Identifying corals on a reef is one of the more fascinating pursuits a diver can undertake. Although they are commonly perceived as rocks or plants by the uninitiated, corals are actually made up of small invertebrate animals called polyps. Hard coral polyps secrete calcium carbonate (limestone) to create their exoskeleton, lending to the branching appearance of these organisms. As each generation of polyps dies, their exoskeleton remains, and because each polyp is so small, hard corals grow at a very slow rate. Let’s take a look at a few kinds of hard corals that are commonly found around the world.
Staghorn Coral
Staghorn coral (Acropora cervicornis) is a branching coral with cylindrical branches ranging from a few centimeters in length to over 6.5 feet. This species exhibits the fastest growth of all known western Atlantic corals, with branches increasing in length by 4-8 inches per year. It is one of the three most important Caribbean corals in terms of its contribution to reef growth and fish habitat.
Table Coral
Table coral (Acropora pulchra) is the same branching type of coral as staghorn coral, but it manifests in broad, flat plates. This shape allows table coral to expose a great deal of its surface area to sunlight, which is vital in keeping the polyps fed and healthy. Although it does feed on plankton, the majority of its sustenance comes from the symbiotic algae living within its tissues.
Brain Coral
Brain coral (Platygyra daedalea) is so named because of its striking resemblance to a human or animal brain. This species has an incredible life span of up to 900 years, and colonies can grow up to three feet in diameter and six feet in height. Although they usually manifest in rounded mounds that look like brains, their shape, and form can vary widely from place to place.
Blue Coral
Blue coral (Heliopora coerulea) derives its name from its distinctive blue skeleton, which takes on the hues of the blue or blue-grey polyps living within tubes in its skeleton. This species is columnar, branching, or plating and typically occurs in shallow tropical waters where reefs are exposed to waves, intertidal zones, and reef flats. It is listed as ‘vulnerable’ by the IUCN.
Pillar Coral
Pillar coral (Dendrogyra cylindricus) builds up toward the surface from the substrate, but does not develop any secondary branching. They can grow up to 8 feet tall, on both flat and sloping sea floors at a depths of 3 to 65 feet. They are one of the few types of hard coral whose polyps can often be seen feeding during the day.
Great Star Coral
Great star coral (Montastraea cavernosa) is a colonial stony coral found in the Caribbean Sea that forms immense boulders and domes, covering as much as 5 feet of shallow sea floor with a single colony. It can be found growing in plate formation at depths ranging from 40 to 100 feet, and is the reigning coral species at these depths. The polyps of the excellent star coral are as large as a human thumb and extend entirely at night to feed.
Elkhorn Coral
Elkhorn coral (Acropora palmata) has long been one of the most important reef-building species in the Caribbean. Once the most abundant coral species in the Caribbean, threats like bleaching, predation, disease, and human activity have dealt a massive blow to its population. Fortunately, this species is also one of the fastest-growing corals on the reef, with the ability to grow as much as four inches in length annually and eventually reach a size of up to twelve feet in diameter!
Sun Coral
Sun coral (Tubastraea) is another stony coral that gets its name from its appearance. When the polyps are fully protruding, they look like little depictions of the sun. Although it’s a hard coral species, it is not of the reef-building type. Rather than deriving energy from photosynthetic symbiotic algae, the nocturnal polyps feed solely on plankton.
