Effect of Ambient Current on Filtration Rate of Sponges

Here, we present published data on the influence of ambient current on the pumping rate of sponges, and subsequently, we make interpretations based on a fluid-mechanical approach for analyzing the sponge–pump system. Ambient water flow past a benthic filter-feeding sponge can give rise to an increased osculum out-flow rate (pumping rate = filtration rate) of the sponge. The mechanism at play is the change in pressure distribution over the sponge. Thus, if ambient flow leads to an increased positive pressure at the ostia inlets and a negative pressure at the osculum outlet, both contribute to a negative backpressure (suction at osculum), which shifts the system characteristic to intersect the pump characteristic at a higher volume flow rate, and this may imply a reduced energy expenditure of the sponge pump. The magnitude of such negative backpressures is estimated for an isolated, upright cylindrical sponge, and based on this, we re-interpret earlier experimental data. However, it is unknown if ambient currents may help the sponge to process the same volume of water at lower energy cost.