Reaction Distance Research Articles

The effects of turbidity and an invasive species on foraging success of rosyside dace (Clinostomus funduloides)

Summary1. Habitat degradation and biological invasions are important threats to fish diversity worldwide. We experimentally examined the effects of turbidity, velocity and intra‐ and interspecific competition on prey capture location, reactive distance and prey capture success of native rosyside dace (Clinostomus funduloides) and invasive yellowfin shiners (Notropis lutipinnis) in Coweeta Creek, North Carolina, U.S.A.2. Increased turbidity and velocity produced significant decreases in the number of prey captured forward of the fish’s location. It is possible that this represents an increase in the amount of energy expended per prey captured.3. We used Akaike’s Information Criterion (AIC) to evaluate competing explanatory models for reactive distance (10 generalised linear models, GLM) and prey capture success (9 generalised linear mixed models, GLMM).4. Reactive distance decreased by 12% with an increase from 2 to 4 conspecifics, whereas a 10 NTU increase in turbidity reduced reactive distance by 9%. Capture success was affected by velocity, dominance and competition, and varied among species. A 6 cm s−1 increase in velocity produced a 28% decline in capture probability; however, dominant fish were 3.2 times more likely to capture a prey item than non‐dominant fish. Yellowfin shiners only were 0.62 times as likely to capture a prey item as rosyside dace. Both intra‐ and interspecific competition reduced capture probability, and fish in high density intraspecific or interspecific trials were 0.46 times and 0.44 times as likely to capture prey, respectively, as fish in two fish intraspecific trials.5. These results suggest behavioural variables are as important as physical factors in determining reactive distance and capture probability by these minnows.

Planktivore vertical migration and shoaling under a subarctic light regime

Visually foraging planktivorous fish are prey of visual predators, and their foraging behaviour may be affected by light levels both in terms of gain and risk. The large seasonal change in day length throughout a subarctic summer at 69°N was used to show the influence of light on diel vertical migration (DVM) and shoaling patterns in a planktivorous fish assemblage consisting of two species ( Coregonus lavaretus and Coregonus albula ). Under the midnight sun in June, night and daytime behaviour was similar, with extensive shoaling and limited DVM. With increasingly darker nights towards autumn, the fish dispersed during the dark hours and showed more extensive DVM. Throughout the changing light regime of both the day and the season, the planktivores consistently chose depths with light levels compatible with visual foraging and reduced predation risk as revealed from reactive distance modelling of coregonids and their salmonid predators. The findings support the hypothesis that behavioural decisions are based on a trade-off between foraging rate and predation risk, and increased predator avoidance behaviour towards autumn suggests that this trade-off is state-dependent.

Combined effects of chemical and visual information in eliciting antipredator behaviour in juvenile Atlantic salmonSalmo salar

Under natural conditions, both young-of-the-year (YOY; 0+ year) and parr (1+ year) Atlantic salmon Salmo salar exhibited strong antipredator behaviour (e.g. increase in latency to resume foraging) following the exposure to damage-released chemical alarm cues relative to a stream water control. Subsequent exposure to a novel visual stimulus had contrasting results. Parr increased their reactive distance to the visual stimulus if they had been previously exposed to a chemical alarm cue, whereas YOY did not. On the other hand, both YOY and parr took significantly longer to resume foraging when exposed to a visual stimulus if they had been previously exposed to a chemical alarm cue than control groups. While YOY and parr differed in the type and intensity of antipredator responses to both chemical and visual stimuli, perhaps due to differential costs and benefits associated with age, both used the chemical and the visual information in a combined manner.

