Fluid dynamics of hairy little legs

Many different types of animals use appendages bearing rows of hairs to capture food or molecules from the surrounding fluid, to locomote, or to move fluids past themselves. The performance of these appendages depends on how much of the fluid that they encounter flows through the gaps between the hairs rather than around the perimeter of the whole array. We have employed mathematical modeling, microcinematography of hairy appendages on small aquatic animals, and flow visualizations around dynamically-scaled physical models to elucidate the factors that determine the leakiness of arrays of hairs. We found that rows of very small hairs function as paddles (i.e. fluid does not flow between neighboring hairs), whereas rows of larger hairs operate like leaky sieves. The size and speed ranges through which the transition in leakiness occurs depends on the geometry of the appendage. Our research has revealed conditions under which there is permission for morphological diversity of hairy appendages with little consequence to performance, versus conditions under which simple changes in speed, size, or mesh coarseness can lead to novel physical mechanisms of operation. We have been studying the mechanisms of food capture by suspension feeders, and of odorant capture by olfactory antennae of various animals.

Selected references on this topic

Fluid Dynamics of Arrays of Hairs:

  • Koehl, M. A. R. (1995) Fluid flow through hair-bearing appendages: Feeding, smelling, and swimming at low and intermediate Reynolds number. In C.P. Ellington T. J. Pedley [eds.], Biological Fluid Dynamics, Soc. Exp. Biol. Symp. 49: 157-182. (review of our work)
  • Cheer, A. Y. L. and M. A. R. Koehl (1987) Paddles and rakes: Fluid flow through bristled appendages of small organisms. J. Theor. Biol. 129: 17-39.
  • Cheer, A. Y. L. and M. A. R. Koehl (1987) Fluid flow through filtering appendages of insects. I.M.A. J. Math. Appl. Med. Biol. 4: 185-199.
  • Koehl, M. A. R. (1992) Hairy little legs: Feeding, smelling, and swimming at low Reynolds number. Fluid Dynamics in Biology. Contemporary Mathematics 141: 33-64.
  • Loudon, C., B. A. Best, and M. A. R. Koehl (1994) When does motion relative to neighboring surfaces alter the flow through an array of hairs? J. Exp. Biol. 193: 233-254.
  • Koehl, M. A. R. (2001) Transitions in function at low Reynolds number: Hair-bearing animal appendages. Math. Meth. Appl. Sci. 24: 1523-1532.
  • Koehl, M. A. R. (2003) Physical modelling in biomechanics. Phil Trans. Roy. Soc. Lond. B. 35: 1589-1596.
  • Koehl, M. A. R. (2004) Biomechanics of microscopic appendages: Functional shifts caused by changes in speed. J. Biomech. 37:789-795.


