Hydrodynamic mediation of predator-prey interactions: differential patterns of prey susceptibility and predator success explained by variation in water flow

In most shallow water marine systems, fluid movements vary on scales that may influence local community dynamics both directly, through changes in the abundance of species, and indirectly, by modifying important behaviors of organisms. We examined how differences in current speed affect the outcome of predator-prey interactions for two species of marine benthic predators (knobbed whelks, Busycon carica, and blue crabs, Callinectes sapidus) foraging on two common prey species (bay scallops, Argopecten irradians, and hard clams, Mercenaria mercenaria). The predators differ in their foraging strategies and prey in their potential escape responses. Predation by blue crabs, highly mobile predators/scavengers that rely upon chemical odors transported in the water column to locate prey, could be strongly affected by changes in current speed and turbulent mixing because their foraging strategy relies on a high degree of spatial integration of prey odor plumes. Whelks, slow moving, predatory gastropods that often forage with their bodies buried in the sediment, may be less susceptible to flow-induced distortion of prey odor plumes because their sluggish movements result in a high degree of temporal integration of prey odors. Bay scallops, relatively mobile bivalves capable of rapid short-distance swimming burst, and hard clams, sedentary bivalves, have been shown to respond to varying degrees to predator odors that are dispersed in the water column. Flow regime for the predator-prey experiments was manipulated in situ using large channels. Predation by blue crabs on both juvenile hard clams and bay scallops decreased with increases in water flow (0-12 vs. 0-30 cm s⁻¹). Whelk predation on bay scallops increased with increases in water flow, whereas predation by whelks on hard clams did not differ between flow regimes. For blue crabs movement decreased at periods of high water flow. Because blue crabs locate prey through chemolocation of water-borne cues, which are diluted rapidly at higher flows, decreases in foraging may result from the inability to successfully detect prey at enhanced flows. Differences in predation by whelks could not be explained by a similar mechanism. Visual observations of foraging whelks revealed no differences in whelk behavior between the two flow regimes. The pattern of higher whelk predation on scallops at enhanced flow is likely to be related to a flow-inhibiting ability of scallops to detect predator approach. Thus, flow enhancement interferes with three of the predator-prey systems but the effect on predator success depends on whether the predator or prey is most affected.     

Prey selection in coastal Eurasian otters Lutra Iutra

Prey preferences and dietary differences between sex and age categories of Eurasian otters were studied in coastal Norwegian habitats Relative to their trapping frequency potential prey species with hard, spiny exoskeletons (crabs and sea urchins) or otherwise tough, spiny integuments (Labridae) were much less frequently found in spraints than fish species with soft integuments Spines did not protect fish with otherwise soft integuments from otter predation The number of non-fish taxa per otter stomach did not vary significantly between otter age categories despite presumed differences in hunting abilities (small cubs large cubs and subadults, adults) Relative frequency of occurrence of crabs and sea urchins was < 5% in the stomachs in each of these otter categories Anadromous, katadromous and freshwater fish species were infrequently eaten The coastal otter population during the study period probably had access to an adequate, and preferred, supply of marine fish prey
At the otter population level no prey size selection was conclusively demonstrated within the range of fish sizes sampled However, fish sizes eaten differed significantly between otter sex and age categories The fish sizes per stomach were on average larger in males than in females, regardless of age Adult males tended to eat the largest fishes Among the self provisioning age categories (subadult and adult otters) fish lengths differed significantly between otter males and females, but not between the otter age categories, and did not covary significantly with otter body length Fish eaten by females with old placental scars (potential mothers of fisheating cubs) were significantly smaller than those eaten by small cubs, provisioned by their mothers     

Molluscan predation by Ovalipes punctatus (De Haan) (Crustacea: Brachyura: Portunidae)

Molluscan predation by the three-spot swimming crab was investigated. The dentition of the heteromorphic chelae allowed crushing, shearing, cutting and holding of prey. Laboratory investigations indicated that small mussels and gastropods were crushed, the larger mussels were prized open, and the foot of the larger gastropods shredded and bits removed. Stomach contents of freshly captured crabs indicated that the crabs are selective carnivores and preferred prey species which are not most abundant in situ (crabs from Kings Beach, Donax serra Röding; crabs from Maitlands River Beach, Bullia rhodostoma Reeve). Ovalipes punctatus (De Haan) foraged on a variety of prey and had no upper prey size limit, but the crabs did show preferences for certain prey sizes. Data indicate that the swimming crabs can effectively utilize the entire mollusc populations on the beaches as prey items.     

