The effect of increasing external flow on 3D orientation of sessile suspension feeder, Vorticella convallaria

Vorticella convallaria are microscopic sessile suspension feeders which live attached to substrates and are ubiquitous in aquatic environments. They depend on a self-generated current to feed and help maintain the health of aquatic ecosystems by consuming bacteria and detritus. They can improve wastewater treatment and bioremediation efforts. Their environmental impact is mediated by their feeding rate. In ambient flow, feeding rates are highly dependent on organism orientation relative to the surface and flow, both due to the superposition of feeding current and ambient flow and due to recirculating eddies at some orientations. We hypothesized that individuals actively orient themselves into positions favorable for feeding. We cultured organisms attached to the bottom surface of a flow chamber and exposed them to unidirectional laminar flow at four speeds with shear rates of 0, 0.5, 1.0, and 1.5 s-1. We recorded the 3D orientation of 18 individuals in each flow speed over a span of 20 minutes using a simultaneous top and side view microscope. We determined that Vorticella’s orientation becomes increasingly tilted downstream as the flow speed increases, but organisms are still able to actively reorient at similar frequencies to still water. At higher shear rates, organisms spent a larger fraction of their time in orientations with reduced feeding rates.