14Coordination of multiple propulsors can provide performance benefits in swimming organisms. 15 Siphonophores are marine colonial organisms that orchestrate the motion of multiple swimming 16 zooids for effective swimming. However, the kinematics at the level of individual swimming 17 zooids (nectophores) have not been examined in detail. We used high speed, high resolution 18 microvideography and particle image velocimetry (PIV) of the physonect siphonophore, 19 Nanomia bijuga, to study the motion of the nectophores and the associated fluid motion during 20 jetting and refilling. The integration of nectophore and velum kinematics allow for a high-speed 21 (maximum ~1 m s -1 ), narrow (1-2 mm) jet and rapid refill as well as a 1:1 ratio of jetting to refill 22 time. Overall swimming performance is enhanced by velocity gradients produced in the 23 nectophore during refill, which lead to a high pressure region that produces forward thrust. 24 Generating thrust during both the jet and refill phases augments the distance travelled by 17% 25 over theoretical animals, which generate thrust only during the jet phase. The details of velum 26 kinematics and associated fluid mechanics elucidate how siphonophores effectively navigate 27 three-dimensional space and could be applied to exit flow parameters in multijet underwater 28 vehicles. 29 30 Summary statement: Colonial siphonophores produce high speed jets and generate forward 31 thrust during refill using a flexible velum to achieve effective propulsion.32 33 34 35The physonect siphonophore Nanomia bijuga is a colonial cnidarian capable of long distance 36 migrations (Robison et al., 1998) as well as short sprints and maneuvering (Costello et al., 2015). 37 As in other physonect siphonophores, multiple swimming units, called nectophores, are 38 organized linearly along a central nectosome (Fig. 1). The nectophores produce pulsed, high 39 speed jets and in N. bijuga the nectophores are coordinated to produce forward swimming, 40 reverse swimming and turning (Mackie, 1964; Costello et al., 2015). The coordination of 41 multiple jets makes N. bijuga a highly effective swimmer that performs extensive diel vertical 42 migrations, travelling 100s of m to the surface each night and then returning to depth during the 43 day (Robison et al., 1998). Due to this pronounced migration behavior, its cosmopolitan 44 distribution (Totton 1965), and the acoustic scattering properties of the gas-filled pneumatophore 45 at the colony tip, N. bijuga is a prominent member of the sound scattering layer in much of the 46 world's oceans (Barham, 1963). 47 Up to this point, studies of swimming in N. bijuga have primarily focused on colony-level 48 coordination between the nectophores, which play different roles during turning and straight 49 swimming depending on their development and position along the nectosome (Costello et al., 50 2015). Newly budded nectophores toward the colony apex (Dunn and Wagner, 2006) are used 51for turning the colony. To achieve this, these nectopho...