Exploring the Persistence of Stream‐Dwelling Trout Populations under Alternative Real‐World Turbidity Regimes with an Individual‐Based Model

AbstractWe explored the effects of elevated turbidity on stream‐resident populations of coastal cutthroat trout Oncorhynchus clarkii clarkii using a spatially explicit individual‐based model. Turbidity regimes were contrasted by means of 15‐year simulations in a third‐order stream in northwestern California. The alternative regimes were based on multiple‐year, continuous monitoring in two streams. Turbidity affected model fish by reducing both their risk of predation and their reactive distance to drifting prey. It did not affect their ability to locate nondrifting food, such as invertebrates on the stream bottom. Under a calibration scenario that assumed trout predominantly consume drifting prey, the less‐turbid real‐world regime produced relatively stable abundance across years (similar to field observations) whereas the more‐turbid regime (under otherwise identical physical conditions) resulted in extinction within the 15‐year simulation period. Additional simulations revealed sensitivity to the relative amounts of prey available via drift versus search feeding and showed that seasonal variation in food availability or strong positive relationships between streamflow and food concentration would not prevent extinction in the high‐turbidity regime under a drift‐feeding‐based food calibration. Extinction of predominantly drift‐feeding trout populations in our simulations contrasts with field observations of salmonid populations that have persisted in moderately turbid regimes. The results highlight the need for better understanding of patterns in the availability of food under turbid conditions and the capability of stream salmonids to use nonvisual cues in feeding.

Use of Benthic Prey by Salmonids under Turbid Conditions in a Laboratory Stream

AbstractThe negative effect of turbidity on the reactive distance of salmonids has been well established. However, determining the consequences of this relationship for overall feeding success remains problematic, as successful foraging by salmonids across a broad range in turbidity has been observed under a variety of conditions. Previous laboratory and field observations suggest that benthic feeding by salmonids in flowing water affects the turbidity dependence of foraging success. Two experiments were conducted in a laboratory stream to quantify benthic feeding success of salmonids across turbidity treatments ranging from 0 to 400 nephelometric turbidity units (NTU). In one experiment, cutthroat trout Oncorhynchus clarkii and coho salmon O. kisutch were offered the same nonliving prey in the drift and on the stream bottom; in the second experiment, cutthroat trout were offered only live oligochaetes moving along the stream bottom. In the first experiment, benthic feeding success of both cutthroat trout and coho salmon at 100 NTU was at least 70% of their feeding performance in clear water (i.e., 0 NTU), whereas neither species fed at 400 NTU. In the second experiment, benthic feeding success of cutthroat trout at 150 NTU was about 35% of their performance in clear water but dropped to near 0% at 200 NTU; no feeding was observed at 400 NTU. Passive integrated transponder tag detections suggested that cutthroat trout activity increased over the range of 0‐150 NTU. Although drift and benthic foraging success decreased over the examined turbidity range, the results suggest that both cutthroat trout and coho salmon are capable of feeding from the benthos in relatively turbid, flowing water. The importance of this ability for salmonid populations will be influenced by the turbidity regime, food availability patterns, and hydraulic complexity of the systems they occupy.

Foraging behavior of bay anchovy larvae, Anchoa mitchilli

Foraging behavior of the bay anchovy larvae, Anchoa mitchilli was characterized by silhouette video photography. Swimming speed, pause frequency and duration, burst duration, reactive distance (RD), horizontal and vertical reaction angle (HRA, VRA), and length of the predation cycle were estimated over a range of sizes and stages of development. First feeding larvae displayed a distinct pause-travel search behavior that included horizontal rotation of their body to the right or left to scan a new volume of water. Pause duration was affected by light intensity in small larvae and significantly decreased in length of time as larvae grew (ANOVA, p < 0.01). Duration of swimming bursts between pauses was short (mean = 0.13 ± 0.03 s). Average swimming speeds increased with size from a mean of 0.50 ± 0.39 BL · s - 1 (body lengths) at first feeding to 2.25 ± 1.27 BL · s - 1 at 12 mm SL. Mean RD of bay anchovy larvae at 10 lux and corrected for vertical angle was 0.65 BL and increased to 0.92 and 1.00 BL at 100 and 250 lux, respectively. The average RD increased in the presence of turbidity from 0.65 to 0.83 BL at 10 lux. Reducing light (250, 100, 10 lux) affected the foraging behavior of the largest larvae (14-19 mm SL) especially at 10 lux where only one attack was observed with or without turbidity. The Horizontal attack angles (HRA) of larvae ranged from 0-114° with 78% of attacks taking place at angles between 20 and 70° while vertical reaction angles (VRA) of larvae ranged from 12-90° with 77% of attacks occurring at < 50°. The average length of the predation cycle decreased with increasing larval size from 2.4 s down to 1.34 s for larger larvae. Bay anchovy larvae frequently aborted attacks (33%) after reacting and approaching prey. Attack success was high even for small larvae (mean = 83%) and approached 100% in the largest size class.

