A distribution ratio value of one indicates that the rate of prey available in the capture zone is equal to that in the water above the sea anemone . A ratio greater than one indicates that more prey were swimming in the water above the sea anemone than were swimming in the water that passed through the sea anemone’s capture zone.Capture efficiency was defined as the proportion of encountered prey that was captured.Trapping efficiency was calculated rather than feeding efficiency because the duration of experiments was short relative to the average ingestion times for sea anemones , thus most captured and retained prey were not ingested during the videos. The duration of the experiments was chosen to minimize the chances that prey would show a decrease in swimming and escape behaviors . Using prey encountered as the denominator when calculating rates for both capture and trapping efficiencies enabled these efficiencies to be compared with published feeding efficiency data for a zooplanktivorous fish . In addition, these efficiencies could be directly related to ecological models that estimate suspension feeding rates from encounter rates. We found that increasing the “strength” of ambient water flow enhanced rates of some steps in the feeding process and decreased others for a passive suspension-feeding predator eating zooplanktonic prey that have strong escape responses. Encounter rates normalized to capture volume depend on the speed of the water moving through the capture zone,dutch bucket for tomatoes and on the turbulence of the flow that stirs the water carrying new prey into prey-depleted water in the capture zone. Therefore, we expected that higher peak wave velocities and turbulence would enhance encounter rates for sea anemones, and this effect was observed for dead copepods.
We also found that encounter rates were greater in strong waves than in weak waves for living copepods, but there was high variation in encounter rates for these swimming prey, thus the difference was not statistically significant. If the tentacles of a predator are deformed by hydrodynamic forces, the volume of the capture zone can be reduced as the velocity of ambient water increases , thereby reducing the rate of encounters in strong waves. However, such deformation of the capture zone did not occur for A. elegantissima in the flow regimes used in our experiments, and all rates were normalized to capture volume. In weak waves more copepods swam above rather than in the capture zone of the sea anemone, whereas in strong waves the copepods were evenly distributed vertically within the water column near the sea anemone. We expected that this difference in vertical distribution would further enhance encounter rates in strong waves compared with weak waves. However our results suggest the variability in the behavior of living copepods plays an important role in shaping encounter rates. We expected that capture rates would be higher for sea anemones feeding in strong waves than in weak waves. We observed that fewer of the copepods passing through the capture zone executed escape maneuvers that avoided the predators’ tentacles in strong waves than in weak waves. Similarly, Heidelberg et al. found that zooplankton could avoid or escape benthic suspension-feeding corals under conditions of slow flow. Likewise, Robinson et al. showed that in weak waves , copepods executed escape maneuvers that enabled them to avoid being captured by a siphon that simulated suction feeding by a predatory fish, whereas in strong waves , the copepods were unable to detect hydrodynamic cues of the siphon and did not swim to avoid capture. Thus, Robinson et al. found higher capture rates for their siphon in strong waves than in weak ones. In our study of A. elegantissima we also found that capture rates were 75% greater in strong waves than in weak waves, but this difference was not significant due to the high variability of encounter rates and the low capture rates of A. elegantissima .
Retention rates were the same for both conditions of flow , and represented less than 0.5% of the prey encountered by a sea anemone. In turbulent and wavy flow, hydrodynamic forces can sweep captured prey off the tentacles of the predator . In slow flow with low turbulence, fewer prey are encountered and captured per time, but a greater proportion of them are retained by the predator than in faster, more turbulent flow. The net result is that the rate of retention of prey of a passive suspension-feeding benthic predator did not change as wave peak velocities and turbulence increased. Nematocysts on the tentacles of sea anemones adhere to prey that contact the tentacles. The adhesive strength of the nematocysts of A. elegantissima was found to be independent of habitat, availability of food, exposure to light, and species of symbiont . This suggests that the lower retention of prey by A. elegantissima in strong waves was not due to physiological differences in the ability of tentacles to hold onto prey, but rather was due to higher hydrodynamic forces dislodging prey in the more rapid flow. Stronger waves increased feeding efficiency for benthic zooplanktivorous fish but had no effect on feeding efficiency for a passive suspension-feeding sea anemone . In both cases, the escape behavior of zooplanktonic prey in slow flow resulted in lower capture rates than in faster flow. As peak velocities and turbulence increased, fewer of the prey moving through the capture zone were stimulated to execute escape maneuvers in response to either type of predator. Although the fish reduced feeding effort and the time spent feeding during a wave cycle, their foraging efficiency improved in stronger waves. These active predators were able to modify their behavior in a way that minimized expenditure of energy for foraging in faster, more turbulent flow, yet their feeding rates increased because fewer of their prey tried to escape capture. In contrast, passive suspension feeders do not swim after their prey and thus probably expend less energy per prey captured than do darting fish. Passive A. elegantissima maintained the same trapping efficiency in both weak and strong waves because, although they capture more prey per unit time in stronger waves, they also lose more of the prey that they catch in the faster, more turbulent flow.
