any of a genus (Noctiluca) of large, spherical, reddish, luminescent dinoflagellates that often occur in vast numbers in the sea, causing the water to glow at night
See noctiluca in American Heritage Dictionary 4
(nŏkˌtə-lo͞oˈkə)
noun
Any of various bioluminescent dinoflagellates of the genus Noctiluca that when grouped in large numbers make the sea phosphorescent.