On the Interaction Between Multiple Paths and Wireless Mesh Networks Scheduler Approaches

Multi-path routing allows building and use of multiple paths for routing between a source-destination pair. This paper investigates the problem of selecting multiple routing paths to provide better reliability and load balancing in wireless mesh networks with stationary nodes. Previous work has investigated the use of additional data redundancy to improve the throughput of the network. In these specific cases, node disjoint-ness property of the multiple paths is required. In this work we investigate multipath routing without packet duplication, and no disjointed paths for achieving better performance in terms of packet delivery rate and low delay. We propose a very simple reactive on-demand distance vector routing protocol. Multiple paths built through this approach are loop-free. In order to better exploit resources redundancy (with the term resources redundancy we mean the possibility to exploit more nodes to send data packets), it is our belief that a routing protocol cannot be independent of the MAC layer. For this reason, we evaluated our routing protocol on four different MAC approaches specifically designed for Wireless Mesh Networks (WMNs). Firstly, we implemented the Coordinated Distributed Scheduler scheme of the Std. IEEE 802.16. Secondly, since some parameters have been left unstandardized in this scheme, we proposed an enhanced version of the CDS, in which a simple and dynamic criterion has been designed to set one of these parameters. Furthermore, we proposed two different scheduling schemes called Randomized- MAC (R-MAC) and Distributed Scheduling Scheme (DSS). We evaluated the impact of multiple paths in respect of the single path on all the scheduler schemes cited above. Results show as the simple routing approach is effective with every MAC protocol considered.

Alteration of sensory abilities regulates the spatial scale of nonlethal predator effects

Many studies have shown that nonlethal predator effects such as trait-mediated interactions (TMIs) can have significant impacts on the structure and function of communities, but the role that environmental conditions play in modulating the scale and magnitude of these effects has not been carefully investigated. TMIs occur when prey exhibit behavioral or physiological responses to predators and may be more prevalent when abiotic conditions increase prey reactions to consumers. The purpose of this study was to determine if turbulence would alter the distance over which prey in aquatic systems respond to chemical cues emitted by predators in nature, thus changing the scales over which nonlethal predator effects occur. Using hard clams and blue crabs as a model predator-prey system, we investigated the effects of turbulence on clam reactive distance to predatory blue crabs in the field. Results suggest that turbulence diminishes clam reactions to predators and that the environmental context must be considered when predicting the extent of indirect predator effects in natural systems.

Physiological and visual refuges in a metalimnion: an experimental study of effects of clay turbidity and an oxygen minimum on fish predation

Summary1. The effectivity of elevated clay turbidity and low‐oxygen concentration in the metalimnion as refuges for chaoborid larvae against fish predation was studied in experimental water columns.2. When 70–80 nephelometric turbidity unit clay turbidity was combined with 3–4 mg L−1 oxygen concentration, prey capture rate by fish (golden orfe Leuciscus idus) was reduced by 74% compared with the control treatment with no refuges. Oxygen and turbidity refuges alone did not significantly reduce the feeding rate.3. All fish in the control treatment dwelled in the metalimnion, but 36% of the fish in the low‐oxygen treatment and 23% of the fish in the turbidity treatment stayed in the epilimnion. In the combined treatment, 54% of the fish were in the epilimnion.4. The results demonstrated that a combination of moderately elevated turbidity and lowered oxygen concentration in the metalimnion is an effective protection against fish predation, while turbidity or oxygen refuge alone are much less effective.5. In the treatment with the combined refuge, oxygen concentration limited the time fish could spend in the metalimnion and turbidity affected the detection of prey through changes in reactive distance.6. Because of the combined effects of turbidity and oxygen refuges, planktivorous fish and phantom midge larvae may co‐occur in clay‐turbid lakes in high densities. Such situation is problematic for biomanipulation, which aims to enhance the grazing rate of zooplankton through reduction of planktivorous fish.

