The algal endosymbiont is a green alga from the genus Nephroselmis. Unlike a fully integrated organelle, the Nephroselmis alga does not divide along with the host cell. When the host cell divides, one of the daughter cells receives the Nephroselmis cell and the other daughter returns to a heterotrophic lifestyle. The latter behaves like a predator until it ingests a green alga. The alga then loses its flagella and cytoskeleton, while the Hatena, now a host, switches to photosynthetic nutrition, gains the ability to move towards light and loses its feeding apparatus.
The name is from a Japanese interjection meaning roughly "enigma", or "how odd".