Q.How important is the presence of air bladder in Pisces?

The air bladder (also called the swim bladder) is a gas-filled sac present in bony fishes, and its importance lies in the control it gives over floating.

• It allows the fish to regulate its buoyancy, that is, to stay at a chosen depth in the water without effort.
• Because a bony fish can adjust the gas in this bladder, it does not have to keep swimming merely to avoid sinking. …

When we talk about the air bladder in Pisces, we are talking about a feature that separates the two great groups of fishes. Consider first the cartilaginous fishes, the Chondrichthyes. They have no air bladder at all, so they cannot control their buoyancy. If such a fish stopped moving it would sink; therefore it is forced to swim continuously simply to keep from going down. This is a real cost, because staying afloat uses energy all the time.

The bony fishes, the Osteichthyes, solve this problem with an air bladder — a gas-filled sac inside the body.

• By adjusting the amount of gas it holds, the fish can change how much it tends to float or sink.
• This lets the fish hover at a chosen depth without effort, holding its position in the water rather than working to stay there. …

Method: The "remove it and see" thought experiment

Imagine surgically removing the air bladder from a bony fish and trace the consequences forward, rather than describing its function directly.

Suppose a bony fish's air bladder were removed. What would immediately change? Without any gas-filled sac to adjust, the fish would lose its ability to fine-tune its density relative to water. As soon as it stopped actively swimming, gravity would pull it down and it would sink — exactly the situation cartilaginous fishes are actually in, since they never had an air bladder to begin with. To stay at a chosen depth, the fish would now have to swim continuously, spending energy purely to counteract sinking, rather than being able to rest, hover, or conserve energy at will. This forced continuous swimming would also limit behaviours like resting motionless on the bottom or ambushing prey from a fixed position, since stopping equals sinking. …