Hydrocharis morsus-ranae

H. morsus-ranae is a free-floating, stoloniferous aquatic plant that can grow to form dense floating mats of interlocking plants which cause many negative environmental and economic impacts. Some of these impacts include displacing native plant species, reducing biodiversity, decreasing water quality and flow, clogging irrigation pumps, impeding recreational activities, and diminishing aesthetic value. H. morsus-ranae is extremely difficult and costly to control, and its ability to form new plants vegetatively has allowed it to spread and proliferate quickly (Scribailo and Posluszny, 1984). The trade and potential escape of H. morsus-ranae through the water garden industry plays a large role in its spread to new locations, as does the transportation of this plant on recreational equipment or by wildlife moving between water bodies (USDA-NAS, 2002; Catling et al., 2003). H. morsus-ranae is declared a noxious weed in parts of the United States (USDA-GRIN, 2002), and is currently established in New York, Vermont, Michigan, and Washington.  

The genus Hydrocharis (family Hydrocharitaceae) contains approximately 15-16 species (Diversity of Life, 2005; ZipcodeZoo, 2008). The genus name comes from the Greek hydro meaning ‘water’ and charis meaning ‘some sort of plant’. The specific epithet is derived from the Latin morsus meaning ‘bite’ and rana meaning ‘frog’, referring to the observation of frogs biting at the leaves, searching for caterpillars that make their protective envelopes from the plant (Catling et al., 2003). The English common name ‘frog-bit’ comes from this relationship. Hydrocharis morsus-ranae was first named by Linnaeus in 1753, and this is still its current accepted scientific name.

H. morsus-ranae is a free-floating, stoloniferous herbaceous perennial aquatic plant that can grow to be 0.1-1.5 m across, with individual rosettes measuring 1-30 cm. H. morsus-ranae is mostly dioecious, though rarely both sexes can occur on the same rosette, albeit on different ramets within a genet (Scribailo and Posluszny, 1984). The roots are usually un-branched, unattached to the substrate, up to 50 cm long, and change colour from green to white as they develop and form numerous long root hairs. The stolons do not readily fragment, though winter buds (turions) are formed on the stolons in autumn, and are ellipsoidal to oval, 5-7 (-9) mm long with a distinct abscission layer. The leathery circular to heart shaped floating leaves are entire, have an indented (cordate) base, often dark purple underneath, are 1.2-6 cm long, 1.3-6.3 cm wide, and have a conspicuous aerenchyma on the undersurface near the mid-vein, with all veins arising from the base. The petioles are slender, 6-14 cm long, with two free lateral stipules up to 2.5 cm long at the petiole base.    The flowers are unisexual, with the male flower arising from a spathe consisting of two bracts, each 1-1.2 cm long on a 0.7-5.5 cm long peduncle. The (1-) 2-5 flowers within each spathe have pedicels up to 4 cm long, white to greenish-white sepals 4-5.5 mm long, broadly obovate to orbicular white petals 9-19 mm long. It has 9-12 stamens, each 2-3.5 mm long in four whorls, with the two outer whorls being fully fertile, and the inner whorls sterile or partly sterile. The pollen is rounded, radiosymetric, without apertures and covered with spines.     The female flowers are borne singly on peduncles up to 9 cm long, with greenish-white sepals 4-5 mm long, obovate to orbicular white-pinkish petals 10-15 mm long, staminodes simple, bifurcate or bifid; stigmas 6, up to 5 mm long, divided to 1/4 - 2/3 of their length. The seeds are approximately 1 mm long, transversely elliptic, covered with stout blunt processes, each with a spiral pattern on the outer surface (Cook and Lüönd, 1982).

H. morsus-ranae is native through much of Europe and parts of temperate Asia (USDA-GRIN, 2002) as well as Africa (Catling et al., 2003). Cook and Lüönd (1982) define its native range as including western and central Europe, extending from Portugal, western France and the British Isles, north to southern Sweden and Finland, and south to northern Italy. There are scattered records in Eastern Europe to 40°E, while it is sparingly represented in Turkey, the Caucasus and south shore of the Caspian Sea. Isolated populations have also been reported from central Siberia and Kazakhstan (Cook and Lüönd, 1982). Reports of H. morsus-ranae from Australia and Japan (Holm et al., 1979) are unreliable and based upon material referable to H. dubia (Catling et al., 2003). H. morsus-ranae has declined or has been extirpated throughout parts of its European range, and is considered a conservation concern in several areas. In the United Kingdom, populations have decreased in their natural habitats, but populations have been reported in canals outside of its native range (Preston and Croft, 1997).  

