Grass Shrimp (Palaemonetes pugio) as a Trophic Link for Methylmercury Accumulation in Urban Salt Marshes.

Grass shrimp (Palaemonetes pugio) represent a potential link in the transfer of methylmercury (MeHg) from salt marsh sediments to transient young-of-the-year (YOY) fish. Across six salt marshes subject to varying degrees of Hg contamination, MeHg concentration in grass shrimp was significantly correlated with MeHg in sediment (p < 0.05, R2 = 0.81). Bioenergetic models show that grass shrimp alone account for 12-90% of MeHg observed in YOY striped bass and 6-22% of MeHg in YOY summer flounder. Direct accumulation of MeHg from grass shrimp to YOY fish increased with MeHg levels in grass shrimp and sediment. However, in the most contaminated salt marshes with the highest levels of MeHg in grass shrimp and sediment, indirect accumulation of MeHg from grass shrimp by YOY summer flounder, whose diet is dominated by benthic forage fish (mummichog), is predicted to plateau because higher concentrations of MeHg in grass shrimp are offset by a lower proportion of grass shrimp in the mummichog diet. Our results demonstrate that grass shrimp are an important trophic link in the bioaccumulation of MeHg in salt marsh food webs and that MeHg accumulation in YOY fish varies with both the concentration of MeHg in salt marsh sediments and benthic food web structure.

Spatial and temporal trends in physiological biomarkers of adult eastern oysters, Crassostrea virginica, within an urban estuary.

Heavy metal contamination and water quality may alter reproductive capacity of oysters in highly urbanized, eutrophic ecosystems. This study assessed physiological biomarkers and heavy metal body burdens in adult oysters, Crassostrea virginica, placed at a highly urban and reference site. Condition index and Vitellogenin-like proteins were significantly different between sites, but protein concentration and activity of the electron transport system were not. Accumulation of Cd and Hg occurred at both sites, and Cd body burden was greater at the urban site. There was a negative relationship between condition index and Cd body burden at the urban site, while no relationship was found between physiological biomarkers and metal burden at the reference site. The results suggest that oyster condition and reproductive potential may be negatively influenced by the biotic and abiotic factors typically found within urban, eutrophic ecosystems.

Examining the relationship between metal exposure (Cd and Hg), subcellular accumulation, and physiology of juvenile Crassostrea virginica.

To assess the toxicity and accumulation (total and subcellular partitioning) of cadmium (Cd) and mercury (Hg), juvenile eastern oysters, Crassostrea virginica, were exposed for 4 weeks to a range of concentrations (Control, Low (1×), and High (4×)). Despite the 4-fold increase in metal concentrations, oysters from the High-Cd treatment (2.4 µM Cd) attained a body burden that was only 2.4-fold greater than that of the Low-Cd treatment (0.6 µM Cd), while oysters from the High-Hg treatment (0.056 µM Hg) accumulated 8.9-fold more Hg than those from the Low-Hg treatment (0.014 µM Hg). This fold difference in total Cd burdens was, in general, mirrored at the subcellular level, though binding to heat-denatured proteins in the High-Cd treatment was depressed (only 1.6-fold higher than the Low-Cd treatment). Mercury did not appear to appreciably partition to the subcellular fractions examined in this study, with the fold difference in accumulation between the Low- and High-Hg treatments ranging from 1.5-fold (heat-stable proteins) to 4.6-fold (organelles). Differences in toxicological impairments (reductions in condition index, protein content, and ETS activity) exhibited by oysters from the High-Cd treatment may be partially due to the nature of how different metals partition to subcellular components in the oysters, though exact mechanisms will require further examination.

Assimilation of elements and digestion in grass shrimp pre-exposed to dietary mercury.

