A 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 forces 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 over 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