PATTERNS OF WHOLE COLONY PREY CAPTURE IN THE OCTOCORAL, ALCYONIUM SIDERIUM.

Colonies of a boreal octocoral, Alcyonium siderium, preferentially catch prey on specific regions of the colony at certain flow speeds of low turbulence. Colonies feeding on brine shrimp cysts capture prey preferentially on the upstream side of the colony under low flow conditions (2.5 cm · s). At intermediate flow speeds (9.0 cm · s), prey capture is uniformly distributed around the circumference of the colonies, while at higher flow speeds (19.0 cm · s), prey capture again becomes asymmetric and downstream polyps capture the most prey. At higher levels of free-stream turbulence, these asymmetric prey capture distributions around the colony disappear; in the vertical direction, prey capture is asymmetric over the surface of the colony at all flow speeds tested, with polyps nearer the top of the colony capturing the most prey only at the lowest speeds. Asymmetrical filtration results from (1) increasing mechanical deformation of polyps into an orientation unfavorable for prey capture with increasing flow speed, and (2) differential prey concentrations in the boundary layer of the colony in the downstream direction. For non-motile particles, the filtration performance of this passive suspension feeder appears governed only by the flow speed and turbulence, the mechanical behavior of the filter elements, and the motion of the particles in the boundary layer of the colony.