For some unknown reason, Clam disappeared from the show after Season 1. He possibly made a cameo in the Season 2 episode Bubble-Cadabra! in the pop song "Abra-Cadabra". There was a clam in the song, but it is uncertain whether it was him or not.
In Season 2, the fish tank that he lives in is empty.
In Season 3, his fish tank is replaced with a globe.