The effect of turbulent waves and prey swimming behavior on suspension feeding by an intertidal sea anemone

Author(s): Robinson, Eve | Advisor(s): Koehl, Mimi A.R. | Abstract: Predators capture prey in complex and variable environments. In the ocean, bottom- dwelling (benthic) organisms are subjected to water currents, waves, and turbulent eddies. For benthic predators that feed on small animals carried in the water (zooplankton), flow not only delivers prey, but can also shape predator-prey interactions. Benthic passive suspension feeders collect prey delivered by movement of ambient water onto capture-surfaces, without actively generating feeding currents. What are the characteristics of flow over benthic suspension feeders and how do these vary over time? How do the environmental fluctuations in flow affect the encounter, capture, and retention of motile zooplanktonic prey (copepods, Acartia spp.; nauplii, Artemia spp.) by passive benthic suspension feeders (sea anemones, Anthopleura elegantissima)? For suspension feeders that can dominate the rocky intertidal, how does the presence of neighbors impact feeding by downstream sea anemones? In Chapter One, I quantify water flow over sea anemones found in a wave-exposed and a wave-protected site. I measured variations in flow habitat using a high-frequency instrument (an acoustic Doppler velocimeter) deployed at many temporal scales to assess the range of conditions in which benthic suspension feeders live. I compare the flow habitat over sea anemones between sites, between heights above the substratum, between the flood and ebb of a daily tidal cycle, the spring and neap of a monthly tidal phase, and between onshore and offshore measures of flow conditions. I show that temporal variation is not as significant a factor as spatial variation between two sea anemone clones, and that waves dominate the flow environment. I found that microhabitats over benthic organisms were disrupted or eroded by incoming waves, so that the velocity over organisms at the leading edge of a rocky intertidal shelf could be estimated using free-stream flow. And I discuss how the offshore measurements of wave height to estimate average onshore conditions are not suitable for predicting localized flow at scales relevant to benthic organisms.In Chapter Two, I compare predator-prey interactions between a benthic sea anemone and an active, lunging fish that both suspension feed on zooplankton prey. In an oscillating flume designed to replicate the characteristics of flow measured over sea anemones in situ, I video-recorded and quantified the rates of predator-prey encounter, capture, and retention in flow regimes with "weak" and "strong" waves. I found that increasing flow did not correspond to increases in encounter rate or capture for prey that swim, and retention rates were a small fraction of the number of prey that pass benthic predators. Faster flow interfered with the ability of the prey to detect predators so feeding efficiency of motile fish increased with higher waves. In contrast, strong waves washed prey off the tentacles of a passive suspension feeder, so feeding efficiency did not improve with waves and that the effect of flow on predation by benthic animals depended on the feeding mode of the predator. In Chapter 3, I examine how zooplankton prey with different swimming behavior affects suspension feeding by solitary predators, and predators with upstream neighbors. The prey used in this study were nauplii (Artemia spp.) that swim with no escape response, dead copepods that have no behavior but are subject to drag, and living copepods (Acartia spp.) that can escape jump to avoid predators. Strong waves enhanced encounter rates for the passive, dead copepods but not for prey that actively swim. There was much variability in the behavior of the live prey. I found that higher encounter rates for passive prey and for solitary sea anemones did not result in higher capture or retention rates. Instead, the behavior of the prey and the presence of neighbors contradict expectation based upon estimates of feeding that use beads or unidirectional flow.