Multi-jet propulsion kinematics, gaits, and scaling of a siphonophore colony*

Many animals swim by squeezing single fluid jets out of a body cavity, one pulse at a time (e.g. squid), but a few fascinating animals propel themselves by ejecting simultaneous jets from multiple cavities. One such animal is the physonect siphonophore, a colonial animal resembling a long chain of connected jellyfish. Here we detail the physonect Nanomia bijuga, which uses its multi-jet locomotion strategy to maneuver with surprising speed and agility. A theoretical fluid dynamics model reveals that the coordinated mechanics of Nanomia's individual jet propulsors result in a whole-colony swimming performance that is greater than the sum of its individuals. The natural kinematics of each propulsor and its flexible nozzle maximize both thrust and propulsive efficiency when compared to simpler kinematic options. Integration of propulsors into a closely packed, streamlined colony reduces cost of transport, gives functional redundancy, and allows for adjustable jetting gaits. The benefits of multi-jet locomotion help explain the wide success of siphonophores, and greater understanding of their biomechanics can guide the design of distributed propulsion vehicles.