Question: How come bull sharks can live in both salt and freshwater?

Katherine Benson answered on 23 Apr 2020:

It’s because their kidneys and other special glands are specially adapted. When in salt water, the bull shark filters the water really well to remove a lot of the salt, moving it to the urine. When in fresh water, the kidneys hold on to some extra salt, so that less gets thrown away in the urine.