Grass shrimp Palaemonetes pugio were fed mercury (Hg)-contaminated oligochaetes for 15 days and analyzed for Hg, cadmium (Cd), and carbon assimilation efficiencies (AE) as well as toxicological end points related to digestion. Disproportionate increases in stable Hg concentrations in shrimp did not appear to be related to partitioning to trophically available Hg in worms. Hg AE by pre-exposed shrimp reached a plateau (approximately 53 %), whereas Cd AE varied (approximately 40-60 %) in a manner that was not dose-dependent. Carbon AE did not differ among treatments (approximately 69 %). Gut residence time was not impacted significantly by Hg pre-exposure (grand median approximately 465 min), however, there was a trend between curves showing percentages of individuals with markers in feces over time versus treatment. Feces-elimination rate did not vary with dietary pre-exposure. Extracellular protease activity varied approximately 1.9-fold but did not exhibit dose-dependency. pH increased over the range of Hg pre-exposures within the anterior (pH approximately 5.33-6.51) and posterior (pH approximately 5.29-6.25) regions of the cardiac proventriculus and Hg assimilation exhibited a negative relationship to hydrogen ion concentrations. The results of this study indicate that previous Hg ingestion can elicit post-assimilatory impacts on grass shrimp digestive physiology, which may, in turn, influence Hg assimilation during subsequent digestive cycles.

Carbon assimilation and digestive toxicity in naïve grass shrimp (Palaemonetes pugio) exposed to dietary cadmium.

Naïve grass shrimp Palaemonetes pugio were pulse-fed cadmium-contaminated meals containing carbon-14, fluorescent or near-infrared markers and analyzed for carbon assimilation efficiency, gut residence time, feces elimination rate, extracellular digestive protease activity or gut pH. Carbon assimilation efficiency (~83%), minimum gut residence time (~435 min) and proventriculus pH (~5.29 to ~6.01) were not impacted significantly by cadmium ingestion. A dose-dependent decrease in feces elimination rate (from ~14.4 to ~6.4 mm h(-1)) was observed for shrimp for 2 h following minimum gut residence time. Protease activities increased ~2.4-fold over the range of dietary cadmium exposures, however, this variation was not dose-dependent. Differential impacts of cadmium exposure on carbon and cadmium assimilation reported previously are consistent with work involving shrimp subjected to chronic field exposure. The influence of ingested cadmium on feces elimination rate may be related to pre-assimilatory impacts on packaging, intestinal transport or release of feces. Protease activities may have been influenced by pre-assimilatory interactions between available cadmium ions in gut fluid and enzyme-secreting cells of the hepatopancreatic epithelium or direct impacts on active enzymes.

Digestive toxicity in grass shrimp collected along an impact gradient.

Ingested pollutants may elicit digestive toxicity following incorporation into consumer tissues. This post-assimilatory toxicity may include tissue damage influencing synthesis of digestive enzymes, gut transit time and absorption of nutrients as well as pollutants by the gut epithelium. This study investigated impacts of chronic field exposure on gut residence time (GRT), feces elimination rate (FER), extracellular digestive protease activities and gut pH in grass shrimp Palaemonetes pugio. Adult shrimp were collected from differentially impacted sites within the New York/New Jersey Harbor Estuary and fed prepared meals containing fluorescent or near-infrared markers and analyzed for digestive toxicity. Relationships between digestive parameters and assimilation efficiencies (AE) for Cd, Hg and organic carbon reported previously were also analyzed. Minimum GRT did not vary significantly for field-collected shrimp, but was positively correlated with Cd, but not Hg or carbon, AE. FER was not impacted by field exposure. Digestive protease activities exhibited a marked decrease in grass shrimp from impacted field sites relative to reference shrimp. Relationships between the assimilation of elements and digestive physiology in field-collected shrimp suggest that digestive plasticity (increasing GRT) may be important in compensating for post-assimilatory digestive toxicity (reduced protease activities) in order to maintain nutrient assimilation. Stress-induced variability in digestive function among grass shrimp populations may, in turn, enhance the assimilation of non-essential elements, such as Cd.

Altered feeding habits and strategies of a benthic forage fish (Fundulus heteroclitus) in chronically polluted tidal salt marshes.

