Particle Capture Mechanism of the Pelagic Tunicate Oikopleura vanhoeffeni

Oikopleurid appendicularians use an external "house" to preconcentrate particles from seawater suspension, but their pharyngeal filter is ultimately responsible for removing suspended particles for ingestion. Although the pharyngeal filter captures submicrometer colloids efficiently, the fluid mechanics of this process have not been investigated. Using video-assisted microscopy, we tracked plastic beads within the pharynx of Oikopleura vanhoeffeni to analyze flow streamlines and flow rates. Impact velocities of particles range from 124 to 1,436μ m s-1, with a mean (± SD) of 559± 292μ m s-1. Reynolds numbers for the filter fibers are of order 10-5. Given the fiber diameters and pore sizes published earlier and our measured impact velocities, current aerosol filtration models predict particle retention spectra that do not differ statistically from those determined empirically for particles between 0.6 and 3μ m in diameter, in contrast ot predictions from a simpler sieving model. We conclude that the pharyngeal filter of O. vanhoeffeni captures particles$>0.6\mu m$in diameter by a combination of sieving and direct interception onto individual fibers. The combination of model predictions of particle capture with particle size distributions from an Arctic polynya suggests that adult O. vanhoeffeni obtains 49, 29, and 18% of its ration by volume from nanoplankton (2-20μ m in diameter), microplankton (20-100μ m), and picoplankton (1-2μ m), respectively, with a possible 4% contribution from submicrometer colloids.