Predatory blue crabs induce byssal thread production in hooked mussels

Abstract. Blue crabs (Callinectes sapidus) prey on hooked mussels (Ischadium recurvum) growing epizoically on oyster clumps in estuaries along the Louisiana coast. In prey size‐selection experiments, blue crabs preferred small mussels (<30‐mm shell length) to larger mussels, possibly because handling time increased with mussel size. When crabs were given a choice of solitary mussels versus mussels in clumps on oysters in the laboratory, mortality was lower by 86% in clumped mussels. However, no size selection by crabs occurred with mussels in clumps, likely because smaller mussels escaped predation in crevices between larger mussels or oysters. When individuals of two size classes of mussels were exposed to water containing the scent of crabs and of mussels consumed by blue crabs, an increase in byssal thread production was induced in all mussels, but byssal thread production rate was higher for small mussels than for large mussels. We conclude that increased predation risk for small mussels has resulted in higher size‐specific production of byssal threads, and that predator‐induced production of byssal threads, which may increase clumping behavior, may reduce their risk of mortality to predatory blue crabs.     

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.     

Parasite modification of predator functional response

Parasite alteration of the host (predator) functional response provides a mechanism by which parasites can alter predator–prey population dynamics and stability. We tested the hypothesis that parasitic infection of a crab (Eurypanopeus depressus) by a rhizocephalan barnacle (Loxothylacus panopei) can modify the crab’s functional response to mussel (Brachidontes exustus) prey and investigated behavioral mechanisms behind a potential change in the response. Infection dramatically reduced mussel consumption by crabs across mussel densities, resulting in a decreased attack rate parameter and a nearly eightfold reduction in maximum consumption (i.e. the asymptote, or inverse of the handling time parameter) in a type II functional response model. To test whether increased handling time of infected crabs drove the decrease in maximum consumption rate, we independently measured handling time through observation. Infection had no effect on handling time and thus could not explain the reduction in consumption. Infection did, however, increase the time that it took crabs to begin handling prey after the start of the handling time experiment. Furthermore, crabs harboring relatively larger parasites remained inactive longer before making contact with prey. This behavioral modification likely contributed to the reduced mussel consumption of infected crabs. A field survey revealed that 20 % of crabs inhabiting oyster reefs at the study site (North Inlet estuary, Georgetown, South Carolina, USA) are infected by the barnacle parasite, indicating that parasite infection could have a substantial effect on the population level crab-mussel interaction.     

Disentangling interference competition from exploitative competition in a crab–bivalve system using a novel experimental approach

In predator–prey relationships such as those between crabs and their bivalve prey, interference competition is a topic of intense investigation as it can have profound consequences on the dynamics of both predator and prey populations. However in laboratory experiments – also those on crab–bivalve systems – workers never adequately disentangled interference competition from exploitative competition, as prey depletion was never compensated. Hitherto, experimental studies on crab–bivalve systems lack direct behavioural observations and have provided only indirect and thus inconclusive evidence of interference competition. We studied interference competition in adult male shore crabs Carcinus maenas that foraged on blue mussels Mytilus edulis. We developed a novel type of experimental tank to replenish each consumed mussel, and thus to keep prey levels constant. We conducted two experiments in which we varied number of crabs (1, 2, 4) and number of mussels (first experiment: 4, 8, 16, 32; second experiment: 8, 32, 128) and directly observed the foraging behaviour of crabs (foraging area=0.25 m²). In the first experiment, feeding rates decreased with increasing crab density only at mussel density 16 because both search time and time spent in agonistic interactions increased. At other mussel densities, variation in crab density did not affect feeding rates, possibly because of low statistical power and the narrow range of mussel densities offered. In the second experiment feeding rates decreased with increasing crab density because crabs spent more time in agonistic interactions and handling their prey. Feeding rates increased with increasing mussel density. Overall, crabs spent on average 14–18% of their foraging time in agonistic behaviours, while on three out of 64 occasions feeding rates decreased because mussels were stolen (kleptoparasitism). Concluding, we have shown that interference competition occurs in absence of prey depletion, while conducting direct behavioural observations aid to identify the behavioural processes that underlie interference competition.     

