Predation by blue crabs, Callinectes sapidus, on rapa whelks, Rapana venosa: possible natural controls for an invasive species?

Blue crabs Callinectes sapidus are voracious predators in Chesapeake Bay and other estuarine habitats. The rapa whelk Rapana venosa is native to Asian waters but was discovered in Chesapeake Bay in 1998. This predatory gastropod grows to large terminal sizes (in excess of 150 mm shell length (SL)) and has a thick shell that may contribute to an ontogenetic predation refuge. However, juvenile rapa whelks in Chesapeake Bay may be vulnerable to predation by the blue crab given probable habitat overlap, relative lack of whelk shell architectural defenses, and the relatively large size of potential crab predators. Feeding experiments using three size classes of blue crab predators in relation to a size range of rapa whelks of two different ages (Age 1 and Age 2) were conducted. Blue crabs of all sizes tested consumed Age 1 rapa whelks; 58% of all Age 1 whelks offered were eaten. Age 2 rapa whelks were consumed by medium (67% of whelks offered were eaten) and large (70% of whelks offered were eaten) blue crabs but not by small crabs. The attack methods of medium and large crabs changed with whelk age and related shell weight. Age 1 whelks were typically crushed by blue crabs while Age 2 whelk shells were chipped or left intact by predators removing prey. Rapa whelks less than approximately 35 mm SL are vulnerable to predation by all sizes of blue crabs tested. Rapa whelk critical size may be greater than 55 mm SL in the presence of large blue crabs indicating that a size refugia from crab predation may not be achieved by rapa whelks in Chesapeake Bay until at least Age 2 or Age 3. Predation by blue crabs on young rapa whelks may offer a natural control strategy for rapa whelks in Chesapeake Bay and other estuarine habitats along the North American Atlantic coast.     

Ecological consequences of ontogenetic shifts in predator diet: Seasonal constraint of a behaviorally mediated indirect interaction

Predators play an important role in structuring assemblages through direct and cascading indirect effects. While there has been recent interest in how the strength and direction of trophic cascades vary spatially, seasonal variability in trophic links is seldom considered. In North Carolina, recruitment-failure of bay scallops typically occurs following the spring but not the fall spawning despite the presence in each of these seasons of predatory blue crabs. One explanation for this pattern is that in the fall, seasonally abundant predators of blue crabs reduce the foraging efficiency of crabs on scallops and thus the overall magnitude of top-down effects. Quantification of bay scallop consumption by blue crabs in closed mesocosms with or without pinfish supported the hypothesis that seasonally abundant adult pinfish indirectly increase survivorship of bay scallop recruits in fall by reducing predation by blue crabs. Despite voracious consumption of bay scallops during both the day and night in mesocosms to which only small blue crabs were added, blue crabs in mesocosms with visually-foraging adult pinfish consumed bay scallops only by night. Juvenile pinfish that dominate estuarine populations in spring did not impede consumption of bay scallops by blue crabs. In mesocosms from which animals could not emigrate, the addition of neither adult nor juvenile pinfish increased the mortality of blue crabs, indicating a behaviorally mediated interaction. Blue crabs restricted by adult pinfish to nocturnal feeding did not compensate for lost feeding time by increasing their night-time consumption of bay scallops. These results strongly suggest that greater survivorship of bay scallops in fall than spring is due to adult pinfish, potential predators of small blue crabs, restricting blue crab foraging to hours of dark. In spring, when pinfish are small and incapable of consuming blue crabs, blue crabs consume bay scallops by day and by night. Such seasonal variation in the number of trophic links in a system may have important evolutionary implications. By timing reproduction to occur in fall when the pinfish-crab-scallop cascade is in operation, bay scallops maximize recruitment.     

