Sharks thrive in salty seas but sink in fresh water, a new study shows.

They use a primitive floating mechanism, an oily liver, which works only in salt water.

Some modern fish have evolved with a liver that allows them to easily float in freshwater environments. They have air livers, which trap air inside a system of chambers. However, sharks have a more ancient and simply constructed liver that is not very suitable for swimming in fresh water.

For a study published online in the Journal of Experimental Biology on Jan. 8, the team theorized that this problem with floating is the main reason why most elasmobranchii have not expanded to freshwater habitats. Only five percent of sharks spend some time in freshwater environments.

The research team used models of freshwater habitats to see what changes in physiology would make a shark able to swim instead of sinking. They found that a shark's liver would need to grow by at least eight times its current volume to allow a shark to float in freshwater.

The researchers studied the bull shark, one species of sharks that dwells in fresh water for part of the time. They found that these sharks ate more in an effort to give their livers maximum volume so that they could float better. However, this would also have the effect of increasing the shark's entire body, creating more weight for the shark to drag around. This would be inefficient.

The team then studied five bull sharks and found that they could float less well than 27 other species of marine sharks. The team concluded that, given the physiognomy of sharks, there is no way for sharks to be as effective swimmers in freshwater situations as they are in the sea.

Researchers previously thought that sharks were ill-adapted to swimming in fresh water for metabolic reasons. This is one of the first in-depth looks into the role of the shark's liver.

Fossil records indicate that sharks used to spend more time in freshwater environments. The researchers behind this study noted that it was not clear what caused sharks to move from fresh water to salt water initially.

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