The food filters of Pegea confoederata salps capture food smaller than its mesh size by optimizing the water flow through their continuously secreted sticky mucus net.

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While it may not seem intuitive that filters can trap particles smaller than the size of their mesh, the fingernail-size marine salp (Pegea confoederata) depends on it for its survival. As the salp pulls the surrounding sea water into its body, it uses muscles to ensure the flow is as calm and orderly as a river on a windless day. By eliminating the effects of turbulence, particles smaller than the mesh, such as bacteria, viruses, and colloidal masses, pass extremely close to the net material. At a certain distance from the net, they adhere to the sticky netting material continuously secreted by the salp. Particles even smaller than bacteria, viruses, and colloidal masses diffuse right into the filter material. The specific fluid mechanical conditions which P. confoederata creates in its filtration systems enable it to trap particles with diameters as small as 0.01 micron (viruses, colloids, etc.) even though the filter mesh measures ~ 1.5 x 6 microns. This adaptation allows the macroscopic salps to survive on a diet of some of the tiniest biological life-forms known.

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"Salps are common in oceanic waters and have higher per-individual filtration rates than any other zooplankton filter feeder…Salps filter feed by rhythmically pumping water into the oral siphon, through the pharyngeal chamber, and out the atrial siphon...Food particles entering the pharyngeal chamber are strained through a mucous net that is continuously secreted and rolled into a food strand that moves posteriorly toward the esophagus...This feeding mechanism results in ingestion of any particles that enter the atrial siphon and adhere to the filtering mesh...At low Re [Reynolds number] (Re << 1), viscous effects prevail and prevent flow separation around filter elements. Filtration in salps operates in this regime, as Re ∼2 × 10−3...Classic principles of low-Re filtration theory show that low-Re filter feeders can collect particles smaller than the mesh spacing by relying on mechanisms other than simple sieving. The primary mechanisms are direct interception of particles traveling on streamlines that come within one particle radius of the filter element, and diffusional deposition caused by Brownian effects or random motility, which deflect particles from streamlines and cause contact with the filter...salps can capture submicrometer particles, and do so at rates that exceed those of larger particles." (Sutherland et al. 2010:15129) 

"Both diffusional deposition and direct interception play a role in determining particle encounter by the filtering mesh [a regularly spaced rectangular feeding mesh with a mean mesh width and length of W=1.5±0.5 μm and L=6.0±1.5μm, respectively], but direct interception is dominant for particle sizes dP > 0.05 μm. For dP = 0.01–0.05 μm (viruses, colloids), diffusion is the primary mechanism of particle encounter, although efficiency is <2%...Because there are substantially higher numbers of small particles in the ocean, these particles can be disproportionately ingested even when encounter efficiencies are relatively low...particles in the 0.01- to 0.1-μm size range (viruses, colloids) are encountered at ~200× the rate of particles in the 0.1- to 1-μm range (submicron particles, bacteria, Pro- chlorococcus;)...A model of simple sieving was an inferior predictor of relative encounter rates and was particularly poor at predicting encounter rates of the smallest particles...measured rates were similar to those predicted by the direct interception model." (Sutherland et al. 2010:15131)

"[T]hese findings suggest that simple sieving is not the sole feeding mechanism for salps, and instead that low Reynolds-number filtering mechanisms play a major and possibly dominant role by enabling salps to capture submicrometer particles...Our model results show that diffusional deposition allows encounter of the smallest particles (dP < 0.05 μm), although very inefficiently. However, a large fraction of submicrometer particles (0.05 μm < dP < W) can be efficiently encountered by direct interception and can largely or entirely satisfy salps’ energetic requirements even if the sticking coefficient # is as small as 0.1...Particles are packaged into membrane-bound fecal pellets that are often incompletely digested and therefore rich in carbon, nitrogen, and phosphorous, and contain trace elements (e.g., Ca and Mg). Fecal pellets sink quickly and are transferred to a longer-lived pool in deeper water, where material is sequestered on time scales of years to centuries." (Sutherland et al. 2010:15132)

Journal article
Filtration of submicrometer particles by pelagic tunicatesProceedings of the National Academy of SciencesAugust 10, 2010
K. R. Sutherland, L. P. Madin, R. Stocker

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