Coral

There are two types of coral: hard corals and soft corals. Hard corals are the foundation – quite literally – of the Great Barrier Reef. Super tiny animals called polyps are the architects of hard corals. Polyps, along with other invertebrate sea creatures such as jellyfish and sea anemones, belong to the group cnidaria (pronounced “nahy-dair-ee-uh” – the “c” is silent). Cnidaria all possess stingers in the tentacles that surround their mouths, which, for polyps, come in handy while feeding. As small fish and plankton float by, a polyp reaches out its tentacles to catch its prey.

Image courtesy Courtesy NOAA/Photographer credit Mr. Mohammed Al Momany, Aqaba, Jordan Carnation coral (Dendronephthya)

Invertebrate means these animals have no spine. Polyps are delicate and highly vulnerable organisms. Similar to creatures that have exoskeletons for protection, nature has also provided an armor system for polyps. Polyps secrete calcium carbonate, which forms a hard external skeleton. The polyps live inside these skeletons, much like a snail lives within a shell. And, like a snail, the polyp can extend itself outside the skeleton, as it might do while feeding. To protect itself, it will retreat inside the hard structure if a predator is nearby.

Image courtesy Courtesy NOAA/Photographer credit Dr. James P. McVey, NOAA Sea Grant Program

Polyps live in large colonies – sometimes numbering in the thousands. During their life cycle, they thrive, form skeletal structures, breed new polyps and eventually die, leaving behind their stony skeletons. As other polyps grow and live, the cycle continues. All the while, the skeletal structures are built up around and upon each other. Like the thousands of stones, wood pieces and bricks that form the Great Wall of China, polyps and the skeletal structures they create help form massive coral reefs.

Yes, we said “help.” It turns out that while polyps and their coral skeletons do the majority of the reef building, there are some other helpers along the way. For example, some common sea-life sub-contractors are mollusks and tubeworms. Mollusks, like polyps, leave something of themselves behind – their shells.

Different polyps create a variety of structures in many shapes and sizes, including brain corals, fan corals, flat plate corals and table corals. If you’ve ever seen living coral, you know it comes in an array of vibrant colors. This comes from a type of algae called zooxanthellae, which has what is called a symbiotic relationship with the coral. This means that the two organisms involved provide something of benefit to the other. In colloquial terms, it is a “you scratch my back and I’ll scratch yours” relationship. In this case, the coral provides protection and access to sunlight to the algae. And, the algae shares food, nutrients and color with the coral. Coral structures that are left behind once a polyp dies are white. This is because the algae, like the polyp, are gone.

Crown of Thorns Currently, more than 20 percent of the world’s coral reefs are at risk of collapsing. A variety of dangers including natural predators, bleaching (when a large area of coral has died at once) and man-made pressures contribute to this risk. Although the Great Barrier Reef has thrived for thousands of years, that doesn’t mean that it is impervious to similar risks and predators. For example, one natural predator of the Great Barrier Reef is the crown-of-thorns starfish. Reportedly, one adult crown-of-thorns starfish can destroy as much as two and one-half square miles of living coral reef in one year. These red-hued starfish are covered in thorny spines. Besides posing a threat to coral, these starfish can be dangerous to divers, too. Puncture wounds inflicted by crown-of-thorns starfish are reportedly quite painful and can cause severe nausea.