The Effect of Contaminated Prey on Feeding,Activity, and Growth of Young-of-the-Year Bluefish, <Emphasis Type="Italic">Pomatomus saltatrix</Emphasis>, in the Laboratory

Young-of-the-year (YOY) bluefish, Pomatomus saltatrix, reside in some contaminated estuaries of the mid-Atlantic bight during their early life history, and as a result of this exposure, they may bioaccumulate high levels of contaminants, including polychlorinated biphenyls (PCBs), pesticides, and methyl mercury. Young-of-the-year bluefish from the Tuckerton, NJ, area of Great Bay (TK) were fed daily in a laboratory with common prey fish, menhaden, and mummichog from two sites: TK (reference) or Hackensack River (HR) (contaminated). Bluefish fed HR prey and the HR prey themselves had significantly elevated concentrations of PCBs, pesticides, and total mercury compared to TK counterparts. The bluefish fed contaminated prey for 4 months displayed significantly reduced feeding, spontaneous activity, and growth compared to the bluefish fed TK prey. Alterations of bluefish behavior and growth from exposure to contaminants may have detrimental effects on migration, overwinter survival, and recruitment success.     

Role of Invasive Green Crabs in the Food Web of an Intertidal Sand Flat Determined from Field Observations and a Dynamic Simulation Model

A benthic focussed food web model of intermediate complexity was developed for the intertidal sand flat in the Kejimkujik National Park Seaside, NS, Canada. The goal was to understand the role of invasive green crabs and to investigate potential effects of its removal on the ecosystem. Inputs of biomass, production, consumption and diet composition for each model compartment were obtained through targeted field collections in summers 2008–2010. Simulation (via Ecosim) was used to develop a data-driven baseline steady-state food web of summer conditions and to evaluate effects of ecosystem perturbations that included moderate and extreme removal of crab biomass, in the presence and absence of migrating shorebirds. In the baseline model, green crabs were important benthic predators and also prey for fishes and birds. Ecosystem properties (e.g. size, energetics, organisation) suggested a stable ecosystem. Scenario analyses predicted that crab removal caused only slight perturbations to overall ecosystem properties, but affected some trophic components. Predators that consumed mainly crabs (e.g. gulls) decreased in biomass with crab removal because diet changes could not compensate for lost crab prey. Predators that consumed crabs and also other prey (e.g. plovers) increased in biomass with crab removal because shared benthic prey increased. Migrating shorebirds had a greater impact on the food web than crab removal. Our study suggests that removal of invasive green crabs will likely not drastically change food web biomass and productivity. This study illustrates how field data can be synthesised with food web models for effective management of coastal ecosystems.     

Intertidal distribution and feeding habits of the mud crab,Eurytium limosum

Average summer densities of the xanthid crab,Eurytium limosum, in an intertidal salt marsh on Sapelo Island, Georgia were in the range of 7.5 to 80.0 individuals m^?2. Crab densities were lowest in wet, lowlying marsh and highest in well-drained creekbank and mussel mound habitats. An analysis of crab stomach contents indicated that feeding occurred mostly around high tide, especially at night. Although the diet included some plant material,Eurytium limosum is primarily predatory and consumed crabs, polychaetes, ostracods, bivalves, and snails. In the laboratory, under simulated low-tide field conditions, both small (11–15 mm carapace width) and large (20–37 mm CW)Eurytium could capture and consume young killifish (Fundulus heteroclitus). Large crabs consumed the entire size range (7–19 mm total length) of larval/juvenile fish offered, but small crabs did not prey upon fish >11.5 mm TL. The potential importance ofE. limosum as a predator on young killifish may not be realized in the field because alternative prey are available and the crabs feed primarily at high tide, when young killifish are dispersed in the water column and are less vulnerable to benthic predators.     

