The Swimming Mechanism of Obelia sp.

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The Swimming Mechanism of Obelia sp.

• Presented by Jill K. Bellavance
• Advised by Dr. Jack Costello
• Biology Department
• Providence College
• Providence, Rhode Island

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Introduction to Obelia

• Cnidaria
• Hydrozoa
• Oblate
• Size

• Distribution
• Temperate oceans
• New England coast from April-November
• Northern and Southern Hemispheres

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Obelia Lifecycle

• www.auz.com/gaia/biosphere/cnidaria/hydrozoa/obeli
  a_cycle.gif

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Obelia medusa
www.lander.edu/rsfox/310Obelia.html
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The Microscopic World

• Low Reynolds numbers
• Boundary layers
• Leakiness

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Reynolds number

• Dimensionless
• Inertial forces per unit volume viscous forces
  per unit volume
• ReUL/V
• Uspeed of moving object
• Llength of object perpendicular to flow
• Vkinematic viscosity of seawater at 20ºC 1.06 x
  10-6 m2/s (Denny, 1993).

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Boundary layers

• Layer of fluid adjacent to solid where viscosity
  slows the relative motion of fluid
• ?1 L/(Re)1/2 (Ellington, 1975)
• Llength of object perpendicular to flow
• ReReynolds number
• ?2 0.4 (L/U)1/2 (Dusenbery, 1992)
• Llength of object perpendicular to flow
• Uwater flow speed

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Leakiness

• Amount of fluid that passes through the tentacles
• (actual inter space) - 2(boundary layer
  thickness)/actual inter space

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Important Points

• Examine fluid dynamics of Obelia
• Devise model for its swimming mechanism
• Test model using particle flow and/or prey
  capture
• Determine Obelias role in the ecosystem

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Methods

• Obelia obtained from kelp fronds collected at
  Fort Wetherill (Jamestown, RI) and cultures from
  University of Rhode Island
• Recorded Obelia swimming using High-speed
  (125-250 FPS) microvideography

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Methods

• Quantitative information
• Bell diameter
• Tentacle width
• Velocity
• Reynolds number
• Boundary layer
• Inter space
• Leakiness

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Reynolds numbers
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Leakiness using ?1 L/(Re)1/2 (Ellington,
1975)
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Leakiness using?2 0.4 (L/U)1/2
(Dusenbery, 1992)
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Swimming Model

• Alternative 1 (using ?1 L/(Re)1/2 )
• The boundary layers overlap close to the base,
  but cease to overlap as the tentacles diverge
• Thus, the tentacles act as paddles close to the
  base and sieves as they diverge (and leakiness
  increases)

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Swimming Model

• Alternative 2 (using ?2 0.4 (L/U)1/2 )
• Essentially, the boundary layers allow little or
  no room for leakiness
• The tentacles act like a solid surface

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Implications

• If Alternative 1
• Particles should go between tentacles from about
  midpoint to tip, but should be unable to pass
  between tentacles from base to midpoint
• Prey capture should occur on the tentacles close
  enough to reach the mouth
• Unsure how prey transfer occurs, but if prey must
  go to the underside of the tentacles, leakiness
  must allow for this transfer

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Implications

• If Alternative 2
• Particles should essentially be unable to pass
  between tentacles, as they act as a solid surface
• Flow visualization does not support this model

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Conclusions

• Obelias cosmopolitan nature
• Two proposed swimming models
• Particle flow and/or prey capture
• Obelias ecological significance

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Acknowledgements

• Thomas and Mary Lou Bellavance
• Dr. Jack Costello
• Dr. Elisabeth Arévalo
• Andrea Bertorelli
• Dorothy Branco
• Alexandra Silveira
• Michael Williams

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Questions??