Evaluating Intertidal Oyster Reef Development in South Carolina Using Associated Faunal Indicators

Eastern oyster (Crassostrea virginica) habitat is increasingly being restored for the ecosystem services it provides rather than solely as a fishery resource. Community‐based projects with the goal of ecological restoration have successfully constructed oyster reefs; however, the habitat benefits of these restoration efforts are usually not assessed or reported. In this study, we examined oyster habitat development at five community‐based oyster restoration sites in South Carolina using oyster population parameters, resident fauna densities, and sedimentation (percent sediment coverage) as assessment metrics. All sites included multiple‐aged reefs (1–3 years old) at the time of the fall 2004 sampling. Resident crabs and mussels were abundant at all five sites and crab assemblages were related to the size structure of the oyster microhabitat. Scorched mussel (Brachidontes exustus) abundances were most frequently correlated with oyster and other resident species abundances. Associations among oysters and resident crabs and mussels were not evident when analyses were conducted with higher level taxonomic groupings (e.g., total number of crabs, mussels, or oysters), indicating that species‐level identifications improve our understanding of interactions among reef inhabitants and oyster populations. Community‐based restoration sites in South Carolina provide habitat for mussels and resident crabs, in some cases in the absence of dense populations of relatively large oysters. Monitoring programs that neglect species‐level identifications and counts of mussels and crabs may underestimate the successful habitat provision that can arise independent of large, dense oyster assemblages.     

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.     

Predation risk predicts use of a novel habitat

Predator–prey interactions are often highly co‐evolved, with selection over time for prey with morphological and behavioral traits that minimize predation risk. Consequently, in many environments prey choose among potential habitats according to their refuge value. It is unclear, however, when presented with new habitats, if prey are able to evaluate the predation risk of these relative to familiar habitats and utilize these in accordance with their value. We tested whether, along the east coast of the USA, native mud crabs Panopeus herbstii utilize the non‐native alga Gracilaria vermiculophylla according to its relative refuge value. Experiments examining predation by blue crabs Callinectes sapidus on mud crabs revealed that the non‐native alga had an intermediate refuge value relative to native oysters, which were the most protective, and unvegetated sediment, which was the least. In subsequent choice experiments, mud crabs selected oysters over alga over unvegetated sediment, in accordance with habitat refuge values. Further, in field experiments, the use of Gracilaria by mud crabs was inversely related to the proximity of the alga to the preferred habitat type, oysters, and was reduced by the presence of a blue crab predator. Consequently, mud crabs are utilizing the non‐native alga Gracilaria in accordance with its intermediate refuge value. The relative refuge value of non‐native vs native habitat‐forming species may provide a baseline expectation against which to measure the speed of learning and opportunism in the response of native prey to novel protective habitats.     

Evaluation of the sample size used for the rapid bioassessment of rivers using macroinvertebrates

Experiments were designed to investigate selective predation by medium (40–55 mm carapace width: CW) and large (55–70 mm CW) Carcinus maenas when feeding on four bivalves of contrasting shell morphology. Size-selection was examined by presenting individual crabs with a wide size range of Mytilus edulis, Ostrea edulis, Crassostrea gigas and Cerastoderma edule. Medium-sized crabs preferred mussels 5–15 mm shell length (maximum shell dimension: SL) and cockles 5–10 mm SL, whereas large crabs preferred mussels 15–25 mm and cockles 10–20 mm SL. Crabs generally showed no preference for any particular size of either oyster species. Species-selection was examined by presenting individual crabs with paired combinations of the four bivalves in various proportions. When offered mussels and oysters simultaneously, both size categories of crabs consistently selected mussels, and food choice was independent of prey relative abundance. By contrast, C. maenas selected mussels and cockles as expected by the frequency in which each size category of crab encountered the preferred size ranges of prey. Crab preference clearly paralleled the rank order of prey profitability, which in turn was mainly determined by prey biomass, suggesting that active selection takes place at some point of the predation cycle. Experiments with epoxy resin models showed that initial reluctance of crabs to attack oysters was not associated with the ultimate energy reward. Moreover, they suggest that foraging decisions are partly based on evaluations of overall prey shape and volume, and that the minimum dimension of the shell constitutes an important feature which crabs recognise and associate with prey value.     

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.     

Predator cue and prey density interactively influence indirect effects on Basal resources in intertidal oyster reefs

Predators can influence prey abundance and traits by direct consumption, as well as by non-consumptive effects of visual, olfactory, or tactile cues. The strength of these non-consumptive effects (NCEs) can be influenced by a variety of factors, including predator foraging mode, temporal variation in predator cues, and the density of competing prey. Testing the relative importance of these factors for determining NCEs is critical to our understanding of predator-prey interactions in a variety of settings. We addressed this knowledge gap by conducting two mesocosm experiments in a tri-trophic intertidal oyster reef food web. More specifically, we tested how a predatory fish (hardhead catfish, Ariopsis felis) directly influenced their prey (mud crabs, Panopeus spp.) and indirectly affected basal resources (juvenile oysters, Crassostrea virginica), as well as whether these direct and indirect effects changed across a density gradient of competing prey. Per capita crab foraging rates were inversely influenced by crab density, but they were not affected by water-borne predator cues. As a result, direct consumptive effects on prey foraging rates were stronger than non-consumptive effects. In contrast, predator cue and crab density interactively influenced indirect predator effects on oyster mortality in two experiments, with trait-mediated and density-mediated effects of similar magnitude operating to enhance oyster abundance. Consistent differences between a variable predator cue environment and other predator cue treatments (no cue and constant cue) suggests that an understanding of the natural risk environment experienced by prey is critical to testing and interpreting trait-mediated indirect interactions. Further, the prey response to the risk environment may be highly dependent on prey density, particularly in prey populations with strong intra-specific interactions.     

