Abstract
During recent decades, Althenia with two species in Eurasia and Africa was accepted as one of four genera of submerged aquatics comprising the family Zannichelliaceae. Molecular phylogenetic data confirmed monophyly of Zannichelliaceae in its traditional circumscription but placed the clade together with Potamogeton and its segregate genera, so that Zannichelliaceae is currently placed in synonymy of Potamogetonaceae. Recent molecular data demonstrated that Eurasian and African species traditionally placed in Althenia s.str. are nested within the Australasian (Australia and New Zealand) Lepilaena. As a result, the two genera were combined under the name Althenia s.l. The present study describes a new species, A. hearnii T. Macfarlane & D.D. Sokoloff from southwestern Western Australia which fills a morphological gap between species traditionally placed in Althenia s.str. and Lepilaena, thus supporting molecular phylogenetic evidence for lumping the two genera. The new species has a polysymmetric circular stigma like Althenia s.str. but 12-sporangiate anther like most species traditionally classified in Lepilaena. Like some other Australian species, Althenia hearnii is dioecious with dimorphic male and female inflorescences. The dimorphism includes the presence of a ligule in leaves within male inflorescences and the absence of a ligule in leaves within female inflorescences as well as strong differences in internode length between male and female inflorescences. A new combination Althenia patentifolia (E.L. Robertson) T. Macfarlane & D.D. Sokoloff is also made in order to allow Althenia to be fully applied in Australia. A revised key to the species of Althenia is provided.