Plants 0.6--10 cm, branched sparingly, bright green. Shoot apex falciform to circinate. Oil cells reduced, with 1 oil body per cell. Leaf scales dimorphic, in 2 lateral and 2 ventral rows along the axis. Specialized asexual reproduction frequent, gemmae multicellular, ventral on the axis. Antheridia marginal along the wing, sunken, single in involucres. Archegonia alternating left and right along the wing axis, single, in subglobose, pyriform to flask-like involucres. Sporophyte seta abbreviated, not elongating; capsule wall 1-stratose; elaters absent but with nurse cells interspersed with immature tetrads. Spores large, dissociated at maturity.
Species 17 (2 in the flora): submerged aquatic in semiarid and arid regions, nearly worldwide, but disjunct, sporadic and local throughout its range; w and sw North America, Mexico, South America, Eurasia, Africa, Australia.
Riella is unique among hepatics in being a submerged aquatic in fresh or brackish water of temporary pools or streams in arid and semiarid regions or rarely in permanent bodies of water. The gametophyte is intolerant of desiccation and capable of surviving for only a short time after water level subsidence. The viability of the genus is insured by spores, which are usually produced a few weeks before the water level subsides. The spores are adapted to survive many years of desiccation and still remain viable, e.g., up to 3 years in R. americana and 13 years in R. capensis (R. A. Studhalter 1932). Riella, then, is an annual hydrophyte that is capable of completing its entire life cycle in less than 3 months. Locating Riella populations is often difficult, as gametophytes are ephemeral and submerged. In fact, many populations of Riella have been discovered accidentally when Riella spores germinated from mud samples that had been collected for scientific purposes unrelated to bryology.
Very few species of Riella exhibit distinguishing vegetative characters. Therefore, taxonomic differentiation within the genus is primarily based on spore and involucre morphologies. Two subgenera have been defined based on involucre characters; Riella subg. Trabutiella Porsild. which has female involucres that bear longitudinally oriented wings (Porsild 1902) and Riella subg. Riella which has smooth, wingless female involucres. The North American species R. americana and R. affinis belong to the subgenera Riella and Trabutiella, respectively.
Perold, S. M. 2000. Studies in the Sphaerocarpales (Hepaticae) from southern Africa. 3. The genus Riella and its local species. Bothalia 30: 125--142. Porsild, M. P. 1902. Sur une nouvelle espèce de Riella (subgen. nov.: Trabutiella) de l’Asie centrale. Bot. Tidsskr. 24: 323--327. Proctor, V. W. 1972. The genus Riella in North and South America: distribution, culture and reproductive isolation. Bryologist 75: 281--289. Thompson, R.H. 1940. A second species of Riella in North America. Bryologist 43: 110--111. Thompson, R.H. 1941. Morphology of Riella affinis. I. Germination of the spore and development of the thallus. Am. J. Bot. 28: 845--855. Studhalter, R. and M.E. Cox. 1941. The lateral leaf scale of Riella americana. Bryologist 44: 19--27. Studhalter, R. and M.E. Cox. 1941. The ventral scale of Riella americana. Bryologist 44: 29--40.