The effect of flow on small-scale interactions between a benthic predator and zooplankton prey are more easily observed in a laboratory flume, where high-speed cameras can capture predator-prey events and prey type and concentration can be controlled. Knowing the flow environment in which these animals live can be used to recreate realistic flow conditions in a flume by matching the characteristics of flow observed over the organisms. Predators seek food under environmental conditions that can alter the outcome of predator-prey interactions. In the ocean, the motion of water varies due to tides, currents, waves, and turbulent eddies. How does this ambient flow impact feeding by marine organisms? Bottom-dwelling , predators that feed on small animals in the water column are dominant components of many marine communities. They play a key role in transporting material from pelagic systems in the water column down to the ocean floor .Visual predators such as burrow-dwelling fish dart out and catch passing plankton, while passive suspension feeders collect food delivered by ambient currents onto capture-surfaces. This study explores the effects of the flow of ambient water on these two contrasting modes of foraging. Passive suspension feeders rely on the motion of the surrounding water to transport prey to capture-surfaces, while active suspension feeders generate currents or actively pass a capture-surface through the water. Variations in the strength of the current can affect the amount of prey delivered to benthic suspension feeders and the ability of those predators to hold onto captured food. In response to flow, active suspension feeders can modify their feeding behavior,nft channel and passive suspension feeders can passively or actively alter their shape or orientation or grow into different configurations .

In shallow coastal habitats rapidly-changing currents, waves, and turbulence can impact feeding by benthic organisms. Currents reach maximumvelocities shoreward then seaward during flooding and ebbing tides, respectively, and minimum velocities at slack high and slack low tides. As waves approach the shore, the orbital motion of the water in the waves is compressed close to the substratum and oscillates back-and-forth on a scale of seconds . Turbulent eddies of different sizes stir the water. Many benthic zooplanktivores live in shallow coastal habitats where they are exposed to the turbulent reversals of flow associated with waves. Feeding rates by passive suspension feeders in unidirectional flow have been studied both theoretically and experimentally, e.g. in soft corals , bryozoans , sea pens , and sea anemones , but only a few experimental studies have explored the effects of waves and turbulence on rates of suspension feeding . The flow of water around benthic zooplanktivores can affect predator-prey interactions at each successive stage of the feeding process: encounter, capture, retention, and ingestion . The rate of encounters with prey is the number of prey that pass through the capture zone of a predator per time. As water velocity increases, more prey are swept past a benthic predator per time. In contrast, oscillating flow due to waves may lead to a predator resampling the same parcel of water, which could become depleted of prey. However, turbulent eddies of different sizes can stir the water and counteract depletion. Rothschild and Osborn modeled the role of turbulence in increasing encounter rates between predators and prey by such mixing, but their focus was on pelagic, not benthic, predators. Although it is informative to know how much food is available to a predator, rate of occurrences of encounters do not necessarily predict feeding rates that depend on the proportion of encountered prey that are captured , retained , and ingested.As prey pass by a predator, the escape behavior of motile planktonic prey that sense a nearby predator can reduce capture rates . Waves and turbulence can mask mechanical signals of the predator in the water and can disperse and dilute chemical signals, thereby inhibiting the ability of prey to detect and avoid the predator .

Retention is the ability of a predator to hold onto captured prey. Retention of a captured particle or organism depends on the stickiness of the predator, the contact area between the predator and prey, the size and shape of the captured item, and the speed of the water, as well as the ability of the captured prey to struggle and dislodge itself. It has been suggested and demonstrated in experiments conducted in unidirectional flow that reduced feeding rates by suspension feeders in rapidly-moving water are caused by drag forces that wash prey off capture-surfaces, but retention of prey in waves has not been analyzed. Ingestion can only occur if a predator is able to successfully retain prey. To understand the mechanisms underlying how turbulence affects the feeding rates of benthic predators that eat zooplankton, we must determine how the flow affects encounter rates , capture rates , and retention rates . If feeding rates scale with flow , rates of encounter, capture, and retention would increase proportionally. Previous studies of benthic zooplanktivorous fish showed that foraging behavior was affected by waves and turbulence . Tube blennies are small tropical fish that live in burrows within coral heads and actively dart out into the water column to capture passing zooplankton such as calanoid copepods. These suction-feeding fishes use vision to identify potential zooplanktonic prey, and then lunge towards the prey in a “predator approach”. The approach is successful when the fish swallows the prey, or unsuccessful when it misses the prey or the prey escapes and swims away. When exposed to increasing turbulence, the blennies reduced foraging effort . When exposed to waves, the blennies only tried to catch prey during the periods of slow flow that occurred as the water in the waves changed direction. However, foraging efficiency improved with increasing turbulence and stronger waves because the ability of evasive prey to detect and avoid predation declined with turbulent and wavy conditions . Although the blennies foraged less frequently, the fish were more successful at capturing prey. For these active zooplanktivores an increase in turbulence and waves interfered both with the predator’s feeding behavior and prey’s escape behavior,hydroponic nft but the net result was an increase in foraging success by the predator.

