Fluid mechanical aspects of suspension feeding

Abstract

Most accounts on suspension feeding assume that mechanical, sievelike filters retain particles from the ambient water Fluid mechanical aspects have been neglected. Suspension feeding is characterized by very low Reynolds numbers. This implies that water processing and particle retention are exclusively determined by viscous forces. Modern filtration theory can therefore be applied to hypotheses on suspension feeding involving mechanical filters. The resistance to water flow through such filters was found to correspond to pressure drops across the filters of about 0.1 to 0.4 mm H,O in flagellates, ciliates, sponges, and ascidians; and of > l mm H,O in copepods and bivalves. These theoretical pressure drops are consistent with the function as filters of ciliate membranelles, pseudopodial collars in flagellates and sponges, and ascidian mucus filters. Ciliary and flagellar water transport operate at very low pressures The pressure drops calculated for copepod second maxillae and bivalve laterofrontal cirri seem to be incompatible with the roles as filters traditionally ascribed to these structures. Recent studies indicate that sievelike models have to b e abandoned in explaining particle retention in copepods and bivalves. Particles seem to be captured by means of mechanisms that do not imply physical interception of the suspended particles Copepods seem to have adopted mechanisms, based on viscous forces, that direct food particles in the surrounding water toward the second maxillae, which eventually capture the parcel of water that contains the particle. In the bivalve gill capture of suspended particles implies transfer from the currents passing through the gill via the interfilamentary spaces to the frontal surface currents along the filaments. Complex patterns of flow arise where the 2 systems of currents meet at the entrance to the interfilamentary spaces. The patterns are characterized by steep velocity gradients which may act on suspended particles and cause them to enter the surface currents, i.e. to b e captured. It remains to be ascertained to what extent fluid mechanics operate in the capture of particles in other metazoan ciliary feeders

Particle capture mechanisms in suspension-feeding invertebrates

“…The foregoing basic flow phenomena are thought to be important in order to understand the different particle capture mechanisms (see also the early discussions by Rubenstein & Koehl 1977, Jørgensen 1983, LaBarbera 1984, Strathmann 1987, Shimeta & Jumars 1991, Koehl 1995, Shimeta & Koehl 1997). In the following we provide supplementary comments to some of the mechanisms covered in the review, now based on biological considerations.…”

“…The basic element for both pumping and filtering is the choanocyte with a flagellum that pumps water through a collar of microvilli (diameter 0.1 µm and spacing 0.3 to 0.4 µm) acting as a sieve that captures free-living bacteria and other particles down to ∼0.1 µm diameter (Bidder 1923, Fjerdingstad 1961a,b, Reiswig 1971, Bergquist 1978, Jørgensen 1983, Simpson 1984, Larsen & Riisgård 1994, Thomassen & Riisgård 1995, Wyeth 1999, Leys & Eerkes-Medrano 2006. The structure of the choanocyte is the same in all sponges andamong the metazoa -sponges are unique in feeding by means of choanocytes.…”

Viscosity of seawater controls beat frequency of water-pumping cilia and filtration rate of mussels Mytilus edulis

“…The lateral cilia provide the power that drives the water flow through a mussel, but for cilia operating at very low Reynolds numbers, and for flows through narrow conduits, the viscosity of the ambient water exerts a crucial mechanical effect ). The hydrodynamic scale is characterized by the dimensionless Reynolds number (Re) = object length (l ) × velocity (u)/kinematic viscosity of seawater (ν) (∼10 -6 m 2 s -1 ), which indicates the relative importance of inertial and viscous forces (Purcell 1977, Jørgensen 1983, Sleigh 1989. In mussels, the mean velocity (u) of the cilia driven through current is about 1 mm s -1 , and the length (l) of a solid perpendicular to the flow is given by the diameter of the lateral cilia (0.2 µm), and thus Re = lu/ν = 0.0002.…”

Form, function and flow in the plankton : jet propulsion and filtration by pelagic tunicates

“…Variables important for filtering include the volume of water and associated particles moving through the filter, the surface area of the filter and the pore size of the filtering mesh. By definition, suspension feeding is the concentration of particles from the surrounding medium by some type of filtration mechanism (Jorgensen 1983). This mode of feeding allows salps to survive and grow large in areas where food particles are relatively small and dilute.…”