Nagi-Asu: A Lull in the Sea Season 1 Episode 3

Akari was rounded up by the older men of the village and Hikari and the others try to stop them. It's there that they first hear the rule that if you are seeing someone from the surface, you will be banished from the village. Hikari, unable to contain his rage, holds a guys-only meeting with Kaname. The girls are thrown out and go to another room, where Manaka finds a red-bellied sea slug. The sea slug is said to tell you the future if you tell it feelings you can't tell anyone else. It's then that Chisaki asks Manaka if she likes Tsumugu-kun...