Have you ever noticed in the begining of "Help Wanted" that Spongebob has a clam in a cage? This means he has another pet! How can this be? Spongebob never notices him/her! He/She also made appearences in "Missing Identity" and "New Digs". I don't know if he/she made any other appearences.

Maybe it's a picture or something.

Trust me it's not a picture.

It's an occassional easter egg they use in the show. On the Complete 1st Season DVD special features, there's an interactive tour where Patchy (if I can remember) revealed that its name is Shelly.