Predator (<Emphasis Type="Italic">Carcinus maenas</Emphasis>) nonconsumptive limitation of prey (<Emphasis Type="Italic">Nucella lapillus</Emphasis>) feeding depends on prey density and predator cue type

Predators often have nonconsumptive effects (NCEs) on prey. For example, upon detection of predator cues, prey can reduce feeding activities to hamper being detected by predators. Previous research showed that waterborne chemical cues from green crabs (Carcinus maenas, predator) limit the dogwhelk (Nucella lapillus, prey) consumption of barnacles regardless of dogwhelk density, even though individual predation risk for dogwhelks decreases with conspecific density. Such NCEs might disappear with dogwhelk density if dogwhelks feed on mussels, as mussel stands constitute better antipredator refuges than barnacle stands. Through a laboratory experiment, we effectively found that crab chemical cues limit the per-capita consumption of mussels by dogwhelks at low dogwhelk density but not at high density. The combination of tactile and chemical cues from crabs, however, limited the dogwhelk consumption of mussels at both dogwhelk densities. The occurrence of such NCEs at both dogwhelk densities could have resulted from tactile cues indicating a stronger predation risk than chemical cues alone. Overall, the present study reinforces the notions that prey evaluate conspecific density when assessing predation risk and that predator cue type affects their perception of risk.     

Recruitment of shore crabsCarcinus maenas on tidal flats: Mussel clumps as an important refuge for juveniles

During the late summer and early fall, juvenile shore crabs (Carcinus maenas L.) occurred in high abundances in mussel clumps scattered on tidal flats of the Wadden Sea. Abundances were much lower on bare tidal flats without mussel clumps and decreased substantially from July to November, whereas numbers in mussel clumps remained high. Large crabs left the tidal flats in early fall, whereas juveniles undertook tidal migrations only in the late fall. In March very few shore crabs were found in the intertidal area. The size of juvenile shore crabs living between mussels did not increase significantly during fall. On the bare tidal flats surrounding the mussels, a size increase was observed. Mussel beds and mussel clumps serve as a spatial refuge for the early benthic phases of juvenile shore crabs. Between mussels they can hide effectively from their epibenthic predators. Juvenile shore crabs do not leave the intertidal area and the mussel habitats before their major predators have left the area. Mussel clumps scattered over the tidal flats may be a critical refuge for juvenile shore crabs settling on tidal flats. Intensified efforts in mussel culturing in the European Wadden Sea during recent decades may have caused an increased abundance of mussel clumps on tidal flats, thus enhancing habitat availability for some mussel-clump inhabitants.     

Refuge utilization and preferences between competing intertidal crab species

Many invertebrates avoid predation risk by seeking and defending refuges that can be in limited supply, producing strong intra- and inter-specific interference competition. Previous experimental studies in central Chile demonstrated that interference competition for refuges is the primary factor driving habitat segregation between the predatory crabs Acanthocyclus gayi and A. hassleri, with the latter species monopolizing galleries inside mussel beds in the mid intertidal zone and limiting A. gayi to rock crevices. Yet, habitat partitioning between rival species can result from differences in habitat preferences and not solely from interference interactions. Moreover, since A. gayi is also known to shelter in turf-forming algae (predominantly Gelidium), which dominates extensive areas in the low intertidal zone, among-sites variation in the turf morphology and abundance could modify habitat preferences and the pattern of inter-specific interactions. We experimentally evaluated refuge habitat preferences of individual, similarly-sized adult A. gayi and A. hassleri in the laboratory, comparing choice patterns across multiple trials with paired combinations of the main refuge types commonly used by crabs in the field: a) mussel galleries, b) rock crevices, c) short algal turf, and d) tall algal turf. Our results showed that both species display a strong ranking of preferences for some refuge habitats over others. In general, mussel galleries were the preferred refuge type for both crab species, but their preference rankings changed depending on turf morphology. When turf was short, A. hassleri and A. gayi made identical refuge choices, strongly preferring mussel galleries over crevices and these over the short turf. In contrast, when the turf was tall A. gayi selected equally the tall turf or mussel galleries, and these were strongly preferred over crevices. A. hassleri, on the other hand, largely ignored tall turf and kept the highest preference for mussel galleries. A field experiment in which crabs were offered to foraging birds demonstrated that A. hassleri is more susceptible than A. gayi to predation by kelp gulls when outside refuges. Differences in patterns of coloration between crabs may underlie between-species differences in predation susceptibility and their habitat choices. These results suggest that the among-site differences in turf morphology (height and shape of fronds), which is largely driven by varying intensity of upwelling, could affect crab preferences for refuge habitats and the relative importance of inter-specific interference competition.     

