This is a really interesting question. Natural sponges are phylum. Some phylum are plant, some animal, it's a very interesting category that is between classifications. In this case, a sponge is an animal phylum. However a natural sponge has no tissues, organs, body symmetry, mouth, heart, nervous system or brain. Without a nervous system, our understanding of existing sentience (and pain perception) is extremely limited. The reason they are classed as animals is mostly because they create sperm-like cells. The natural kingdom is so complex! I suppose it's a bit like asking about yeast - albeit a micro-organism. If I were you I'd read about the functions of a sponge and make up your own mind - or to consider which use is better for the plant (an artificial sponge or a natural sponge).