The turbulent life of copepods: effects of water flow over a coral reef on their ability to detect and evade predators

The heterogeneous distribution of water flow over structurally complex environments, such as coral reefs, may play an important role in the interactions between copepods and plankti- vorous fish by interfering with the copepods' ability to detect and evade predators. The escape response and capture rates of the copepod Acartia tonsa were examined in laboratory flumes that created both unidirectional and oscillatory flow conditions similar to those found near coral reefs. Two turbulent regimes were produced in each flume: 'smooth' flow was formed using a grid colli- mator and 'rough' flow was generated by placing a branched coral skeleton upstream of the flume's working section. A predator was simulated by a fixed siphon to generate a stimulatory flow field. Copepod detection of the siphon was measured as the distance from the siphon tip to where an escape response was initiated. This reactive distance remained the same in low-flow conditions as in still water, but was reduced by 25% at higher flow speeds, indicating a decline in the copepods' ability to detect velocity gradients formed by the siphon. Rough turbulence regimes intensified the effect of current speeds, resulting in an even shorter copepod reactive distance. Capture rates of copepods by the siphon increased with current speed, wave motion, and in rough flow, while the capture rates of non-evasive prey, Artemia nauplii, did not vary with flume conditions. The differ- ences in capture rates between evasive and non-evasive prey suggest that behavioral shifts in cope- pod escape thresholds may account for increases in predation by reef-dwelling fishes observed in hydrodynamically complex coral environments.

A review of flatfish behavior relative to trawls

Trawls harness the innate avoidance behavior of fish to affect their capture. As such, effective bycatch reduction relies, in part, upon knowledge of behavioral differences between target and non-target species. The behavior of flatfish during herding, net entry and passage through trawls differs substantially from that of many roundfish. These differences result from the unique body morphology of flatfish and the constraints this morphology places upon their natural predator avoidance and evasion tactics. Paramount, in this regard, is the intimate association between flatfish and the seafloor. Flatfish utilize a detection minimization strategy that combines burial, highly evolved cryptic capabilities and low activity. Additionally, the maximum sustained swimming speeds and endurance of flatfishes are low compared to most roundfish. As a result, flatfish typically respond to trawl ground-gear at shorter distances, remain closer to the seafloor during herding and herd in the net mouth for a shorter time, prior to net entry, than roundfish. As is the case for roundfish species, light and temperature influence flatfish reactivity and herding behavior. At low ambient light levels herding behavior in flatfish breaks down, as fish initially respond to ground-gear by rising off the bottom, which removes them from the ground-gear's zone of influence. Similarly, at low temperatures some flatfish display reduced herding. Differences in behavior between flatfish and roundfish have spurred development of selective flatfish trawls, with low rise head ropes and/or open or large mesh intermediates that allow rising roundfish to exit the net opening, while flatfish remain close to the net floor, passing into the net and back to the codend. Lastly, a conceptual design for a counter-herding device is presented, which takes advantage of the longer reactive distances of roundfish, and their tendency to herd farther of the seafloor, thereby allowing them to be herded out of the path of the net, while not influencing the normal inward herding of flatfish.

