These are Cypridina hilgendorfii. They belong to a class of organisms called ostracods - a type of crustacean. Too tiny to make a meal of, humans have nonetheless found a use for them. When you piss them off, they secrete a substance that glows a beautiful blue color. This species is native to Japan, and is locally called umi-hotaru, or sea firefly. They were used in WWII by Japanese soldiers to provide light to read maps and messages.
|C. hilgendorfii in action. The jar is from the 1940s and yet the chemicals remain active.|