Ultra-sun: The sand that accumulates inside its shell eventually becomes a pearl. But the pearl gets in the way, so it spits it out and discards it.Moon: This Pokémon’s tongue is always hanging out. It uses its tongue with great dexterity to dig up sand from the seabed in its search for food.Alpha-sapphire: At night, this Pokémon uses its broad tongue to burrow a hole in the seafloor sand and then sleep in it. While it is sleeping, Shellder closes its shell, but leaves its tongue hanging out.
Prof. Lotus' Notes: When Shellder clamps on to a Slowpoke tail, they both evolve into a Slowbro. When it does the same on the head, Slowpoke will evolve into Slowking. Slowking is considered one of the most intelligent creatures in the Pokemon universe. This further strenthens the idea that they have a symbiotic relationship. Cloyster's shiny form is Golden in colour I believe this is because of Cloyster's ties to luxury. You'll find a lot of Golden Oyster Shell trinkets.