In its non-native range, H. morsus-ranae is currently established in southeastern Canada, and it is known primarily from southeastern Ontario and western Quebec. It has also established in northern New York, northern Vermont, and southeastern Michigan in the eastern United States. H. morsus-ranae has also recently been reported as established in Washington, and is the first population recorded in the western coast of the United States (USDA-NAS, 2002).

The first recorded introduction of H. morsus-ranae into Canada was in 1932 when an intentional introduction for horticultural purposes was made at an aquatic pond in the arboretum of the Central Experimental Farm in Ottawa (CWS, 2003). Earlier introduction of the plant is also possible (Catling and Porebski, 1995). The original plants, or possibility seeds, came from the Zürich Botanical Garden in Switzerland (Dore, 1968; CWS, 2003). The population grew at the original site without incident until 1939, when Minshall (1940) noticed that it had spread to nearby sections of the Rideau Canal and Brown’s Inlet, a nearby artificial pond with underwater connections to the Canal. By 1952, a population was collected from the Ottawa River at Montreal Island, either as a result of floating plant material travelling and establishing downstream, or perhaps the result of a separate introduction from specimens which escaped confinement from the Montreal Botanic Garden or McGill University (Minshall and Scarth, 1952; Dore, 1968; Catling and Dore, 1982). In 1953 it had spread towards the exit of the Rideau Canal and along the shores of the Ottawa River, being found in the main channel of the Rideau River by 1957. During 1960 H. morsus-ranae was found at various locations along the Ottawa River around Montreal, as well as much further downstream at Lake St. Peter (CWS, 2003). By 1967 the plant had also spread upstream in the Rideau River to at least Merrickville (Dore, 1968), and continued to spread along the Rideau and Ottawa Rivers, as well as into connected tributaries and isolated wetlands (Reddoch, 1976). By 1970, the total extent of North American occurrence was a stretch of 340 km of major waterways (Catling and Dore, 1982). H. morsus-ranae was reported along the Lake Erie shoreline in 1976 (USGS-NAS, 2002). By 1980 it had extended southwest to Lake Ontario and northeast to Quebec City (Catling et al., 2003). H. morsus-ranae has recently continued to spread throughout much of southern Ontario.

H. morsus-ranae was first discovered in the United States in 1974 from the Oswegatchie River, just off the St. Lawrence River in northern New York (USGS-NAS, 2002). By the early 1980s, populations had spread and increased at several inland sites south of the St. Lawrence River, and into bays and marshes of Lake Ontario by the 1990s (Catling and Dore, 1982).   H. morsus-ranae was first discovered in Vermont during 1993 at the northern portion of Lake Champlain near the town of Grande Isle, and by 1999-2000 had spread to the southern portion of the lake near Benson, Orwell, and West Haven, Vermont, as well as Mill Bay in eastern New York (USDA-NAS, 2002).     In Michigan, unidentified plants were first observed in 1996 during dredging of a slough at Lake St. Clair, and within two years the plants had become abundant throughout the marsh and formed dense mats in cut ponds. In 2000 the plants were identified as H. morsus-ranae, and established populations were observed in marshes of both the Detroit River and Lake St. Clair, both of which flow into Lake Erie (USDS-NAS, 2002).   H. morsus-ranae was first discovered in the western coast of the United States in 2001, where an established population was discovered in Snohomish County, Washington, in the wetlands surrounding Meadow Lake (USGS-NAS, 2002; Catling et al., 2003).

H. morsus-ranae is continuing to expand its range and become more abundant. H. morsus-ranae is a popular water garden and aquarium plant, and the ability to order this plant over the Internet and through mail order gives it the ability to travel to all parts of the world (Catling et al., 2003). It has escaped confinement and has been intentionally or accidentally introduced on several occasions outside of its native range. In the locales to which it has been introduced, it has often become the dominant plant species, out-competing and displacing native species that depend on the ecosystem (Catling et al., 2003). The ability of H. morsus-ranae to reproduce vegetatively facilitates its spread and colonization; it is a highly competitive plant which is capable of a maximum rate of spread of 15.6 km/year, and has already spread over an area of 644 km from point of origin (Catling et al., 2003). Although its initial spread had been confined to interconnected waterways, it has since found ways to spread into isolated and unconnected wetlands and waterways, facilitated by dispersal on boats, trailers, and wildlife (Catling et al., 2003). Within the next ten to 20 years, it is anticipated that H. morsus-ranae will continue to move westward through the Great Lakes basin towards the Midwest, and become established in the northern Midwest and prairie regions of the United States (Catling and Porebski, 1995; Catling et al., 2003) 

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H. morsus-ranae looks similar to Limnobim spongia, commonly referred to as American frog-bit (UFL-IFAS, 2002). H. morsus-ranae can be distinguished from L. spongia by having relatively less developed aerenchyma on the undersurface of the leaf, relatively longer leaf lobes, leaf veins on either side of the mid-vein less ascending, free stipules in pairs, roots usually un-branched, and stolon buds developing one instead of multiple roots initially (Catling and Dore, 1982). 

