3. The architects of a reef

Coral reefs are tropical ecosystems but show global patterns. The Caribbean has about 60 reef-building coral species, while Southeast Asia has nearly 1,000, this number broadly diminishing with distance east and west from the Southeast Asian region. Diversity of corals also diminishes broadly with distance north and south of the equator. While basic patterns exist, there are several kinds of reef in the same sense that there are different kinds of forests, sometimes forming near-monocultures, sometimes with more diverse mixtures of species. Their key to success is that they house vast numbers of captive dinoflagellates that photosynthesize in a close symbiosis, which explains how these complex ecosystems persist in the absence of substantial fields of large, visible seaweeds. All deposit limestone in its aragonite form, in a way characteristic to each species, which has been used for distinguishing between species. The basic unit of a coral, the polyp, reproduces sexually, but more importantly by asexual budding, which allows for the growth of large colonies of polyps, all clones. Numerous other organisms have crucial associations with the coral polyp: bacteria and archaea especially, the whole forming what is now termed the coral holobiont. Aside from photosynthesis, corals have nematocysts in their tentacles to capture zooplankton food. Corals compete for space using these stinging cells also, amongst other methods. On any reef, soft corals are numerous, especially in the Caribbean, though these do not deposit limestone rock. Calcareous algae are crucial reef-building components too, particularly in the shallows.