Fish have built-in mechanisms that allow them to move up and down and side to side in their environment.
To ascend, a fish must reduce its overall density by increasing its volume without significantly increasing its mass. Most fish do this with something called a swim bladder. A swim bladder is just an expandable sac, like a human lung. To reduce its overall density, a fish fills the bladder with oxygen collected from the surrounding water via the gills. When the bladder is filled with this oxygen gas, the fish has a greater volume, but its weight is not greatly increased. When the bladder is expanded, it displaces more water and so experiences a greater force of buoyancy. When the bladder is completely inflated, the fish has maximum volume and is pushed to the surface.
When the bladder is completely deflated, the fish has minimum volume and sinks to the bottom of the environment they are in. To stay at a particular level, a fish fills its bladder to the point at which it displaces a volume of water that weighs what the fish weighs. In this case, the forces of buoyancy and gravity cancel each other out, and the fish stays at that level.
Most fish rise and sink using this method, but not all do. Some species don’t need a swim bladder because they spend all their life skimming along the floor. Other fish, such as rays and sharks, ascend and descend by propelling themselves forward. Just as in an airplane, the movement of fluid under the fins creates lift, which pushes the fish upward.
How Fish Breathe
The oxygen that fish breathe is not the oxygen in H2O. Instead, fish are breathing O2 (oxygen gas) that is dissolved in the water. Many different gases dissolve in liquids, and we see an example all the time in carbonated beverages. In these beverages, there is so much carbon dioxide gas dissolved in water that it rushes out in the form of bubbles. Fish “breathe” the dissolved oxygen out of the water using their gills. Extracting the oxygen is not very easy, air has close to 20 times more oxygen in it than the same volume of water. Plus, water is a lot heavier and thicker than air, so it takes a lot more work to move it around. The main reason why gills work for fish is the fact that fish are cold-blooded, which reduces their oxygen demands. Warm-blooded animals like whales breathe air like people do because it would be hard to extract enough oxygen using gills.
Did You Know
The name “piranha” is derived from the Tupi Indian language, native to Brazil. It’s a combination of the Tupi word pira, or “fish,” and ranha, meaning “tooth.”
As fish age, their scales grow larger in size.
The study of fish is called ichthyology, from the Greek words for “fish” and “discourse.”
Other animals that live in water, such as frogs in the tadpole stage, also have gills and backbones, but fish are the only backboned animals with two-chambered hearts.
The largest fish is the whale shark, which is estimated to reach a length of 60 feet or more.
The sailfish is the fastest fish in the ocean, cruising at speeds of up to 68 mph.
Sharks, skates, and rays belong to the Chondrichthyes class of fish. These fish have no bones; their skeletons are made of cartilage.