Glossostigma cleistanthum is a low-growing aquatic plant that has begun appearing in lakes of the Northeast. In collaboration with Don Les from the University of Connecticut, The Connecticut Agricultural Experiment Station Invasive Aquatic Plant Program (CAES IAPP) has identified 19 populations of the plant in North America: six in Connecticut, one in Rhode Island, one in Pennsylvania and 11 in New Jersey. Because the plant is small, easily overlooked and easily confused with other small aquatic plants, it is likely that Glossostigma cleistanthum occurs more widely than is currently known.
Glossostigma cleistanthum generally occurs in lakes, although the first North American location was in Hamburg Cove, a freshwater tidal wetland in Connecticut, where the species was collected in 1992. It usually is found on sandy sediment but can occur on silt and gravel as well. Lakes where it occurs generally have very clear water, and this determines how deep the plants will grow. It has been found in as much as 4 m of water in one very clear lake. Water chemistry analysis shows that Glossostigma cleistanthum occurs in lakes with lower pH, lower alkalinity, lower conductivity and lower phosphorus levels than Connecticut lakes in general. This is a species that, unlike many invasive aquatic plants, will invade oligotrophic waters.
Plants spread rhizomatously along the sediment, producing paired spatulate leaves that are generally 1-3 cm long. Glossostigma cleistanthum occurs both submerged and emersed on the shore. When emersed, it produces tiny open-pollinated flowers on pedicles that are about as long as the leaves ; when submerged, Glossostigma cleistanthum produces cleistogamous (self-fertilized) flowers on very short stalks. Cleistogamous flowers are very abundant and can be seen easily along the rhizomes. Few of these produce mature fruits, but those that do contain abundant seeds. Emergent Glossostigma cleistanthum plants are annuals and die back in the winter. Submerged plants, however, are perennial; they remain green through the winter, continuing to grow and produce flowers. Because of this perennial submerged habit, there is no reason to believe the spread of Glossostigma in the Northeast will be limited by cold winter temperatures.
Glossostigma cleistanthum commonly occurs in densities of 10,000-25,000 plants per square meter, covering large areas of lake bottom and shoreline , . Densities can exceed 100,000 plants per square meter over small areas . In Mansfield Hollow Reservoir, Connecticut, the species occurs in 91 discrete patches and covers about 12,000 square meters of lake bottom.
Because of the plants’ small size, they are unlikely to interfere with recreational uses of New England lakes. The plants may pose a serious ecological threat, however, because they have shown an ability to invade natural, undisturbed lakes and now occur with a number of species that appear on lists of endangered species in Connecticut and/or New Jersey, including Eriocaulon parkeri, Crassula aquatica, Lilaeopsis chinensis, Limosella subulata, Myriophyllum tenellum and Heteranthera multiflora.
Molecular analysis to identify the species is continuing. The genus has seven or eight species, all native to Australia, New Zealand, India and east Africa. It is believed that the previous identification of the North American plants as Glossostigma diandrum is in error, but positive identification has not yet been made. One species, Glossostigma elatinoides, is popular in the aquarium trade. This is not the species that occurs in North America, but the invasive Glossostigma cleistanthum probably was introduced with aquarium plants.