Adult Grass Shrimp Research Articles

Demographic and Trophic Analysis of Adult Grass Shrimp (Palaemonetes pugio) from Living Shoreline and Natural Tidal Marshes in the Chesapeake Bay

With extensive and ongoing human development in coastal areas, effective shoreline-protection strategies against erosion and rising sea level that also provide natural ecosystem services will become increasingly important. Constructed living shoreline marshes can serve as successful management solutions when implemented correctly, but studies comparing these created estuarine habitats to their natural marsh counterparts are needed to assess their ecological functionality. We compared adult Palaemonetes pugio (Daggerblade Grass Shrimp) population demographics and diets from 13 pairs of living shoreline and natural marsh sites in the southwestern portion of the Chesapeake Bay. No significant differences were observed in demographics including abundance, weight, length, number of gravid females, and egg abundance and quality. Gut-content analysis detected significantly more terrestrial debris in shrimp from living shorelines, but the omnivorous dietary needs appeared met in all marsh sites. Overall, shrimp populations differed more among site pairs from different shorescape settings than between created and natural marshes from the same settings, suggesting that living shorelines are approaching ecosystem functionality similar to natural marsh habitats. However, the observed negative effect of agricultural land use on shrimp abundance and the discovery of microplastics in gut contents from all sites sampled indicate the need for further consideration of human impacts in both constructed and natural coastal habitats.

Comparative toxicity of pyrethroid insecticides to two estuarine crustacean species, Americamysis bahia and Palaemonetes pugio

Pyrethroid insecticides are widely used on agricultural crops, as well as for nurseries, golf courses, urban structural and landscaping sites, residential home and garden pest control, and mosquito abatement. Evaluation of sensitive marine and estuarine species is essential for the development of toxicity testing and risk-assessment protocols. Two estuarine crustacean species, Americamysis bahia (mysids) and Palaemonetes pugio (grass shrimp), were tested with the commonly used pyrethroid compounds, lambda-cyhalothrin, permethrin, cypermethrin, deltamethrin, and phenothrin. Sensitivities of adult and larval grass shrimp and 7-day-old mysids were compared using standard 96-h LC50 bioassay protocols. Adult and larval grass shrimp were more sensitive than the mysids to all the pyrethroids tested. Larval grass shrimp were approximately 18-fold more sensitive to lambda-cyhalothrin than the mysids. Larval grass shrimp were similar in sensitivity to adult grass shrimp for cypermethrin, deltamethrin, and phenothrin, but larvae were approximately twice as sensitive to lambda-cyhalothrin and permethrin as adult shrimp. Acute toxicity to estuarine crustaceans occurred at low nanogram per liter concentrations of some pyrethroids, illustrating the need for careful regulation of the use of pyrethroid compounds in the coastal zone.

Toxicity of the mosquito control insecticide phenothrin to three life stages of the grass shrimp (Palaemonetes pugio)

Phenothrin is a synthetic pyrethroid used as a contact insecticide in mosquito control programs. This study compared the toxicity of phenothrin to adult, larval and embryonic grass shrimp (Palaemonetes pugio) and examined oxidative stress responses in adult and larval grass shrimp. The adult 24-h LC50 was 0.341 μg/L (95 % confidence intervals 0.282–0.412) and the 96-h LC50 was 0.161 μg/L (95 % CI 0.128–0.203 μg/L). The larval 24-h LC50 was 0.50 μg/L (95 % CI 0.441–0.568) and the 96-h LC50 was 0.154 μg/L (95 % CI 0.139–0.170 μg/L). In the presence of sediment, the 24-h LC50 was 6.30 μg/L (95 % CI 5.00–7.44 μg/L) for adults and 0.771 μg/L (95 % CI 0.630–0.944) for larvae. The sublethal biomarkers glutathione and lipid peroxidase (LPx) were examined after 96-h phenothrin exposure at five concentrations, and there were no statistically significant differences in these levels in adults or larvae compared to controls. There was a significant downward trend in larval LPx levels. This research confirms that phenothrin is highly toxic to grass shrimp and suggests that both adult and larval grass shrimp are appropriate life stages for risk assessments.