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.     

Broken pieces: can variable ecological interactions be deduced from the remains of crab attacks on bivalve shells?

Shell fragmentation patterns that result from attacks by durophagous predators on hard‐shelled marine invertebrates are a rich source of indirect evidence that have proved useful in interpreting predation pressure in the fossil record and recent ecology. The behaviour and effectiveness of predators are known to be variable with respect to prey size. It is less well understood if variable predator–prey interactions are reflected in shell fragmentation patterns. Therefore, we conducted experimental trials to test the behavioural response of a living crab, Carcinus maenas, during successful predatory attacks on the blue mussel Mytilus edulis on two prey size categories. Further, we examined resultant shell fragments to determine whether specific attack behaviours by C. maenas could be successfully deduced from remaining mussel shells. In contrast to previous studies, we observed no significant differences in attack behaviour by the predators attributable to prey size. In most experimental predation events, crabs employed an ad hoc combination of five mechanisms of predation previously described for this species. We identified seven categories of shell breakage in predated mussels, but none of these were unambiguously correlated with specific attack behaviour. Combined attack behaviours may produce shell breakage patterns that have previously been assumed to be attributable to a single behaviour. While specific patterns of shell breakage are clearly attributable to durophagy, the results of this study provide important insights into the limitations of indirect evidence to interpret ecological interactions.     

Personality interacts with habitat quality to govern individual mortality and dispersal patterns

Individual phenotypic differences are increasingly recognized as key drivers of ecological processes. However, studies examining the relative importance of these differences in comparison with environmental factors or how individual phenotype interacts across different environmental contexts remain lacking. We performed two field experiments to assess the concurrent roles of personality differences and habitat quality in mediating individual mortality and dispersal. We quantified the predator avoidance response of mud crabs, Panopeus herbstii, collected from low‐ and high‐quality oyster reefs and measured crab loss in a caging experiment. We simultaneously measured the distance crabs traveled as well as the stability of personalities across reef quality in a separate reciprocal transplant experiment. Habitat quality was the primary determinant of crab loss, although the distance crabs traveled was governed by personality which interacted with habitat quality to control the fate of crabs. Here, crabs on low‐quality reefs rapidly emigrated, starting with the boldest individuals, and experienced modest levels of predation regardless of personality. In contrast, both bold and shy crabs would remain on high‐quality reefs for months where bolder individuals experienced higher predation risk. These findings suggest that personalities could produce vastly different population dynamics across habitat quality and govern community responses to habitat degradation.     

A comparison of the predatory impacts of an invasive and native crab species using a functional response approach

The European green crab (Carcinus maenas) is invasive on the West coast of North America, but the ecological consequences of this invasion remain poorly understood. Comparative functional response analysis has arisen as a method of elucidating ecological consequences of invasive species by comparing the impact of these species to native analogues. Through comparative functional response experiments of green crabs and native red rock crabs (Cancer productus) we found that green crab predation increased asymptotically (Type II functional response) when fed increasing densities of Pacific oysters (Magallana gigas), while red rock crab predation displayed a sigmoidal (Type III) response. At high oyster densities red rock crabs consume more Pacific oysters than green crabs do, due to their reduced handling time, though green crabs consume more Pacific oysters relative to their size than red rock crabs. However, compared to red rock crabs, green crabs consume more oysters at low prey densities, which implies that they have a larger, potentially destabilizing impact on low densities of Pacific oysters. As green crabs continue to spread across the West coast of North America, Pacific oysters will face increased predation pressure. Our results show the advantage of using functional response analysis to compare density dependent predation between an invasive species and a native species to predict the ecological consequences of invasions.