Responses in feeding ecology of a benthic forage fish, mummichogs (Fundulus heteroclitus), to altered prey resources were investigated in chronically polluted salt marshes (the Arthur Kill-AK, New York, USA). The diet niche breadth of the AK populations of mummichogs was significantly lower than that of the reference population, reflecting reduced benthic macroinfaunal species diversity. Most of the AK populations also had 2-3 times less food in their gut than the reference population. This disparity in gut fullness among the populations appeared to be partly due to ingested prey size shifts; some of the AK populations ingested fewer large prey than the reference population. Furthermore, benthic assemblages were strongly associated with sediment-associated mercury; gut fullness of the AK populations also significantly decreased with increasing mercury body burdens. These results indicate that chronic pollution may have directly (chemical bioaccumulation) and indirectly (reduced prey availability) altered the feeding ecology of mummichogs.

Metal intracellular partitioning as a detoxification mechanism for mummichogs (Fundulus heteroclitus) living in metal-polluted salt marshes.

Intracellular partitioning of trace metals is critical to metal detoxification in aquatic organisms. In the present study, we assessed metal (Cd, Cu, Pb, and Zn) handling capacities of mummichogs (Fundulus heteroclitus) in metal-polluted salt marshes in New York, USA by examining metal intracellular partitioning. Despite the lack of differences in the whole body burdens, partitioning patterns of metals in intracellular components (heat-stable proteins, heat-denaturable proteins, organelles, and metal-rich granules) revealed clear differential metal handling capacities among the populations of mummichogs. In general, mummichogs living in metal-polluted sites stored a large amount of metals in detoxifying cellular components, particularly metal-rich granules (MRG). Moreover, only metals associated with MRG were consistently correlated with variations in the whole body burdens. These findings suggest that metal detoxification through intracellular partitioning, particularly the sequestration to MRG, may have important implications for metal tolerance of mummichogs living in chronically metal-polluted habitats.

Relative importance of multiple environmental variables in structuring benthic macroinfaunal assemblages in chronically metal-polluted salt marshes.

In this study, we assessed importance of sediment-associated trace metals in structuring benthic macroinfaunal assemblages along multiple environmental gradients in chronically polluted salt marshes of the Arthur Kill - AK (New York, USA). More than 90% of benthic macroinfaunal communities at the northern AK sites consisted of a considerably large number of only a few polychaete and oligochaete species. Approximately 70% of among-site variances in abundance and biomass of benthic macroinfaunal communities was strongly associated with a few environmental variables; only sediment-associated mercury consistently contributed to a significant proportion of the explained variances in species composition along natural environmental gradients (e.g., salinity). Although sediment-associated copper, lead, and zinc were substantially elevated at some of the AK sites, their ecological impacts on benthic macroinfaunal communities appeared to be negligible. These findings suggest that cumulative metal-specific impacts may have played an important role in structuring benthic macroinfaunal communities in chronically polluted AK ecosystems.

Biodiversity loss in benthic macroinfaunal communities and its consequence for organic mercury trophic availability to benthivorous predators in the lower Hudson River estuary, USA.

Organic mercury such as methylmercury is not only one of the most toxic substances found in coastal ecosystems but also has high trophic transfer efficiency. In this study, we examined implications of chronically altered benthic macroinfaunal assemblages for organic mercury trophic availability (based on organic mercury intracellular partitioning) to their predators in the Arthur Kill-AK (New York, USA). Despite low species diversity, both density and biomass of benthic macroinvertebrates in AK were significantly higher than those at the reference site. Disproportionately high biomass of benthic macroinvertebrates (mostly polychaetes) in the northern AK resulted in a more than twofold increase ('ecological enrichment') in the trophically available organic mercury pool. These results suggest that altered benthic macroinfaunal community structure in AK may play an important role in organic mercury trophic availability at the base of benthic food webs and potentially in mercury biogeochemical cycling in this severely urbanized coastal ecosystem.

Relevance of intracellular partitioning of metals in prey to differential metal bioaccumulation among populations of mummichogs (Fundulus heteroclitus).

