Passive Suspension Feeding by an Octocoral in Plankton Patches: Empirical Test of a Mathematical Model.

Feeding rate in the octocoral, Alcyonium siderium, was investigated as a function of colony size, flow speed, and prey concentration. The feeding rate decreases with time in high prey concentrations. A model of passive suspension feeding is formulated that successfully predicts feeding behavior. At low prey concentrations, the model predicts a linear feeding response as particle flux or colony size increases. The dominant constraint on feeding is the "handling time" required to transfer prey from tentacle to pharynx and to re-extend the tentacle. The time constant of prey capture shows no relation to particle flux, in agreement with the model. Another constraint, the "filtration time," is inversely related to colony size and flow speed. Filtration time becomes important only during feeding in sparse prey concentrations, when feeding rate is proportional to flow speed, colony size, and prey concentration. In the field, Alcyonium colonies reduce filtration time by orienting at right angles to the dominant flow direction. Feeding efficiency on prey patches is low and inversely related to flow speed, colony size, and prey concentration. Feeding in patches is not a simple process for this octocoral, because colonies will "saturate" with prey before all polyps have successfully captured a single prey item.