Particle capture mechanism of the pelagic tunicate Oikopleura vanhoefleni

Abstract

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 1,436 µm s‒1, with a mean (±SD) of 559±292 µm 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 3 µm in diameter, in contrast to predictions from a simpler sieving model. We conclude that the pharyngeal filter of O. vanhoeffeni captures particles >0.6 µm in diameter by a combination of sieving and direct interception onto individual fibers. The combination of model predictions of particle capture with particle size dis.tributions from an Arctic polynya suggests that adult O. vanhoeffeni obtains 49, 29, and 18% of its ration by volume from nanoplankton (2–20 µm in diameter), microplankton (20–100 µm), and picoplankton (1–2 µm), respectively, with a possible 4% contribution from submicrometer colloids.

Mammoth grazers on the ocean's minuteness: a review of selective feeding using mucous meshes

“…After conveyance through the food-concentrating filter, particles reach the pharyngeal filter, which has a left-skewed retention efficiency curve that declines below approximately 3 mm for larger species (O. vanhoeffeni) [87,88] and approximately 1-2 mm for smaller species (O. dioica and F. borealis) [89]. However, gut content analysis and experiments with live prey indicate that appendicularians can consume small bacteria and large viruses (less than 0.3 mm) [90][91][92].…”

A self‐cleaning biological filter: How appendicularians mechanically control particle adhesion and removal

“…Historically, appendicularians have been assumed to feed non-selectively (Bedo et al 1993;Acuña et al 1996;Gorsky et al 1999). Our results, however, show that size-dependent adhesion may cause selective particle retention by the appendicularian house, with smaller particles being more likely to remain adhered to the FCF (Fig.…”

“…To test if pulsatile flow alone can explain the detachment process, we can simplify the system of the FCF to two forces: the drag force for detachment (F d ) and the adhesion force (F a ). Because particle collection by appendicularians occurs in laminar flow at low Reynolds numbers (Morris and Deibel 1993;Acuña, Deibel, and Morris 1996), we can use a modified Stokes drag, which increases linearly with particle size as:…”

“…Appendicularians (Phylum: Chordata, Subphylum: Tunicata) are a class of globally abundant, planktonic marine grazers that can filter-feed on particles several orders of magnitude smaller than themselves, down to picoplankton (0.2-2 lm) (Acuña et al 1996;Gorsky and Fenaux 1998). Appendicularians have both an internal filter (the pharyngeal filter) and an extracellular mucous filtration apparatus called the house (Fig.…”

“…It is a spherical extracellular secretion that the animal lives inside and uses to concentrate prey particles from seawater. Appendicularian filtration shares the fundamental particle interception mechanisms employed by numerous filter-feeders, including sieving, direct interception, and diffusional deposition (Acuña et al 1996).…”