Sharks in the fresh water of Nicaragua

The largest lake of Central America, in southwest Nicaragua.
 



The freshwater lake contains fish, such as tuna and sharks,
usually found only in salt water because it was part of the Caribbean Sea
until land masses rose around it in prehistoric times.


Lake Nicaragua, 3,089 sq mi (8,001 sq km), c.100 mi (160 km) long
and up to 45 mi (72 km) wide.
It is drained into the Caribbean Sea by the San Juan River.

Sharks that lives in Lake Nicaragua is a species of shark that has been adapting for a
long time in the lake since it's being seperated from the sea millions of years ago.



How Sharks Survive in Fresh Water

For most shark species, spending a day in fresh water would be like
placing a human on the moon without a spacesuit.
It could not survive due to the inhospitable surrounding environment.
A process called osmosis is central to the problem.
Osmosis is when a fluid moves through a semi-permeable membrane from a solution
with a low solute concentration to a solution with a higher solute concentration,
until there is an equal concentration of liquid on both sides of the membrane.
The dissolved substances, in this case, primarily involve sodium and chloride.  

Since sharks evolved in salt water, they tend to have very salty bodies.
Even sharks in fresh water contain more than twice the amount of salt and chloride
as more common freshwater fishes.
In theory, they should burst like an overfilled water balloon, given the osmosis effect,
but they have come up with an effective answer to the problem -- they urinate a lot.  

Thomas Thorson studied bull sharks living in Lake Nicaragua and found these huge fish
take in a lot of extra water, as expected, but they excrete much of it as dilute urine,
at a rate of over 20 times that of typical saltwater sharks.
That means their kidneys must work extra hard, utilizing additional energy.
Like people who become accustomed to life in low oxygen regions, however,
sharks in fresh water appear to adapt to what would seem to be formidable conditions.




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