Particle capture mechanism of the pelagic tunicate Oikopleura vanhoefeni *

Oikopleurid appendicularians use an external “house ” to preconcentrate particles from seawater suspen-sion, 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, WC tracked plastic beads within the pharynx of Oikopleura vanhoefini to analyze flow streamlines and flow rates. Impact velocities of particles range from 124 to 1,436 pm s-l, with a mean (&SD) of 559+292 pm s-l. Reynolds numbers for the filter hbcrs are of order 10p5. Given the fiber diameters and pore sizes publishec. 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 3 pm in diameter, in contrast to predictions from a simpler sieving model. We conclude that the pharyngeal filter of 0. vanhoefini captures particles>0.6 pm in diameter by a combination of sieving and direct intercepticn onto individual fibers. The com-bination of model predictions of particle capture with particle size dirtributions from an Arctic polynya suggests that adult 0. vanhoefini obtains 49, 29, and 18 % of its ration by volume from nanoplankton (2-20 pm in diameter), microplankton (20-l 00 brn), and picoplankton (1-Z pm), respectively, with a possible 4 % contribution from subnncromcter colloids.