Elodea is a small genus of probably 15 species but species delimitation is still controversial. All species are native in the New World. Some have become noxious environmental weeds outside their native distribution range.
Former Belgian records of Elodea callitrichoides (Rich.) Caspary (syn.: E. ernstiae St John) turned out to be in error for E. nuttallii (as shown by molecular and morphometric studies; see Vanderpoorten & al. 2000). Some rare forms of the latter with spreading leaves (not recurved) much resemble Elodea callitrichoides. Elodea nuttallii is distinguished by smaller sepals (1,6-2,5 mm vs. 3,1-4,3 mm but flowers are probably never produced in Elodea callitrichoides in western Europe!). Moreover, the adventitious root-tips are whitish to greyish-green (vs. reddish in Elodea callitrichoides). The latter has been confirmed from several European countries and its presence in Belgium cannot be excluded.
A putative hybrid between Elodea canadensis and E. nuttallii has been recorded in the British Isles (Wurzell 1992). It must have been introduced as such, since hybridisation is not possible in Europe.
- Leaves usually more than 1,8 mm wide at middle (0,8-2,3 mm wide 0,5 mm behind apex), obtuse to subacute at apex, spreading (not strongly recurved) === 1. Elodea canadensis
- Leaves usually less than 1,7 mm wide at middle (0,2-0,8 mm wide 0,5 mm behind apex), acute to acuminate at apex, usually strongly recurved (more rarely spreading) === 2. E. nuttallii
Cook C.D.K. & Urmi-König K. (1985) A revision of the genus Elodea (Hydrocharitaceae). Aquatic Botany 21: 111-156.
Delay J. & Petit D. (2008) Les espèces aquatiques invasives du Nord de la France et leur biologie. Bull. Soc. Bot. N. Fr. 61(1-4): 5-22.
St John H. (1961) Monograph of the genus Egeria Planchon. Darwiniana 12: 293-307.
St John H. (1962) Monograph of the genus Elodea. Part 1. Western North America. Research Studies Washington State University 30(2): 19-44.
St John H. (1963) Monograph of the genus Elodea (Hydrocharitaceae). Part 3. The species found in the northern and eastern South America. Darwiniana 12: 639-653.
St John H. (1964) Monograph of the genus Elodea (Hydrocharitaceae). Part 2. The species found in the Andes and western South America. Caldasia 9: 95- 113.
St John H. (1965a) Monograph of the genus Elodea (Hydrocharitaceae). Part 4. The species of eastern and central North America. Rhodora 67: 1-35.
St John H. (1965b) Monograph of the genus Elodea (Hydrocharitaceae). Summary. Rhodora 67: 155-180.
Vanderpoorten A., Lambinon J. & Tignon M. (2000) Morphological and molecular evidence of the confusion between Elodea callitrichoides and E. nuttallii in Belgium and northern France. Belg. J. Bot. 133: 41-52.
Wurzell B. (1992) Aliens and adventives. Plant records. BSBI News 60: 34-36.