Habitat context influences predator interference interactions and the strength of resource partitioning

Despite increasing evidence that habitat structure can shape predator–prey interactions, few studies have examined the impact of habitat context on interactions among multiple predators and the consequences for combined foraging rates. We investigated the individual and combined effects of stone crabs (Menippe mercenaria) and knobbed whelks (Busycon carica) when foraging on two common bivalves, the hard clam (Mercenaria mercenaria) and the ribbed mussel (Geukensia demissa) in oyster reef and sand flat habitats. Because these species co-occur across these and other estuarine habitats of varying physical complexity, this system is ideal for examining how habitat context influences foraging rates and the generality of predator interactions. Consistent with results from previous studies, consumption rates of each predator in isolation from the other were higher in the sand flat than in the more structurally complex oyster reef habitat. However, consumption by the two predators when combined surprisingly did not differ between the two habitats. This counterintuitive result probably stems from the influence of habitat structure on predator–predator interactions. In the sand-flat habitat, whelks significantly reduced their consumption of their less preferred prey when crabs were present. However, the structurally more complex oyster reef habitat appeared to reduce interference interactions among predators, such that consumption rates when the predators co-occurred did not differ from predation rates when alone. In addition, both habitat context and predator–predator interactions increased resource partitioning by strengthening predator dietary selectivity. Thus, an understanding of how habitat characteristics such as physical complexity influence interactions among predators may be critical to predicting the effects of modifying predator populations on their shared prey.     

Native species vulnerability to introduced predators: testing an inducible defense and a refuge from predation

To manage the impacts of biological invasions, it is important to determine the mechanisms responsible for the effects invasive species have on native populations. When predation by an invader is the mechanism causing declines in a native population, protecting the native species will involve elucidating the factors that affect native vulnerability. To examine those factors, this study measured how a native species responded to an introduced predator, and whether the native response could result in a refuge from predation. Predation by the green crab, Carcinus maenas, has contributed to the decline in numbers of native soft-shell clams, Mya arenaria, and efforts to eradicate crabs have proven futile. We tested how crab foraging affected clam burrowing, and how depth in the sediment affected clam survival. Clams responded to crab foraging by burrowing deeper in the sediment. Clams at shallow depths were more vulnerable to predation by crabs. Results suggest soft-shell clam burrowing is an inducible defense in response to green crab predation because burrowing deeper results in a potential refuge from predation by crabs. For restoring the native clam populations, tents could exclude crabs and protect clams, but when tents must be removed, exposing the clams to cues from foraging crabs should induce the clams to burrow deeper and decrease vulnerability. In general, by exposing potential native prey to cues from introduced predators, we can test how the natives respond, identify whether the response results in a potential refuge, and evaluate the risks to native species survival in invaded communities.     

The Impact of Physiological Demands on Foraging Decisions under Predation Risk: A Test with the Whelk Acanthina monodon

We study whether and how physiological demands affect foraging decisions under predation risk, by evaluating the effect of starvation on the rate of food consumption and prey‐size preferences and the potential trade‐off between starvation and predation risk on foraging behavior in the whelk Acanthina monodon, a gastropod inhabiting the intertidal rocky shores of central Chile. These whelks appear to adjust their foraging strategy to physiological (nutritional) demand and predation risk. Starvation reduced the effect of predation risk on the rate of food consumption by A. monodon. Thus, in the absence of the predator sea star, the rate of food consumption by starved and satiated whelks was similar. When a predator was present, starved whelks fed faster than satiated whelks. Our results indicate that foraging behaviour represents an integrated and hierarchical response to environmental conditions and the physiological conditions of the forager.     

Optimal foraging versus shared doom effects: interactive influence of mussel size and epibiosis on predator preference

In the western Baltic Sea, the highly competitive blue mussel Mytilus edulis tends to monopolize shallow water hard substrata. In many habitats, mussel dominance is mainly controlled by the generalist predator Carcinus maenas. These predator-prey interactions seem to be affected by mussel size (relative to crab size) and mussel epibionts.There is a clear relationship between prey size and predator size as suggested by the optimal foraging theory: Each crab size class preferentially preys on a certain mussel size class. Preferred prey size increases with crab size.Epibionts on Mytilus, however, influence this simple pattern of feeding preferences by crabs. When offered similarly sized mussels, crabs prefer Balanus-fouled mussels over clean mussels. There is, however, a hierarchy of factors: the influence of attractive epibiotic barnacles is weaker than the factor ‘mussel size’. Testing small mussels against large mussels, presence or absence of epibiotic barnacles does not significantly alter preferences caused by mussel size. Balanus enhanced crab predation on mussels in two ways: Additional food gain and, probably more important, improvement in handling of the prey. The latter effect is illustrated by the fact that artificial barnacle mimics increased crab predation on mussels to the same extent as do live barnacles.We conclude that crab predation preferences follows the optimal foraging model when prey belong to different size classes, whereas within size classes crab preferences is controlled by epibionts.     