  • Koehl, M. A. R. (1996) Small-Scale fluid dynamics of olfactory antennae. Mar. Fresh. Behav. Physiol. 27: 127-141.
  • Mead, K.S., M.A.R. Koehl, and M.J. O'Donnell (1999) Stomatopod sniffing: the scaling of chemosensory sensillae and flicking behavior with body size. J. Exp. Mar. Biol. Ecol. 241: 235-261.
  • Loudon, C. and M. A. R. Koehl. (2000) Sniffing by a silkworm moth: Wing fanning enhances air penetration through and pheromone interception by antennae. J. Exp. Biol. 203: 2977-2990.
  • Mead, K. S. and M. A. R. Koehl (2000) Stomatopod antennule design: The assymmetry, sampling efficiency, and ontogeny of olfactory flicking. J. Exp. Biol. 203: 3795-3808.
  • Koehl, M. A. R. (2001) Fluid dynamics of animal appendages that capture molecules: Arthropod olfactory antennae. pp. 97-116 In, Computational Modeling in Biological Fluid Dynamics. L. Fauci and S. Gueron [eds.], IMA Series #124.
  • Goldman, J. A. and M. A. R. Koehl (2001) Fluid dynamic design of lobster olfactory organs: High-speed kinematic analysis of antennule flicking by Panulirus argus. Chemical Senses 26: 385-398.
  • Stacey, M., K. S. Mead, and M. A. R. Koehl (2002) Molecule capture by olfactory antennules: Mantis shrimp. J. Math. Biol. 44: 1-30.
  • Crimaldi, J. P., M. A. R. Koehl, and J. R. Koseff (2002) Effects of the resolution and behavior of olfactory appendages on the chemical signals they intercept in a turbulent odor plume. Environ. Fluid Mech. 2: 35-63.
  • Koehl, M. A. R., J. R. Koseff, J. P. Crimaldi, M. G. McCay, T. Cooper, M. B. Wiley, and P. A. Moore (2001) Lobster sniffing: Antennule design and hydrodynamic filtering of information in an odor plume. Science 294: 1948-1952.
  • Mead, K. S., M. B. Wiley, M. A. R. Koehl and J. R. Koseff (2003) Fine-scale patterns of odor encounter by the antennules of mantis shrimp tracking turbulent plumes in wave-affected and unidirectional flow. J. Exp. Biol. 206: 181-193.
  • Koehl, M. A. R. (2003) Physical modelling in biomechanics. Phil Trans. Roy. Soc. Lond. B 358: 1589-1596.
  • Koehl, M. A. R. (2006) The fluid mechanics of arthropod sniffing in turbulent odor plumes. Chem. Senses 31: 93-105.
  • Reidenbach, M. A., N. George, and M. A. R. Koehl (2008) Antennule morphology and flicking kinematics facilitate odor sampling in the spiny lobster, Panulirus argus. J. Exp. biol. 211: 2849-2858.
  • Koehl, M. A. R. (2011) Hydrodynamics of sniffing by crustaceans. In: Chemical Communication in Crustaceans. T. Breithaupt and M. Theil [eds], Springer Verlag, pp. 85-102.
  • Reidenbach, M.A. and M. A. R. Koehl (2011) The spatial and temporal patterns of odors sampled by lobsters and crabs in a turbulent plume. J. Exp. Biol. 214: 3138-3153.
  • Schuech, R., M.T. Stacey, M. F. Barad, and M. A. R. Koehl (2011) Numerical simulations of odorant detection by biologically inspired sensory arrays. IOP Journal of Bioinspiration and Biomimetics. doi:10.1088/1748-3182/7/1/016001.
  • Waldrop, L. D., M. Hann, A. Henry, A. Kim, A. Punjabi, and M. A. R. Koehl (2015) Ontogenetic changes in the olfactory antennules of the shore crab, Hemigrapsus oregonensis, maintain sniffing function during growth. Journal of the Royal Society Interface. DOI: 10.1098/rsif.2014.1077
  • Waldrop, L. D. Reidenbach, M. A., and Koehl, M. A. R. (2015) Flexibility of Crab Chemosensory Sensilla Enables Flicking Antennules to Sniff. Biol. Bull. 229: 185–198.
  • Waldrop, L. D. and M. A. R. Koehl (2016) Do terrestrial hermit crabs sniff? Air flow and odorant capture by flicking antennules. J. Roy. Soc. Interface 13: 20150850. doi.org/10.1098/rsif.2015.0850

Suspension Feeding:

  • Rubenstein, D. I. and M. A. R. Koehl (1977) The mechanisms of filter feeding: Some theoretical considerations. Amer. Natur.111: 981-994.
  • Koehl, M. A. R. (1977) Water flow and the morphology of zoanthid colonies. Proc. Third Int. Coral Reef Symp. I. Biology, pp. 437-444.
  • Koehl, M. A. R. (1981) Feeding at low Reynolds number by copepods. Lectures in Mathematics in the Life Sciences 14: 89-117.
  • Koehl, M. A. R. (1983) The morphology and performance of suspension-feeding appendages.J. Theor. Biol. 105: 1-11.
  • Koehl, M. A. R. and J. R. Strickler (1981) Copepod feeding currents: Food capture at low Reynolds number. Limnol. Oceanogr. 26: 1061-1073.
  • Koehl, M. A. R. (1984) Mechanisms of particle capture by copepods at low Reynolds number: Possible modes of selective feeding. pp. 135-160, In D. L. Meyers and J. R. Strickler [eds.], A.A.A.S. Selected Symposium #85: Trophic Interactions Within Aquatic Ecosystems. Westview Press.
  • Sebens, K. P. and M. A. R. Koehl (1984) Predation on zooplankton by two benthic anthozoans, Alcyonium siderium (Alcoynacea) and Metridium senile (Actiniaria), in the New England subtidal. Mar. Biol. 81: 255-271.
  • Childress, S., M. A. R. Koehl, and M. Miksis. (1987) Scanning currents in Stokes flow and the efficient feeding of small organisms. J. Fluid Mech. 177: 407-436.
  • Shimeta, Jeff and M. A. R. Koehl (1997) Mechanisms of particle selection by tentaculate suspension feeders during encounter, retention, and handling. J. Exp. Mar. Biol. Ecol. 209: 47-73.
  • Koehl, M.A.R. (1998) Small-scale hydrodynamics of feeding appendages of marine animals. Oceanography. 11: 10-12.
  • Koehl, M.A.R. (2004) Biomechanics of microscopic appendages: Functional shifts caused by changes in speed. J. Biomech. 37: 789-795
  • Roper, M., M. J. Dayel, R. E. Pepper, N. King, and M. A. R. Koehl, (2013) Cooperatively generated stresslet flows supply fresh fluid to multicellular choanoflagellate colonies. Physical Review Letters 110: 228104. DOI: 10.1103/PhysRevLett.110.228104.
  • Robinson, H.E., C. M. Finelli, and M. A. R. Koehl (2013) Interactions between benthic predators and zooplanktonic prey are affected by turbulent waves. Integrative Biology 53: 810-820.

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