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Common Name: giant salvinia; aka: Kariba weed, African pyle, aquarium watermoss, koi kandy


Taxonomy: Family-Salviniaceae; Division-Polypodiophyta (Pteridophyta), true ferns. Salvinia represents a single genus in this family of remarkably adapted water ferns. Ten species of Salvinia occur worldwide, seven originate in the Neotropics, including Salvinia molesta. None are native to North America.

Identification: Floating, rootless aquatic fern. Consists of horizontal stems that float just below the water surface, and produce at each node, a pair of floating or emergent leaves. Floating and emergent leaves are green in color and ovate to oblong in shape. Plants bear a third leaf that is brown, highly divided and dangles underwater. Submersed leaves are commonly mistaken as roots. They may grow to great lengths, and by creating drag, act to stabilize the plant. Upper surfaces of green leaves are covered with rows of white, bristly hairs. The stalks of each divide into four thin branches that soon rejoin at the tips to form a cage. The resulting structures resemble tiny eggbeaters. Cage-like hairs may be damaged on mature leaves, thereby not appearing true to this description. Young, unfolding leaves will, however, reveal intact structures. These specialized hairs create a water repellent, protective covering.

Habitat: Quiet water of lakes and ponds, oxbows, ditches; slow flowing streams and rivers, backwater swamps, marshes and rice fields. Under optimal conditions (light, temperature and nutrient) in the laboratory, plant populations have been found to double in size every 2-4 days (Gaudet, 1973). Under favorable natural conditions, biomass doubled in about one week to 10 days (Mitchell and Tur 1975; Mitchell 1979). A single plant has been described to cover forty square miles in three months (Creogh 1991-1992). Biomass weights of live plants approach those recorded for water-hyacinth (Eichhornia crassipes) (Mitchell 1979). Salvinia molesta is strictly a freshwater species, not tolerating brackish or marine environments. In experimental trials, salinity above 7 parts per thousand (ppt) retarded growth and damaged plant tissues. Higher salt concentrations proved lethal. Plants maintained at 11 ppt were killed after 20 hours exposure. At 20 ppt, mortality resulted in less than 1.5 hours. Full strength seawater (34 ppt) killed plants in 30 minutes (Divakaran et al 1979).

Threat: Giant salvinia has the potential to alter aquatic ecosystems in several ways. Rapidly expanding populations can overgrow and replace native plants. Resulting dense surface cover prevents light and atmospheric oxygen from entering the water. Meanwhile, decomposing material drops to the bottom, greatly consuming dissolved oxygen needed by fish and other aquatic life (Thomas and Room 1986).

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