Most of the sharks are marine. However, only the shark named the giant shark can live in freshwater and has the character which goes back to the river. It is this research to have pursued why such a thing is possible.
The bull shark is a kind of shark which lives widely in the tropics and temperate zone. It also lives in rivers around Okinawa and is called Hironaka.
It might be able to be said that it is the most dangerous kind in the shark because it has the character which advances to the freshwater region on that though it is a troublesome existence that the temper is rough because of omnivorous, and the example of attacking man is known.
The species can be commonly inhabited in freshwater areas, and there are examples of freshwater lakes that go thousands of kilometers back in the Mississippi, Zambezi, and Amazon rivers.
The maximum length is 1 meter and the weight exceeds 1 kg. Well, the shark belongs to the cartilage fish as well as the ei and the gin shark, etc. Humans are hard-bone vertebrates, but cartilage fish and hard-boned animals are said to have branched out about 450 million years ago.
Cartilage fish, in order to adapt to high salt and osmotic pressure of seawater, to prevent dehydration by storing urea in the body.
Most of the cartilage fish are marine species, and freshwater cartilage fish are only known only slightly as freshwater rays (potamotrigone) in the Amazon basin. There is a lot of “wide salt” fish which can live in both seawater and freshwater in the hard bonefish, but it is few in the cartilage fish.
Its slight wide-salt cartilage fish is known to continue to hold high concentrations of urea in the body even if placed in freshwater. The research team of the Institute of Atmospheric and Oceanic Sciences, The University of Tokyo, Riken, and Okinawa Churaumi Aquarium conducted an experiment to migrate the bull shark raised at The Okinawa Churaumi Aquarium from seawater to freshwater.
Then, it is said that various genes are activated in the kidney, sodium chloride and urea are reabsorbed by the kidney, and thin urine is discharged.