For passive suspension feeders dependent on flowing water to deliver prey, do increases in turbulence and stronger waves similarly impact capture rates and feeding efficiency? The effects of unidirectional flow on feeding rates of passive suspension-feeders are well studied . By quantifying feeding rates, only the retention or ingestion stage of the feeding process is observed, while the impacts of flow on encounter and capture of prey are obscured. Research examining the mechanisms used in passive suspension-feeding to encounter, capture, retain, and ingest prey has been carried out on non-motile “prey” and suggests that higher velocities of flow lead to higher rates of encounters and captures . Experiments with corals feeding on motile planktonic prey demonstrated that evasive swimming behavior by prey reduced capture rates in low flow and in waves . The research reported here examined how levels of turbulence and speed of waves affected each stage of the feeding process used by benthic suspension feeders eating zooplankton. The objective of this study was to measure how the trapping of motile zooplanktonic prey by passive benthic suspension feeders is affected by the “strength” of ambient flow across the predators. We addressed this question using sea anemones, Anthopleura elegantissima , which are abundant on intertidal rocky shores , and which eat a variety of zooplankton, including those with strong escape responses such as copepods . In this study we used calanoid copepods as model prey organisms because they are an important component of the diets of many benthic suspension-feeding organisms , and because their swimming behavior in response to various conditions of flow is well-characterized . We examined how the turbulent and wavy flow observed in shallow coastal habitats affect encounter, capture, and retention rates of zooplanktonic prey by a passive suspension-feeding sea anemone. Our goal was to compare the effects of turbulence and waves on predator-prey interactions between passive suspension feeders and actively-escaping zooplanktonic prey with the effects of similar ambient flow on interactions between benthic fish and such prey. All individuals of Anthopleura elegantissima were collected from Horseshoe Cove, in the Bodega Marine Reserve along the Sonoma Coast in California , during October 2012 and May 2013. Sea anemones from one clone were gently peeled from the rock using a butter knife, and each individual was placed in a separate plastic bag filled with air. The bags were kept in a cooler at 10- 15°C and transported to the University of California Berkeley . The anemones were maintained for ten days in a 19-liter aquarium where they were placed on a suspended plastic mesh substratum to prevent attachment to the aquarium walls. In a temperature-controlled cold room kept at 10-15 °C, the aquarium had recirculating filtered seawater with a salinity of 35‰.

The sea anemones were exposed to a photoregime of a 12 hours dark and 12 hours light provided by full-spectrum fluorescent bulbs . Sea anemones 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. Zooplankton were collected from the Bridge Tender Marina in Wilmington, North Carolina , using a plankton net . Samples were diluted in seawater, aerated, and used within 12 hours of capture. Individual calanoid copepods, Acartia spp., were selected using Pasteur pipettes, and held in beakers with bottoms made of Nitex mesh that were submerged in filtered and UV-treated seawater. Before experiments, copepods were dyed red to make the organisms easy to visualize in videos. To dye the plankton, the mesh beaker was submerged in a solution of Neutral Red for 20 minutes . Copepods were videotaped while swimming in still sea water at 15°C in an aquarium before and after being stained. The trajectories of the copepods were digitized with ImageJ , and the behaviors were categorized and measured using Python .Swimming speed, duration, and direction measured from copepod trajectories in still water were not significantly different between undyed copepods and dyed copepods . For control experiments that used dead prey, copepods were heat-shocked after the dye treatment. Laboratory experiments using an oscillating flume were conducted at the University of North Carolina Wilmington. A motor-controlled piston drove FSW back and forth through a U-shaped flume with a sealed working section that was 50 cm long, 10 cm wide, and 10 cm tall . In some cases copepods were captured on the far side of the observed tentacles. If a copepod carried in the flow “disappeared” behind an illuminated tentacle and did not re-emerge, we assumed that it was captured. When this occurred, the tentacles were observed carefully in subsequent frames of the video and in every case the captured copepod became visible when the tentacles moved, the copepods fluttered into view during peak velocities, or the copepods washed off the tentacles. In addition, aerial-view photos of each sea anemone in still water were taken directly after the experiment and captured copepods were noted. No discrepancies occurred between the total number of captured copepods counted by the end of the experiment and copepods observed on the tentacles once the experiment was complete. To quantify the vertical distribution of copepods in the water column, and thus the relative availability of prey in the sea anemone’s capture zone, a distribution ratio was calculated for prey in strong and weak wave regimes. The number of copepods per time that passed through the area above a sea anemone was counted in each video . The ratio described the rate at which swimming copepods passed above the copepod in the ambient flow, relative to the rate at which swimming copepods were carried through the capture zone.