Foraging by Two Estuarine Fishes, <Emphasis Type="Italic">Fundulus heteroclitus</Emphasis> and <Emphasis Type="Italic">Fundulus majalis</Emphasis>, on Juvenile Asian Shore Crabs (<Emphasis Type="Italic">Hemigrapsus sanguineus</Emphasis>) in Western Long Island Sound

The Asian shore crab, Hemigrapsus sanguineus, is a recent and particularly successful introduction to the east coast of the USA. Little research has been done on the utilization of Asian shore crabs for food by native species, a potential form of biocontrol. Over a 4-year period, we examined the gut contents of cogeners, Fundulus heteroclitus and Fundulus majalis, collected from two embayments in western Long Island Sound for the presence of juvenile H. sanguineus. Frequency (percent) of occurrence of food items in the guts of both species varied over year and study site. Asian shore crabs were consumed more often by F. heteroclitus than by F. majalis, but predation pressure by both species was low. Only 13% of F. heteroclitus and 7.7% of F. majalis found with food in their guts had ingested Asian shore crab remains. Of those, 1/3 had consumed whole crabs; the rest had only autotomized appendages in their guts. The mean carapace width of juvenile Asian shore crabs ingested by F. heteroclitus was 3.59 ± 2.22 mm (N = 33). Results of our study on killifish predation support the hypothesis that H. sanguienus abundance is partly explained by reduced impact of native predators (i.e., the “enemy release hypothesis”). Predation pressure of other potential enemies on both planktonic and benthic stages of the Asian shore crab must be investigated, however, to understand the full impact of predation on H. sanguineus population dynamics.     

Predation and Functional Responses of Carcinus maenas and Cancer magister in the Presence of the Introduced Cephalaspidean Philine orientalis

An increasing number of examples suggest that interactions among introduced species are ecologically important and relevant to the management of invaded systems. We investigated the potential for the introduced cephalaspidean sea slug Philine orientalis to interfere with the feeding of the introduced European green crab (Carcinus maenas) and the native Dungeness crab (Cancer magister). We observed co-occurrence of crab species and P. orientalis at field sites in Bodega Harbor and Tomales, San Pablo, and San Francisco Bays. In laboratory and field experiments, we determined whether crab feeding was suppressed by P. orientalis and the duration of this suppression for individual crabs. We also used foraging response models to explore changes in the feeding rate of crabs with varying densities of P. orientalis and small bivalve prey. We found that P. orientalis deterred predation by green and Dungeness crabs on small clams in laboratory feeding trials, but not in field experiments with green crabs and P. orientalis. Foraging models predicted that P. orientalis would only affect crab feeding in the field under specific conditions of crab, P. orientalis, and prey densities. These foraging models bridged an important gap between lab and field experiments and allowed us to predict how changes in species abundances at two trophic levels might alter the importance of crab suppression by P. orientalis.     

The necessity for intertidal foraging by estuarine populations of subadult dungeness crab,Cancer magister: Evidence from a bioenergetics model

Complex intertidal habitats characteristic of northeastern Pacific coastal estuaries provide critical nursery environments for young-of-the-year Dungeness crab,Cancer magister, yet their role in supporting subsequent year classes remains unclear. SubadultC. magister (40–130 mm; 1+ and >1+ year classes), which reach densities as high as 4,300 crabs ha^?1 in subtidal channels during low tides, migrate during flood tides from subtidal refuges into intertidal habitats to forage. As with other brachyuran species that undertake extensive tidally-driven migrations, intertidal foraging may contribute significantly to the energy budget of subadultC. magister. In order to explore the energetic incentive for intertidal migrations by subadult crabs, we developed an ontogenetically-based bioenergetics model for crabs within Willapa Bay, Washington. The model showed that energetic demand varied spatially across the bay, with the highest average energetic demand of a population of subadult crabs (2.13×10⁶ kJ ha^?1) occurring in a habitat stratum termed lower side channel (LSC) and characterized by relatively little subtidal area and extensive intertidal flats. Comparison of model results with subtidal prey production revealed that the latter could not satisfy subadultC. magister energetic demands, especially in LSC where modeled crab predation depleted subtidal prey biomass within 17 simulation days. We estimate that 1 ha of subtidal crabs from LSC would minimally require an additional 1.6 ha of intertidal area to satisfy energetic demands without depleting prey biomass. Our model results support the assertion thatC. magister make regular migrations to forage on productive intertidal flats, and suggest that intertidal foraging may contribute significantly to the diet of subadult crabs in coastal estuaries.     

