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.

Over-the-counter (OTC) drugs are medicines that may be sold to a customer without a prescription. The term "over-the-counter" is somewhat counter-intuitive, since these items can often be found on the shelves of stores and bought like any other packaged product in some countries in contrast to prescription drugs which are more likely to literally be passed over a counter from the pharmacist to the customer. Some medicines considered safe in general terms may be available in general stores, supermarkets, gas stations etc. The rules vary considerably from country to country.

As a general rule, over-the-counter drugs have to be primarily used to treat a condition that does not require the direct supervision of a doctor and must be proven to be reasonably safe and well-tolerated. OTC drugs are usually also required to have little or no abuse potential, although in some areas drugs such as codeine are available OTC (usually in strictly limited formulations or requiring paperwork or identification to be submitted during purchase). One of the oldest OTC drugs is aspirin

Over time, drugs that prove themselves safe and appropriate for self-medication, may be switched from prescription to OTC. An example of this is diphenhydramine (Benadryl) which once required a prescription but now is available OTC nearly everywhere. Diphenhydramine is a deliriant, nevertheless, most recreational drug users find its effects uncomfortable rather than exciting. More recent examples are cimetidine and loratadine in the United States, and ibuprofen (Herron Blue/Nurofen) in Australia.

Recently many U.S. drugstores have begun moving products containing pseudoephedrine into locations where customers must ask a pharmacist for them. A prescription is not required; the change is allegedly being made in an effort to reduce methamphetamine production. Since the passage of the Methamphetamine Precursor Control Act, the purchase of pseudoephedrine in the United States is restricted and the identity of the purchaser is required to be obtained and recorded. In addition, pseudoephedrine itself is a mild stimulant- somewhere between caffeine and ephedrine. Nonetheless, these products are still considered OTC since no prescription is required.

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)

For more information (background, pictures, experiments and references): Daphnia, Over-the-Counter Drug