Foraging strategies and prey switching in the California sea otter

Summary Southern sea otters (Enhydra lutris nereis), in recovering from near extinction, are gradually extending their range to include areas from which they have been absent for more than one hundred years. This study took advantage of the otters' relatively sudden arrival in the area near Santa Cruz, California, to monitor their prey selection in the first two years of residence there. Foraging observations revealed that sea urchins (Strongly-locentrotus franciscanus) were heavily preyed upon initially, but virtually disappeared from the diet after one year of sea otter residence. The disappearance of sea urchins was accompanied by an increased use of kelp crabs (Pugettia producta) and the appearance of clams (Gari californica) in the otters' diet. Abalones (Haliotis rufescens) and cancer crabs (Cancer spp.) remained fairly stable as dietary items throughout the two year period. An electivity index was used to quantify sea otter preferences, which corresponded closely with a ranking scheme based on energy intake/unit foraging time calculated for each major prey species. As predicted by optimal foraging theory, sea otters prefer food species of high rank and replace depleted dietary items with those of next highest rank. The process of dietary switching was analyzed with respect to foraging success rates, and it appears that poor success rates, associated with predation on an increasingly rarer prey species (sea urchins), drive sea otters to hunt for different prey. Both patch selection and search image formation appear to function in this process. The potential effects on community structure and stability of predators exhibiting a preference for the most profitable prey are discussed.     

Crab shell-crushing predation and gastropod architectural defense

The shell-breaking behavior of the crabs Ozius verreauxii Saussure 1853 and Eriphia squamata, Stimpson 1859 from the Bay of Panama is described. The master claws of both these crabs are well designed for breaking shells. Small shells, relative to the size of a crab predator, are crushed by progressively breaking off larger segments of a shell's apex, while larger shells are peeled by inserting a large dactyl molar into the aperture of a shell and progressively chipping away the lip of the shell.

Heavy gastropod shells are shown to be less vulnerable to crab predators than lighter shells, and narrow shell apertures and axial shell sculpture are demonstrated to be architectural features that deter crab predation. The incidence of architectural features which deter crab predation appears to be higher for smaller gastropod species than for larger gastropods which are too large for most crab predators. Large fish predators prey upon both gastropods and shell-crushing crabs. To avoid fish predators, both these prey groups seek refuge under rocks when covered by the tide. Fish predation thus appears to enforce a close sympatry between smaller gastropods and their crab predators.     

Seeigel und ihre Beutegreifer im Mittelmeer

Defensive delicacies: Sea urchins and their predators in the Mediterranean Sea Despite their various defense mechanisms, sea urchins always attract predators that are able to counter these mechanisms. In the Mediterranean Sea, these predators are often invertebrates, such as gastropods, decapods, and seastars, but also fish, including sea breams. Cassid gastropods use mucus to agglutinate the defensive spines and pedicellaria, and drill the calcareous tests with the aid of sulfuric acid. Large crustaceans, such as spiny lobsters and spider crabs, crush the tests of sea urchins with their armored claws and walking legs. Starfish ingest small sea urchins completely, or evert their stomachs to digest the urchins extra‐intestinally. Fish, especially sea breams, first bite off the spines and then crush the urchins test with their specialized teeth. In some cases, recognizable traces, like drill holes, scratch marks, indentations, or breakage patterns remain on the urchins hard parts allowing these events to be reconstructed in the fossil record.     

