10: Sharks, alligators and goldfish

Sharks and alligators and goldfish, oh my!

What, if anything, does all of this tell us about life-as-we-know-it? Here is one small example.

Below is a table of osmolarities associated with environments and organisms:

seawater: 1 OsM freshwater: 0.001 OsM
shark: 1.075 OsM goldfish: 0.293 OsM
alligator: 0.278 OsM
mammal: 0.330 OsM

So a shark is a little “saltier” than its environment, and a freshwater fish is a lot saltier (inside) than the water around it. Why?

Sharks do not generally “drink”. Instead they get freshwater from the seawater around them. Because their insides are a little saltier than the water around them, freshwater flows INTO their cells from their surroundings. The opposite of what would happen to you or me, who would get pickled if we spent too long in the ocean.

On the other hand, mammals are a lot saltier than their environment. But when you drink water, your body fluids become more diluted, so water flows INTO your cells. That’s why everyone wants you to drink 8-10 glasses a day of water. And by the same token, if you put a shark into freshwater, the results would NOT be pretty. (The difference in OsM between shark and freshwater is 1.074, vs. 0.292 for goldfish and freshwater.)