Acute toxic effects of endosulfan sulfate on three life stages of grass shrimp, Palaemonetes pugio

In this study, the toxicity of endosulfan sulfate, the primary degradation product of the insecticide endosulfan, was determined in three life stages of the grass shrimp (Palaemonetes pugio). After 96 h exposure to endosulfan sulfate, the grass shrimp adult LC50 was 0.86 μ g/L (95% CI 0.56–1.31), the grass shrimp larvae LC50 was 1.64 μ g/L (95% CI 1.09–2.47) and the grass shrimp embryo LC50 was 45.85 μ g/L (95% CI 23.72–88.61 μ g/L). This was compared to the previously published grass shrimp 96-h LC50s for endosulfan. The toxicity of the two compounds was similar for the grass shrimp life stages with adults more sensitive than larvae and embryos. The presence of sediment in 24h endosulfan sulfate–exposures raised LC50s for both adult and larval grass shrimp but not significantly. The USEPA expected environmental concentrations (EEC) for total endosulfan and endosulfan sulfate and the calculations of risk quotients (RQ) based on the more sensitive adult grass shrimp 96-h LC50 clearly show that environmental concentrations equal to acute EECs would prove detrimental to grass shrimp or other similarly sensitive aquatic organisms. These results indicate that given the persistence and toxicity of endosulfan sulfate, future risk assessments should consider the toxicity potential of the parent compound as well as this degradation product.

Acute effects of the antibiotic oxytetracycline on the bacterial community of the grass shrimp, Palaemonetes pugio

The toxicity of oxytetracycline (OTC) was evaluated in adult grass shrimp, Palaemonetes pugio. Initially, static acute (96 h) toxicity tests were conducted with shrimp exposed from 0 to 1,000 mg/L OTC. A calculated lethal concentration 50% value of 683.30 mg/L OTC (95% confidence interval 610.85-764.40 mg/L) was determined from these tests, along with a lowest-observable-effect concentration of 750 mg/L and no-observable-effect concentration of 500 mg/L. Moreover, chronic sublethal effects of OTC exposure on grass shrimp intestinal bacterial population were assessed using doses from 0 to 32 mg/L OTC. The total viable counts in digestive tract content had levels between 5.2 and 1x10(4) colony-forming units per gram of tissue at times 0 and 96 h, respectively. Aeromonas hydrophila were the most resistant isolates (27.78%) to OTC exposure. Vibrio alginolyticus showed significant positive growth following exposure to OTC, whereas other bacterial species abundance declined over time. A total of 268 bacterial isolates were screened using antibiotic resistance analysis from a library containing 459 isolates. Among the tested isolates from the OTC treatments, 15.4% were resistant to OTC and 84.6% were OTC sensitive. Oxytetracycline was generally not consistently quantifiable with liquid chromatography-mass spectroscopy technique in shrimp homogenates. The only peak detected was at the 32 mg/L dose of OTC at 96 h. Nevertheless, OTC had a significant biological effect on the bacterial population. Antibiotic resistance to five other antibiotics (penicillin G, sulfathiazole, trimethoprim, trimethoprim and sulfamethoxazole, and tetracycline) was strongly associated with OTC exposures. The present study indicates that OTC toxicity effects in P. pugio and changes in the shrimp microbial community would only be expected under special circumstances.

Temperature and salinity effects on the toxicity of common pesticides to the grass shrimp, Palaemonetes pugio

This study investigated the effects of increased temperature and salinity, two potential impacts of global climate change, on the toxicity of two common pesticides to the estuarine grass shrimp, Palaemonetes pugio. Larval and adult grass shrimp were exposed to the fungicide chlorothalonil and the insecticide Scourge® under standard toxicity test conditions, a 10°C increase in temperature, a 10 ppt increase in salinity, and a combined increased temperature and salinity exposure. Toxicity of the fungicide chlorothalonil increased with temperature and salinity. Toxicity of the insecticide Scourge® also increased with temperature; while increased salinity reduced Scourge® toxicity, but only in adult shrimp. These findings suggest that changes in temperature and salinity may alter the toxicity of certain pesticides, and that the nature of the effect will depend on both the organism's life stage and the chemical contaminant. Standard toxicity bioassays may not be predictive of actual pesticide toxicity under variable environmental conditions, and testing under a wider range of exposure conditions could improve the accuracy of chemical risk assessments.

