Why Limnobium is now Hydrocharis
Hydrocharis, commonly called Frogbit, are unique aquatic plants as they only develop floating leaves rather than submersed leaves. They typically have round to heart-shaped leaves and small white flowers. Frogbit species thrive in calm freshwater environments, where their fine roots absorb nutrients from the water.
This small genus of aquatic plants has a wide distribution across tropical and temperate regions of the world. Originally this genus was comprised of just three species: Hydrocharis chevalieri endemic to central Africa, H. morsus-ranae of Europe and H. dubia native to Asia. Two plants from the America's were, until recently, not included in the Hydrocharis genus - instead called Limnobium laevigatum and Limnobium spongia.
Recent research has shed light on the evolutionary history within Hydrocharis and Limnobium to better understand their relationships. By analysing their DNA, particularly in their plastomes, it was discovered that Limnobium laevigatum fits well within the Hydrocharis group, supporting the idea of merging Limnobium with Hydrocharis. One species, the African endemic H. chevalieri, appears quite distinct from the others and might be moved to a separate genus down the track.
In light of this recent phylogenomic analysis, the formerly distinct Limnobium laevigatum, as well as its close relative Limnobium spongia, have been reclassified as Hydrocharis laevigata and Hydrocharis spongia. This reclassification is supported by both morphological similarities and genetic evidence, with a particularly close relationship between Hydrocharis laevigata and Hydrocharis dubia. The placement of Hydrocharis laevigata within the Hydrocharis genus underscores its evolutionary proximity to other Frogbit species and highlights the relationship between these aquatic plants.
Hydrocharis originated in Europe and Central Africa, diversifying during the Miocene epoch - as some Hydrocharis species ended up in America, this indicates they dispersed through the Bering Land Bridge that once connected Asia and America during the Miocene.