 H. morsus-ranae may also be confused with other round, floating leaves plants such as Nymphaea spp., Nuphar spp., and Brascenia schrebii. H. morsus-ranae differs from these aquatics by having rounded leaves with five prominent veins and converging primary veins. H. morsus-ranae seedlings may be distinguished from the floating duckweeds Lemna minor and Spirodela polyrhiza by its roots, which arise from a rosette base or base of a leaf petiole rather than directly from the undersurface of the leaf (Catling et al., 2003).   

H. morsus-ranae prefers shallow, slow moving waters, inhabiting quiet edges of rivers, lakes, ponds, sheltered bays, open marshes, and wetlands (UFL-IFAS, 2002; USDA-NAS, 2002). It is also found growing in canals, beaver dams, swamps, and irrigation ditches. It tolerates a wide range of climatic conditions, and the genotypes established in North America also appear to have a broad tolerance. It is currently established in two major eco-zones of Canada and five eco-regions (Catling et al., 2003). Organic substrate is necessary for development, and it does not tolerate waters with a mineral substrate, such as clay pits of fishponds. H. morsus-ranae favours calcium-poor waters, often occurring on peaty soils, and often occurs in mesotrophic waters, though usually not found in oligotrophic conditions (Cook and Lüönd, 1982).  

Genetics 

North American populations of H. morsus-ranae have a chromosome number of 2n=28 as reported by material collected by Dore and analyzed by G.A. Mulligan from the Rideau Canal in Ottawa, Ontario (Catling et al., 2003). Several other authors report 2n=28 for Eurasian material (Löve, 1980; Dvorak, 1989). H. morsus-ranae posses one pair of long sub-metacentric chromosomes, the rest being medium length, metacentric, acrocentric, or short metacentric (Cook and Lüönd, 1982). Meiosis is reported to be regular. Dvorak (1989) provides additional information on chromosome morphology.     

Reproductive Biology 

H. morsus-ranae reproduces primarily vegetatively by means of strong stolons and the productions of turion winter buds. It is estimated that a single plant can form approximately 100 to 150 turions (Dore, 1968; Scribailo and Posluszny, 1984).

H. morsus-ranae also has the ability to reproduce sexually, though reproduction by seeds is rarely reported (Catling et al., 2003), and probably is of limited importance in the spread of the species (Scribailo and Posluszny, 1984). The fruit is a globose berry containing up to 74 seeds, with an average of 26-42 seeds (Scribailo and Posluszny, 1985).   

Physiology and Phenology 

In late summer and early autumn, turions are formed along the stolon and then separate from the plant in late autumn, sinking towards the bottom where they remain dormant over winter for seven months (Dore, 1968). Their release from dormancy requires several weeks of chilling, with a temperature of approximately 5°C being optimal (ISSG, 2005). In the spring the turions rise towards the surface, germinate from late April to early May, and grow into small floating rosettes (Catling et al., 2003). Rosettes are well developed or fully grown by mid-May. By early June most plants have developed into three rosettes joined by stolons, and by mid to late June will often have six rosettes. Flowering is regulated by photoperiod, though it is very erratic and small fluctuations in temperature can readily influence timing. Generally, the peak flowering period in North America ranges from mid-July to mid-August. 

H. morsus-ranae is eaten by grass or amur carp (Ctenopharyngodon idella Val.), ducks, water birds, and rodents. It is a food plant for a number of insects and water snails (Catling and Dore, 1982), as well as host to a number of rusts, smuts and molds (Catling et al., 2003). In Europe it is reported as being consumed by beavers (Castor fiber) (Catling et al., 2003).    

H. morsus-ranae has limited water flow in irrigations systems and canals (Catling et al., 2003). In addition, the loss of recreational and aesthetic value associated with H. morsus-ranae can also cause a decline in waterfront property values, as well as possible declines in tourism related revenue for the community.

H. morsus-ranae can form dense mats that impede recreational activities such as boating, fishing, swimming, water skiing, canoeing, and kayaking. In addition, unsightly mats of vegetation decrease aesthetic values. These declines in recreational and aesthetic values can decrease tourism, which can be a major source of livelihood within the community. 

Infestations of aquatic invasive species are often first reported at boat launches, and these areas should be monitored frequently in order to eradicate or control new invasions at an early stage. Users should inspect all recreational equipment before leaving any water body, and any visible plants, animals, or sediment should be removed. In addition, rinsing gear with hot water or steam may help in removing any additional non-visible organisms.