Cephalopods such as squid use jet propulsion for rapid escape from predators; they use other mechanisms for slow swimming. The jet is produced by ejecting water 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.[1] Sea hares (gastropod molluscs) employ a similar method, but without the sophisticated neurological machinery of cephalopods they navigate somewhat more clumsily.[1]

Some teleost fish have also developed jet propulsion, passing water through the gills to supplement fin-driven motion.[7]: 201

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.[8]

Scallops and cardiids,[9] siphonophores,[10] tunicates (such as salps),[11][12] and some jellyfish[13][14][15] also employ jet propulsion. The most efficient jet-propelled organisms are the salps,[11] which use an order of magnitude less energy (per kilogram per metre) than squid.[16]

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