The effect of Southwestern Atlantic burrowing crabs on habitat use and foraging activity of migratory shorebirds

Southwestern Atlantic estuaries (Southern Brazil to Northern Patagonia, Argentina) are characterized by the presence of an intertidal burrowing crab Chasmagnathus granulata. This crab species is an important bioturbator which lives in large assemblages and excavates semi-permanent burrows that affect sediment characteristics. Our observations showed that distribution of the crabs in the Mar Chiquita coastal lagoon, Argentina (37°45′S, 57°26′W) affected habitat use and feeding success of migratory shorebirds. During the migratory season the two-banded plover (Charadrius falklandicus) foraged more frequently inside crab beds, and yellowlegs (Tringa flavipes and T. melanoleuca) fed more freqeuntly outside crab beds. Focal observations on the feeding behavior of the white-rumped sandpiper (Calidris fuscicollis) and the two-banded plover inside and outside crab beds showed that the plover was a visual searcher and captured more prey inside crab beds, and the white-rumped sandpiper was a tactile feeder. Although consumption rates (prey min^?1) did not differ between sites, their efficiency (prey probe^?1) inside crab beds was less. These differences were probably related to changes in sediment characteristics and prey behavior, which vary with crab activity. Burrowing crabs alter the suitability of intertidal habitats used by shorebirds in southwestern Atlantic estuaries. We believe that the same process could be occurring with other burrowing curstaceans such as thalassinidean shrimps in other estuaries of the world and could have important implications for management of flats for shorebirds. *** DIRECT SUPPORT *** A01BY090 00007     

<Emphasis Type="Italic">Deepwater Horizon</Emphasis> Oil Spill Impacts on Salt Marsh Fiddler Crabs (<Emphasis Type="Italic">Uca</Emphasis> spp.)

The Deepwater Horizon oil spill was the largest marine oil spill in US waters to date and one of the largest worldwide. Impacts of this spill on salt marsh vegetation have been well documented, although impacts on marsh macroinvertebrates have received less attention. To examine impacts of the oil spill on an important marsh invertebrate and ecosystem engineer, we conducted a meta-analysis on fiddler crabs (Uca spp.) using published sources and newly available Natural Resources Damage Assessment (NRDA) and Gulf of Mexico Research Initiative (GoMRI) data. Fiddler crabs influence marsh ecosystem structure and function through their burrowing and feeding activities and are key prey for a number of marsh and estuarine predators. We tested the hypothesis that the spill affected fiddler crab burrow density (crab abundance), burrow diameter (crab size), and crab species composition. Averaged across multiple studies, sites, and years, our synthesis revealed a negative effect of oiling on all three metrics. Burrow densities were reduced by 39 % in oiled sites, with impacts and incomplete recovery observed over 2010–2014. Burrow diameters were reduced from 2010 to 2011, but appeared to have recovered by 2012. Fiddler crab species composition was altered through at least 2013 and only returned to reference conditions where marsh vegetation recovered, via restoration planting in one case. Given the spatial and temporal extent of data analyzed, this synthesis provides compelling evidence that the Deepwater Horizon spill suppressed populations of fiddler crabs in oiled marshes, likely affecting other ecosystem attributes, including marsh productivity, marsh soil characteristics, and associated predators.     

