Giant salvinia, Salvinia molesta. A serious exotic, invasive pest species now widespread throughout the warmer regions of the US. It is a rootless, floating fern that can block sunlight and reduce dissolved oxygen levels. The "roots" are in reality modified leaves. Listed as a Federal Noxious Weed under the Plant Protection Act, which makes it illegal in the U.S. to import or transport between states without a permit. Photo Caddo Lake, TX.