Chimney spacing in encrusting bryozoan colonies (Membranipora membranacea):: Video observations and hydrodynamic modeling

Closely packed zooids in encrusting bryozoan colonies pump water from ambient pressure above the colony to the higher base pressure prevailing under the lophophore canopy. This higher pressure drives the flow of filtered water against friction in the canopy region to either the rim of the colony or to chimneys located in the interior. The larger the distance from a given zooid to such locations the higher the base pressure and the lower the filtration rate of the zooid, as expressed by the backpressure characteristic of zooids. This hydrodynamic problem is modeled to deter-mine distributions of base pressures and feeding conditions of zooids in a colony. It is hypothesized that, as a colony increases in size, a chimney will develop in a given area when the feeding rate becomes less than a certain minimal value, found to be of the order of 30% of that of the isolated zooid. This in turn determines the likely spacing between chimneys in large colonies. The concept of the area specific mean feeding rate has been introduced, and model calculations seem to indicate that colonies often operate collectively near the maximal value of this quantity. In support for the suggested hydrodynamic model video observations were made on Membranipora membranacea which forms large continuous layers on fronds of seaweed.