Water water everywhere
Narayani Subramanian
"Water water everywhere and not a drop to drink", claims the poem by Samuel Taylor Coleridge. It talks about how man cannot drink seawater. One of the first survival instructions for people who get stranded in a remote island is to never drink seawater, however thirsty you get. Seawater is toxic to humans. If there is too much salt in your body, your kidneys cannot function properly and that can be deadly.
What about the animals that live in salty water? What about their kidneys?
Let us take fishes as an example and see what happens to them in two different types of enviroment : Freshwater and Seawater.
As you may have read in school, solutions that have a high concentration of a soluble substance (solute) are called hypertonic. Solutions with low concentrations of solute are called hypotonic. In this case, the solute is salt. So seawater is a hypertonic solution and freshwater is a hypotonic solution.
In a hypertonic solution, that is seawater, the cells inside the fishes tend to shrink by losing their liquid. In a hypotonic solution that is freshwater, fish cells tend to swell up by taking in all the water and bloat. Both these scenarios are deadly for the fish cells, as swelling or shrinking leads to cell death. Fishes keep themselves safe by a process called "osmoregulation" which is the act of balancing the salt and water content of the body in comparison to the outside environment. 
By logic, we can understand shrinking cells can be saved by addition of more water and swollen cells can be saved by water removal.Marine fishes drink a lot of seawater to prevent their cells from shrinking. While this maintains the water balance, this brings an additional problem: the incoming seawater has a lot of salt, which is dangerous for the body. The fishes have a lot of mechanisms to get rid of the salt. They filter out the excess salt inside the plasma with the help of their gills and kidneys. There is an enzyme called Sodium Potassium ATPase which helps in pumping the excess salt to the outside environment through the gills. The kidneys also help in filtering out the excess salt through urine. 
In freshwater fishes, water-salt regulation happens through excessive urination. They need to concentrate the salt content inside the body to compensate for the low salinity environment. So they keep producing dilute urine in excessive amounts throughout the day. They can urinate a volume of upto one-third of their body weight per day!
Ok... now the question arrives.
What about the fishes that migrate to other environments? We have heard of marine salmon going to rivers for spawning, and freshwater eels migrating to the ocean. If they are hard wired to either remove or add water into the body, how do they manage to survive when the salinity suddenly changes?
Nature has its tricks. These are what the scientists call "Euryhaline" species, or species with a tolerance for a wide range of salinities. They can regulate their internal mechanisms according to the external environment whenever it  is necessary, as long as the salinity transition is gradual. The gradual transition is achieved as the fishes migrate slowly into the new environment. 
While this works for fishes, drinking of seawater is not a common behaviour in marine mammals. They maintain water balance through metabolic processes. They aslo have mechanisms to reduce loss of water, thereby maintaining a stable level of liquids and salt inside their body.
How do you think seasnakes evolved the ability to maintain the water and salt balance? Write to JM and let us know!