by Jennifer Zoon
In the wee hours of the morning, just off the shores of Puerto Rico, invertebrate keeper Mike Henley scours the seafloor for an animal too small to be seen with the naked eye. A broken, hand-sized branch of elkhorn coral (Acropora palmata) materializes in the darkness, dangling over the edge of a cavern. He delicately scoops it up and returns to shore with his find. Hiding inside is a tiny creature that is critical to coral conservation: a golden-brown alga named zooxanthella.
Hours earlier, Henley had spent part of the evening capturing elkhorn coral gametes. But parent corals send their babies off without zooxanthellae—an essential part of their care package. “Zooxanthellae provide each coral with 90 percent of its food,” explains Henley. “Without them, coral would never survive.”
Zooxanthella mimics a plant, but it is actually a single-celled animal. When this alga reaches maturity, it sprouts two propulsion limbs, called flagella, and searches for a place to live. It makes itself at home on a coral, jellyfish, or anemone and settles in the stomach or skin of its host. Zooxanthella and its cousin, plankton, are among the most abundant—and most consumed—animals in the ocean.
Corals eat zooxanthellae but don’t digest them. Instead, they snap up the one-celled organisms’ byproducts, turning them into calcium carbonate (for its hard skeleton), proteins, carbohydrates, and even fat. In return, zooxanthellae receive carbon dioxide—an essential ingredient for photosynthesis —and protection from predators. It’s a symbiotic relationship —both species win.
Henley has a specific process for collecting zooxanthellae. At a beach lab, he uses a water pick to blast the coral branch with seawater. The coral strips away like a banana peel, shedding layers and even chunks of flesh. When little but bare bones remains, he places the coral in a spinning, water-filled tub called a centrifuge, which breaks up the coral. As the skeleton sinks to the bottom, zooxanthellae cling together and rise to the surface, where Henley gathers them in vials.
Zooxanthellae contain photosynthetic pigments that give corals their vibrant colors. Unfortunately, the type of zooxanthellae that elkhorn consumes easily succumb to stress. Exposure to air during low tides, too much or too little salt in the water, too much sun, fertilizer runoff, and even the clarity of the surf can disturb the sensitive animals.
A week after capturing coral gametes in Puerto Rico, Henley returns to the Invertebrate Exhibit’s lab at the National Zoo and places his catch in a settling tank. After a week of growth, the gametes have morphed into wormlike larvae that crawl along the tiles. Those that settle anchor themselves and transform again, this time into the familiar polyp shape. Only then are zooxanthellae added. As they swim about, the tentacles of stealthy polyps snatch them out of the water.
“Under a microscope, you can see these little green zooxanthellae moving around in the polyp’s stomach,” Henley marvels. “It’s really amazing!”
-- JENNIFER ZOON is an editorial intern for Smithsonian Zoogoer.