Sponges are primitive multicellular animals that live in water. All adult sponges are sessile (fixed to one spot), most being attached to hard surfaces such as rocks, corals, or shells. More than 4,500 living species are known. Although some species occur in freshwater, the vast majority are marine, living mainly in shallow tropical waters. Sponges have an amazing power of regeneration: they are capable of growing into a new individual from even the tiniest fragment of the original body.