concept

Jet propulsion

Moving by drawing water into a cavity and forcing it out through a narrow opening: the momentum of the ejected water pushes the animal the other way. Thrust scales with mass flow times exit velocity, but the energy bill scales with velocity squared — so a small aperture forces the animal to throw a little water very fast, and most of the work ends up in the wake rather than in the body. That is why squid and jellyfish accelerate ferociously and travel expensively, and why the tsyong reserves its siphons for the seconds that decide a hunt.