Habitat- and sex-related differences in a small carnivore’s diet in a competitor-free environment

The alien invasive American mink Neovison vison is fully established in the low species richness and competitor-free environment of Iceland. This study documents the diversity as well as seasonal and sexual variation in the diet of mink in Iceland based on stomach contents. Seasonal changes mainly reflected variation in abundance of migratory birds and wood mice Apodemus sylvaticus. In comparison with mink elsewhere in similar habitats, the mink in Iceland consumed more fish and birds and fewer mammals, which is in accordance with local availability. This reinforces evidence of opportunistic foraging. Females generally ate more fish and fewer birds than males and this might be explained by their smaller body size and possible limitation in catching larger birds. Mink in coastal habitats showed greater sexual differences in diet than mink in riparian habitats, probably reflecting less prey diversity in riparian habitats than coastal ones. This study is an input towards explaining the ecological consequences of sexual size dimorphism and supports the hypothesis that generalist species might be successful invaders due to their capability in utilising new and diverse resources. The mink in Iceland can be regarded as a model for a small-bodied semi-aquatic carnivore away from the confounding effects of inter-specific competition.     

Behavioural responses of the lizard Pedioplanis l. lineoocellata to overgrazing

We studied the effects of overgrazing on the foraging behaviour of the lizard Pedioplanis l. lineoocellata (Spotted Sand Lizard), a sit-and-wait forager, in habitats of differing vegetation states to determine the effects of habitat degradation on this species. At high grazing intensity where vegetation cover and diversity is low, the lizard P. lineoocellata moves more frequently, spends more time moving and covers larger distances than in habitats where vegetation cover and diversity is high. These behavioural changes in movement patterns can be explained by less abundant prey in habitats with low vegetation cover and diversity. Although morphology, phylogeny and physiology of P. lineoocellata should constrain the change in foraging behaviour, the species has modified its foraging strategy from sit-and-wait to actively foraging. We assume that this behavioural flexibility of P. lineoocellata is a buffer mechanism enabling the species to use and survive in degraded (unfavourable) habitats.     

The diet composition of immature loggerheads: Insights on trophic niche, growth rates, and fisheries interactions

For immature animals, diet quality and composition influence expression of life history traits such as growth rates and ultimately life stage duration and age to maturity. Circumglobally distributed loggerhead turtles (Caretta caretta) exhibit a multi-decade immature stage that generally occupies neritic habitats and is characterized by slow growth and an omnivorous diet. Although adult nesting populations are geographically distinct, foraging areas for immature loggerheads show a high degree of mixing of individuals that originate from multiple nesting stocks. Furthermore, despite their generalist foraging ecology, immature loggerheads have been observed to supplement their natural diets with fish from fishery discards and/or caught in fishing gear. However, whether trophic opportunism results in variation in loggerhead growth rates within or among feeding areas has not been investigated. In Core Sound, North Carolina (NC), USA, immature loggerheads demonstrate highly variable size-specific growth rates, in contrast to other studies that report discernible somatic growth functions in immature marine turtles. To determine whether inter-individual variation in growth rates at this site was due to variation in diet composition, and specifically variation in consumption of fish, we analyzed carbon and nitrogen stable isotope ratios of loggerhead blood plasma and of tissue samples of putative loggerhead prey, as well as commercially important fish species. Our results indicated that growth rates were not related to trophic levels at which individual turtles fed, but rather probably reflected inter-individual variation in overwintering or foraging behavior (i.e. nearshore vs. offshore). Furthermore, loggerhead diets were highly diverse, and comprised mainly blue crabs and/or whelks, as well as small proportions of cannonball jellies. Fish were unimportant dietary components for loggerheads. Although loggerheads in NC do not appear to feed on fish catch or discards, immature turtles showed dietary preferences for prey items that are also valuable to or are commonly taken as bycatch in commercial fisheries (e.g. blue crabs and whelks, respectively) in the region. Thus, the status of these prey items/fishery stocks as well as trends in loggerhead populations should be monitored to mitigate potential competitive interactions between fisheries activities and loggerhead turtles.     

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.     

