Sharks may seem to be invincible, yet they have some significant weaknesses. While some dolphins succumb to their attacks, acting as a group they can kill a shark by ramming it. Sharks
also have to keep moving otherwise they die. 
Now it has been discovered that fresh water is their enemy. They get dehydrated and cannot remain alert. Breeding is out of the question in non-salty water. A terrific battle for survival ensues as sharks continually keep sinking.
The reason for this is that sharks do not have air bladders. This "device" enables fish to swim at mid-depths. Sharks use their oily livers to stay afloat - very inefficient. They have to work 50 per cent harder when they leave the sea.
Freshwater species of shark have refined the evolution of their livers. The organs are flatter than their ocean cousins. There is a penalty for a flatter liver. A fatter body means a less sleek body which makes river and estuary species slower. Even with fatter livers, they live close to the bottom of the water.
✴ Biology by Ty Buchanan ✴
