noun
sea sparkle; Noctiluca scintillans
A bioluminescent dinoflagellate that causes the sea to glow at night. Often seen in warm coastal waters.
夏の夜、海岸で夜光虫が青く光るのを見た。
On a summer night, I saw sea sparkles glowing blue at the shore.
From 夜光 (yakō, 'noctilucent') + 虫 (chū, 'bug/insect'). The name describes its luminescent property.