Saltatory search in a lateral line predator

The dwarf scorpionfish Scorpaena papillosa detected the hydrodynamic signals produced by prey with the mechanosensory lateral line. This species displayed a pause and move search pattern that is consistent with a saltatory search. The pause phase of the search cycle was probably used to detect prey because pauses often ended early in order to initiate an approach at prey and prey were detected throughout the search space. The move phase of the search cycle repositioned the fish so that it moved approximately a third of the reactive distance. Move distance was found to be the most important factor in gaining novel search space. Turning was shown to be relatively unimportant in gaining novel search space with a high frequency of low turn angles made by the fish. The dwarf scorpionfish, however, exhibited a spiralling or looping pattern over a search path exhibiting a turn bias towards either the left or right. The dwarf scorpionfish adopted a search behaviour that is consistent with a saltatory search and efficient for lateral line predation.

The influence of turbidity on three‐spined stickleback foraging

Three‐spined sticklebacksGasterosteus aculeatuswere used as a model to test for the effects of turbidity on visual‐predator foraging. While the number of feeding lunges and number of prey consumed did not change with increasing turbidity, reactive distance to prey decreased.

Turbidity Affects Foraging Success of Drift‐Feeding Rosyside Dace

AbstractThe effects of suspended sediment on nongame fishes are not well understood. We examined the effects of suspended sediment (i.e., turbidity) on reactive distance and prey capture success at spring–autumn (12°C) and summer (18°C) temperatures for rosyside dace Clinostomus funduloides in an artificial stream. Experimental turbidities ranged from 0 to 56 nephelometric turbidity units (NTUs) at the two temperatures, and sample sizes for each turbidity–temperature combination ranged from 5 to 10 fish. All fish were tested at a single, randomly selected turbidity. Nonlinear regression (dosage–response curves) detected a negative relationship between turbidity and reactive distance at both spring–autumn (R2 = 0.96) and summer (R2 = 0.90) temperatures. Turbidity also had a strong negative effect on capture success at both spring–autumn (R2 = 0.88) and summer (R2 = 0.70) temperatures. Two‐way analysis of variance showed that turbidity and temperature were significant in all models (P &lt; 0.0001) and that there were no significant interactions. The median effective concentrations (EC50s, i.e., the concentrations required to elicit a response equal to 50% of the maximum response observed) for reactive distance regressions were 9.9 NTUs in spring and autumn and 9.2 NTUs in summer. Data from moderately impacted streams in the region indicate that these streams possess turbidities that exceed the EC50s approximately 50% of the time. These results suggest that turbidity negatively affects the foraging behavior of rosyside dace at even low to intermediate levels (&gt;9 NTUs) and that these effects may vary seasonally.

Depth related trends in proximate composition of demersal fishes in the eastern North Pacific

The proximate chemistry of the white muscle and liver of 18 species of demersal fish from the eastern North Pacific was studied to determine trends with depth, locomotory mode and buoyancy mechanism, foraging strategy and to elucidate energetic strategies. Data for 24 species from shallow water were taken from the literature and included for analysis of muscle water content. Benthopelagic species, primarily gadiforms, have significantly larger lipid-rich livers than benthic species. The benthopelagic species may use this lipid to add buoyancy, but it is also used as energy storage. Buoyancy mechanism was directly related to proximate composition. Fishes using gasbladders had normal muscle composition. The two species of benthopelagic fishes without gasbladders have either very high muscle lipid content ( Anoplopoma fimbria) or gelatinous muscle ( Alepocephalus tenobrosus) to aid in achieving neutral buoyancy. The macrourid, Albatrossia pectoralis, has a very small gasbladder and also has gelatinous muscle. Both of these benthopelagic fishes with gelatinous muscle feed on pelagic organisms. Gelatinous muscle was also found in two flatfishes that inhabit the oxygen minimum zone. For these fishes, high water content may serve to lower metabolic costs while maintaining large body size. Scavengers such as Coryphaenoides armatus and Coryphaenoides acrolepis have lipid rich livers and others such as A. fimbria and Pachycara sp. have high and variable muscle lipid content. Thus foraging mode also acts to influence proximate composition. Several depth-related trends in proximate composition were found. White muscle water content increased significantly with depth, and all four gelatinous species occurred at bathyal depths. This adds evidence in support of the hypothesis that decreasing light levels shorten reactive distances and relax the selective pressure for high locomotory capacity. In addition significant declines in liver protein content were observed, suggesting that the rates of metabolism in this organ also decline with depth. There was little evidence for food availability affecting proximate composition. There were no significant changes in either muscle or liver lipid or caloric density with depth. Total lipid stores actually increased significantly, but they were driven primarily by the abyssal scavenger C. armatus suggesting that foraging strategy rather than depth may be the most important factor determining total lipid stores.