Intracellular partitioning of trace metals is critical to metal tolerance in aquatic organisms and may also influence metal trophic transfer in ecosystems. In this study, we tested the relevance of metal (Cd, Cu, Pb, and Zn) intracellular partitioning in prey as an indicator of metal trophic availability to benthic forage fish, mummichogs (Fundulus heteroclitus), in chronically metal-polluted salt marshes in New York, USA. Two common prey of mummichogs in the study area, Palaemonetes pugio and Nereis acuminata, generally stored increasingly higher proportions of non-essential metals (particularly Pb) in insoluble (less trophically available) cellular components, as the whole body burdens increased. In contrast, intracellular partitioning of essential metals (Cu and Zn) in invertebrate prey varied relatively little among sites. Differential Cd and Pb intracellular partitioning patterns within P. pugio among sites were significantly associated with Cd and Pb whole body burdens in mummichogs, respectively (i.e., prey-driven bioreduction of metals), while bioaccumulation of Cu and Zn in mummichogs was similar among populations. The findings in this study suggest that metal intracellular partitioning within prey may be partially responsible for metal trophic availability to a predator in metal-polluted habitats, while there was also evidence that some predator-dependent processes may offset differential trophic availabilities from prey.

Assimilation and subcellular partitioning of elements by grass shrimp collected along an impact gradient.

Chronic exposure to polluted field conditions can impact metal bioavailability in prey and may influence metal transfer to predators. The present study investigated the assimilation of Cd, Hg and organic carbon by grass shrimp Palaemonetes pugio, collected along an impact gradient within the New York/New Jersey Harbor Estuary. Adult shrimp were collected from five Staten Island, New York study sites, fed (109)Cd- or (203)Hg-labeled amphipods or (14)C-labeled meals and analyzed for assimilation efficiencies (AE). Subsamples of amphipods and shrimp were subjected to subcellular fractionation to isolate metal associated with a compartment presumed to contain trophically available metal (TAM) (metal associated with heat-stable proteins [HSP - e.g., metallothionein-like proteins], heat-denatured proteins [HDP - e.g., enzymes] and organelles [ORG]). TAM-(109)Cd% and TAM-(203)Hg% in radiolabeled amphipods were approximately 64% and approximately 73%, respectively. Gradients in AE-(109)Cd% ( approximately 54% to approximately 75%) and AE-(203)Hg% ( approximately 61% to approximately 78%) were observed for grass shrimp, with the highest values exhibited by shrimp collected from sites within the heavily polluted Arthur Kill complex. Population differences in AE-(14)C% were not observed. Assimilated (109)Cd% partitioned to the TAM compartment in grass shrimp varied between approximately 67% and approximately 75%. (109)Cd bound to HSP in shrimp varied between approximately 15% and approximately 47%, while (109)Cd associated with metal-sensitive HDP was approximately 17% to approximately 44%. Percentages of assimilated (109)Cd bound to ORG were constant at approximately 10%. Assimilated (203)Hg% associated with TAM in grass shrimp did not exhibit significant variation. Percentages of assimilated (203)Hg bound to HDP ( approximately 47%) and ORG ( approximately 11%) did not vary among populations and partitioning of (203)Hg to HSP was not observed. Using a simplified biokinetic model of metal accumulation from the diet, it is estimated that site-specific variability in Cd AE by shrimp and tissue Cd burdens in field-collected prey (polychaetes Nereis spp.) could potentially result in up to approximately 3.2-fold differences in the dose of Cd assimilated by shrimp from a meal in the field. The results of this study also suggest that chronic field exposure can impact mechanisms of metal transport across the gut epithelium that do not influence carbon assimilation. Differences in the assimilation and subcellular partitioning of metal may have important implications for metal toxicity in impacted shrimp populations.

Relating disparity in competitive foraging behavior between two populations of fiddler crabs to the subcellular partitioning of metals.

