Jet propulsion is thrust produced by passing a jet of matter (typically air or water) in the opposite direction to the direction of motion. By Newton's third law, the moving body is propelled in the opposite direction to the jet. It is most commonly used in the jet engine, but is also the most common means of spacecraft propulsion.
A number of animals, including cephalopods, sea hares, arthropods, and fish have convergently evolved jet propulsion mechanisms.
Jet propulsion is most effective when the Reynolds number is high - that is, the object being propelled is relatively large and passing through a low-viscosity medium.
In biology, the most efficient jets are pulsed, rather than continuous: at least when the Reynolds number is greater than 6.
A jet engine is a reaction engine that discharges a fast moving jet of fluid to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets and pump-jets. In general, most jet engines are internal combustion engines but non-combusting forms also exist.
Jet propulsion in cephalopods is produced by water being exhaled through a siphon, which typically narrows to a small opening to produce the maximum exhalent velocity. The water passes through the gills prior to exhalation, fulfilling the dual purpose of respiration and locomotion. Sea hares (gastropod molluscs) employ a similar means of jet propulsion, but without the sophisticated neurological machinery of cephalopods they navigate somewhat more clumsily.
Some teleost fish have also developed jet propulsion, passing water through the gills to supplement fin-driven motion.
In some dragonfly larvae, jet propulsion is achieved by the expulsion of water from a specialised cavity through the anus. Given the small size of the organism, a great speed is achieved.
Scallops and cardiids, siphonophores, tunicates (such as salps), and some jellyfish also employ jet propulsion. The most efficient jet-propelled organisms are the salps, which use an order of magnitude less energy (per kilogram per metre) than squid.