I imagine that you must have heard of this, but it is too good to not pipe up with....the slipper shell snail! (which is the main study of one of the labs in my department). Lives in stacks attached to rocks and things. the ones on the top of the stack are male; the ones in the middle of the stack are intersex, and the ones on the bottom of the stack (which are also the oldest) are female. as the females age and die, and new males join the stack from the top, each snail ends up progressing from male to female.