RIELLA SPECIES
The genus Riella (Riellaceae, Sphaerocarpales) includes some 24 species of thalloid aquatic liverworts with unique morphological and ecological features among hepatics.
The thallus is composed of an axis and a unistratose, dorsal, flat, undulate or helicoid blade (generally referred to as the thallus wing), making the thallus bilaterally symmetrical in the plane of the wing. The thallus apex is usually falciform or circinate, especially in young individuals.
Antheridia develop in single rows along the margin of the wing, in its sinuses or rarely near the axis, whereas archegonia develop along the thallus axis. The sporophyte is enclosed within a sac-like, globose, gametophytic structure (involucre), as characteristic of the Order Sphaerocarpales (Figure 1). The spores are variously ornamented with spines, papillae, etc., and provide the most informative set of characters used for the identification of the species. Riella includes both monoicous and dioicous species (Figure 2).
These species are clasified within two subgenera of Riella (Porsild 1902) based on the morphology of the female involucres. Subgenus Riella includes taxa with smooth or papillose involucres whereas subgenus Trabutiella Porsild (1902: 327) includes taxa with lamellar longitudinal expansions (wings) on the female involucres (Figure 3).
a) Subgenus Trabutiella
Subgenus Trabutiella is less diversified with three to six taxa having been recognised therein depending on authors, some of which have been taxonomically debated (Segarra-Moragues et al. 2014). Includes taxa with longitudinal wings on female involucres.
- Riella affinis
- Riella heliospora
- Riella cossoniana
- Riella echinata
- Riella mediterranea
- Riella gamundiae
b) Subgenus Riella
Species of the subgenus Riella: includes taxa with smooth or papillose female involucres.
- Riella helicophylla (under review)
- Riella notarissi
- Riella choconensis (under review)