Diet composition and foraging ecology of U.S. Pacific Coast groundfishes with applications for fisheries management

Determining the prey composition and foraging habitats of U.S. Pacific Coast groundfishes are specified management directives that have not received much scientific attention. To address this knowledge gap, we conducted a meta-analysis of the feeding ecology of 18 commercially important species and their life stages during a recent review of Pacific Coast groundfish essential fish habitat. A Major Prey Index was developed to evaluate relative importance among 47 prey taxa. Based on this metric, unidentified teleosts, euphausiids, and brachyuran crabs were the most important prey groups. When 14 generalized prey categories were used, fishes represented the dominant taxon (mean % weight or volume = 32.3) followed by shrimps (11.5), crabs (10.0), and euphausiids (9.5). PERMANOVA results indicated that species-specific differences were the primary source of dietary variability among tested variables (life stage, functional group, taxonomic group). Pacific Coast groundfishes mainly were characterized as mesopredators with estimated trophic levels ranging from 3.4 to 4.2. Foraging habitats differed significantly among functional (benthic, demersal, pelagic) and taxonomic (elasmobranch, roundfish, rockfish, flatfish) groups. Using hierarchical agglomerative cluster analysis, we identified a significantly distinct trophic guild that consumes mainly polychaetes and hard-shelled molluscs (juvenile, juvenile–adult Dover Sole; juvenile–adult English Sole) and another that specializes on euphausiids (juvenile Pacific Hake; juvenile–adult Darkblotched Rockfish). Our findings filled substantial data gaps in the trophic ecology and habitat-based management of commercially important species and can be used to inform future reviews of Pacific Coast groundfish essential fish habitat.     

Diet reveals links between morphology and foraging in a cryptic temperate reef fish

Predators select prey so as to maximize energy and minimize manipulation time. In order to reduce prey detection and handling time, individuals must actively select their foraging space (microhabitat) and populations exhibit morphologies that are best suited for capturing locally available prey. We explored how variation in diet correlates with habitat type, and how these factors influence key morphological structures (mouth gape, eye diameter, fin length, fin area, and pectoral fin ratio) in a common microcarnivorous cryptic reef fish species, the triplefin Helcogrammoides cunninghami. In a mensurative experiment carried out at six kelp‐dominated sites, we observed considerable differences in diet along 400 km of the Chilean coast coincident with variation in habitat availability and prey distributions. Triplefins preferred a single prey type (bivalves or barnacles) at northern sites, coincident with a low diversity of foraging habitats. In contrast, southern sites presented varied and heterogeneous habitats, where triplefin diets were more diverse and included amphipods, decapods, and cumaceans. Allometry‐corrected results indicated that some morphological structures were consistently correlated with different prey items. Specifically, large mouth gape was associated with the capture of highly mobile prey such as decapods, while small mouth gape was more associated with cumaceans and copepods. In contrast, triplefins that capture sessile prey such as hydroids tend to have larger eyes. Therefore, morphological structures co‐vary with habitat selection and prey usage in this species. Our study shows how an abundant generalist reef fish exhibits variable feeding morphologies in response to the distribution of potential habitats and prey throughout its range.     

Optimal foraging and risk of claw damage: How flexible are shore crabs in their prey size selectivity?

Understanding the behavioural mechanisms that underlie prey size preference of predators is an essential component of unravelling the processes that govern predator-prey dynamics. In marine systems, despite being able to consume larger and more profitable prey, many molluscivorous predators show a preference for smaller, less profitable prey, most likely to minimize the risk of damaging feeding extremities. Here we assessed the flexibility of this prey size preference. We observed that shore crabs (Carcinus maenas) that were food deprived, and which were offered mussels (Mytilus edulis) of different sizes in dichotomous preference tests, preferred smaller, less profitable mussels. The same result was observed for crabs foraging with a conspecific competitor. Only crabs that were conditioned to feed on the larger, most profitable mussels shifted their prey size preference and ranked the most profitable mussels as highest. Although shore crabs showed flexibility in prey size preference, through which they would be able to cope with environmental variability, our results in general emphasize preference for smaller prey. We discuss the possibility that crabs maximize their long-term feeding rate, in which case it can be optimal to select these smaller mussels.     

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.     

