This is showing my age, but when the first chapter actually said Koto-Hono-Umi, and again when Mezashi's chapter featured them — well, every time the second-years show up as a triad, I think delightedly, "KonoHonoUmi!" and then immediately a Cartoon Network promo from back when they were 90% old Hannah-Barbera reruns gets stuck in my head:
https://youtu.be/nEc4gDFHEBw