In coastal regions, the primary source of flow-agitated bioluminescence is dinoflagellates. These single-celled organisms are common members of the plankton—tiny marine plants, animals or bacteria that float on or near the ocean’s surface. Bioluminescent dinoflagellates range in size from about 30 µm to 1 mm, and are found in all the world’s oceans. Occasionally they are found in high concentrations, resulting in red tides, so called because the high abundance of organisms discolors the water. If the dinoflagellates are luminescent, there can be spectacular displays of bioluminescence at night. For example, in October 2011 there was an intense red tide of the dinoflagellate Lingulodinium polyedrum in San Diego leading to some amazing video footage.
Red tides are usually hard to predict and don't last too long. But in some areas, such as the so-called bioluminescent bays of Puerto Rico, brilliant bioluminescence persists throughout the year and makes for an amazing personal experience as you kayak or swim through the warm water.
Laboratory experiments have provided insight into the role that dinoflagellate bioluminescence plays in marine ecology. Dinoflagellate flashes cause a startle response in their predators, disrupting their feeding behavior and resulting in a decrease in grazing rate by reducing the number of dinoflagellates consumed. Dinoflagellate bioluminescence is also thought to act as a “burglar alarm” to attract a secondary predator that threatens to eat the primary predator. When handled by a predator, the dinoflagellate cell is triggered to flash by the imparted mechanical stress. But any mechanical stress of sufficient magnitude, such as the forces in waves, surge, or by swimming animals (including us) can also trigger the luminescence.
The cellular regulation of dinoflagellate bioluminescence is complex and only partially understood, but the luminescent chemistry is ultimately caused by a drop in pH due to an influx of protons within the cell. The time from stimulus to light emission is less than 20 ms, making it one of the most rapid cellular processes known. Each dinoflagellate cell can produce more than one flash, which lasts on the order of 100 ms. However, the bioluminescence capacity of a cell is depleted once all its available luciferin has been oxidized. But during the next day it can recharge the chemicals to flash again the following night.