Bioenergetic effects of aqueous copper and cadmium on the grass shrimp, Palaemonetes pugio

Adult grass shrimp ( Palaemonetes pugio) were exposed to either aqueous copper (ranging from 7.54 to 41.29 μg Cu 2+/L) or cadmium (2.48– 6.55 μg Cd 2+/L) for 14 days in laboratory experiments to quantify effects on survival and bioenergetic processes, including respiration, somatic growth, energy (lipid) storage, and food consumption. The lowest observed effect concentrations for mortality were 41.29 μg Cu 2+/L or 6.55 μg Cd 2+/L, expressed as free metal ion concentrations. Both copper and cadmium caused a decrease in respiration rate at concentrations of 7.54 to 41.29 μg Cu 2+/L or 6.55 μg Cd 2+/L. Exposure to copper (≥ 27.03 μg Cu 2+/L) resulted in negative somatic growth (i.e., weight loss). Cadmium exposure (6.17 μg Cd 2+/L) caused a decrease in growth rate, relative to the control, but growth remained positive. Nonpolar lipid content and food consumption were not significantly affected by exposure to either copper or cadmium. Our results suggest that both copper and cadmium result in overall metabolic depression, decreasing energy allocation to both maintenance and production.

Lethal and sublethal effects of the pyrethroid, bifenthrin, on grass shrimp (Palaemonetes pugio) and sheepshead minnow (Cyprinodon variegatus)

This study investigated the lethal and sublethal effects of the pyrethroid insecticide bifenthrin on adult and larval grass shrimp, Palaemonetes pugio, and adult sheepshead minnows, Cyprinodon variegatus. The effects were determined by conducting 96-h aqueous static renewal tests and 24-h static tests with sediment. Oxidative stress biomarkers, lipid peroxidation, glutathione, and catalase were also assessed. The 96-h aqueous LC50 value for adult shrimp was 0.020 μ g/L (95% CI: 0.015–0.025 μ g/L) and for larval shrimp was 0.013 μ g/L (95% CI: 0.011–0.016 μ g/L). The 96-h aqueous LC50 for adult sheepshead minnow was 19.806 μ g/L (95% CI: 11.886–47.250 μ g/L). The 24-h sediment LC50 for adult shrimp was 0.339 μ g/L (95% CI: 0.291–0.381 μ g/L) and for larval shrimp was 0.210 μ g/L (95% CI: 0.096–0.393 μ g/L). The oxidative stress assays showed some increasing trends toward physiological stress with increased bifenthrin concentrations but they were largely inconclusive. Given the sensitivity of grass shrimp to this compound in laboratory bioassays, additional work will be needed to determine if these exposure levels are environmentally relevant.

Effects of the statin antihyperlipidemic agent simvastatin on grass shrimp, Palaemonetes pugio

This study investigated lethal effects (i.e., survival) and sublethal effects (glutathione, GSH; lipid peroxidation, LPx; cholesterol, CHL; and acetylcholinesterase, AChE) of the antihyperlipidemic drug simvastatin on larval and adult grass shrimp (Palaemonetes pugio). The 96-h LC50 test for larvae resulted in an estimated LC50 of 1.18 mg/L (95% confidence interval 0.98-1.42 mg/L). The adult 96-h LC50 was >10.0 mg/L. GSH and AChE levels for both the larvae and the adults were not significantly affected by simvastatin exposure. LPx levels in the larvae were significantly higher than controls in the lowest and the highest simvastatin exposures. In adult grass shrimp, LPx levels were highest in the three lowest simvastatin exposures. CHL levels were significantly reduced in larvae at the highest simvastatin exposure level of 1 mg/L while adult CHL was not affected. Both lethal and sublethal effects associated with simvastatin exposure were only observed at concentrations well above those reported in the environment.