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.     

Effects of substrate on interactions between juvenile sea scallops (Placopecten magellanicus Gmelin) and predatory sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say)

We investigated the effect of substrate (glass bottom, sand, granule, pebble) on predation of juvenile sea scallops (Placopecten magellanicus) by sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus) at two prey sizes (11-15 mm and 24-28 mm shell height), and two prey densities (10 and 30 scallops per aquarium) in laboratory experiments. Specifically, we quantified predation rate and underlying behaviours (proportion of time a predator spent searching for and handling prey, encounter rate between predators and prey, and various outcomes of encounters). We detected a significant gradual effect of particle size of natural substrates on sea star predation: specifically, predation rate on and encounter rate with small scallops tended to decrease with increasing particle size (being highest for sand, intermediate for granule, and lowest for pebble). Substrate type did not significantly affect predation rates or behaviours of sea stars preying on large scallops or of rock crabs preying on either scallop size classes. Other factors, such as prey size and density, were important in the scallop-sea star and scallop-rock crab systems. For example, predation rate by sea stars and crabs and certain sea star behaviours (e.g. probability of consuming scallops upon capture) were significantly higher with small scallops than with large scallops. As well, in interactions between small scallops and sea stars, predation rate and encounter rate increased with prey density, and the proportion of time sea stars spent searching was higher at low prey density than high prey density. Thus, substrate type may be a minor factor determining predation risk of seeded scallops during enhancement operations; prey size and prey density may play a more important role. However, substrate type still needs to be considered when choosing a site for scallop enhancement, as it may affect other scallop behaviours (such as movement).     

Recognizing biotic breakage of the hard clam, Mercenaria mercenaria caused by the stone crab, Menippe mercenaria: An experimental taphonomic approach

The ability to assign lethal traces left on prey to particular durophagous predators enhances our understanding of predation pressure in the fossil record. To determine whether stone crabs (Menippe mercenaria Say 1818) leave diagnostic traces in the act of feeding on hard clams (Mercenaria mercenaria Linnaeus 1758), live clams were offered to crabs in laboratory aquaria over several months and the fragments produced during predation were examined for diagnostic breakage patterns. These fragments were then compared both macroscopically and using scanning electron microscopy to the fracture patterns produced by tumbling clams in a rock tumbler which simulated breakage during transport in the surf zone, and crushing clams using an Instron which simulated breakage resulting from sediment compaction. Fossil specimens of Mercenaria mercenaria were also examined to determine whether the criteria for recognizing predation traces generated experimentally could be recognized. While not all acts of predation produce diagnostic traces, when larger fragments (greater than 50% shell remaining) are produced during feeding, predatory-diagnostic breakage ranges from 70 to 80%. Macroscopic breakage patterns generated during the predation experiments were also present in fossil specimens. Damage caused by abiotic mechanisms (tumbling and crushing) is highly unlikely to be confused with damage produced by this predator.     

Foraging Decision Rules and Prey Species Preferences of Northwestern Crows (Corvus caurinus)