     

Energy flow to two abundant consumers in a subtropical oyster reef food web

Oyster reefs are among the most threatened coastal habitat types, but still provide critical habitat and food resources for many estuarine species. The structure of oyster reef food webs is an important framework from which to examine the role of these reefs in supporting high densities of associated fishes. We identified major trophic pathways to two abundant consumers, gray snapper (Lutjanus griseus) and crested goby (Lophogobius cyprinoides), from a subtropical oyster reef using stomach content and stable isotope analysis. The diet of gray snapper was dominated by crabs, with shrimp and fishes also important. Juvenile gray snapper fed almost entirely on oyster reef-associated prey items, while subadults fed on both oyster reef- and mangrove-associated prey. Based on trophic guilds of the gray snapper prey, as well as relative δ¹³C values, microphytobenthos is the most likely basal resource pool supporting gray snapper production on oyster reefs. Crested goby had omnivorous diets dominated by bivalves, small crabs, detritus, and algae, and thus were able to take advantage of prey relying on production from sestonic, as well as microphytobenthos, source pools. In this way, crested goby represent a critical link of sestonic production to higher trophic levels. These results highlight major trophic pathways supporting secondary production in oyster reef habitat, thereby elucidating the feeding relationships that render oyster reef critical habitat for many ecologically and economically important fish species.     

Salt marsh shoreline geomorphology influences the success of restored oyster reefs and use by associated fauna

Restoration is increasingly implemented as a strategy to mitigate global declines in biogenic habitats, such as salt marshes and oyster reefs. Restoration efforts could be improved if we knew how site characteristics at landscape scales affect the ecological success of these foundation species. In this study, we determined how salt marsh shoreline geomorphologies (e.g. with variable hydrodynamic energy, fetch, erosion rates, and slopes) affect the success of restored intertidal oyster reefs, as well as how fauna utilize restored reefs and forage along marsh habitats. We constructed oyster reefs along three marsh shoreline geomorphologies in May 2012: 1) “creek” (small‐fetch, gradual‐sloped shoreline), “ramp” (large‐fetch, gradual‐sloped shoreline), and “scarp” (large‐fetch, steep‐sloped shoreline). Following recruitment, oyster spat density was greatest on ramp reefs; however, 2 years later, the highest adult oyster densities were found on creek reefs. Total nekton and blue crab catch rates in trawl nets were highest in the creek, while piscivore catch rates in gill nets were highest along the scarp shoreline. We found no difference in predation on snails in the salt marsh behind constructed reef and nonconstructed reference sites, but there were more snails consumed in the creek shoreline, which corresponded with the distribution of their major predator—blue crabs. We conclude that oyster reef construction was most successful for oysters in small‐fetch, gradual‐sloped, creek environments. However, nekton abundance did not always follow the same trends as oyster density, which could suggest constructed reefs may offer similar habitat‐related functions (prey availability and refuge) already present along existing salt marsh borders.     

Impact scenario for the invasive red king crab <Emphasis Type="Italic">Paralithodes camtschaticus</Emphasis> (Tilesius, 1815) (Reptantia,Lithodidae) on Norwegian,native, epibenthic prey

Large invasive predators like the king crab, Paralithodes camtschaticus, deserve particular attention due to their potential for catastrophic ecological impact on recipient communities. Conspicuous, epibenthic prey species, such as the slow growing commercial scallop Chlamys islandica, are particularly exposed to the risk of local extinction. A research program integrating experiments and field monitoring is attempting to predict and track the impact of invasive king crab on scallop beds and associated fauna along the north Norwegian coast. The claw gape of the crab shows no limitations in handling the flat-bodied scallop. However, the potential impact of the crab on scallop may depend on the availability of other calcified prey associated with scallop beds, such as the sea star, sea urchin, and blue mussel, all species recorded in the diet of P. camtschaticus. To address this issue, a laboratory experiment on foraging behaviour of P. camtschaticus was conducted. The experimental results show that all size classes of red king crab prefer scallops, but small juveniles and medium sized crabs demonstrate active selection for starfish (Asterias rubens) that equals or surpasses the electivity of the large crab. The selection of sea urchin (Strongylocentrotus droebachiensis) and blue mussel (Mytilus edulis) is slightly positive or neutral for the three crab size classes. These results suggest that scallop beds with a rich associated fauna are less vulnerable to red king crabs predation and possibly more resilient than beds with few associated species. Also, crab size distribution is likely relevant for invasion impact, with increasing abundance of small and medium sized crabs being detrimental for alternative calcified prey associated with scallop beds. Successive stages of crab invasion will see an acceleration of scallop mortality rates associated with (i) decreasing availability of alternative prey, due to protracted predation pressure intensified by recruitment of juvenile crabs, and (ii) increased number of large crabs. Estimates of crab density and intake rates suggest that the accelerated loss rates will eventually endanger scallop beds persistence.