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

Abstract

Appendicularians are ubiquitous marine grazers that use tangential filtration to collect micron and submicron prey. The food-concentrating filter (FCF) is the primary determinant of appendicularian prey selectivity, but the precise means by which it concentrates and conveys particles to the pharyngeal filter remain poorly understood. We used high-speed videography to examine the mechanics of the FCF of Oikopleura dioica with high resolution to better understand how filter structure, hydrodynamics, and animal behavior affect feeding selectivity. We observed that filtration occurs through the process of serial adhesion and detachment of particles, and in this respect, differs from an industrial tangential filtration system. We demonstrate how filter elasticity and the hydrodynamics of pulsatile flow caused by the kinematics of the animal's tail contribute to the filter's self-cleaning abilities. We present experimental observations showing that smaller particles (3 μm) adhere significantly more to the filter than larger ones (10 μm), but that, overall adhesion is quite low for an array of particle sizes (3–20 μm). This study provides a mechanistic basis for predicting the effects of appendicularian grazing on particle size spectra in the upper ocean.