Sharks!!!

Before dinosaurs roamed the earth, sharks were swimming the seas. For 420 million years, sharks have been evolving and adapting to their environments. One of the most essential aspects of this evolution has been the development of osmoregulation. There are three primary environments in which osmoregulators live: terrestrial, freshwater, and marine. Because sharks primarily live in saltwater, they live in the marine environment. This environment can pose a problem to many animals, as it makes it hard for them to achieve homeostasis, the state at which the ratio of solutes to solvent inside the body equals the ratio of solutes to solvents outside of the body. It is one of the most basic goals of any animal to acheive homeostasis, and thus, over time, each species of animal has developed a unique way of balancing the ratios. A shark is an osmoconformer, as are most animals that live in the marine environment. This means that it adapts its internal environment to have homeostasis with that of the outside. The bodies of sharks are especially high in two substances, urea and trimethylamine N-oxide. Urea is commonly found in urine, and trimethylamine N-oxide is commonly found in decomposing animals. Both of these compounds allow a shark to be isotonic. Any excess salt in the body is removed through the urine. Thus, unlike most animals that live in salt water, sharks do not drink the water that they live in. Instead, they change their concentration gradient by storing large amounts of chemicals inside of their own bodies. This allows can absorb water straight from the ocean and into their own cells. However, this prevents most sharks from living in fresh water, with few exceptions. One of these exceptions would be the bull shark. A bull shark has adapted its kidneys so that when the shark moves into freshwater, the kidneys expel less salt and more urea, allowing the shark to retain more of the salt necessary to reach homeostasis. Overall, the ability of a shark to osmoconform has been vital to their survival for millions of years.

Fun facts:
These facts do not relate to osmoregulation, but they relate to sharks and are quite interesting!
1. The skeleton of a shark is made entirely of cartilage
2. Great White sharks can jump ten feet into the air!
3. Certain species of sharks will stop breathing if they stop moving
4. Great White sharks eat 11 tons of food in one year!
5. A mere 20 of the 350 species of sharks worldwide have attacked humans
6. On average, people kill 73,000,000 sharks each year
7. Magnets can repel sharks
8. Sharks are able to hear the fish that they eat from 800 feet away
9. Sharks can smell one drop of blood in an area of water equivalent to that of an olympic pool (660,000 gallons)
10. The smallest type of shark is eight inches long!

An image of the pygmy shark, the world's smallest shark

Sources:
http://www.thebiohub.blogspot.com
http://en.wikipedia.org/wiki/Shark
http://www.sharks.org.za/osmoregulation.html
http://dsc.discovery.com/sharks/top-shark-facts-10.html
http://dsc.discovery.com/sharks/top-shark-facts-03.html
http://dsc.discovery.com/sharks/top-shark-facts-01.html
http://dsc.discovery.com/sharks/top-shark-facts-04.html
http://dsc.discovery.com/sharks/top-shark-facts-08.html
http://dsc.discovery.com/sharks/top-shark-facts-07.html
http://en.wikipedia.org/wiki/Olympic-size_swimming_pool
http://dsc.discovery.com/sharks/top-shark-facts-09.html
http://static.howstuffworks.com/gif/great-white-shark-1.jpg
http://static.howstuffworks.com/gif/pygmy-shark-2.jpg
http://en.wikipedia.org/wiki/Bull_shark
http://www.sharksavers.org/en/education/shark-biology-behavior/385-how-bull-sharks-can-live-in-fresh-water-through-clever-osmoregulation.html