Evaluating salt marsh restoration in Delaware Bay: The response of blue crabs,<Emphasis Type="Italic">Callinectes sapidus</Emphasis>, at former salt hay farms

Marshes are important habitats for various life history stages of many fish and invertebrates. Much effort has been directed at restoring marshes, yet it is not clear how fish and invertebrates have responded to marsh restoration. The blue crab,Callinectes sapidus, uses marsh habitats during much of its benthic life. We investigated the response of blue crabs to marsh restoration by comparing crab abundance (catch per unit effort), mean size and size frequency distribution, sex ratio, and molt stages of crabs in recently restored marshes that were former salt hay farms to that of adjacent reference marshes with similar physical characteristics in the mesohaline portion of Delaware Bay. Field sampling occurred monthly (April–November) in 1997 and 1998 using replicate daytime otter trawls in large marsh creeks and weirs in smaller intertidal marsh creeks. Blue crabs were either equal or more abundant, the incidence of molting was in most months similar, and population sex ratios were indistinguishable in restored and reference marshes, suggesting that the restored areas attract crabs and support their growth. Site location had a greater effect on the sex ratio of crabs such that marshes closer to the mouth of the bay contained a higher percentage of adult female crabs. In each annual growing season (April–July), the monthly increase in crab size and, in some months (June–July), the incidence of molting at the restored sites was greater than the reference sites, suggesting that the restored sites may provide areas for enhanced growth of crabs. These results suggest that blue crabs have responded positively to restoration of former salt hay farms in the mesohaline portion of Delaware Bay.     

Detrital subsidy to the supratidal zone provides feeding habitat for intertidal crabs

Beach-cast wrack of marine origin is considered a spatial subsidy to the marine-terrestrial transition zone. We found that the wrack line on sand and gravel beaches of Vancouver Island was frequented by intertidal purple shore crabs,Hemigrapsus nudus (Dana 1851) and densely colonized by detritivorous talitrid amphipods. Amphipods spend the day buried in sand and forage on beach wrack during the night.H. nudus were found in supratidal wrack putches immediately after nightly high tides in field censuses, but spent most of the day and ebb tides either submerged subtidally or hidden underneath intertidal rocks and boulders. In feeding trials, intertidal shore crabs were capable of preying on talitrid amphipods. We considerH. nudus an omnivore feeding on both fresh and decaying macroalgae as well as animal prey. Although living supratidally, amphipods were significantly preferred over intertidal littorine snails by foraging shore crabs. Handling time of amphipods was significantly shorter than for littorine snails. While amphipods had a reduced risk of predation byH. nudus when buried in the sand, foraging undern eath wrack patches did not reduce predation pressure on amphipods by shore crabs. Rates of amphipod consumption by shore crabs were higher at darkness than daylight. In addition to an apparent day-night rhythm, tidal height and time elapsed since previous high tide had a significant influence on shore crab density wrack. We conclude that beach-cast wrack acts as a spatial subsidy by virtue of providing a valuable food source to talitrid amphipods, which are in turn consumed by shore crabs that ride the nightly high tide into supratidal wrack patches to reduce the risk of passing bare sand on theiry way to a feeding habitat rich in valuable prey.     

Feeding habits of red drum (Sciaenops ocellatus) in Galveston Bay, Texas: Seasonal diet variation and predator-prey size relationships

Feeding habits, seasonal diet variation, and predator size-prey size relationships of red drum (Sciaenops ocellatus) were investigated in Galveston Bay, Texas through stomach contents analysis. A total of 598 red drum ranging from 291–763 mm total length were collected and their stomach contents analyzed during fall 1997 and spring 1998. The diet of red drum showed significant seasonal patterns, and was dominated by white shrimp (Penaeus setiferus) during fall and gulf menhaden (Brevoortia patronus) during spring. Blue crab (Callinectes sapidus) was an important component of red drum diets during both seasons. Significant differences existed between prey types consumed during fall and spring as red drum diet reflected seasonal variation in prey availability. Predictive regression equations were generated to estimate original carapace width of blue crabs from several measurements taken from carapace fragments recovered in red drum stomachs. Regressions were highly significant (r²>0.97) and increased the number of blue crabs with size information nearly three fold. Predator size-prey size relationships were determined for red drum feeding on white shrimp, gulf menhaden, and blue crab. Although regression slopes were statistically significant, prey sizes increased only slightly with increasing red drum size. Comparisons of prey sizes consumed by red drum with sizes occurring in the field indicate that red drum feed in nearshore shallow water habitats, which serve as nursery areas for many juvenile fishes and crustaceans. Our findings demonstrate that red drum feed on several prey species of commercial and recreational value and may have important effects on estuarine community structure.     

