Egeria naias Planch., a Newly Naturalized Aquatic Species in China

SU Fan, GUO Yanan, WANG Ruijiang

（South China Botanical Garden, Chinese Academy of Sciences, Guangzhou 510650, China）

Abstract：【Objective】The study was conducted to well understand the diversity of wetland plants so as to provide important data support for the conservation and utilization of plant resources in Guangdong.【Method】Field investigation and literature consulting helped us make the checklist of wetland plants in Guangdong.【Result】Egeria naias, a recently naturalized aquatic species in China, was recorded for the first time for local flora.Morphological and biological characteristics were described and photographed in detail for further taxonomic identification.The potential threat of E. naias to the local wetland ecosystem was also evaluated according to its ecological adaptability and propagation characteristics.【Conclusion】This new information may provide not only valuable data for the study of the flora in Guangdong but also the important references for prevention and control of the exotic plants in Guangdong.

Key words: alien species; Egeria; Hydrocharitaceae; taxonomy

EgeriaPlanch.［1］, a small genus including three species of the family Hydrocharitaceae, is native to southeastern Brazil, Uruguay, Paraguay, and Argentina in South America［2］.TheEgeriaspecies are perennial plants; leaves in whorls of 5 or more,sessile; flower solitary and unisexual, dioecious,staminate flowers with 9 stamens and pistillate flowers with 3 styles and 3 staminodes; fruits ovoid,dehiscing irregularly; seeds fusiform, mucilaginous.The shape of the filaments and nectaries of the male flowers and staminodia of the female flowers are the main characters for species classification［3］.Due to its high adaptability to the local environment, fast growth of biomass and good nutritional values,Egeria densaPlanch.［1］, sometimes as well asElodea canadensisMichaux［4］andHydrilla verticillata（L.f.） Royle［5］, has been used as food for aquaculture in South China probably since 1920s.It was therefore listed in the Flora of China as cultivated plant［6］.

During our field investigation into the aquatic and wetland plants in the Jinjiang river basin, Jiangmen Municipality, China, an interesting species with whitish flowers floating on the water surface attracted our attention.It has very soft stem, linear, whorled and serrated cauline leaves with dented margin, and solitary flowers with 3 green sepals and 3 white petals,projected to water surface by slender and elongated hypanthium.The morphological comparison revealed that this plant should beEgerianaiasPlanch., also called “Narrow Leaf Anacharis”or“Narrow Leaf Elodea”, a native species in Argentina, Bolivia,Brazil, Paraguay, Southern Cone and Uruguay, etc.of South America but becoming established in nature of China.E.naiashas apparently not persisted long and has not been listed for the flora of China［6-7］.

1 Naturalization record of Egeria naias Planch.

Ann.Sci.Nat., Bot., Sér.3, 11: 80.1849.Type: Brazil, Province de Minas Gera?s, prope S.Romao, June 1840, G.Gardner 5238 （holotype: K,barcode K000587182, image!; isotypes: BM, barcode BM000938273, image!; G, barcode G00098219,image!; K, barcode K000587183, image!）

Submerged, dioecious aquatic herbs.Stems 1.5-2.0 mm in diameter; internodes 1.0-10 mm long.Leaves usually recurved, 8-25 mm×0.8-2.0 mm,linear or linear lanceolate, in whorls of 4（-8）at sterile nodes, margin visibly serrated.Staminate flowers not seen.Pistillate flowers: spathe united and free at the apex, ca.3 mm long, axillary; flower solitary in each spathe, sessile but flower parts raised above the water by the slender threadlike, elongating to 3.5-4 cm hypathium; sepal lobes 3, ca.3×1.5 mm, elliptic,boat-shaped, reflexed; petals 3, free, ca.7×4 mm,elliptic, rounded at apex, white, spreading; nectaries 3, ca.0.3 mm long; staminodia 3, ca.1 mm long, free,antesepalous, filiform, simple, papillate; styles trifid to base, antepetalous; stigma lobes deeply 2-cleft and papillate, filiform, ca.2 mm long, white; ovary inferior.Fruit fusiform, sessile, translucent.Seeds 9 per capsule, ca.0.5×0.35 mm, ellipsoidal, slightly bent at the chalazal end; micropyle elongate and beak-like（Fig.1）.

Phenology: Flowering & fruiting in November to December.

Voucher specimen: Shuijian, Nancun village,Datian town, Enping City, Jiangmen Municipality,Guangdong Province.CHINA.November 24, 2019,N22°16′07.54″, E112°11′37.64″, alt.43 m, Su Fan, Zhou Xinxin & Guo Yanan 1671 （IBSC）.

2 Phenology observation

Although only the pistillate flowers ofE.naiashas been observed in field, its characteristic morphology is congruent with that of the genus.In addition, it was reported that male flowers usually out-number females in the native wild populations ofE.naias, as well asE.densa［8］, but the situation seems to be contrary in China.This species is apparently different fromE.densaby its leaves with 4（-8）-whorled （vs.4）at sterile nodes, frequently recurved （vs.spreading）,and very conspicuously （vs.not obviously by eyes）serrated margin.The flowering period of bothE.densaandE.naiasis usually from September to December,sometimes can extends to next March, but rarely to April in the countries of South Hemisphere, according to the record of the specimens.Therefore, they have little difference in terms of flowering season.

3 Invasive risk evaluation

E.naiashas been widely used as aquarium plants and become one of the most frequently cultivated aquatic plants in the tropical fishery industry.In Europe, it was also introduced for cultivating［8］.It is probably released via aquarium dumping, as the occurance ofE.densa, which has become a serious invasive weed in some areas of the United States and Canada［2］and showed much frequently negative effects on water uses［9］.Same asE.densa, the fragmentation of double nodes inE.naias can sprout new stems and roots and crowd out native plant species too［10］.The possibility of future larger-scale interference by the dense growth and free-floating of this aquatic weed with fisheries, recreational use,transport and hydro-electricity generation suggests a need for continued monitoring of weed distribution and abundance, and investigation of appropriate management measures［11］.Under this circumstance,according to the present growing situation in South China, we may make an alert to the biosafety of the aquatic environment in China.