Salvinia, commonly called watermosses, is a genus of free-floating aquatic ferns in the family Salviniaceae, order Salviniales. Around twelve species are recognized, distributed across tropical and subtropical regions of North America, Mexico, the West Indies, Central America, South America, Eurasia, Africa (including Madagascar), and South Borneo.
These small plants lack true roots. Instead they float on the water surface with creeping, branched stems bearing hairs on leaf-surface papillae. Leaves arise in trimerous whorls: two are green, flat, and floating, while the third is finely dissected, petiolate, and pendent beneath the surface — functioning like a root. Salvinia shares the heterosporous condition of all ferns in Salviniales, producing spores of two differing sizes, but stands apart from all other ferns in leaf development: the upper surface of each floating leaf is morphologically abaxial (what would normally be the underside). The genus is closely related to Azolla, the mosquito fern, and modern sources place both genera in Salviniaceae, though each was historically assigned its own family.
At least three species — Salvinia molesta, S. herzogii, and S. minima — are considered likely hybrids, as their sporangia are consistently found empty. Salvinia cucullata holds the distinction of being one of only two fern species for which a reference genome has been published.
The best-known member is giant salvinia (Salvinia molesta), native to South America but now a widely established invasive weed in warm climates globally. It can double its dry weight in roughly two and a half days and forms dense, light-blocking mats over still water that devastate aquatic ecosystems. The tiny weevil Cyrtobagous salviniae has been deployed with considerable success as a biological control agent. Salvinia's hydrophobic trichomes — fine hairlike structures on the leaf surface — repel water but not oil, and the plants have been studied as candidates for absorbing oil spills, becoming saturated within about thirty seconds of contact.
Etymology
The genus name Salvinia honors Anton Maria Salvini (1653–1729), a 17th-century Italian naturalist and classical scholar. The name was formally published in 1754 by French botanist Jean-François Séguier in his work Plantae Veronenses, a catalogue of plants found in the vicinity of Verona.
Distribution
Salvinia is predominantly tropical in distribution, occurring across North America, Mexico, the West Indies, Central America, South America, Eurasia, Africa (including Madagascar), South Borneo, and Asia. Fossil evidence suggests the genus had an even broader distribution during the Tertiary.
Ecology
Salvinia species occupy still or slow-moving freshwater habitats, floating freely on the surface and forming dense mats in favorable conditions. Giant salvinia (S. molesta) is one of the world's most problematic aquatic invasive plants, capable of doubling its dry weight in roughly 2.5 days and smothering waterways by blocking light and reducing dissolved oxygen. The weevil Cyrtobagous salviniae has been used successfully for biological control of this species. The hydrophobic trichomes covering floating leaves repel water but absorb oil rapidly, a property explored for oil-spill remediation. The genus is heterosporous, with sporangia borne on submerged, rootlike leaves.
Taxonomy Notes
Salvinia is placed in the family Salviniaceae, order Salviniales, class Polypodiopsida — the leptosporangiate ferns. It is one of only two extant genera in Salviniaceae, the other being Azolla (mosquito ferns); each was formerly placed in its own family. At least three Salvinia species (S. molesta, S. herzogii, S. minima) are regarded as probable allopolyploid hybrids based on the consistent sterility of their sporangia. Salvinia leaf development is unique among ferns: the adaxial (upper) surface of floating leaves is morphologically abaxial.
