their zooxanthellae cannot photosynthesise) will start to die after a few months irrespective of how much food they have.
The main points about algal symbiosis are:
- Corals acquire their zooxanthellae either directly from the parent colony or through infection of free-swimming or newly-settled larvae.
- Uptake of non-parental zooxanthellae in early life may be by random chance, giving different advantages to different colonies: some colonies might be infected with temperature tolerant symbionts, others with more productive ones. This hypothesis has yet to be confirmed.
- Zooxanthellae readily change in abundance depending on conditions such as season, position on the coral and light level (as seen in corals experimentally transplanted to different water depths).
- More than one genetic type of zooxanthellae can occupy a single colony.
- The abundance of genetic types varies geographically on any scale.
- Some genetic types facilitate a faster growth rate than others.