Categorization of similar prey types and the application of decision rules by dietary generalists can enhance the efficiency of foraging decisions and facilitate the inclusion of novel prey types in the diet. While considerable research attention has been directed toward investigation of these concepts in invertebrates, few have assessed categorization and decision rules used by generalist vertebrate predators. In this study, we experimentally investigated decision rules and prey preferences of northwestern crows (Corvus caurinus) feeding on littleneck clams (Tapes philippinarum) and whelks (Nucella lamellosa). We presented crows with three species‐size combinations: small clams (2.0–2.9 cm length) paired with large whelks (4.0–4.9 cm), small clams paired with medium whelks (3.0–3.9 cm), and large clams (4.0–4.9 cm) with large whelks. Profitability estimates based on observations of crows feeding on these prey species indicated that clams were always the more energetically profitable option; however, in prey choice trials crows consistently selected the heavier prey species, regardless of differences in profitability. These results show that crows apply a general decision rule according to which they select heavier prey items when feeding on hard‐shelled prey requiring similar handling techniques, and that while such decision rules may approximate optimal choices they may not always follow predictions based solely on prey profitability. We discuss these results in the context of behavioural flexibility of generalist predators, and predicting impacts of intertidal avian predators on prey populations.     

Partitioning mechanisms of Predator Interference in different Habitats

Prey are often consumed by multiple predator species. Predation rates on shared prey species measured in isolation often do not combine additively due to interference or facilitation among the predator species. Furthermore, the strength of predator interactions and resulting prey mortality may change with habitat type. We experimentally examined predation on amphipods in rock and algal habitats by two species of intertidal crabs, Hemigrapsus sanguineus (top predators) and Carcinus maenas (intermediate predators). Algae provided a safer habitat for amphipods when they were exposed to only a single predator species. When both predator species were present, mortality of amphipods was less than additive in both habitats. However, amphipod mortality was reduced more in rock than algal habitat because intermediate predators were less protected in rock habitat and were increasingly targeted by omnivorous top predators. We found that prey mortality in general was reduced by (1) altered foraging behavior of intermediate predators in the presence of top predators, (2) top predators switching to foraging on intermediate predators rather than shared prey, and (3) density reduction of intermediate predators. The relative importance of these three mechanisms was the same in both habitats; however, the magnitude of each was greater in rock habitat. Our study demonstrates that the strength of specific mechanisms of interference between top and intermediate predators can be quantified but cautions that these results may be habitat specific. An erratum to this article can be found at     

Biogeographic variation in behavioral and morphological responses to predation risk

The expression of prey antipredator defenses is often related to ambient consumer pressure, and prey express greater defenses under intense consumer pressure. Predation is generally greater at lower latitudes, and antipredator defenses often display a biogeographic pattern. Predation pressure may also vary significantly between habitats within latitudes, making biogeographic patterns difficult to distinguish. Furthermore, invasive predators may also influence the expression of prey defenses in ecological time. The purpose of this study was to determine how these factors influence the strength of antipredator responses. To assess patterns in prey antipredator defenses based upon geographic range (north vs. south), habitat type (wave-protected vs. wave-exposed shores), and invasive predators, we examined how native rock (Cancer irroratus) and invasive green (Carcinus maenas) crab predators influence the behavioral and morphological defenses of dogwhelk (Nucella lapillus) prey from habitats that differ in wave exposure across an ~230 km range within the Gulf of Maine. The expression of behavioral and morphological antipredatory responses varied according to wave exposure, geographic location, and predator species. Dogwhelks from areas with an established history with green crabs exhibited the largest behavioral and morphological antipredator responses to green crabs. Dogwhelk behavioral responses to rock crabs did not vary between habitats or geographic regions, although morphological responses were greater further south where predation pressure was greatest. These findings suggest that dogwhelk responses to invasive and native predators vary according to geographic location and habitat, and are strongly affected by ambient predation pressure due to the invasion history of an exotic predator.     

Predator biomass determines the magnitude of non-consumptive effects (NCEs) in both laboratory and field environments

Predator body size often indicates predation risk, but its significance in non-consumptive effects (NCEs) and predator risk assessment has been largely understudied. Although studies often recognize that predator body size can cause differing cascading effects, few directly examine prey foraging behavior in response to individual predator sizes or investigate how predator size is discerned. These mechanisms are important since perception of the risk imposed by predators dictates behavioral responses to predators and subsequent NCEs. Here, we evaluate the role of predator body size and biomass on risk assessment and the magnitude of NCEs by investigating mud crab foraging behavior and oyster survival in response to differing biomasses of blue crab predators using both laboratory and field methods. Cues from high predator biomass treatments including large blue crab predators and multiple small blue crab predators decreased mud crab foraging and increased oyster survival, whereas mud crab foraging in response to a single small blue crab did not differ from controls. Mud crabs also increased refuge use in the presence of large and multiple small, but not single small, blue crab predators. Thus, both predator biomass and aggregation patterns may affect the expression of NCEs. Understanding the impact of predator biomass may therefore be necessary to successfully predict the role of NCEs in shaping community dynamics. Further, the results of our laboratory experiments were consistent with observed NCEs in the field, suggesting that data from mesocosm environments can provide insight into field situations where flow and turbulence levels are moderate.     

