Ichthyology (fish buoyancy): Identify the anatomical feature that prevents most bony fishes from sinking. Full question: Most fish do not sink in water primarily because of which internal structure? Choose the correct option.

Concept
Most bony fishes (teleosts) possess a swim bladder—a gas-filled sac that regulates buoyancy via gas secretion/resorption. Cartilaginous fishes (e.g., sharks) lack a swim bladder and partly offset density with large oil-rich livers.

Why others are incorrect
Bird-like air sacs are absent; spongy bone air does not control buoyancy; fatty liver is species-specific and not the general teleost mechanism; stomach inflation is not a controlled buoyancy organ.

Final Answer
Swim bladder (air bladder).