Reconstruction of Late Pleistocene palaeoecology of the Upper Narmada valley (Central India) using fossil communities

Pleistocene deposits of Narmada valley are exposed in the Jabalpur and Narshingpur districts of Central India and have yielded microvertebrates (rodents, lizards and fish) as well as invertebrates (gastropods and pelecypods). The Late Pleistocene fossil-bearing horizons are from two fossil localities, Bhedaghat and Devakachar. Earlier report of Homo erectus from the valley strengthens palaeoecological interpretations. Here, we combine our study of microfossils with the earlier work on large mammals in our tentative reconstruction of palaeoecology of the area. The most common microfossil communities belong to stream, pond–pond bank, wooded grassland and arid to semi-arid habitats.     

Foraging behavior and energetics of Great Egrets and Snowy Egrets at interior rivers and weirs

ABSTRACT.   Wading birds may use different foraging methods and prey capture techniques in particular habitats or under specific conditions. We measured foraging behavior and its energetic costs for Great Egrets ( Ardea alba ) and Snowy Egrets ( Egretta thula ) at two weirs (small overflow dams that raise water levels in a stream or river) and in two naturally flowing rivers in Kansas in May and June 2000 and 2005. We observed 99 randomly selected birds (38 Great Egrets and 61 Snowy Egrets) for 1513 min, and noted strike rate, prey capture rate, capture efficiency, prey size, and social interactions. In addition, 30 of these birds were observed for 504 min to estimate ambulation velocities and foraging energetics. Both species had higher strike rates and prey capture rates in rivers, but caught larger fish at weirs. Capture efficiency was higher for Snowy Egrets at weirs, but did not differ between microhabitats for Great Egrets. Snowy Egrets had higher rates of conspecific aggression at weirs than in rivers, but little aggression was documented for Great Egrets. Established algorithms suggest that, while foraging in rivers, Snowy Egrets had similar costs for changes in velocity. Changes in ambulation velocity for Great Egrets were greater at weirs than rivers. For both species, the percentage of time spent standing was twice as high at weirs as in rivers. Both species also used low-cost foraging strategies at weirs that yielded larger fish, so net energetic gains at weirs were higher than in rivers. Weirs appear to be more important to Snowy Egrets than to Great Egrets. Estimates of energy gains and expenses provide valuable predictive power for understanding egret behavior.     

Review of the effects of within-patch scale structural complexity on seagrass fishes

Studies on the effects of within-patch scale structure of seagrass habitats on predator–prey fish interactions and abundance/habitat use patterns were reviewed. Most laboratory experiments have employed chase-and-attack predators, usually resulting in lower foraging efficiency in (denser) seagrass. However, a few laboratory procedures employed alternative foraging tactics, resulting in no differences in prey mortality rates. Field studies did not always result in lower prey mortality rates in seagrass habitats. Accordingly, it is premature to conclude that seagrass presence is almost always negatively related to predator foraging efficiency or that increasing seagrass abundance is usually associated with a decrease in predator efficiency. Because several categories of predator and prey fishes occur in seagrass habitats, further studies are needed with all of these predator–prey combinations, in order to fully clarify predator–prey fish interactions in association with seagrass structure. Seagrass fishes have been shown to respond to alterations in seagrass structure in various ways: seagrass height and/or density reduction or clearance resulted in decreased abundance of some species but increases or no change in others. Some explanations have been proposed, not all mutually exclusive, for these phenomena. Although within-patch scale processes have been well studied, room exists for improvement. For example, predator–prey fish interactions in relation to varying within-patch scale complexity is not yet fully understand. The relationships of patch size, edge effects and within-patch scale complexity also still remain unclear. Further studies, which add to the clarification of within-patch scale process, will in turn improve our understanding of larger spatial scale processes.     

Exploitation and recovery of a sea urchin predator has implications for the resilience of southern California kelp forests

