The fluid mechanics of barnacle feeding

Suspension feeders separate organic particles from the aquatic medium. Suspension feeding is the most common type of foraging in the ocean and spans from microscopic unicellular marine organisms to whales. Marine food webs are ultimately based on the concentration of particles by suspension feeding organisms, and the mechanisms they utilize are highly varied. The ocean is typically a dilute suspension of organic particles, forcing suspension feeders to evolve effective suspension feeding mechanisms. In coastal areas of all the major oceans, barnacles are among the most common benthic invertebrates, and they capture different particles in a highly dynamic environment. They rely on specialized appendages which are swept through the water to capture a range of particulate matter including phytoplankton, detritus, and highly evasive zooplankton. This diversity of particles they capture as well as the highly variable environment they inhabit suggest that barnacles have a unique and specialized feeding mechanism. However, the feeding current and filter mechanism of barnacles has so far remained poorly understood.

This thesis aims to provide a mechanistic understanding of barnacle feeding across a range of particles and environmental flow conditions. It is based on three collaborative papers each focusing on a different aspect of the feeding mechanism. First, we study the feeding current generated by barnacles in quiescent water and how they are able to catch evasive prey. Second, we investigate the effect of body size on the feeding current and prey size specific clearance rates of barnacles. Finally, we examine barnacle feeding behavior and flow around the barnacle filter in ambient flow. We used high speed videography, particle image velocimetry and fluid mechanical modelling to gain insight into feeding mechanism of three species of barnacles.

We found that barnacles are able to capture evasive prey in quiescent water by generating a highly asymmetrical feeding current, enabling the transport of evasive prey through regions of low deformation. We also found that the development of asymmetrical feeding flows in quiescent water develops as barnacles mature, and that newly settled juveniles are unlikely to rely on evasive zooplankters. We found that the switching between feeding modes is gradual rather than sudden in response to ambient flow. Finally, we found that barnacles are more active and gain an advantage in terms of their clearance rate in ambient flow, though counterintuitively, ambient flow does not increase their chances of capturing evasive prey. In these aspects this thesis provides new mechanistic insights into barnacle feeding mechanisms and the ecological role they play in coastal ecosystems.