Like fireflies, seas can glow in the dark. Certain bodies of water in the world—such as Altona Lagoon in St. Croix, a U.S. Virgin Island—can emit a sharp, bright blue color when disturbed. These bodies of water emit a glowing blue color due to their high concentration of microorganisms called dinoflagellates. These microorganisms are phytoplankton and often float to the surface during the day to absorb sunlight, with the highest concentration remaining at the surface around the time of full moons, and return to the floor during the night to absorb nutrients (performing photosynthesis during the day).

The phytoplankton produce light through a chemical reaction triggered by physical disturbance. When disturbed, oxygen combines with a compound called luciferin in the presence of an enzyme named luciferase, which creates a quick and flashing glow inside the phytoplankton’s special organelles called scintillons. They have this reaction as an evolutionary trait in hopes of scaring predators away, but this trait has also had the side effect of interfering with military stealth, specifically during World War I.

The phytoplankton typically concentrate themselves in warmer bodies of water. The bodies of water typically have excess nutrients—which can cause eutrophication, which is when a body of water is deprived of dissolved oxygen and typically leads to the inability to sustain marine life—caused by agricultural runoff or natural upwelling. However, certain species of marine life, such as jellyfish, are typically found in these areas. The jellyfish typically undergo the same chemical reaction to produce light, but can use that for mating or attracting prey.

Bioluminescence is common within deep-sea creatures as well, with some estimating that nearly 76% to 90% of deep-sea creatures are capable of producing their own light, rendering it essential to their survival. Some even use this bioluminescence as a weapon: the vampire squid, for example, ejects bioluminescent mucus rather than ink to startle and confuse predators while it escapes. Other sea creatures use bioluminescence to attract prey, similar to the jellyfish, such as anglerfish (horrifying creatures; I invite you to look them up or rewatch Finding Nemo ;).

All in all, bioluminescence is an interesting natural occurrence that developed through evolution. If you ever get the chance, visit a bioluminescent lake. It requires decent planning to visit during times when there are peak numbers of phytoplankton—but if timed right, it’s a sight to behold!