Hydrilla (waterthyme) is a genus of aquatic plants, usually treated as containing just one species — Hydrilla verticillata, but some botanists divide it into several species. It is native to the cool and warm waters of the Old World in Asia, Africa, and Australia, with a sparse, scattered distribution; in Australia it occurs from the Northern Territory to Queensland and New South Wales. The stems grow up to 2 m long. The leaves are arranged in whorls of two to eight around the stem, each leaf 5–20 mm long and 0.7–2 mm broad, with serrations or small spines along the leaf margins; the leaf midrib is often reddish when fresh. It is monoecious (sometimes dioecious), with male and female flowers produced separately on a single plant; the flowers are small, with three sepals and three petals, the petals 3–5 mm long, transparent with red streaks. It reproduces primarily vegetatively by fragmentation and by rhizomes and turions (overwintering), and flowers are rarely seen. They have air spaces to keep them upright. Hydrilla has a high resistance to salinity compared to many other freshwater aquatic plants. It can grow up to an inch a day, producing dense mats of vegetation along the bottom of lakes and rivers. As it grows up to the water's surface, these mats can become several feet thick.
Taxonomy and naming Hydrilla closely resembles some other related aquatic plants, including Egeria densa and Elodea canadensis. Synonyms include H. asiatica, H. japonica, H. lithuanica, and H. ovalifolica.
Ecology Hydrilla verticillata is negative allelopathic to the common hornwort (Ceratophyllum demersum) and prickly hornwort (C. muricatum), that is, it produces compounds that inhibit growth of the latter two species. As aquatic macrophytes, Hydrilla play critical roles in the ecosystem. They influence nutrient cycles and the ecology of the body of water, as well as the sediments. Hydrilla interacts with other organisms, supplying food and nutrients as well as habitats and shelters. Hydrilla can have negative impacts in aquatic communities. When abundant, they affect dissolved oxygen levels, which can lead to decline in populations of fish, invertebrates, and other plant species.
Status as an invasive plant
Introduction
Hydrilla is naturalized and invasive in the United States following release in the 1950s and 1960s from aquariums into waterways in Florida, due to the aquarium trade. It is now established in parts of southern Canada and in the United States from Connecticut to Texas, and also in California. By the 1990s control and management were costing millions of dollars each year. The plant was introduced when a Florida West Coast aquarium dealer shipped live Hydrilla from Sri Lanka under the common name "Indian star-vine." These plants were considered unsatisfactory and were dumped into a canal near Tampa Bay, where they flourished. By 1955, the plants found their way from Tampa to Miami as they were transported for cultivation and pet trade sale. It is believed that several undocumented cases of accidental or careless releases followed, as there was extensive spread of the Hydrilla throughout Florida and the southeastern United States.
Importance of accurate identification Accurate identification of Hydrilla verticillata is essential for regulatory reporting, ecological monitoring, and management planning. Misidentification may lead to unnecessary treatment of native vegetation or delayed response to invasive spread. Hydrilla is frequently confused with Elodea canadensis and Egeria densa due to similar submerged growth form.
Problems
Due to the combination of herbicide resistance, high growth rates, high ecological adaptability, dispersion ability, and low resources required, Hydrilla is able to invade almost every region of the world and spread at an alarming rate, affecting the ecosystem in a negative manner. The high photosynthesis rate of Hydrilla leads to the depletion in dissolved carbon dioxide during daytime, which raises the pH, and to higher concentrations of oxygen. At night, the oxygen is used for oxidative phosphorylation, resulting in anoxia, and carbon dioxide is replenished, which in turn lowers the pH. As an invasive species in Florida, Hydrilla has become the most serious aquatic weed problem for Florida and most of the U.S. Because it was such a threat as an invasive species, one of the first cost-effective broadscale herbicide controls developed was fluridone. Unfortunately, this single-use herbicide resulted in fluridone-resistant Hydrilla. "As Hydrilla spread rapidly to lakes across the southern United States in the past, the expansion of resistant biotypes is likely to pose significant environmental challenges in the future." Hydrilla populations have caused economic, environmental, and ecological damage. Hydrilla is known to be an aggressive and competitive plant, out-competing and displacing native species, such as pondweeds and eelgrass. Hydrilla has thus created monocultures, areas dominated by a single species, rather than having a balance among many species, as in a normal ecosystem. In Australia, Hydrilla can become invasive if the nutrient levels are raised in disturbed ecosystems, though is not generally known to be problematic. Hydrilla can host a biofilm of the cyanobacteria Aetokthonos hydrillicola, which can produce the brominated neurotoxin aetokthonotoxin — the causative agent of avian vacuolar myelinopathy, a fatal brain wasting disease of waterfowl and raptors.
Management
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