Coral Biology

Corals are invertebrate animals belonging to the phylum Cnidaria (along with the jellyfishes and sea anemones), with stinging cells (or cnidocytes), as a distinguishing characteristic. Their basic body plan and shape are quite simple ― a vase-like sac (known as a polyp) with a hole in one end (the mouth), surrounded by stinging tentacles that can capture food. During feeding, a coral polyp will extend its tentacles out from its body and wave them in the water current, where they encounter small plankton or other food particles and capture prey with their tentacles’ stinging cells.

Coral polyps can extract abundant calcium from surrounding seawater and use this to create a hardened structure for protection and growth. Coral reefs are created by millions of tiny polyps forming large carbonate structures, and are the basis of a framework and home for hundreds of thousands, if not millions, of other species. Coral reefs form the largest biological structures on earth; they are only ones distinctly visible from space.

The coral reefs as we know them today have evolved on earth over the past 200 to 300 million years. Over this history, corals have developed a highly evolved form of symbiosis, or a mutually-beneficial living arrangement between coral animals and tiny single-celled plants, known as zooxanthellae. Inside the tissues of each coral polyp live these microscopic, single-celled algae. These two groups ―animal and plant ― share space, gas exchange and nutrients to survive.

Therefore, while corals are indeed animals, they functionally act like plants in many respects, explaining why reef-building corals are confined to living so near the surface of the water where they are easily reached by sunlight. The symbiosis between plant and animal also underlies the colors of corals, so appealing to divers on a reef. Light is important in driving corals to compete for space on the sea floor, and so constantly pushes the limits of their physiological tolerances in a competitive environment among so many different species. However, it also makes corals highly susceptible to environmental stress.

Many coral species reproduce once or twice each year. Most of coral species spawn by releasing eggs and sperm into the water, but the period of spawning varies from one species to another. Other coral species, with limited distribution, are brooders. A fertilized egg forms a larva known as a planula. This baby coral looks like a little tiny jellyfish and floats around near the surface at first, and then lower in the water column until it finds a suitable space to call home – usually a hard surface to which it can attach.

In general, massive corals tend to grow slowly, increasing in size from 0.5 cm to 2 cm per year. However, under favorable conditions (high light exposure, consistent temperature, moderate wave action), some species can grow as much as 4.5 cm per year. In contrast to the massive species, branching colonies tend to grow much faster, and under favorable conditions, these colonies can grow vertically by as much as 10 cm per year.