This study reveals the importance of both the behavior of the prey and the flow of ambient water in determining the predation rates of benthic predators. In slow water feeding rates on non-swimming or weakly swimming zooplanktonic prey might be higher than on prey with strong escape responses. Furthermore, studies of feeding by benthic predators on passive particles that have no swimming behavior might overestimate feeding rates. Likewise, feeding studies of shallow-water benthic predators carried out in flumes with steady-state water flow that does not mimic the waves and turbulence to which such predators are exposed in nature could yield unrealistic feeding rates because actively-swimming prey might be able to avoid predators more readily in steady flow with less turbulence and no back-and-forth flow of waves, and the ability of predators to hold on to captured prey exposed to steady drag forces might be different from their retention abilities when prey are exposed to the pulsatile hydrodynamic forces in turbulent waves. Predators capture prey in complex and variable environments. In the ocean, bottomed welling organisms are subjected to water currents, waves, and turbulent eddies. For benthic predators that feed on small animals carried in the water , flow not only delivers prey but can also shape predator-prey interactions.Turbulent flow can stir the fluid environment, enhance prey delivery ,blueberry grow pot reduce the ability of the prey to detect and avoid predation , or wash prey off capture surfaces . How does flow impact predator-prey interactions between a benthic suspension feeder and zooplanktonic prey? Studies of passive suspension feeding have been done experimentally in unidirectional flow for corals , bryozoans , sea pens , and sea anemones , but few studies have examined the effects of waves and turbulence on suspension feeding . Experiments have generally focused on the consumption of non-motile prey, yet actively swimming zooplankton can contribute significantly to the diet of passive suspension feeders . Brine shrimp neutrally-buoyant cysts, or hatched nauplii have been used as live prey . A small number of studies of benthic suspension-feeding predators that used zooplankton prey suggest prey swimming and escape responses might impact capture rates. Although Artemia spp. nauplii can swim, they do not exhibit escape behavior. Previous research on this ubiquitous feeding strategy has been useful in estimating how much suspension feeding can contribute to ecological links between pelagic and benthic communities. A common measure of interactions between predator and prey are encounter rates . For sedentary predators that rely on ambient water to deliver food, encounter rates are dependent on the ambient flow and prey behavior. Humphries suggested the efficiency of particle capture might be higher than estimated for filter feeders in low flow. Chapter 2 suggests that retention is not 100% as is often assumed, and can be a small fraction of the initial encounter rate . The presence of neighbors affects flow around benthic suspension feeders. Okamura found that the feeding rate of an encrusting bryozoan colony was enhanced in the presence of a neighboring colony. The feeding current from the upstream colony drew currents closer to the substratum so that downstream zooids captured more prey.
Passive suspension feeders do not generate their own feeding current. However, ambient flow over a bumpy surface of organisms can generate eddies from which extended tentacles can capture prey from turbulent wakes . The objective of this study was to measure how ambient water flow, prey swimming behavior, and the presence of neighbors affect predation by benthic passive suspension feeders. I addressed this question using sea anemones, Anthopleura elegantissima , which live in wave-dominated flow habitats , feed on a variety of zooplankton that exhibit different swimming behaviors, and live in dense colonies surrounded by conspecifics . In this study we used prey with different swimming behaviors: 1) the calanoid copepod Acartia spp., which has a well-characterized escape response , 2) heat-killed Acartia spp., which are non-swimming prey with the same size, shape, and drag as living Acartia spp., and 3) nauplius larvae of Artemia spp., which are swimming prey with no escape behavior. We examined the effects on feeding of the peak water velocities and turbulent kinetic energy of the wavy ambient water flow, prey swimming and escape maneuvers, and upstream and downstream neighbors. Understanding how zooplankton swimming and the effect of neighboring suspension feeders in realistic flow conditions can contribute to predictions about the link between pelagic and benthic communities based upon flow and prey type. All sea anemones, Anthopleura elegantissima, were collected in October 2012 and May 2013 from Horseshoe Cove, in the Bodega Marine Reserve along the Sonoma Coast in California . The clone from which sea anemones were selected was the same bed over which flow measurements were collected . Sea anemones that were next to one another and positioned away from the edges of the clone were selected. Since A. elegantissima forms genetically identical polyps by binary fission, adjacent sea anemones were likely from the same clone though genetic testing was not performed. Sea anemones were gently peeled from the rock, and each individual was placed in an air-filled plastic bag. The bags were transported to the University of California Berkeley in a cooler kept at 10-15°C. The anemones were housed in a 19-L aquarium filled with recirculating filteredsea water with a salinity of 35‰. The aquarium was kept in a temperature-controlled cold room at 10-15 °C and exposed to a photoregime with 12 hours dark and 12 hours light provided by full-spectrum fluorescent bulbs . The sea anemones were placed on a suspended plastic mesh substratum to prevent attachment to the aquarium walls, and were fed hatched Artemia spp. nauplii once a day, but were not fed 24 hours before use in flume experiments. For flume experiments, sea anemones were transported to the University of North Carolina Wilmington via overnight delivery. Individual sea anemones were placed in plastic bags that were filled with oxygen. The bags were packed into a Styrofoam cooler over a base of ice packs and a middle cushioning layer of newsprint. Upon arrival sea anemones were removed from the plastic bags and housed under aquarium conditions identical to those previously described.