When they light up, it's as if someone has flicked on a switch — the sea suddenly fills with tiny blue orbs.
These glowing lifeforms are umi-hotaru (sea fireflies). But though they're called "fireflies" they're unlike their terrestrial cousins. These are ostracods — a kind of micro-crustacean known as "seed shrimp" that emit light using the organic compound luciferase.
In the daylight, umi-hotaru look like plankton or minute chia seeds floating in the water, but at night you can see the luminescent chemicals activating in their light organ. The glow is strong enough that Japan collected umi-hotaru during World War II and ground them into a light-emitting powder to help soldiers check maps at night.