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Suspension Feeding

Suspension Feeding. Mostly mechanics and foraging theory. Terms. Definition — feeding on particles by removing them from suspension Active — create own feeding currents Passive — use ambient fluid motion and (or) gravity Beware of classifications/dichotomies

wynter-clemons
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Suspension Feeding

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  1. Suspension Feeding Mostly mechanics and foraging theory

  2. Terms • Definition — feeding on particles by removing them from suspension • Active — create own feeding currents • Passive — use ambient fluid motion and (or) gravity • Beware of classifications/dichotomies • Nobody make a living by swallowing seawater. Good particles must be concentrated.

  3. Phytoplankton Detritus Bacteria Protists Animals More than one of the above One or more from column A, plus something acquired in another feeding mode (e.g., deposit feeding or osmotrophy) Suspension, like Deposit, Feeding Says How, not What

  4. Passive & Active SF Many benthic phyla Many benthic phyla In the plankton, thecosome pteropods In the plankton, copepods, salps, some fishes, etc.

  5. More terms and concepts • BEWARE of “volume filtered” or “volume cleared.” • It comes from the practice of measuring C(t). If you know C (x) and C (x + y), then from the geometric mean concentration over the interval y and the time y and the volume of the experimental container you can calculate what volume has been cleared of cells in that time. • Just because you can do the calculation does not mean that the animal actually filters that volume.

  6. Where to forage • Planktonic suspension feeders, where C is high, but observation is that they forage where production is high. • Benthic suspension feeders where Cxu is high • Benthic suspension feeders can’t chase patches

  7. What Particles to Take • For feeders on living organisms, take particles larger than the mean size • For detritivores, take smaller particles and ones lower in specific gravity • If sorting is moderately expensive, show partial preference • Sorting may be an issue for benthic suspension feeders that experience high concentrations of poor foods

  8. How Fast to Feed Filter only fast enough to keep the gut full Recall the digestion lectures and reprints

  9. Cautions • Aerosol filtration theory (Rubenstein and Koehl 1977) has a different goal and flow geometry • The flow is often unbounded in filtration of hydrosols by suspension feeders • Beware of early aquatic applications that focus on efficiency of encounter and fail to use excess particle density

  10. Flow and Collector Geometry

  11. Direct Interception The only mechanism that does not cross streamlines

  12. Inertial Impaction Fl= 2Culs lc . ls< rc Fl= 2Curc lc . ls≥ rc

  13. Gravitational Deposition

  14. Diffusional Deposition

  15. Issues • The mechanisms can be interactive rather than additive (but often one will be so dominant that it does not matter). • The mechanisms are linear in particle concentration. • The velocity you need is a face velocity, not a bulk velocity. • The concentration you need is local to the collector. • Per particle, bacteria are hard to encounter by any mechanism.

  16. Fenchel Closes the Microbial Loop (Bombannes 1982)

  17. But leaves out fluid motion Shimeta and Jumars (1993) added shear, and showed — as is true for most suspension feeders and sit-and-wait predators — that an intermediate shear rate maximizes rate of ingestion (effective encounter).

  18. Exceptions to the ~ 5-10 µm rule for bacterivory • Tunicates that use very fine meshes and so have a small pressure drop (and other thin mucus strand makers) • Viruses • Shear from decaying turbulence pushes it up • Ability to use ambient flow (benthos) • FW daphnids (charge effects?)

  19. For benthos, encounter and ingestion rates hard to match • Local u and C poorly known • Re often > 1 • Vertical gradients are strong, and food can be depleted near the bed by dense assemblages of suspension feeders. • Unsteady, active motions can be important for encounter • Calculated rates do “match” for one brittlestar species and some protists

  20. Bivalves are a mess • There are no intelligible, mechanistic models of encounter. • Particle detection and unsteady motion is involved but not modeled. • Controversy over mechanisms and rates has raged for > 20 yr

  21. Medium-scale flow issues • Rejection (exhalent) jets — fast & high • Induced flow (Venturi effect) • Lee feeding • Vortex trapping • Induced resuspension • Location relative to obstacles & bedforms • Depleted boundary layers

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