Effects of environmental stress on blue crab (<Emphasis Type="Italic">Callinectes sapidus</Emphasis>) harvests in Chesapeake Bay tributaries

Trotline blue crab (Callinectes sapidus) fishermen in the Patuxent, Chester, and Choptank tributaries of Chesapeake Bay set their gear in areas where the water quality, characterized by average mid-channel bottom dissolved oxygen, varies across the river systems and over the fishing season. Two harvest production models are developed to capture the potential effects of this source of environmental stress. One model treats the impact of water quality as influencing the availability of crabs to the gear, while the second treats the impact as another source of mortality. Both models are estimated assuming that dissolved oxygen has no effect on crab harvests above an upper threshold of 5 mg I⁻¹. Contrary to the mortality model, the availability model produces estimates that are consistent with our prior beliefs that productivity will be negatively impacted by poor water quality. To determine the percentage of the available crab stock in an area that will be harvested by a given amount of gear under different water quality conditions a simulation of the availability model is developed. The simulations show that a decline in average mid-channel bottom dissolved oxygen in the Patuxent River to 4 mg I⁻¹ may lead to a 48% decline in the percentage of the blue crab population in the area that will be harvested with the same amount of fishing effort.     

Food habits of red drum and spotted seatrout in a restored mangrove impoundment

Benthic resource utilization by, red drum (Sciaenops ocellatus) and spotted seatrout (Cynoscion nebulosus) was studied in a restored, mangrove-rimmed impoundment (Cabbagehead Bayou) of Upper Tampa Bay, Florida, and in a nearby, natural site of unaltered tidal regime (Double Branch Bay). Diets of fish captured from August 1990 to May 1992 were determined from stomach content analysis. Simultaneously, food availability was evaluated by sampling benthic macroinvertebrates, mobile decapods, and small fish. Red drum and spotted seatrout utilized the restored habitat 1 yr after it was opened to tidal influence. Both species also were collected in the natural mangrove. Although there were noted differences in benthic assemblages between the two sites, red drum and spotted seatrout exhibited flexibility in diet, feeding on abundant and accessible prey. The high abundance of microcrustacea, such as amphipods, on detritus accumulated in the restored habitat constituted a main food resource for both fish species. Major food items in the diet of small (<200 mm) red drum were amphipods, mysids, and nereid and arenicolid polychaetes. Large (200–590 mm) red drum fed on polychaetes, xanthid crabs, palaemonid shrimp, and small fishes. Spotted seatrout preyed primarily upon mysids, shrimp, and small fishes, and to a lesser extent, upon a nereid polychaete. Our findings on fish feeding in a restored mangrove impoundment indicated that the detrital-associated benthic community is utilized by reinvading fish within a short time period, suggesting that not only habitat but food resources were augmented by the reopening of this wetland.     

