This is the seed of a red mangrove (Rhizophora mangle) (there are red, black, white and buttonwood varieties in the Everglades.) It starts out pretty small, and that bit hanging down grows to 12 inches or so. Then the whole thing falls into the water and floats along. That bulb up there at the top soaks up water until the bulb sinks and the whole thing floats upside down. When it hits mud, specialized cells in those 'petals' you see at the top grip the muck and the plant starts growing.