MadSci Network: Zoology
Query:

Re: How do Bull Sharks adapt to fresh water?

Date: Mon Nov 19 21:07:47 2001
Posted By: Alastair Lyon, Science Information Officer
Area of science: Zoology
ID: 1005670737.Zo
Message:

Hi Annia,

I don't know about the experiment, but I can tell you a little about the 
magic of Bull sharks (Carcharhinus leucas).

Because marine fish (not sharks) constantly have water leaking out of 
their system, which they maintain themselves by drinking large amounts of 
water and extruding the excess salt via special cells on their gills. 
Marine fish are HYPOsmotic - they're not as salty as the seawater.

Sharks have a different strategy. They're actually slightly saltier but 
very similar to the surrounding water (because of stored urea). They take 
on water as they need it, and extra water is "dumped overboard" via the 
kidneys. BUT, if a shark went into a river, it would be overwhelmed by the 
influx of freshwater.

Bullsharks and some stingrays have special strategies to overcome this - 
giving them an advantage of being able to use freshwater and estuarine 
environments to find food. Bull Sharks can adjust their urea level (within 
limits!)- so they can reduce it to a close equivalent to that of the water 
they are in, so the influx of fresher water into their system is minimal 
and can be dealt with by the kidneys.

I hope this helps clear your mind what's going on. 

cheers,

Alastair


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