Quantifying Tidal Movements of the Shore Crab <Emphasis Type="Italic">Carcinus maenas</Emphasis> on to Complex Epibenthic Bivalve Habitats

Many subtidal predators undertake regular tidal migrations into intertidal areas in order to access abundant prey. One of the most productive habitats in soft bottom intertidal systems is formed by beds of epibenthic bivalves such as blue mussels (Mytilus edulis) and Pacific oysters (Crassostrea gigas). In the Dutch Wadden Sea, these bivalves might face substantial predation pressure by the shore crab (Carcinus maenas), which increased considerably in numbers during the last 20 years. However, the quantification of this species on bivalve beds is challenging, since most methods common for quantifying animal abundance in marine habitats cannot be used. This study investigated the potential of two methods to quantify the abundance of C. maenas on 14 epibenthic bivalve beds across the Dutch Wadden Sea. The use of the number of crabs migrating from subtidal towards intertidal areas as a proxy of abundance on bivalve beds yielded unreliable results. In contrast, crabs caught with traps on the beds were correlated with the abundance assessed on the surrounding bare flats by beam trawl and therefore provided usable results. The estimates, however, were only reliable for crabs exceeding 35 mm in carapace width (CW). The application of these estimates indicated that crab abundances on bivalve beds were influenced by the biogenic structure. Beds dominated by oysters attracted many large crabs (> 50-mm CW), whereas abundances of medium-sized crabs (35–50-mm CW) showed no relationship to the oyster occurrence. The combination of traps and trawls is capable of quantifying crab abundance on bivalve beds, which offers the possibility to study biotic processes such as predator-prey interactions in these complex structures in more detail.     

Effects of size and temperature on mud crab,Panopeus herbstii,predation on hard clams,Mercenaria mercenaria

Predation byPanopeus herbstii onMercenaria mercenaria was significantly affected by temperature, and the size of predators and prey items. LargerP. herbstii opened more clams and preyed more successfully on larger clams than did smaller crabs. Increase in seed clam size and decrease in water temperature significantly reduced predation. Clam size appeared to be more important than crab satiation in reducing predation rate. Planting larger seed clams in cooler months should help to improve clam survival by reducing the impact ofP. herbstii in culture operations.     

Kepone®: Toxicity and bioaccumulation in blue crabs

Two long-term studies were conducted to determine toxicity, uptake and depuration of Kepone in blue crabs (Callinectes sapidus). In the first, Kepone was administered to crabs in seawater (0.03 or 0.3 μg Kepone/I) or food (eastern oyster,Crassostrea virginica, containing 0.25 μg/g Kepone). Uptake of Kepone in 28 days was primarily through the contaminated oysters. When these crabs were held in Kepone-free seawater and fed Kepone-free oysters for 28 days, no loss of the insecticide was evident. There were adverse effects on molting and survival in crabs fed oysters that contained 0.25 μg/g Kepone. A second study was conducted to determine: (1) the depuration of Kepone over a 90-day period in blue crabs fed oysters from the James River, Virginia (containing 0.15 μg/g Kepone); and (2) the effects of Kepone on molting and survival of blue crabs fed James River oysters or laboratory-contaminated oysters that contained 0.15 or 1.9 μg/g Kepone. Crabs fed Kepone-contaminated oysters followed by a diet of Kepone-free oysters for 90 days had detectable concentrations of the insecticide in tissues. Also, blue crabs that ate oysters containing Kepone in concentrations similar to those found in oysters from the James River, died or molted less frequently than crabs fed Kepone-free oysters meats.     

Using Sr isotopes to trace the geographic origins of Chinese mitten crabs

Identifying the geographic origins of Chinese mitten crabs is important for food safety and fair market competition. In this study, we used strontium (Sr) isotopes as a tool to trace the geographic origins of Chinese mitten crabs. Chinese mitten crabs, water, and different types of crab feed were collected from four different lakes for Sr isotope analyses. The results showed that the Sr isotope compositions of the different parts from one single crab were consistent within error, indicating that any piece of a crab could be used to represent the Sr isotope characteristics of the whole crab. The Sr isotope compositions of Chinese mitten crabs from the same lake were consistent within the analytical error, and the values were similar to the Sr isotope composition of the water from the same lake. However, the Sr isotope compositions of water and crabs from different lakes are significantly different. Therefore, the Sr isotope compositions of Chinese mitten crabs are mainly controlled by lake water composition, while the impact of feed is limited. This study provides an effective method for tracing the geographic origins of Chinese mitten crabs.

