Northern Prairie Wildlife Research Center
Hutchinson (1959) placed the genus Ruppia in the family Ruppiaceae. Kartesz and Kartesz (1980) place the genus in the family Zosteraceae. They recognize three North American (including Greenland) species of Ruppia, (R. anomala, R. cirrhosa and R. maritima) and list nine varieties of R. maritima. Older North American floras, phytogeographical studies, and waterfowl food habit studies often differentiated R. occidentalis ("western wigeongrass") from R. maritima. Many taxonomists now consider the plant a variety of R. maritima. Morphological variations of the plant caused by the environment may impose taxonomic problems in interior North America (Hammer and Heseltine 1988). In Europe, the genus is considered a member of the family Potamogetonaceae and two species (R. cirrhosa and R. maritima) are recognized (Verhoeven 1975, 1979). These species are separated by morphology and chromosome number (Reese 1962) and the salinity, depth, and water permanence of the wetlands they inhabit (Verhoeven 1975, Verhoeven and Van Vierssen 1978b). Australia has four species (R. maritima, R. megacarpa, R. polycarpa and R. tuberosa) that are also placed in the Potamogetonaceae (Bayly and Williams 1973; Brock 1982a; Jacobs and Brock 1982). Other species, varieties and forms of this taxonomically and nomenclaturally confused genus are recognized from similar habitats in other parts of the world (Verhoeven 1979). Van Vierssen et al. (1981) urges taxonomic study of the genus on a global scale.
Wigeongrass (McAtee 1935) is sometimes spelled "widgeongrass" or "widgeon-grass," but I have maintained the cited common name because of the officially-accepted (American Ornithologists' Union 1983) common names, American and Eurasian wigeon (Anas americana and A. penelope). Other common or colloquial names for wigeongrass include "ditch-grass," "duck grass," "fines," "niggerwool," "peter-grass," "puldoo-grass," "sea grass," "swan grass," "tassel grass," "tasselweed," "tasselpondweed," and "zhebes" (McAtee 1915, 1939; Setchell 1924;Ferguson Wood 1959).
Most of the information on wigeongrass in this report applies to the genus, or Ruppia maritima s.l. (i.e., sensu lato, meaning the species in its widest sense), except where references show that differences in morphology, growth form, habitat, or other features exist among the six aforementioned species. In these cases, I follow the lead of Verhoeven (1979), and present data only for Ruppia maritima s.s. (i.e., sensu stricto, the species in its narrowest sense). Ruppia maritima s.s. from northern Europe normally has 2n = 20 chromosomes, but some southern populations have 2n = 40 (Van Vierssen et al. 1981; Aedo and Fernandez Casado 1988).
Fossil Ruppia pollen from the North American Pleistocene (Martin 1963) and R. maritima drupelets from the Holocene (Pierce and Tiffney 1986) are known.
Although often found with the seagrasses, wigeongrass (Fig. 1) is not a true marine plant, but considered a freshwater species with a pronounced salinity tolerance (Zieman 1982). Verhoeven (1979) considers Ruppia to have little competitive strength outside its rather well-defined ecological niche and states that its survival is inhibited by competition in certain freshwater and marine habitats that would otherwise be physically suitable. Even in suitable habitats, frequency and biomass of wigeongrass varies greatly, both temporally and areally (Davis et al. 1985).
Ruppia maritima s.l. behaves as an annual (vegetation perishes) in habitats subject to drought, lethal increases in salinity, or other extremes, or as a perennial (at least some vegetative parts grow year around) in deeper, more stable environments (Richardson 1980; Bigley and Harrison 1986). For wigeongrass behaving as an annual, Bigley and Harrison (1986) describe its demography as "rapid production and early death of ramets [individual plants of clones] after production of seeds." Richardson (1980) noted that, early in the growing season, plants with annual growth habit likely have an affinity for areas of low salinity attributable to their requirement of rapid germination and fruit production before salinity maximums occur. In more saline waters, he found forms with perennial growth habit.
Plants in some wetlands alternate between perennial and annual life cycles (Koch and Seeliger 1988). Setchell (1924) believed perennial forms grew mostly in areas subject to tidal conditions that left plants exposed or covered with shallow to deep water, whereas annual forms grew in shallow ponds with less tidal influence. In culture, however, plants thought to be annuals flourished throughout the year and produced abundant fruit.
Plants from more stable environments generally are taller, have wider leaves, and produce fewer sexual propagules (Verhoeven 1979). These plants also have longer flower peduncles so pollination occurs at, rather than under, the water surface, and larger, stronger root systems that allow vegetative hibernation.
The growth form of wigeongrass, as seen in culture experiments, is also dependent on sediment chemistry. Plants supplied with low levels of inorganic nutrients in a washed-sand substrate grew many short shoots from an extensive network of rhizomes and roots, whereas those supplied with organic nutrients grew as long vertical shoots from reduced belowground parts (Pulich 1989).
In shallow sites, wigeongrass concentrates much leaf area just above the bottom (Wetzel et al. 1981). In deeper waters, plants often grow in a form termed by Hutchinson (1975) as parvopotamid - that is, a higher aquatic plant, rooted in sediment, perennially submersed except inflorescences, and having long stems and small, mostly undivided leaves. Luxuriant parvopotamid growth results in dense leaves, branches, and inflorescences in the upper part of the water column, but much thinner density of stems and widely-spaced leaves below. Vegetative density of the upper part increases with falling water levels (Verhoeven 1980a). Verhoeven (1980a) recognized three horizontal growth patterns in Ruppia-dominated communities in Europe: dense monospecific beds, mosaics of sharply delimited patches, and beds of mixed species that formed patches often touching or penetrating each other. He also found several examples of horizontal zonation, where plants tended to order along gradients of depth, substrate, or exposure. The most common pattern was where short-lived forms of wigeongrass occupied nearshore areas (with temporarily or intermittently exposed water regimes) and perennial forms inhabited deeper offshore areas in a mosaic pattern with sago pondweed (Potamogeton pectinatus). Many other growth forms for aquatic macrophytes have been described (Hutchinson 1975). Communities supporting wigeongrass are noted for a poverty of such forms; most European stands assume the parvopotamid form, but are sometimes mixed with filamentous algal, charid, and zosterid forms (Verhoeven 1980a).
Classifications of Ruppia-dominated stands by the Braun-Blanquet (1931) phytosociological system popular in Europe (e.g. Gillner 1960; Westhoff and Van der Maarel 1973; Beeftink 1977) will not be outlined here.
Worldwide distributional records for Ruppia taxa show that representatives of the genus occur on all continents of the world and on many islands. The northern limit is about 69 degrees N, the southern limit is at least 55 degrees S, and the altitudinal limit is as at least 3800 m above sea level (Verhoeven 1979).