Why do nautiluses float and sink?

Answer: Gas-filled shell chambers

Inflating body with water — Wrong. Nautiluses can pump water in/out of shell chambers, but buoyancy control primarily uses gas (nitrogen, oxygen) in the sealed chambers.

Gas-filled shell chambers ✓ — Correct! Nautilus shells have up to 30 chambers separated by walls. As the nautilus grows, it adds new chambers and seals off old ones. By adjusting gas and liquid amounts in chambers (using osmosis), it controls buoyancy—rising or sinking without swimming! It's a natural submarine ballast system!

Swimming muscles power movement — Wrong. Nautiluses do swim using jet propulsion, but depth control comes from adjusting gas in shell chambers, not muscle power.