Salvinia molesta D.S. Mitchell

Common Name: giant salvinia, 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.

Salvinia molesta belongs to a group of closely related Neotropical species that share the feature of eggbeater type hairs.  Named the “Salvinia auriculata complex”, the members include S. auriculata Aublet, S. biloba Raddi, S. herzogii de la Sota, and S. molesta Although subtle differences have been found among the members of the group, sporocarps are generally needed to tell these species apart.

Sporocarps develop in elongated chains among the submersed leaves. Salvinia molesta is known for it’s of egg-shaped sporocarps that end in a slender point.  Mature plants can produce large quantities of sporocarps, which are actually outer sacs that contain numerous sporangia.  However, the sporangial sacs are usually empty of microscopic spores or with only a few deformed remnants.  Being a pentaploid species, Salvinia molesta demonstrates irregularities during meiosis that prevent spore formation and result in functionally sterile plants (Loyal and Grewal 1966).

Three growth forms have been described where individual leaves can range from a few millimeters to 4 centimeters in length. During early colonization small leafed, thin plants lie flat on the water surface.  As populations expand, leaves curl at the edges in response to self-competition. Later a vertical leaf position is attained as mature plants press into tight chains to form mats of innumerable floating plants (Mitchell and Thomas 1972; Mitchell and Tur 1975).

Identification Keys: Mitchell and Thomas 1972; Forno 1983.

Line Drawings: Guided Tour of Salvinia molesta

Reproduction: Salvinia molesta effectively reproduces through vegetative means. Stems fragment spontaneously as plants mature.  New branches develop from apical and lateral buds. Each node harbors up to five serial lateral buds (Lemon and Posluszny 1997), adding to the high potential for growth and dormancy. Salvinia molesta will withstand periods of stress, both low temperature and dewatering, through latent buds.

Habitat: Quiet water of lakes and ponds, oxbows, ditches; slow flowing streams and rivers, backwater swamps, marshes and rice fields.

Impact of Introduction: Salvinia molesta, one of the world’s most noxious aquatic weeds, is notorious for dominating slow moving or quiet freshwaters (Mitchell et al 1980). Its rapid growth, vegetative reproduction and tolerance to environmental stress make it an aggressive, competitive species known to impact aquatic environments, water use and local economies.

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 demonstrates tolerance to freezing air temperatures, but cannot withstand ice formation on the water surface (Whiteman and Room 1991).

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).

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).

At Swinney Marsh, Texas, local fishermen have found it impossible to cast into water covered with dense mats of giant salvinia and are abandoning spots once fished for bass, crappie and sunfish. Giant salvinia clogs water intakes to interfere with agricultural irrigation and electrical generation. Many infested farm ponds in Texas lie on creeks that drain tributaries heavily depended on for agricultural irrigation.


Author: C.C. Jacono
Updated: 25 Feb. 2003

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