title for learn-about section of A SNAIL'S ODYSSEY
  Colonial life
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Research study 1

graph comparing drag forces of water current on natural colonies of the hydroid Abeitinaria sp. versus colonies that have been experimentally stiffened by coating with cyanoacrylate cement and other plasticsA design conflict exists in hydroid colonies, as in other passive suspension-feeders, between maximising surface area for feeding and minimising drag-related forces.  A study at Friday Harbor Laboratories, Washington on drag forces in the hydroid Abeitinaria sp. shows that flexibility in the colony reduces drag from a square- to a first-power dependence.  Thus, because of overall colony flexibility, drag forces are reduced to a linear function of velocity.  Moreover, the advantages of colony flexibility extend through to the polyp, with flow velocity past the polyp being reduced considerably over an order of magnitude.  Thus, flexibility both reduces the chance of the colony being torn off in currents as well as reducing the disruptive effects of current on feeding.  The graph on the Left shows the difference in drag forces acting on a normal colony versus a colony-preparation that has been experimentally stiffened. Note that the graph comparing drag forces on polyps of the hydroid Abeitinaria sp.: natural flexible colonies vs. colonies that have been experimentally stiffened with cyanoacrylate cement and acrylic plasticforces acting on a normal live colony are directly proportional to mainstream current velocity, while those acting on the stiffened colonies are proportional to the square of mainstream velocity.

As mainstream velocity increases in a normal colony, the polyps flex and the tentacles stream into areas of lower velocities, and this may enable capture of food particles in eddy currents.  Note in the graph on the Right that local flow velocities in natural colonies remain constant at about 2-3cm . m-2 photograph of an Abeitinaria-type colonial hydroidover a 17 cm . m-2 range of mainstream velocities.  In comparison, variation in velocities at the polyp level in the stiffened colonies is much greater.  The authors remark that this finding is a previously unappreciated consequence of colony flexibility in hydroids.  Harvell & Labarbera 1985 Biol Bull 168: 312.

NOTE   the colony is stiffened by drying, threading a steel wire up the central axis, impregnating with cyanoacrylate cement, and coating with 5 coats of aerosol acrylic plastic



Abeitinaria-type colonial hydroid 1X

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photograph of a hydroid colony taken from a video

CLICK HERE to see a video of a hydroid colony growing amongst some old parchment-tubeworm tubes. The video is not particularly exciting, but does show something of the ramified growth form of what may be a single hydroid individual.

NOTE the video replays automatically

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