sea sparkle; Noctiluca scintillans
A bioluminescent dinoflagellate that causes the sea to glow at night. Often seen in warm coastal waters.
On a summer night, I saw sea sparkles glowing blue at the shore.
Katakana spelling, common in biological contexts or for emphasis.
From 夜光 (yakō, 'noctilucent') + 虫 (chū, 'bug/insect'). The name describes its luminescent property.