Immunological detection of winter flounder (Pseudopleuronectes americanus) eggs and juveniles in the stomach contents of crustacean predators

Predation on the early life history of fish is an important factor regulating year-class strength. Verifying predation events, however, is difficult when analyses rely on visually identifying the remnants of partially digested fish in the stomachs of suspected predators. The objective of this study was to assess the utility of using immunological assays to detect the presence of winter flounder eggs and juveniles (Pseudopleuronectes americanus) in the gut contents of sand shrimp (Crangon septemspinosa) and green crab (Carcinus maenas). After defining assay capabilities, the stomach contents of field-collected shrimp and crabs were examined to determine if these predator-prey relationships occur under natural conditions. Winter flounder-specific antisera developed and used in this study successfully identified homologous antigens (egg or juvenile flounder extracts) without appreciably cross-reacting with antigenic material from predators or nontarget prey. Moreover, antisera detected flounder eggs 10.8-16.4 h after initial feeding by various sized shrimp, and identified juvenile flounder 9.4 and 7.8 h after initial ingestion by shrimp and crabs, respectively. Immuonological dietary analysis of decapod crustaceans collected from Niantic River, Connecticut, revealed that C. septemspinosa and C. maenas are potentially important predators on the early life stages of winter flounder. The temporal trends and magnitude of flounder predator-induced mortality was affected primarily by the spatial and temporal overlap between predator and prey (egg mortality), and the size-dependent relationships underlying crustacean and flatfish predator-prey interactions (juvenile mortality).     

Prey selection and the functional response of sea stars (Asterias vulgaris Verrill) and rock crabs (Cancer irroratus Say) preying on juvenile sea scallops (Placopecten magellanicus (Gmelin)) and blue mussels (Mytilus edulis Linnaeus)

Predators in nature include an array of prey types in their diet, and often select certain types over others. We examined (i) prey selection by sea stars (Asterias vulgaris) and rock crabs (Cancer irroratus) when offered two prey types, juvenile sea scallops (Placopecten magellanicus) and blue mussels (Mytilus edulis), and (ii) the effect of prey density on predation, prey selection, and component behaviours. We quantified predation rates, behavioural components (proportion of time spent searching for prey, encounter probabilities) and various prey characteristics (shell strength, energy content per prey, handling time per prey) to identify mechanisms underlying predation patterns and to assess the contribution of active and passive prey selection to observed selection of prey. Sea stars strongly selected mussels over scallops, resulting from both active and passive selection. Active selection was associated with the probability of attack upon encounter; it was higher on mussels than on scallops. The probability of capture upon attack, associated with passive selection, was higher for mussels than for scallops, since mussels can not swim to escape predators. Sea stars consumed few scallops when mussels were present, and so did not have a functional response on scallops (the target prey). Rock crabs exhibited prey switching: they selected mussels when scallop density was very low, did not select a certain prey type when scallop density was intermediate, and selected scallops when scallop density was high relative to mussel density. The interplay between encounter rate (associated with passive selection) and probability of consumption upon capture (associated with both active and passive selection) explained observed selection by crabs. Scallops were encountered by crabs relatively more often and/or mussels less often than expected from random movements of animals at all scallop densities. However, the probability of consumption varied with scallop density: it was lower for scallops than mussels at low and intermediate scallop densities, but tended to be higher for scallops than mussels at high scallop densities. When mussels were absent, crabs did not have a functional response on scallops, but rather were at the plateau of the response. When mussels were present with scallops at relatively low density, crabs exhibited a type II functional response on scallops. Our results have implications for the provision of protective refuges for species of interest (i.e., scallops) released onto the sea bed, such as in population enhancement operations and bottom aquaculture.     

