Common name: giant salvinia
Synonyms and Other Names: Kariba weed, African pyle, aquarium watermoss, koi kandy, giant salvinia, aquarium watermoss, Australian azolla, waterfern, water spangles, giant azolla
available through www.itis.gov
Identification: Free floating, aquatic fern. Consists of horizontal stems that float just below the water surface, and produce a whorl of three leaves (technically fronds) at each node. The upper pair of floating or emergent leaves are green in color and ovate to oblong in shape. The lower submerged leaf is brown, highly divided and resembles and functions as a root. The lower leaves may grow to great lengths (up to 25 cm), and by creating drag, act to stabilize the plant (Mitchell and Tur 1975; Room 1983).
Upper surfaces of green fronds are covered with rows of white, bristly hairs (papillae) (Mitchell 1972), which divide into four thin branches that soon rejoin at the tips to form a cage. The resulting structures resemble tiny eggbeaters. This characteristic eggbeater structure can reliably distinguish S. molesta from the morphologically similar S. minima that has unjoined hairs (Wunderlin and Hansen 2011). These specialized hairs create a water repellent, protective covering (Mitchell and Thomas 1972).
Salvinia molesta belongs to a group of closely related Neotropical species that share the feature of eggbeater type hairs (Mitchell 1972; Mitchell 1979). 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 (Forno 1983; Mitchell 1972).
Sporocarps develop in elongated chains among the submersed leaves. Salvinia molesta is known for its 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, S. molesta demonstrates irregularities during meiosis that prevent spore formation and result in functionally sterile plants (Loyal and Grewal 1966; Mitchell 1979).
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).
Size: Paired fronds or leaves 2-4 cm long and 1-6 cm wide
Native Range: southeast Brazil (Forno 1983)
Interactive maps: Point Distribution Maps
Puerto Rico &
Table 1. States with nonindigenous occurrences, the earliest and latest observations in each state, and the tally and names of HUCs with observations†. Names and dates are hyperlinked to their relevant specimen records. The list of references for all nonindigenous occurrences of Salvinia molesta are found here.
Table last updated 6/7/2018
† Populations may not be currently present.
Ecology: Reproduction is strictly vegetative as spores are sterile (Mitchell 1979). Under favorable natural conditions, biomass can double in about one week to 10 days (Mitchell and Tur 1975; Mitchell 1979). Biomass weights of live plants approach those recorded for floating waterhyacinth (Eichhornia crassipes (Mart.) Solms) (Mitchell 1979).
Salvinia molesta demonstrates tolerance to freezing air temperature, but cannot withstand ice formation on the water surface except when dense mats protect the underlying plants (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) slowed growth and damaged plant tissues. Higher salt concentrations proved lethal. Plants maintained at 11 ppt were killed after 20 hours exposure. Full strength seawater (35 ppt) killed plants in 30 minutes (Divakaran et al. 1980).
Means of Introduction: The first establishment outside of its native range was in Sri Lanka in 1939 (Room 1990), by the Botany Department at the University of Colombo. The species continued to be introduced to other warm regions of the world intentionally as an aquarium and water garden plant, and unintentionally as a contaminant in shipments of other aquatic plants (Oliver 1993; Nelson 1984). Once established in a new region, the plant is likely spread as a hitchhiker on boats, trailers and other recreational gear. Local movement between waterbodies may be facilitated by birds and aquatic mammals (Mitchell and Thomas 1972).
Spread will continue through natural drainage and flow in river and stream systems. In lakes and large water-bodies upright well buoyed leaves are effectively dispersed by wind and currents to infest new coves.
Status: Populations established in the southeast and southwest U.S., including Guam, Hawaii, and Puerto Rico. Extirpated from colder climate states (Connecticut, District of Columbia, Kansas, Missouri, Virginia) and eradicated from Northern Alabama and California, and parts of the Carolinas, Florida, and Texas. More than 99% of the Pascagoula River population was killed by storm surge salinity or by being deposited on land during Hurricane Katrina in 2005, thus leaving about 2 hectares of S. molesta distributed over ~20 sites (Fuller et al. 2010).
Impact of Introduction: Salvinia molesta has the potential to alter aquatic ecosystems in several ways. Rapidly expanding populations can overgrow and replace native plants resulting in dense surface cover that prevents light and atmospheric oxygen from entering the water. Decomposing plant material drops to the bottom, consuming dissolved oxygen needed by fish and other aquatic life (Divakaran et al. 1980). Flores and Carlson (2006) noted a 2.5 fold increase in dissolved oxygen by removing 90% of S. molesta at five east Texas sites.
Floating mats of S. molesta can be up to one meter thick (Whiteman and Room 1991), impeding navigation, reducing water flow and interfering with fishing and recreational activities (Mitchell and Thomas 1972). This could be significant in areas where economic or subsistence fishing is important (Mitchell et al. 1980). Like dense floating mats of waterhyacinth, dense floating mats of S. molesta support secondary colonizing plants, leading to the formation of floating islands or tussocks (McFarland et al. 2004; Mitchell et al. 1980).
Few have researched the beneficial effects of S. molesta. Due to its floating habit, S. molesta was utilized to treat sewage and industrial effluent (Finlayson et al. 1982). Asian countries have supplemented livestock fodder with S. molesta, and utilized it as a compost and mulch (Oliver 1993; Thomas and Room 1986).
Thayer, D.D., I.A. Pfingsten, C.C. Jacono, M.M. Richerson, and V. Howard
Revision Date: 4/4/2018
Peer Review Date: 3/18/2016
Thayer, D.D., I.A. Pfingsten, C.C. Jacono, M.M. Richerson, and V. Howard, 2018, Salvinia molesta Mitchell: U.S. Geological Survey, Nonindigenous Aquatic Species Database, Gainesville, FL, https://nas.er.usgs.gov/queries/FactSheet.aspx?speciesID=298, Revision Date: 4/4/2018, Peer Review Date: 3/18/2016, Access Date: 6/20/2018
This information is preliminary or provisional and is subject to revision. It is being provided to meet the need for timely best science. The information has not received final approval by the U.S. Geological Survey (USGS) and is provided on the condition that neither the USGS nor the U.S. Government shall be held liable for any damages resulting from the authorized or unauthorized use of the information.