Introduction to Hard Corals of the Indo-Pacific

Although corals are the foundation of the marine environment, divers often overlook coral diversity. We tend to lump many coral species together under the term “coral reef.” There are nearly 670 species of reef-building corals in the Indo-Pacific, however. The epicenter lies in the aptly named Coral Triangle, located in the coral triangle between the Philippines, Solomon Islands and Indonesia.

In this series we’ll help you identify seven different coral-growth forms and some common genera of coral in each group. If you’re new to coral identification, look at our Coral Biology: Part I and Coral Biology: Part II articles. These will help you learn the basics of coral biology, as well as some helpful terms for identification.

Hard-coral construction

Hard corals, as the name implies, are corals which build a hard calcium-carbonate skeleton. Each coral colony is comprised of an animal called a polyp. As the polyp grows, it creates a protective calcium-carbonate skeleton around its body called a corallite, which becomes the polyps’ home. As the skeleton builds up, it becomes what we call the coral reef.

Polyps are either colonial, with hundreds of polyps making up a single coral colony, or solitary, with a single large polyp surrounded by a hard skeleton. Polyps can also be tiny. The smallest is less than .03-inch (1 mm) across, while the largest polyp can grow up to 12 inches (30 cm) across.

You’ll find hard corals in sunny, tropical waters because they need light to survive. Zooxanthellae, a photosynthetic marine algae, lives inside the corals’ tissue. The coral provides a home and protection for the algae and, in exchange, these algae absorb sunlight and produce energy for the polyp.

Coral bleaching

Polyps can also get energy from ingesting plankton and food particles. But, since they’re fixed in place, their energy from food consumption is limited. Coral polyps rely on sunlight to survive. Without the energy created by the zooxanthellae, they will die.

Coral bleaching happens when too much light or elevated water temperatures stress the polyps. In response, the stressed polyp expels the zooxanthellae, which gives the coral its color. When the zooxanthellae disappear, corals bleach, turning stark white. If water temperatures don’t fall quickly enough, the polyp will die without this important symbiont.

Most corals are made up of hundreds to thousands of tiny, identical polyps. However, there is one growth of corals which remain a single polyp like the Acanthophyllia above, which is over 10 inches wide.

Plating corals can grow in thin, overlapping tiers. Always swim well above these corals to avoid damaging the delicate plates.

These corals are commonly known as brain corals — for obvious reasons.

While the coral photographed above is massive in size, the growth form “massive” refers to any corals that form large boulders. Massive coral must start somewhere, so even a coral the size of a baseball can be considered massive if it falls into the species category.

As the name implies, encrusting coral encrusts along rocks in a thin sheet that can help stabilize and cement loose substrate together.

Branching corals can have long, dispersed branches or short compact branches.

Branching corals can have long, dispersed branches or short compact branches.

An Acropora branch showing axial and radial corallites with polyps.

Coral growth forms

Recognizing hard-coral growth forms is one of the most effective ways to narrow down genus and species. Corals grow in several unique forms, including branching, encrusting, plating, meandering, massive, and solitary. Within these groups there are large and small polyp types, as well as various corallite arrangements. Identifying coral growth forms also goes a long way to knowing what you’re looking at. Doing so can enrich your diving experience immensely. Stay tuned as we begin the series by examining branching corals of the Indo-Pacific.