Invasive species cause large-scale loss of native California oyster habitat by disrupting trophic cascades

Although invasive species often resemble their native counterparts, differences in their foraging and anti-predator strategies may disrupt native food webs. In a California estuary, we showed that regions dominated by native crabs and native whelks have low mortality of native oysters (the basal prey), while regions dominated by invasive crabs and invasive whelks have high oyster mortality and are consequently losing a biologically diverse habitat. Using field experiments, we demonstrated that the invasive whelk’s distribution is causally related to a large-scale pattern of oyster mortality. To determine whether predator–prey interactions between crabs (top predators) and whelks (intermediate consumers) indirectly control the pattern of oyster mortality, we manipulated the presence and invasion status of the intermediate and top trophic levels in laboratory mesocosms. Our results show that native crabs indirectly maintain a portion of the estuary’s oyster habitat by both consuming native whelks (density-mediated trophic cascade) and altering their foraging behavior (trait-mediated trophic cascade). In contrast, invasive whelks are naive to crab predators and fail to avoid them, thereby inhibiting trait-mediated cascades and their invasion into areas with native crabs. Similarly, when native crabs are replaced with invasive crabs, the naive foraging strategy and smaller size of invasive crabs prevents them from efficiently consuming adult whelks, thereby inhibiting strong density-mediated cascades. Thus, while trophic cascades allow native crabs, whelks, and oysters to locally co-exist, the replacement of native crabs and whelks by functionally similar invasive species results in severe depletion of native oysters. As coastal systems become increasingly invaded, the mismatch of evolutionarily based strategies among predators and prey may lead to further losses of critical habitat that support marine biodiversity and ecosystem function. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Interaction strength between a predator and dangerous prey: Sinistrofulgur predation on Mercenaria

The lack of direct empirical evidence of predator evolution in response to prey adaptation is a fundamental weakness of the arms race analogy of predator-prey coevolution. I examined the interaction between the predatory busyconine whelk Sinistrofulgur sinistrum and its bivalve prey Mercenaria mercenaria to evaluate whether reciprocal adaptation was likely in this predator-prey system. Thick-lipped whelks use their shell lip to chip open the shell of their prey, often resulting in breakage to their own shell. Thus, hard-shelled prey, such as Mercenaria, may be considered dangerous because they are able to inflict damage to the predator as a consequence of the interaction. The strength of interaction between whelks and their bivalve prey was viewed by regressing predator performance (the incidence of shell breakage in encounters with prey) on prey phenotype (a function of size). Interaction with Mercenaria of varying sizes has strong and predictable consequences (r²=0.946; p=0.028) for Sinistrofulgur. Predators that select large, thick bivalve prey increase the likelihood that their shell lip will be broken in the process of attempting to open their prey. Ecological consequences of feeding-induced breakage may include reduced growth rate, reproductive success, and survivorship. These results suggest that natural selection should favor predator phenotypes that reduce feeding-induced breakage when interactions with damage-inducing prey occur.     

Modeling changes in predator functional response to prey across spatial scales

Extrapolation of predator functional responses from laboratory observations to the field is often necessary to predict predation rates and predator-prey dynamics at spatial and temporal scales that are difficult to observe directly. We use a spatially explicit individual-based model to explore mechanisms behind changes in functional responses when the scale of observation is increased. Model parameters were estimated from a predator-prey system consisting of the predator Delphastus catalinae (Coleoptera: Coccinellidae) and Bemisia tabaci biotype B (Hemiptera: Aleyrodidae) on tomato plants. The model explicitly incorporates prey and predator distributions within single plants, the search behavior of predators within plants, and the functional response to prey at the smallest scale of interaction (within leaflets) observed in the laboratory. Validation revealed that the model is useful in scaling up from laboratory observations to predation in whole tomato plants of varying sizes. Comparing predicted predation at the leaflet scale, as observed in laboratory experiments, with predicted predation on whole plants revealed that the predator functional response switches from type II within leaflets to type III within whole plants. We found that the magnitude of predation rates and the type of functional response at the whole plant scale are modulated by (1) the degree of alignment between predator and prey distributions and (2) predator foraging behavior, particularly the effect of area-concentrated search within plants when prey population density is relatively low. The experimental and modeling techniques we present could be applied to other systems in which active predators prey upon sessile or slow-moving species.     