Maternal body burdens of methylmercury impair survival skills of offspring in Atlantic croaker ( Micropogonias undulatus)

Methylmercury (MeHg), the organic form of mercury, bioaccumulates easily through the food chain. Fish in high trophic levels can accumulate substantial levels of MeHg and transfer it to their developing eggs. Here, the effects of maternally derived MeHg on the planktonic larval stage of Atlantic croaker were investigated. Adult Atlantic croaker were fed MeHg-contaminated food at three levels: 0, 0.05, and 0.1 mg kg −1 day −1 for 1 month. Fish were then induced to spawn and MeHg levels in the eggs were measured (0.04–4.6 ng g −1). Behavioral performance of exposed and control larvae was measured at four developmental stages: end of yolk absorption (yolk), end of oil absorption (oil), and 4 and 11 days after oil absorption (oil + 4 and oil + 11). Behaviors analyzed included survival skills related to foraging and predator evasion: routine behavior (rate of travel, active swimming speed, net-to-gross displacement ratio, and activity) and startle response to a visual and a vibratory stimulus (responsiveness, reactive distance, response distance, response duration, average response speed, and maximum response speed). Maternally transferred MeHg induced concentration-dependent effects on survival skills. Statistical and simulation models applied to predict the ecological consequences of the behavioral effects suggested that maternal transfer of MeHg may substantially lower survival of planktonic stage larvae compared to unexposed larvae. These results also imply that larvae of top predatory fish species, such as blue marlin, may suffer mortality through maternal transfer of MeHg.

Visual field of cultured striped trumpeter Latris lineata (Teleostei) larvae feeding on rotifer prey

The visual field of striped trumpeter Latris lineata larvae fed rotifer prey was determined from analysis of feeding behaviour in the horizontal plane. The visual field was forward and laterally directed, characterised by maximum reactive distances (distance at which the predator first detects and reacts to the prey) of 5.07 mm and 5.25 mm on days 13 and 17 post-hatching, respectively, 97% of mean larval length. This confirmed the predicted horizontal visual field, forward and laterally directed, derived from higher cone cell densities in the dorso-temporal and medial regions of the retina compared with ventral regions. The visual field of prey detection expanded laterally with ontogeny as a wider range of reactive angles was used by 17 day-old than 13 day-old larvae. Larvae displayed a saltatory searching pattern, periodically stopping to scan for prey throughout the visual field, and exhibited a side-to-side movement of the head as they approached and stopped, prior to striking at a detected prey item. Larvae on day 17 post-hatching terminated 35% of feeding sequences at the pre-strike position, at a mean distance from prey of 0.58 mm.

