Daphnia magna with eggs
Daphnia are small, planktonic crustaceans, between 0.2 and 5 mm in length. Daphnia are members of the order Cladocera, and are one of the several small aquatic crustaceans commonly called water fleas because of their saltatory swimming style (although fleas are insects and thus only very distantly related). They live in various aquatic environments ranging from acidic swamps to freshwater lakes, ponds, streams and rivers.
The lifespan of a Daphnia does not exceed one year and is largely temperature dependent. For example, individual organisms can live up to 108 days at 3°C while some organisms live for only 29 days at 28°C. A clear exception to this trend is during the winter time in which harsh conditions limit the population in which females have been recorded to live for over six months. These females generally grow at slower rate but in the end are larger than ones under normal conditions.
Sometimes Daphnia may be used in certain environments to test the effects of toxins on an ecosystem. This makes Daphnia an indicator species, particularly useful in that area because of its short lifespan and reproductive capabilities. Because they are nearly transparent, their internal organs are easy to study in live specimens (an example might be to study the effect of temperature on the heart rate of these ectothermic organisms). They are often fed to tadpoles or small species of amphibians such as the African Dwarf Frog, Hymenochirus biettgeri. Daphnia are also a popular live food in tropical and marine fish keeping.
Several water flea species are considered threatened. The following are listed as vulnerable by IUCN: Daphnia nivalis, Daphnia coronata, Daphnia occidentalis, and Daphnia jollyi. Some species are halophiles, and can be found in hypersaline lake environments.
A herbicide is used to kill unwanted plants. Selective herbicides kill specific targets while leaving the desired crop relatively unharmed. Some of these act by interfering with the growth of the weed and are often synthetic "imitations" of plant hormones. Herbicides used to clear waste ground, industrial sites, railways and railway embankments are non-selective and kill all plant material with which they come into contact. Some plants produce natural herbicides, such as the genus Juglans (walnuts). They are applied in total vegetation control (TVC) programs for maintenance of highways and railroads. Smaller quantities are used in forestry, pasture systems, and management of areas set aside as wildlife habitat.
Herbicides are widely used in agriculture and in landscape turf management. In the U.S., they account for about 70% of all agricultural pesticide use.
Certain herbicides affect metabolic pathways and systems unique to plants and not found in animals making many modern herbicides among the safest crop protection products having essentially no effect on mammals, birds, amphibians or reptiles.
Some herbicides cause a range of health effects ranging from skin rashes to death Template:Ref needed. The pathway of attack can arise from intentional or unintentional direct consumption, improper application resulting in the herbicide coming into direct contact with people or wildlife, inhalation of aerial sprays, or food consumption prior to the labeled pre-harvest interval. Under extreme conditions herbicides can also be transported via surface runoff to contaminate distant water sources. Most herbicides decompose rapidly in soils via soil microbial decomposition, hydrolysis, or photolysis. Some herbicides are more persistent with longer soil half-lives Template:Ref needed. Other alleged health effects include chest pain, headaches, nausea, and fatigue.
All organic and non-organic herbicides must be extensively tested prior to approval for commercial sale and labeling by the Environmental Protection Agency. However, because of the large number of herbicides in use, there is significant concern regarding health effects. Some of the herbicides in use are known to be mutagenic, carcinogenic or teratogenic.
Some herbicides may have therapeutic uses. There is current research into the use of herbicides as an anti-malaria drug that targets the plant-like apicoplast plastid in the malaria-causing parasite Plasmodium falciparum.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For more information (background, pictures, experiments and references): Daphnia, Herbicides