Vallisneria L.
Species delimitation is still controversial within the genus Vallisneria. Haynes (2000), based on Lowden (1986), only recognized two species, one native mainly to the New World (V. americana Michaux), the other to the Old World (V. spiralis L.). According to Nelson (1986) Vallisneria counts ca. 10 species in the warm-temperate regions of the world, three of which are in cultivation as aquarium plants in Europe: V. americana, V. gigantea Graebn. and V. spiralis. Recent molecular studies seem to confirm a narrow species concept and several new species were recently described (Les & al. 2008). The genus probably includes at least 15 species now but there seem to be very few morphological features that reliable distinguish these species.
Since all species is cultivation are superficially similar, records of Vallisneria should be carefully checked, preferably using molecular markers. More species and even hybrids can be expected. Based on molecular data Japanese records of invasive Vallisneria proved to belong to V. australis S. W. L. Jacobs & D. H. Les and a hybrid of V. spiralis and native V. denseserrulata Makino (Wasekura & al. 2016).
In the absence of molecular data Nelson (1986) is followed in the present account. A critical reassessment, however, could shed new light on the identity of the plants currently found in the wild in Belgium.
1 Leaves up to 30 mm wide === Vallisneria americana
Leaves at most 10 mm wide === V. spiralis
Literature:
Haynes R.R. (2000) Hydrocharitaceae. In: Flora of North America Editorial Committee (eds.), Flora of North America, vol. 22. Oxford University Press, New York-Oxford: 26-38.
Jacobs S.W.L. & Frank K.A. (1997) Notes on Vallisneria (Hydrocharitaceae) in Australia, with description of two new species. Telopea 7(2): 111-118.
Les D.H., Jacobs S.W.L., Tippery N.P., Chen L., Moody M.L. & Wilstermann-Hildebrand M. (2008) Systematics of Vallisneria (Hydrocharitaceae). Syst. Bot. 33(1): 49-65. [available online at: http://hydrodictyon.eeb.uconn.edu/people/les/Manuscript_Files/Syst_Bot_33(49)%5B2008%5D.pdf]
Lowden R.M. (1982) An approach to the taxonomy of Vallisneria L. (Hydrocharitaceae). Aquatic Bot. 13: 269-298. [available online at: http://opensample.info/an-approach-to-the-taxonomy-of-vallisneria-l-hydr...
Nelson E.C. (1986) Vallisneria. In: Walters S.M. & al. (eds.), The European Garden Flora, vol. 1. Cambridge University Press, Cambridge: 116.
Wasekura H., Horie S., Fujii S. & Maki M. (2016) Molecular identification of alien species of Vallisneria (Hydrocharitaceae) species in Japan with a special emphasis on the commercially traded accessions and the discovery of hybrid between nonindigenous V. spiralis and native V. denseserrulata. Aquatic Bot. 128: 1-6.