I don't think they "die" I believe the portion of the sponge that is exposed to air will die, and if that area is too great it can cause the rest of the sponge to die. I have read posts about people getting sponges from the LFS where the LFS took the sponge completely out of the water, causing certain pieces to die off, but the hobbyist removed the dead areas with an xacto knife and the sponge recovers.