Salps, a type of pelagic tunicate, are centimeter-sized oceanic animals that move and feed by rhythmically pumping water through their barrel-shaped, gelatinous bodies. As seawater passes through their mucous-mesh filters, they capture plankton and other particles for sustenance. This process removes organic matter from upper ocean levels and packages it into waste pellets that sink to lower levels. Previous studies surmised that salp filtration operates like a simple sieve, trapping particles larger than the mesh. We hypothesized that this model may not fully explain salp filtration and that salps may, in fact, filter submicrometer (microscopic) particles that are abundant in their ocean habitats.

 

To accurately observe and measure the salp mesh, we used epifluorescence microscopy to highlight its structure under the microscope. To assess flow pattern and speed, we placed salps in a tank of seawater with titanium dioxide particles. The particles were then illuminated using a sheet of laser light, allowing us to track and videotape their movements. Additionally, we conducted feeding experiments using polystyrene microspheres (tiny styrofoam beads) to investigate salp filtration rates of different sized particles at different concentrations. Lastly, we employed mathematical models to predict salp encounter and capture rates of various particle sizes in ocean waters. 

 

Key Findings:

• The simple-sieve model previously used to describe salp grazing does not adequately account for salp grazing of submicrometer particles.

• Our direct interception model explains salps’ ability to capture submicrometer particles at rates higher than larger particles.

• The additional surface area of small particles (less than 1µm) may make them more easily digestible by salps than larger particles. 

• Particles smaller than the mesh size can fully satisfy salp energy needs.

• Salps remove tiny particles of organic matter from ocean waters and package them into larger, denser particles that sink to the ocean floor. 

 

Our research suggests that current models of salp filtering mechanisms are incomplete as they fail to account for salp grazing of submicrometer particles. In contrast, data from our experiments suggest that submicrometer particles play a central role in salp feeding. These results lead us to believe that salp feeding removes large amounts of carbon-rich particulate from the seawater and sequesters it in the ocean depths. Additional research is needed to investigate the potentially wide-ranging implications of these findings with respect to biogeochemical cycles and climate change. 

Sutherland, Kelly R., Laurence P. Madin, and Roman Stocker. "Filtration of Submicrometer Particles by Pelagic Tunicates." Proceedings of the National Academy of Sciences 107, no. 34 (August 2010): 15129-15134. https://doi.org/10.1073/pnas.1003599107.

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Photo attribution: Salp colony

By Lars Plougmann. CC BY-SA 2.0