Behavioral changes in aquatic organisms such as reduced prey capture and decreased mobility have been linked to exposure to contaminants in the field. The purpose of this study was to compare competitive foraging and dominance behaviors of two populations of the fiddler crab, Uca pugnax, and to examine the relationship between tissue metal residues and observed differences in behavior. Foraging behavior (number of total scoops and scoops on a protein-rich patch) and dominance behavior (percentage of successful attacks) of fiddler crabs from an impacted site (Meredith Creek, New York) and a reference site (Tuckerton, New Jersey) were compared in the laboratory. Tuckerton (Tk) crabs were found to have twice the number of total scoops (70 vs. 38 scoops, p < 0.05) and three times the number of scoops on patch (34 vs. 10 scoops, p < 0.05) than Meredith (Me) crabs. No difference was observed between crab populations in the number of successful attacks (i.e., fights over the protein-rich patch). Analyses of total metal body burdens and metals associated with various subcellular fractions showed that, when compared with Tk crabs, Me crabs had higher levels of Ag, Cd, Cu, Ni, and Se in the heat-denatured proteins (HdeP) (i.e., enzymes). Metal bioaccumulation can have inhibitory effects on enzymes, which play an essential role in the regulation of various biochemical, metabolic, and physiological activities in crustaceans. This study suggests that there is a relationship between the accumulation of metals in HdeP (i.e., enzymes) and impairment of competitive foraging behavior in fiddler crabs. Additionally, this study shows that when compared with dominance behavior, foraging behavior is a more sensitive indicator of metal exposure and might be used as an end point in ecotoxicology studies.

Bioenhancement of cadmium transfer along a multi-level food chain.

Previous studies have shown that metal partitioned to a subcellular compartment containing trophically available metal (TAM) is readily available to predators and may be enhanced by increased binding of metal to heat-stable proteins (HSP - e.g., metallothioneins). The aim of the current investigation was to determine the influence of TAM on the trophic transfer of Cd along an experimental, three-level food chain: Artemia franciscana (brine shrimp)-->Palaemonetes pugio (grass shrimp)-->Fundulus heteroclitus (mummichog). P. pugio were fed for 7 days on A. franciscana exposed to Cd in solution (including (109)Cd as radiotracer) and subjected to subcellular fractionation or fed to F. heteroclitus. An HSP-driven increase in the percentage of Cd associated with TAM (TAM-Cd%) in A. franciscana exposed to 1 muM Cd resulted in a bioenhancement (i.e., a greater than linear increase with respect to A. franciscana exposure) of Cd trophic transfer to P. pugio. Increased dietary Cd exposure did not affect TAM-Cd% in P. pugio nor trophic transfer to F. heteroclitus.

Linking metal bioaccumulation of aquatic insects to their distribution patterns in a mining-impacted river.

Although the differential responses of stream taxa to metal exposure have been exploited for bioassessment and monitoring, the mechanisms affecting these responses are not well understood. In this study, the subcellular partitioning of metals in operationally defined metal-sensitive and detoxified fractions were analyzed in five insect taxa. Samples were collected in two separate years along an extensive metal contamination gradient in the Clark Fork River (MT, USA) to determine if interspecific differences in the metal concentrations of metal-sensitive fractions and detoxified fractions were linked to the differences in distributions of taxa relative to the gradient. Most of the Cd, Cu, and Zn body burdens were internalized and potentially biologically active in all taxa, although all taxa appeared to detoxify metals (e.g., metal bound to cytosolic metal-binding proteins). Metal concentrations associated with metal-sensitive fractions were highest in the mayflies Epeorus albertae and Serratella tibialis, which were rare or absent from the most contaminated sites but occurred at less contaminated sites. Relatively low concentrations of Cu were common to the tolerant taxa Hydropsyche spp. and Baetis spp., which were widely distributed and dominant in the most contaminated sections of the river. This suggested that distributions of taxa along the contamination gradient were more closely related to the bioaccumulation of Cu than of other metals. Metal bioaccumulation did not appear to explain the spatial distribution of the caddisfly Arctopsyche grandis, considered to be a bioindicator of metal effects in the river. Thus, in this system the presence/ absence of most of these taxa from sites where metal exposure was elevated could be differentiated on the basis of differences in metal bioaccumulation.