Do fish feel thirsty?

Gummy Bear Osmosis Experiment

To help explain omoregulation, you can use the classic gummy bear osmosis experiment.

The gummy bear represents a fish. The outside represents the semi-permeable skin of a fish, and the inside represents the inside of the fish.

You’ll need

Several gummy bears or other gummy sweets.

Water

Salt

Two containers

Instructions

Half fill both containers with plain water.

Add salt to the second container until it’s hard to dissolve any more.

Freshwater fish

Place a gummy bear in the plain water container for a few hours. It should increase in size.

The concentration of water in the container is higher than the concentration of water inside the gummy bear, so water moves from the container into the bear by osmosis.

This is why fish living in freshwater don’t drink and have to excrete urine almost constantly. If they didn’t, they would swell up and maybe even burst!

Saltwater Fish

Carefully place the swollen gummy bear in the salty water and leave for a few hours. This time, you should find the gummy bear shrinks.

If you made the water salty enough, the concentration of water inside the bear should be higher than that of the salty water, and so water moves from the bear into the water, making the bear shrink.

Saltwater fish have to drink constantly, as water is continually moving out of their body into the salty ocean.