What is the buoyancy organ in many bony fishes?

• A. Sirenia
• B. Swim Bladder
• C. Porifera
• D. Polychaeta

Answer: (B)

Buoyancy in many bony fishes is controlled by a gas-filled organ called the swim bladder. This structure lets the fish adjust its overall density so it can hover at a chosen depth without swimming. When the bladder fills with gas, the fish becomes more buoyant and rises; when gas is removed, it sinks. Different species achieve gas adjustment in different ways: some have a duct that connects the bladder to the gut, so air can be gulped from the surface (physostomous fishes); others regulate gas exchange with the blood through a gas gland and a gas-absorbing organ (physoclistous fishes). This swim bladder feature is typical of many bony fishes and is not found in groups like Sirenia (mammals) or invertebrates such as Porifera and Polychaeta.