(I apologize ahead of time for any misspelled, misplaced or mixed up Pokemon names.)
There is an episode where Ash Brock and Misty are with a Professor Westwood (I forgot the title). During the episode, the professor is trying to answer two questions:
1) Why does a Shellder clamp onto a Slopoke, causing it to evolve to Slobro?
2) Why does the shape of Shellder's shell change from a clam-style to spiral shaped when it does clamp onto Slopoke?
Near the end of the episode, the professor discovers and explains why Shellder clamps onto Slopoke's tail. However, he still hasn't found out why Shellder changes shape, yet no one mentions that.