Swimming and filtration in the ocean by jet-propelled salps
Dr. Kelly R. Sutherland
Postdoctoral Scholar in Bioengineering, California Institute of Technology
MECHANICAL ENGINEERING SEMINAR
Engineering Sciences Building, ESB 1001
Thursday, October 4th, 4.00 p.m.
Salps are barrel-shaped marine organisms that are common in the open ocean and swim using a pulsed jet. Among salp species, there are a variety of body shapes and swimming styles that correspond to differences in ecological function. Dye visualization via bluewater SCUBA techniques and laboratory Digital Particle Image Velocimetry (DPIV) were used to describe jet wake structure and swimming performance variables including thrust, drag and propulsive efficiency among three salp species (Pegea confoederata, Weelia (Salpa) cylindrica, Cyclosalpa sp.). Locomotion by each species was achieved using vortex ring ring propulsion. Different combinations of swimming speed and hydrodynamic efficiency were observed and can be considered in light of metabolic constraints and ecological roles. Though nature does not strive for optimality, this work shows the value of a comparative approach for understanding how underlying structure and mechanism influence performance.
During swimming, the same fluid that propels the salp forward also contains food particles, which are captured on a mucous mesh as fluid passes through the mostly hollow body. Though salps are centimeters in length and swim at speeds of ~1-10 cm s-1, filtration occurs on a fine, mucous mesh (fiber diameter ~0.1 μm) at low velocity (1.6 cm s−1) and is thus a low Reynolds number (Re ~10−3) process. A model of particle capture efficiency by a rectangular mesh was used to estimate particle capture rates on the salp filtering mesh based on realistic oceanic particle concentrations. Particle feeding experiments using 0.5, 1 and 3 µm fluorescent polystyrene microspheres were then performed to test the theoretical model. Results from both the model and from experiments showed that smaller particles are captured at considerably higher rates than larger particles. Though particles smaller than mesh openings (1.4 µm) are expected to supply substantially less carbon than larger particles, they can still completely satisfy salp energetic needs. By removing different sized particles with nonuniform efficiency and packaging them into fast-sinking fecal pellets, salps have the potential to structure oceanic particle size spectra.