My glamorous cousin went scuba diving and was kind enough to send me some pictures of the balloon fish. This friendly little fish puffs up when threatened into a little ball of spikes. http://www.youtube.com/watch?v=hrxmYVY1yo8&feature=related .
I wonder if the makers of Gremlins had a pictre of this fish on their inspiration board. Gizmo could definitely be a cousin.