Rediscovery of Althenia (Potamogetonaceae) in Portugal, 168 years after the last collection

Althenia (Potamogetonaceae) is a small genus of aquatic angiosperms from saline environments that includes the most recently evolved seagrass, Althenia marina. One or two species occur in Europe, both rare and considered Data Deficient at the regional level. We report the first records of Althenia in Portugal since 1853: two populations in the Algarve and a third in central Portugal. As in previous studies, all Portuguese populations include plants with and without visible nerves in the leaf sheaths, a character used to distinguish between A. orientalis and A. filiformis. We tentatively recognize a single Althenia species in Eurasia, pending further studies. We recommend further searches of suitable habitat, as Althenia species are small and easily overlooked, as well as monitoring and safeguarding of known populations through in situ and ex situ conservation.