Paddling our kayaks into the darkness of Mosquito Bay, we felt skeptical.

“I don’t see anything,” I said to my traveling companion. 

“Me neither,” he replied, squinting into the black water. 

“Just wait,” our guide said. “There’s still some light in the sky. Let’s get a little farther out.”

As we obediently followed, something flickered beneath the water’s surface. Then, like magic, they appeared—tiny, blue-green sparkles, a few at first, then thousands twinkling in the wake of our boats and the slice of our paddles. When we reached the middle of the Bay, we climbed out of our kayaks and splashed into the water, cloaking our bodies in shimmering light. Snapped fingers ignited a burst of electric blue, and splashing limbs seemed to radiate stardust. When I lifted my arm out of the water, jeweled droplets slid down my skin. It felt like we were swimming among the stars.

Photo by Steph Goralnick (www.sgoralnick.com )

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The sparkles weren’t magic, they were an example of bioluminescence, the production of light by living creatures. Our glowing companions were Pyrodinium bahamense, a type of single-celled algae called dinoflagellates. These microscopic creatures feed themselves via photosynthesis, and move through the water using a pair of long, tail-like flagellums. In Mosquito Bay (commonly called the bio bay), on the island of Vieques east of Puerto Rico, there are estimated to be between 500,000 and 750,000 dinoflagellates per cubic gallon of water—that’s about 6,000 per tablespoon!

Such high concentrations create a vivid underwater light show, which results from a simple reaction occurring inside the organism: the chemicals luciferin and luciferase reacting with oxygen to generate light. Such bioluminescence is much more common in oceans than on land, and creatures such as insects, bacteria, jellyfish, squids, fish, and worms can generate their own light. For the dinoflagellates, any physical disturbance (waves, a rain storm, our kayaks’ paddles) will trigger the reaction. Each resulting flash lasts only a moment, but produces a light one hundred times the creature’s size! By comparison, if humans could do the same our glow would reach to the top of the Washington Monument.

It might seem dangerous to spotlight your tiny presence in a vast, dangerous ocean. But in fact, researchers believe the reasons behind bioluminescence are just the opposite. For dinoflagellates like those in Mosquito Bay, studies suggest that bioluminescence is a way to frighten off predators. Other organisms employ bioluminescent for similar “predator alarms,” and may also use their light to lure prey, attract mates, and to navigate.

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