Riccia fluitans is a liverwort (phylum Marchantiophyta), one of three evolutionary lines of small, non-vascular “bryophyte” plants (those that lack water conducting cells, reproduce by alternating generations, and conduct photosynthesis in the haploid generation). The mosses (Bryophyta) and the hornworts (Anthocerotophyta) complete the three groups of bryophytes.
Riccia fluitans is the only truly aquatic liverwort in Florida. Plants have been collected from scattered locales in the northern, central, and southern regions. Unlike most other liverworts, it floats freely in tangled clumps or mats below the water surface. The plants are thallose (not differentiated into stem and leaf but green flattened strands), several cells thick, that branch dichotomously (by open forking) many times. Each strand is notched at the tip. Strands of Riccia fluitans measure less than a millimeter wide and lack functional roots. Rhizoids – thin colorless, single-celled strands – have little function as roots, but may help anchor thalli to the mud during water recession. Very rarely, if ever, is Riccia fluitans found with reproductive features.
Unlike most aquatic byrophytes which are biological indicators of clean, well-oxygenated and fast-flowing aquatic systems, Riccia flutians is common to turbid, slow-moving streams and ponds, including those affected by eutrophication. This species occurs in all major continents of the world and is recognized for its aromatic volatile oils which, stored in specialized organelles, may protect against predation.