The Hydrocharitaceae is a small but morphologically diverse family of fully aquatic monocots. About eighty species are included in about fifteen genera, excluding the genus Najas which has occasionally been suggested to belong in this family (Tanaka et al., 1997). Hydrocharitaceae includes both freshwater and marine species, such as seagrasses (Enhalus, Thalassia and Halophila), eelgrasses (Hydrilla, Vallisneria) and oxygen weeds (Egeria, Elodea, Lagarosiphon). Depending on the species, Hydrocharitaceae may live their lives partially or entirely submerged.
A major factor in the diversity of this family derives from the diversity of pollination methods. Hydrocharitaceae may be monoecious (separate male and female flowers, but on the one plant) or dioecious (separate male or female plants). Many of the genera in this family are insect-pollinated (entomophilous), with flowers protruding from the water, and comparison with related families suggests that this is the ancestral condition for the family. However, more that one lineage of Hydrocharitaceae has evolved to take advantage of their home environment by becoming water-pollinated (hydrophilous). Such species may be epihydrophilous, with flowers borne at the water surface, or hypohydrophilous, with flowers completely underwater. The genera Vallisneria, Lagarosiphon, Nemachandra and Enhalus have become epihydrophilous in a way that no other aquatic flowering plant has - rather than releasing pollen into the water like other hydrophilous plants, the entire male flower is released to float on the surface of the water until it reaches a female flower. Remarkably, while this process is unique to Hydrocharitaceae, genera with detaching flowers do not form a single clade - instead, the process has evolved at least three times within the Hydrocharitaceae (Tanaka et al., 1997, who did not test the position of Nemachandra).
Hydrocharitaceae also have other reproductive options open to them. Vegetative reproduction is common among plants, a factor which gardeners have profited from for generations. The Hydrocharitaceae are vegetative reproducers par excellence, with broken-off fragments all too ready to reroot and establish themselves. In the case of the Florida seagrass Halophila johnsonii, the only marine plant listed as endangered in the United States, vegetative reproduction may be the only thing keeping it going. Only female flowers have ever been recorded for this species, and seed production has never been recorded. While some have suggested that Halophila johnsonii may reproduce apomictically (parthenogenetically for the zoologically-inclined), York et al. (2008) demonstrated that this is probably not the case - ovules and gametes are produced in the same manner as other sexually-reproducing Halophila species. Theoretically, pollination of these ovules should be entirely possible - but somehow, the males have all disappeared, and the Halophila johnsonii females are waiting for a pollinator that will never come.
Unfortunately, this propensity for vegetative propagation is also the dark side of Hydrocharitaceae. Widely propagated as aquatic ornamentals or for aquaria and fish tanks, Hydrocharitaceae have found it all too easy to escape their alloted positions and invade exotic waterways. In New Zealand, three species of oxygen weed (Egeria densa, Lagarosiphon major and Elodea canadensishave become leading invasives, despite the fact that, for all three, individuals of almost invariably only a single sex are present in the wild, rendering sexual reproduction nonexistent (Healy & Edgar, 1980). In 1968, power production at a hydroelectric station on the Waikato River was brought to a halt by dense Lagarosiphon major blocking the station's water intake. Elodea canadensis achieved even greater invasions:
The heavy stand in Lake Rotoroa, Nelson Lakes National Park is noteworthy: between 1965 and 1971 the plant formed a virtually complete marginal weed-bed to a depth of 8.5 m, with stems to 6 m high. Here, in terms of dry matter per m2, the amount of weed herbage is significantly higher than that recorded for any other freshwater macrophyte community elsewhere in the world. (Healy & Edgar, 1980)
Healy, A. J., & E. Edgar. 1980. Flora of New Zealand vol. III. Adventive cyperaceous, petalous and spathaceous monocotyledons. P. D. Hasselberg, Government Printer: Wellington (New Zealand).
Tanaka, N., H. Setoguchi & J. Murata. 1997. Phylogeny of the family Hydrocharitaceae inferred from rbcL and matK gene sequence data. Journal of Plant Research 110 (3): 329-337.
York, R. A., M J. Durako, W. J. Kenworthy & D. W. Freshwater. 2008. Megagametogenesis in Halophila johnsonii, a threatened seagrass with no known seeds, and the seed-producing Halophila decipiens (Hydrocharitaceae). Aquatic Botany 88: 277-282.