Q."Presence of air bladder is a common feature of bony fishes". Justify in one point.

Think of a scuba diver wearing a buoyancy vest. By adding or letting out a little air, the diver can hover at a chosen depth without paddling — neither sinking to the bottom nor bobbing to the surface. A bony fish carries its own version of that vest inside its body: the air bladder, sometimes called the swim bladder.

The air bladder is a gas-filled sac found in the bony fishes (Osteichthyes). Its job is to let the fish regulate its buoyancy — that is, to control whether it floats up, sinks down, or stays level in the water. By managing the gas inside this sac, a bony fish can hold its position at any depth without having to swim continuously just to keep from sinking.

The importance of this becomes clear the moment you compare bony fishes with cartilaginous fishes (Chondrichthyes):

• Bony fishes have an air bladder, so they can float effortlessly and rest at a chosen depth.
• Cartilaginous fishes lack an air bladder. Without it they cannot control their buoyancy, so they would sink if they stopped moving. This forces them to swim constantly to stay afloat.