Behavioural mechanisms of sea stars (Asterias vulgaris Verrill and Leptasterias polaris Müller) and crabs (Cancer irroratus Say and Hyas araneus Linnaeus) preying on juvenile sea scallops (Placopecten magellanicus (Gmelin)), and procedural effects of scallop tethering

We compared predation rates and behaviours of sea stars (Asterias vulgaris and Leptasterias polaris) and crabs (Cancer irroratus and Hyas araneus) preying on juvenile sea scallops (Placopecten magellanicus, 25-35 mm shell height) in laboratory. These predatory species co-occur with sea scallops on the sea bed of the Gulf of St. Lawrence, Canada, and limit scallop survival in seeding operations. We also examined, under controlled conditions, the effect of tethering scallops on predator-prey interactions. Predation rates, time budgets and encounter behaviours observed for A. vulgaris and C. irroratus preying on free (untethered) scallops were comparable to previous studies. C. irroratus were more effective predators as they consumed 3.1 scallops predator^− 1 day^− 1, although they spent only 0.9% of their time searching for prey. A. vulgaris consumed 0.9 scallops predator^− 1 day^− 1 and spent 7.6% of their time searching. Sea stars L. polaris had a lower predation rate (0.02 scallop predator^− 1 day^− 1) than A. vulgaris. The frequent avoidance behaviour of L. polaris and its low ability to capture scallops support the notion that scallops are not a main component of this sea star's diet. Crabs H. araneus had similar predation rates (1.3 scallops predator^− 1 day^− 1) and behaviours to C. irroratus, although the probability of consumption upon capture was affected by relatively high numbers of rejections and post-capture escapes of scallops. As expected, the tethering procedure increased predation rate of L. polaris (about 19 times higher), but surprisingly did not significantly affect that of A. vulgaris. Examination of behaviours indicated that A. vulgaris offered tethered scallops tended to have a higher probability of capture, but spent less time searching for prey (possibly because satiation was reached) than A. vulgaris offered free scallops. Predation rates and behaviours of both crab species were not affected by tethering, since encounter rate was the primary determinant of crab-scallop interactions. Identification and quantification of behaviours underlying the predation process allowed us to mathematically model predator-related mortality for the four predator species.     

Behavior and aquatic plants as factors affecting predation by three species of larval predaceous diving beetles (Coleoptera: Dytiscidae)

Predation among aquatic invertebrate predators can have important effects on patterns of exclusion and coexistence in aquatic habitats, especially if these predators also act as intraguild predators. Such patterns may be explained by variation in predator foraging mode and in the extent and overlap of habitat use. Predaceous diving beetles (Coleoptera: Dytiscidae) are abundant in isolated bodies of water and are effective predators on many aquatic organisms, including other dytiscids. The under-investigated role of hunting behavior and habitat use in altering outcomes of predation under different plant densities may offer insights into patterns of coexistence among larval dytiscids. I performed experiments that quantified behavior of larvae of three common genera of dytiscids that share common prey and then measured predation among genera in the presence or absence of aquatic plants. Behavioral analyses concluded that there were significant differences in foraging modes, with Dytiscus primarily exhibiting sit-and-wait tactics, Graphoderus engaging in active, open water searching, and Rhantus displaying combinations of these behaviors. Predation among larvae was common and occurred when predators were larger than the prey, with no indication of prey preference. Incidence of predation among generic combinations depended on the presence of plants and appeared to be related to behavioral differences among genera. The presence or absence of plants and differences in larval behavior may help to mitigate predation by reducing negative interactions in natural aquatic systems. These results have implications for IGP interactions and may be one of the explanations for the observed richness of this group of predators within aquatic habitats.