Resilient Corals

Pat drills into a coral colony on a previous expedition to Bermuda.
Photo: T. DeCarlo

Press the trigger. The drill screams, the bit whirls, air bubbles erupt. The pitch drops as the bit digs into the coral. Coral tissue spews, and the fish swoop in for a snack. It all happens quick as we begin drilling a core of coral skeleton.

As SCUBA divers, we are always so careful to not touch the coral. They are delicate organisms. An accidental kick with a fin can cause some real damage.

Yet, the corals always recover from our drilling. We drill into a relatively small area on the surface of the colony, careful not to damage the rest of the colony. A coral is made of many thousands of coral polyps, which together form a colony. Each polyp is a separate animal, which looks like a mouth with tentacles waving around searching the water for plankton to munch on. But all of the polyps in a colony are clones, genetically
identical.

Pat pulls a skeleton core from a colony. The core is white

because it is only the former skeleton, not the living tissue.
Only the very top of the core is living coral. Photo: A. Cohen    

Only the very outer surface – the width of your little finger or less – of a coral is living. Underneath this thin veneer of life, all that remains is the former coral skeleton. The polyps constantly build new skeleton on top of old, growing outward. The older skeleton becomes buried within the colony, left behind by polyps that have built the colony taller and taller.

After we remove our skeleton core, we fill the hole with a cement plug and secure it with nontoxic underwater epoxy. This serves two purposes. Plenty of reef creatures – like octopus, urchins, and some fish – would find that our drill holes make perfect homes. Our cement plugs not only keep these creatures out, but they also provide a surface for the coral to grow over.

Come back a few months later and the cement will be only partly visible. New coral polyps cover the outer edges. Give it a year, and the plug is invisible, buried beneath the living surface of the colony. 

A coral colony recovering from drilling. The cement plug

on the right was emplaced 6 months prior to the photo

and is partly grown over. The cement plug on the left

is freshly epoxied to the colony surface. Photo: A. Cohen

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Tom DeCarlo

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