On the occurrence of Riella affinis M. Howe & Underwood (Marchantiopsida, Sphaerocarpales) in the Sahara Desert (Africa)

A small population of Riella affinis M. Howe et Underwood, a rare aquatic liverwort species belonging to the subgenus Trabutiella, has been located in the Tassili n'Ajjer (Algeria, central Sahara desert). This is the first confirmed record of this disjunct tropical species from North Africa. The study of living material shows that its reproductive biology seems especially well adapted to stochastic habitats found in seasonal pools. Although it is likely to be of Gondwanic origin, the present distribution of R. affinis is, at least in some cases, best explained by events of dispersal favoured by strong winds. Its presence in the Sahara desert is probably a relic from biome shifts associated with wetter periods of the Holocene.