EGERIA DENSA PLANCHÓN (HYDROCHARITACEAE) : A NEW ANGIOSPERMIC RECORD FOR BANGLADESH

Hydrocharitaceae of Bangladesh is represented by 6 genera namely, Hydrilla, Blyxa, Nechamandra, Vallisneria, Ottelia and Hydrocharis (Khan and Halim, 1987). Under these genera 9 species have been reported so far. Another member of Hydrocharitaceae, Egeria densa Planchón is native to the coast of southeastern Brazil through Argentina. The plant was also cultivated as an aquarium plant and is now considered naturalized in the Eastern United States (Cook and Urmi-König, 1984). The plant material was collected through a limnological expedition carried out in a remote natural lake of Bangladesh namely Bogakain situated in the hilly Bandarban district (Khondker et al., 2010). Details of the sampling location together with the preliminary limnological condition of the lake has been presented in Khondker et al. (2010). Bogakain, an eutrophic hilly natural lake has a vast littoral. The sample was collected on 10 March 2010 from 1 m depth near the shore of the lake as submerged vegetation and transplanted in a concrete house (1 × 0.5 m, depth 0.40 cm) in the Botanical Garden, Department of Botany, University of Dhaka. Some fresh materials were preserved in 4% formaldehyde and few herbarium sheets of the material were prepared and preserved in the Hydrobiology and Limnology Laboratory, Department of Botany, University of Dhaka. The specimen has been identified as Egeria densa Planchón with the help of Cook and Urmi-König (1984), Haramoto and Ikusima (1988), Roberts et al. (1999), Haynes (2000), Yarrow et al. (2009) and Morgan (2010). Egeria densa Planchón was not reported earlier from the areas that now fall under the territory of Bangladesh in the relevant literatures, viz. Hooker (1888), Prain (1903), Heinig (1925), Raizada (1941), Datta and Mitra (1953), Sinclair (1955), Mia and Khan (1995) and Rahman (2004 a, b). Hence, it is reported here as a new record for Bangladesh. A detailed taxonomic account along with illustrations of the species has been prepared based on the fresh specimen.

Hydrocharitaceae of Bangladesh is represented by 6 genera namely, Hydrilla, Blyxa, Nechamandra, Vallisneria, Ottelia and Hydrocharis (Khan and Halim, 1987).Under these genera 9 species have been reported so far.Another member of Hydrocharitaceae, Egeria densa Planchón is native to the coast of southeastern Brazil through Argentina.The plant was also cultivated as an aquarium plant and is now considered naturalized in the Eastern United States (Cook and Urmi-König, 1984).
The plant material was collected through a limnological expedition carried out in a remote natural lake of Bangladesh namely Bogakain situated in the hilly Bandarban district (Khondker et al., 2010).Details of the sampling location together with the preliminary limnological condition of the lake has been presented in Khondker et al. (2010).Bogakain, an eutrophic hilly natural lake has a vast littoral.The sample was collected on 10 March 2010 from 1 m depth near the shore of the lake as submerged vegetation and transplanted in a concrete house (1 × 0.5 m, depth 0.40 cm) in the Botanical Garden, Department of Botany, University of Dhaka.Some fresh materials were preserved in 4% formaldehyde and few herbarium sheets of the material were prepared and preserved in the Hydrobiology and Limnology Laboratory, Department of Botany, University of Dhaka.The specimen has been identified as Egeria densa Planchón with the help of Cook and Urmi-König (1984), Haramoto and Ikusima (1988), Roberts et al. (1999), Haynes (2000), Yarrow et al. (2009) and Morgan (2010).
A detailed taxonomic account along with illustrations of the species has been prepared based on the fresh specimen.
(Figs 1-2) Common names: Brazilian elodea, Brazilian waterweed, Common water weed, Dense water weed, Egeria, Leafy elodea, South American water weed.A submerged, dioecious, freshwater perennial herb, rooted between 1 and 2(7) m below water surface.Stems trailing, elongate, slender, 2-3 mm thick, single or sparingly branched, up to 2 m long, they form dense mat over the water surface, producing adventitious roots at double-nodes.Stem fragments having double-node gives out roots and continue growth, forming dense monospecific stands covering extensive areas.Leaves and stems generally bright green, short internodes frequently give plant very leafy appearance, may persist as fragments, drifting in water column.Leaves 3-6(-8) whorled, densely arranged towards the apex, linear to oblong, cuneate, up to 1.6 × 0.3 cm, minutely serrated, apex acute.Tubers absent.The present specimen has been identified based on leaf and canopy characters, unbranched adventitious roots, buds and canopy like branching arising at double-nodes (Fig. 1 and 2) and absence of tubers.
Ecology: E. densa found in lentic and lotic waters that forms dense monospecific stands which restrict water movement, trap sediment, and cause fluctuations in water quality.
Distribution: E. densa is distributed in Brazil, Argentina, Uruguay, Alaska, Hawaii, has invaded New Zealand (Coffey and Clayton, 1986), Japan (Hamabata and Kobayashi, 2002), in a high altitude lake in Colombia (Carrillo et al., 2006), Chile (Cook and Urmi-König, 1984) and Australia (Roberts et al., 1999) and numerous areas across Europe (Dutartre et al., 1999).In the United States, E. densa has invaded lakes and ponds across most East Coast states, from New England (Vermont, New Hampshire, Massachusetts, Connecticut) south to Florida.In the Western US, it has heavily infested areas of the California delta as well as many lakes in Oregon and Washington.Most recently three water bodies near Moscow and Boise, Idaho were found and treated for infestations (Morgan, 2010).
Notes: Egeria typically looks larger and leafier than Hydrilla and in the former genus, the majority of biomass is located near the water surface (Fig. 1F).Leaves curved downwards in Egeria.On the other hand, Hydrilla leaves are narrow and straight.No tuber formation occurs in Egeria, but potato-like tubers found in Hydrilla which remains attached to the roots in the mud.Double nodes present in Egeria, which produce lateral buds, branches, and adventitious roots.Double nodes typically spaced along stems at 6-12 node intervals in Egeria (Cook and Urmi-König, 1984).Only fragments with a double node develop into new plant.In Hydrilla double node is absent.All these differences are shown in Fig. 2.

Fig. 1 A
Fig. 1 A-F.A, Habit of E. densa with long stem (just after collection from the lake), B. Habit after culturing in a concrete house, C. A double-node (2 adjacent nodes separated by a short internode), D. Leaves found to be present at nodes, E. Lateral buds, branches and adventitious roots growing from a doublenode region of the stem, F. Plant canopy in the natural habitat (Lake Bogakain).