This is not at all what I study, but it seems possible to me that sponges have evolved a tolerance for low oxygen conditions in the billion or so years since they first arose. It's a common mistake to think that because something is morphologically very similar to an ancient form (a so called "living fossil"), that it hasn't undergone any evolution. As long as it has been reproducing and mutating, better adapted versions have always had differential success, hence they've evolved, even if the shape hasn't changed.