salps

Research at the Experimental Hydrodynamics Laboratory

Propulsive performance of pelagic tunicates (salps)
A. H. Techet with K. R. Sutherland & L. Madin (WHOI)

Summary: Salps, or pelagic tunicates, are common gelatinous organisms in oceanic waters. They swim by jet propulsion, drawing fluid in through an oral siphon and ejecting the fluid through an atrial siphon at the opposite end of the body. Muscle bands encircling the cylindrical body drive a pulsed jet that propels the animal forward and also creates a feeding and respiratory current. Salps are the only jet propelled creatures that have opposing anterior and posterior openings, allowing for unidirectional flow and forward and reverse swimming. These organisms show a range of swimming styles based on speeds, pulse rates, and degree of musculature. Some of these distinct swimming styles are driven by individual morphology and chain architecture. Fast swimming forms have higher pulse rates and streamlined, linear morphologies. Slower swimmiforms tend to have slower pulse rates and less hydrodynamic shapes. Salps have a two-part life-cycle composed of asexual, solitary individuals and sexual chains of aggregates. Asexual, solitary forms give rise to smaller chains of aggregates. There are tens to hundreds of sexually reproducing individuals in each chain and chains can be tens of meters long.

Methods: Salps were collected and observed at the Liquid Jungle Lab off Pacific Coast of Panama, Veraguas province (7° 50' N, 81° 35' W) during January of 2007, 2008 and 2009. Bluewater diving techniques were used for collection and field observations. Propulsive jet wakes of salps were visualized using two approaches: 1) Particle Image Velocimetry (PIV) in the laboratory and 2) fluorescein dye during night SCUBA dives. Particle motions for PIV as well as dye wakes were recorded with a Sony HDR-HC7 high definition camcorder (1440x1080 pixels, 30 fps).
Results: Salps, or pelagic tunicates, are common gelatinous organisms in oceanic waters, which swim and maneuver by jet propulsion. Salps can reverse direction quickly but reversing the flow of fluid through their bodies by generating impulsive vortex rings (figure 1c). The vortex ring jets are formed by drawing water through incurrent (oral) and ejecting it through the excurrent (atrial) siphons at the opposite end of the body. In the sexual part of their life-cycle, the salps are found in aggregate, or chained together. These aggregates appear to coordinate movement in response to external stimuli to allow a coordinated maneuver for even a relatively long aggregate chain.

( Images courtesy of Kelly R. Sutherland )

The images at the left shows an asexual, solitary salp (Cyclosalpa sp.) with a developing sexual chain of aggregates. Dye visualization of the evolution of the jet wake is shown in the vertical sequence of images. During a pulse cycle, the oral siphon closes and circular muscle bands are contracted to eject water out the atrial siphon and propel the animal forward. The result is a pulsing jet that produces cyclical changes in swimming velocity, or unsteady swimming. With each pulse, a vortex ring and trailing wake is generated. The species shown is a slow swimmer (mean speed = 4.8 cm s-1) with slow pulse rates (0.7 Hz). SCUBA divers acquired the images using an HD camcorder in an underwater housing and fluorescein dye ejected from a micropipette.

The image at right shows PIV measured velocity and vorticity fields in the wake of a pulsing Cyclosalpa sp. solitary. There is a central jet and vortex ring indicated by negative (blue) and positive (red) vorticity, associated with clockwise and counterclockwise vortices, respectively. The salp is shown in white. PIV was conducted in a laboratory tank with a 532 nm continuous laser and an HD camcorder (30 Hz). The outline of the salp body is masked in white.

References

Sutherland, K. R., Madin, L., & Techet, A. H. (2008) In situ visualization of the propulsive jet wakes produced by pelagic tunicates, Gallery of Fluid Motion Poster, 61st Annual Meeting of the APS Division of Fluid Dynamics.

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last updated: Aug 2009