August 11, 2020

Bioluminescent sea fireflies along the shore of Okayama, Japan

Glow with the flow

Scientists call the bioluminescent crustaceans washing over these rocks Vargula hilgendorfii, and here in Japan they're commonly known as umi-hotaru. They're visible at night in shallow sea waters but are only found along the coasts of southern Japan, although other bioluminescent species can be seen glowing in the waters of the Gulf of Mexico, the Caribbean Sea and off the USA’s Californian coast. The glow is a result of a chemical reaction in their bodies and may be a self-defence mechanism to ward off predators.

Although these tiny sea fireflies are about the size of a sesame seed, as a group they project an impressive aura. Their glow intensifies depending on how salty the water is and on other physical stimuli. As well as lighting up the shores at night, the crustaceans also do their bit to clean up the beach by eating sandworms and dead fish. That's one less thing for beachgoers to worry about during a glimmering seaside walk.