Coral Biology

Corals are living organisms in the phylum Cnidaria. There are two main types of corals: soft corals and stony corals (also called ‘hard’ corals). Stony corals of the order Scleractinia are the corals primarily responsible for reef formation through the production and secretion of calcium carbonate (e.g., CaCO₃, or limestone). Most reef-building corals have a symbiotic relationship with photosynthetic dinoflagellate algae called zooxanthellae (or Symbiodiniaceae, previously referred to as Symbiodinium). The relationship is considered mutualistic, where the coral provides a protected environment, carbon dioxide (CO₂) and nutrients (nitrogen and phosphorus) to the algae, and the algae in turn provide oxygen (O₂) and carbon to the coral through photosynthesis. While coral polyps can feed on plankton, they get about 95% of their nutritional requirements from symbiosis with the zooxanthellae.

Coral colonies are considered modular organisms because they consist of repeated morphological units, or polyps. The way these polyps are arranged dictates the different coral colony growth forms, or morphologies. Common descriptive terms include branching, columnar, massive, foliose, laminar, encrusting, and free-living. ref

Many clonal polyps on a branching Porites colony in Australia. Photo © Margaux Hein

Corals can reproduce through asexual and sexual reproduction. Asexual reproduction occurs through budding, where a coral polyp divides into clones forming two polyps, and fragmentation, where pieces of coral colonies break or detach and then resettle in a new location on the reef under suitable conditions. New and genetically unique coral colonies are formed through sexual reproduction. There are two modes of sexual reproduction: broadcast spawning (in which coral colonies release sperm and eggs in the water) and brooding (in which fertilization occurs internally).