The Reef Builders

by S. F.

Primary reef builders live a Jekyll-and-Hyde existence: benign plants by day, indiscriminate plankton feeders by night.

The Dr. Jekyll component consists of thousands of single-celled zooxanthellae algae that are embedded within the living tissue of a coral polyp, an animal no bigger than the nail of a pinky finger. By day, when the animal element recedes deep into its limestone stronghold, its algal partner rises to the surface to bathe in sunlight.

By night, the animal withdraws the algae; a stinging, multitentacled mouth emerges that feeds on any tiny planktonic animal. The union between plant and animal makes the coral polyp a miniature, self-sustaining ecosystem. The polyp generates waste carbon dioxide and ammonia, which nourishes the algal component. The algae, in return, donate waste oxygen to the polyp and manufacture organic compounds and complex sugars that are absorbed by the coral tissues as food.

In a process that has been carefully studied but is not fully understood, the partnership captures calcium carbonate from seawater and converts it into hard limestone. The polyp secretes this skeleton continuously, remaining on the surface as the reef mass grows.

The living tissue of the polyp extends from its home to join with neighboring polyps. Thus the reef*s entire surface is covered with a living mat of tissue.

Ocean Planet Exhibition Floorplan

gene carl feldman ( (301) 286-9428
Judith Gradwohl, Smithsonian Institution (Curator/Ocean Planet)