Calanoid copepod escape behavior in response to a visual predator

Calanoid copepods typically exhibit escape reactions to hydrodynamic stimuli such as those gener- ated by the approach of a predator. During the sum- mers of 2000, 2001 and 2004, two small calanoid species, Temora turbinata Dana, 1849 and Paracalanus parvus Claus, 1863 were exposed to a visual predatory Wsh, the blenny Acanthemblemaria spinosa Metzelaar, 1919, and their predator-prey interactions were recorded using both high-speed and standard video- graphic techniques. Copepod escape reaction compo- nents, including swimming pattern, reactive distance, turning rate, and jump kinetics, were quantiWed from individual predation events using motion analysis tech- niques. Among the observed escape reaction compo- nents, diVerences were noted between the species' swimming patterns prior to attack and their response latencies. Temora turbinata was a continuous cruiser and P. parvus exhibited a hop-and-sink swimming pat- tern. During periods of sinking, P. parvus stopped beating its appendages, which presumably reduced any self-generated hydrodynamic signals and increased perceptual abilities to detect an approaching predator. Response latency was determined for each copepod species using a hydrodynamic stimulus produced by a 1 ms acoustic signal. Response latencies of T. turbinata were signiWcantly longer than those of P. parvus. Despite some apparent perceptual advantages of P. parvus, the blenny successfully captured both species by modifying its attack behavior for the targeted prey.

Spectral sensitivity of the two-spotted gobyGobiusculus flavescens(Fabricius): a physiological and behavioural study

Microspectrophotometry of Gobiusculus flavescens photoreceptors revealed a single rod visual pigment (lambdamaxat 508 nm) and the three cone pigments (lambdamax 456, 531 and 553 nm). The cone population was dominated by identical double cones containing the middle-wave-sensitive (MWS) pigment, but with a small number of non-identical MWS/LWS (long-wave-sensitive) and identical LWS double cones. Small populations of large single cones also contained either the MWS or LWS pigment. The short-wave-sensitive (SWS) pigment was found in small single cones. Lens transmission was great reduced below 410 nm. The spectral sensitivity of the behaviourally determined reaction distance (RD) to prey at a high irradiance level (0.5 micromol m-2 s-1) correlated with the maximum sensitivity of the MWS cones, both peaking around 530 nm. However, at a lower irradiance level (0.015 micromol m-2 s-1) such a correlation was not so apparent. The RD was greatly reduced, though still maintaining a peak around 530-550 nm, but with a relatively smaller reduction in RD at shorter wavelengths. Optomotor behaviour displayed a somewhat similar spectral sensitivity to the RD responses at the higher light intensity. However, the peak was at slightly longer wavelengths at 550 nm, suggesting a greater input from LWS cones to the optomotor response.

CLAMMING UP: ENVIRONMENTAL FORCES DIMINISH THE PERCEPTIVE ABILITY OF BIVALVE PREY

The lethal and nonlethal impacts of predators in marine systems are often mediated via reciprocal detection of waterborne chemical signals between consumers and prey. Local flow environments can enhance or impair the chemoreception ability of consumers, but the effect of hydrodynamics on detection of predation risk by prey has not been investigated. Using clams as our model organism, we investigated two specific questions: (1) Can clams decrease their mortality by responding to predators? (2) Do fluid forces affect the ability of clams to detect approaching predators? Previous research has documented a decrease in clam feeding (pumping) in response to a neighboring predator. We determined the benefits of this behavior to survivorship by placing clams in the field with knobbed whelk or blue crab predators caged nearby and compared mortality between these clams and clams near a cage-only control. Significantly more clams survived in areas containing a caged predator, suggesting that predator-induced alterations in feeding reduce clam mortality in the field. We ascertained the effect of fluid forces on clam perception of predators in a laboratory flume by comparing the feeding (pumping) behavior of clams in response to crabs and whelks in flows of 3 and 11 cm/s. Clams pumped significantly less in the presence of predators, but their reaction to blue crabs diminished in the higher velocity flow, while their response to whelks remained constant in both flows. Thus, clam reactive distance to blue crabs was affected by fluid forces, but hydrodynamic effects on clam perceptive distance was predator specific. After predators were removed, clams exposed to whelks took significantly longer to resume feeding than those exposed to blue crabs. Our results suggest that prey perception of predators can be altered by physical forces. Prey detection of predators is the underlying mechanism for trait-mediated indirect interactions (TMIIs), and recent research has documented the importance of TMIIs to community structure. Since physical forces can influence prey perception, the prevalence of TMIIs in communities may, in part, be related to the sensory ability of prey, physical forces in the environment that impact sensory performance, and the type of predator detected.