Why do some fish live in freshwater and others only in seawater?
Quick Answer
The difference is mostly about osmoregulation: balancing water and salt in the body. Freshwater fish and marine fish face opposite problems, so their kidneys, gills, and hormones are adapted differently.
The Full Story
Freshwater fish live in water that is less salty than their bodies, so water tends to enter their tissues while salts are lost. Marine fish face the opposite problem: seawater is saltier, so they tend to lose water and gain salt. To survive, freshwater fish usually excrete lots of dilute urine and actively absorb salts, while marine fish often drink seawater and remove excess salt through their gills and kidneys. Salmon are famous because they are anadromous, meaning they migrate between fresh and salt water during their life cycle. That migration requires major physiological changes controlled by hormones.
Key Facts
YouTube Angle
Suggested video title for this topic:
"Why fish don’t all live in the same kind of water"