     

A Test of Bycatch Reduction Devices on Commercial Crab Pots in a Tidal Marsh Creek in Virginia

The effectiveness of bycatch reduction devices (BRDs) on commercial pots designed to capture blue crabs Callinectes sapidus was tested in the York River on Felgates Creek (37.2667 N, −76.5850 W) over the period 4 June through 31 July 2009. For each of 10 pairs of pots, one had BRDs affixed to all four entrance gapes and the other had none. Pots were baited approximately once each week but were sampled for blue crabs and bycatch 6 of 7 days each week for the duration of the study. More than one fourth of 1,643 total crabs were caught on the first day after baiting, and for these 7 days, no statistical difference was detected between either the number or size of legal-size crabs caught in BRD versus non-BRD pots. Of 51 Malaclemys terrapin and 44 fish caught as bycatch throughout the study, all but three fish were captured in non-BRD pots. BRDs exclude bycatch and may reduce incidental mortality of crabs in pots that are not tended regularly.     

Effects of the intertidal burrowing crab<Emphasis Type="Italic">Chasmagnathus granulatus</Emphasis> on infaunal zonation patterns,tidal behavior,and risk of mortality

Rhythmic movements in response to tidal cycles are characteristic of infaunal inhabitant of intertidal soft-bottoms, allowing them to remain in the area with best living conditions. The effect of bioturbators as modifier of local environmental conditions and thus of gradients in intertidal habitats, has not been investigated yet. The Atlantic estuarine intertidal areas are dominated by the burrowing crabChasmagnathus granulatus that generates strong environmental heterogeneity by affecting the physical-chemical characteristics of the sediment. The comparison between intertidal areas with and without crab shows that sediments in the crab beds remain more humid, softer, and homogeneous across the intertidal and along the tidal cycle than areas without crabs. The densities of infauna were higher at high intertidal zones in crab beds than in similar areas without crabs. Infaunal organisms performed vertical movements into the sediment following the tidal cycle that were always of higher magnitude in habitats without crabs. Infaunal species tend to spend most of the time buried into the sediment in the crab bed. Migratory shorebirds use the Atlantic estuarine environments as stopover or wintering sites. They feed (mainly on polychaetes) in the low intertidal zones of both habitats (with and without crabs), but they also feed in the upper intertidal of the crab bed; polychaete per capita mortality rate is higher in the upper part of the crab bed. Environmental heterogeneity produced by crab disturbance has an effect on the infaunal behavior, risk of mortality, and the zonation pattern. This is another example of the ecosystem engineering ability of a burrowing intertidal species.     

The influence of introduced European green crabs (<Emphasis Type="Italic">Carcinus maenas</Emphasis>) on habitat selection by juvenile native blue crabs (<Emphasis Type="Italic">Portunus pelagicus</Emphasis>)

The European green crab (Carcinus maenas) is a highly successful marine invader, having established populations in a number of areas outside its natural range in the last 100 years. In South Australia,C. maenas can be abundant on intertidal mud flats, which are used by juveniles of the native blue swimmer crab (Portunus pelagicus) and could have the potential to cause substantial negative effects on this species. The influence of adult blue and green crabs on habitat selection by juvenile blue crabs was tested to determine if they responded to both predators in a similar fashion. The presence of predators did not influence habitat selection by juvenile blue crabs in either laboratory or field experiments, but juvenile behavior in the selected habitat did differ between the two adult species. Many more crabs buried themselves beneath the substrate when adult conspecifics were present than when adult green crabs were present. Burying in the presence of adult green crabs was no more frequent than when predators were absent. It remains to be determined if this makes juvenile blue crabs more vulnerable to predation by green crabs than by adults of their own species, or if the difference in response is because green crabs pose a different or lesser threat.