Bioluminescent plankton in the waves at Littlestone on Sea, near Dungeness, Kent, June 2021, at night. The Milky Way core over Dungeness point and the power station.

Most bioluminescent zooplankton don’t glow in the dark themselves. Instead, they squirt globs of glowing chemicals into the water.

Many zooplankton use these glowing gobs of goo as a defense against predators. A fish that’s trying to catch a copepod will be confused by the bright flash of light; it will try to attack the bioluminescent goo instead. That gives the copepod time to escape.

Some zooplankton also use bioluminescence to attract a mate.