Rigid Hornwort
Ceratophyllum demersum is a species of Ceratophyllum.
It is a submerged, free-floating aquatic plant, with a cosmopolitan distribution, native to all continents except Antarctica.
It grows in lakes, ponds, and quiet streams with summer water temperatures of 15-30 °C and a rich nutrient status. In North America, it occurs in the entire US and Canada, except Newfoundland.
It excretes substances that inhibit the growth of phytoplankton and cyanobacteria (blue-green algae). Its dense growth can outcompete other underwater vegetation, leading to loss of biodiversity.
It is also a popular aquarium plant.