What natural object inspired the structure of a submarine ballast tank?

The blowfish or argonaut octopus inspired the structure of submarine ballast tank

Explanation:

A submarine can be usually referred to as a submersible warship that is constructed in such a way that it can be operated at a great depth below the sea or oceans, and is mainly used for defense purposes.

This submarine is comprised of a ballast tank, and as it dives, the ballast tanks are occupied by water and the air present inside this ballast tank is allowed to move out until the overall density becomes larger than the surrounding water, resulting in the sinking of the submarine.

This ballast tank structure was inspired by the aquatic fish namely the blowfish or argonaut, whose body resembles the shape of a submarine.