Common salvinia, also known as water spangles; Salvinia minima. It's odd, lantern-shaped hairs prevent the leaves from becoming wet by causing water to bead on the surface of the hairs. This highly specialized aquatic fern is native to Central and South America, but has been introduced widely. In the US, Bermuda and Puerto Rico it is considered a potentially invasive species, but not as serious a threat as the larger, related Salvinia molesta. Photo taken in San Juan, Puerto Rico.