Corals are carnivorous invertebrates that permanently attach themselves to the ocean floor. They are related to sea anemones, and have a base structure called a polyp. A coral polyp is a tiny soft-bodied organism that looks like a tube closed at one end. The open end serves as the polyp’s mouth and is surrounded by a ring of tentacles. Each tentacles contains stinging cells, nematocysts, that allows the polyp to capture small organisms that swim too close.
Coral polyps are naturally translucent, and many coral reefs that live in warm waters get their colors from algae they host called zooxanthellae. Zooxanthellae are plant-like organisms that reside within the coral’s tissue and have a symbiosis relationship with the coral. The coral benefits as the algae produce oxygen, removes wastes, and supply product from photosynthesis. Meanwhile the algae make use of the coral’s metabolic waste and the sun’s energy for photosynthesis. This symbiosis relationship is what allows the shallow water corals to grow and form structures we know as coral reefs.
Similar to jellyfishes, corals have multiple reproductive strategies and can reproduce either asexually or sexually. They use asexual reproduction to increase the size of the colonies; this can occur either through budding or fragmentation. Budding occurs when a new organism develops from an outgrowth or bud due to cell division at one particular site. The new polyp remains attached as it grows, separating from the parent only when it is mature. Fragmentation occurs when a piece of the parent coral is broken off, and the broken part of the colony establishes a new colony. Corals can also form new colonies by sexual reproduction. Sexual reproduction increases genetic diversity and starts new colonies far from the parent.
Corals which we are most familiar with are the so-called stony or hard corals, and they compose most tropical reefs. Their hard exterior is a result of polyps secreting a hard outer layer of calcium carbonate (limestone) that attaches to a rock or the dead skeleton of other polyps. These polyp colonies grow and die in an endless cycle over time and produce the shape of corals that we are familiar with. Additionally, because of this cycle of growth, death, and rebirth of individual polyps, coral colonies can live for a long time.
There are other types of corals, but the family tree of animals which we group corals in is complicated and would take a long time to explain thoroughly. Thus we will be sticking to stony corals which are the primary victim of coral bleaching.