Size-structured predator–prey interactions can be altered by the history of exploitation, if that exploitation is itself size-selective. For example, selective harvesting of larger sized predators can release prey populations in cases where only large individuals are capable of consuming a particular prey species. In this study, we examined how the history of exploitation and recovery (inside marine reserves and due to fisheries management) of California sheephead (Semicossyphus pulcher) has affected size-structured interactions with sea urchin prey in southern California. We show that fishing changes size structure by reducing sizes and alters life histories of sheephead, while management measures that lessen or remove fishing impacts (e.g. marine reserves, effort restrictions) reverse these effects and result in increases in density, size and biomass. We show that predation on sea urchins is size-dependent, such that the diet of larger sheephead is composed of more and larger sized urchins than the diet of smaller fish. These results have implications for kelp forest resilience, because urchins can overgraze kelp in the absence of top-down control. From surveys in a network of marine reserves, we report negative relationships between the abundance of sheephead and urchins and the abundance of urchins and fleshy macroalgae (including giant kelp), indicating the potential for cascading indirect positive effects of top predators on the abundance of primary producers. Management measures such as increased minimum size limits and marine reserves may serve to restore historical trophic roles of key predators and thereby enhance the resilience of marine ecosystems.     

Structural complexity and fish body size interactively affect habitat optimality

Predator–prey interactions are strongly influenced by habitat structure, particularly in coastal marine habitats such as seagrasses in which structural complexity (SC) may vary over small spatial scales. For seagrass mesopredators such as juvenile fishes, optimality models predict that fitness will be maximized at levels of SC that enhance foraging but minimize predation risk, both of which are functions of body size. We tested the hypothesis that in eelgrass (Zostera marina) habitat, optimal SC for juvenile giant kelpfish (Heterostichus rostratus), an abundant eelgrass mesopredator in southern California, changes through ontogeny. To do this, we quantified eelgrass SC effects on habitat associations, relative predation risk, and foraging efficiency for three size classes of juvenile giant kelpfish. We found that habitat selection differed with fish size: small fish selected dense eelgrass, whereas larger fish selected sparse eelgrass. Small kelpfish experienced the lowest relative predation risk in dense eelgrass but also had higher foraging efficiency in dense eelgrass, suggesting that dense eelgrass is selected by these fish because it minimizes risk and maximizes potential for growth. Surprisingly, larger kelpfish did not experience lower predation risk than small kelpfish. However, larger kelpfish experienced higher foraging efficiency in sparse eelgrass vs. dense eelgrass, suggesting that they select sparse eelgrass to maximize foraging efficiency. Our study highlights that trade-offs between predation risk and foraging can occur within a single habitat type, that studies should consider how habitat value changes through ontogeny, and that seagrass habitat value may be maximal when within-patch variability in SC is high.     

Predation by Callinectes sapidus (Rathbun) within Spartina alterniflora (Loisel) marshes

This study examined predation by the blue crab, Callinectes sapidus Rathbun, within intertidal Spartina alterniflora (Loisel) marshes of Dauphin Island, Alabama. Species and size preferences displayed by the predator when foraging within the marsh were investigated using nektonic, epifaunal, and infaunal prey populations including Fundulus similis Baird and Girard, Littorina irrorata Say, and Geukensia demissa Dillwyn.

Short-term field experiments involving the use of predator inclusion cages, in which the relative abundances of all prey species and the density of macrophyte vegetation were manipulated, indicated that mean mortality differed significantly among species. Blue crabs exhibited a distinct species preference for Littorina, and to a lesser extent, for Fundulus. However, the predator rarely choses infaunal individuals. Within predator inclusion cages, size selection by the crabs among three size classes of each prey was evident for Littorina and Fundulus but not for Geukensia. Blue crabs tended to select intermediate-sized snails and large fish while not exhibiting a size preference for infaunal bivalves.

In the marsh, mean percentage of the Littorina population within the 14–18 mm size class exhibited an increased mortality as compared to two other size classes, which was negatively correlated with increasing tidal height. Such a relationship may have been due to a decreasing gradient of crab predation associated with increasing tidal height. Geukensia size class distributions showed little evidence of differences along the tidal height gradient. No data are available for Fundulus, a mobile species which would not experience such differential predation along a marsh gradient.

In comparing crab predation patterns among prey species, it is apparent that Callinectes utilizes prey species differentially. Such differential utilization may be based on optimization of energy yield and minimization of energy expenditure. Thus, the preference of blue crabs for nektonic and epifaunal prey is hypothesized to be the result of a smaller energy expediture as a result of the crab's visual evaluation of these prey. Infaunal prey species (e.g., Geukensia) require a greater energy investment because of the necessity of excavating the prey item. Such prey also allow little selection by size because of being cryptic.