Toxicity of the antimicrobial compound triclosan and formation of the metabolite methyl‐triclosan in estuarine systems

Triclosan, a commonly used antimicrobial compound, has been measured in aquatic systems worldwide. This study exposed marine species to triclosan to examine effects primarily on survival and to investigate the formation of the degradation product, methyl-triclosan, in the estuarine environment. Acute toxicity was assessed using the bacterium Vibrio fischeri, the phytoplankton species Dunaliella tertiolecta, and three life stages of the grass shrimp Palaemonetes pugio. P. pugio larvae were more sensitive to triclosan than adult shrimp or embryos. Acute aqueous toxicity values (96 h LC50) were 305 microg/L for adult shrimp, 154 microg/L for larvae, and 651 microg/L for embryos. The presence of sediment decreased triclosan toxicity in adult shrimp (24 h LC50s were 620 microg/L with sediment, and 482 microg/L without sediment). The bacterium was more sensitive to triclosan than the grass shrimp, with a 15 min aqueous IC50 value of 53 microg/L and a 15 min spiked sediment IC50 value of 616 microg/kg. The phytoplankton species was the most sensitive species tested, with a 96 h EC50 value of 3.55 microg/L. Adult grass shrimp were found to accumulate methyl-triclosan after a 14-day exposure to 100 microg/L triclosan, indicating formation of this metabolite in a seawater environment and its potential to bioaccumulate in higher organisms. Triclosan was detected in limited surface water sampling of Charleston Harbor, SC at a maximum concentration of 0.001 microg/L, substantially lower than the determined toxicity values. These findings suggest triclosan poses low acute toxicity risk to estuarine organisms; however, the potential for chronic, sublethal, and metabolite effects should be investigated.

Effects of the anti-fouling herbicide Irgarol 1051 on two life stages of the grass shrimp, Palaemonetes pugio

This study investigated lethal and sublethal effects (glutathione, lipid peroxidation, cholesterol, and acetylcholinesterase) of the anti-fouling herbicide Irgarol 1051 on larval and adult grass shrimp (Palaemonetes pugio). The 96-hour LC50 test for larvae resulted in an estimated LC50 of 1.52 mg/L (95% confidence interval [CI] 1.26–1.85 mg/L). The adult 96-h LC50 was 2.46 mg/L (95% CI = 2.07–2.93 mg/L). Glutathione, lipid peroxidation, cholesterol and acetylcholinesterase levels were not significantly affected in adult grass shrimp by exposure of up to 3.00 mg/L irgarol. Lipid peroxidation and acetylcholinesterase levels in the larvae were significantly higher than controls in the highest irgarol exposures of 1.0 and 2.0 mg/L, respectively. Cholesterol levels were significantly reduced in larvae in all four irgarol concentrations tested while glutathione levels were not significantly affected in larvae. Both lethal and sublethal effects associated with irgarol exposure were only observed at concentrations well above those reported in the environment.

Effects of coal combustion residues on survival, antioxidant potential, and genotoxicity resulting from full-lifecycle exposure of grass shrimp (Palaemonetes pugio Holthius)

Coal combustion residues (CCRs), largely derived from coal-fired electrical generation, are rich in numerous trace elements that have the potential to induce sublethal effects including oxidative stress, alterations in antioxidant status and DNA single strand breaks (SSB). CCRs are frequently discharged into natural and man-made aquatic systems. As the effects of CCRs have received relatively little attention in estuarine systems, the estuarine grass shrimp, Palaemonetes pugio, was chosen for this study. Grass shrimp were exposed in the laboratory to CCR-enriched sediments and food over a full life cycle. Survival to metamorphosis was significantly reduced in CCR-exposed larvae (17+/-4 versus 70+/-13% in the controls) but not in the juveniles or adults. The COMET assay, a general but sensitive assay for genotoxicity, was used to quantify DNA SSB in the adults. Total antioxidant potential was examined to assess the overall antioxidant scavenging capacity of CCR-exposed and non-exposed adult grass shrimp. Grass shrimp exposed to CCR significantly accumulated selenium and cadmium compared to unexposed shrimp, although an inverse relationship was seen for mercury accumulation. Chronic CCR exposure caused DNA SSB in hepatopancreas cells, as evidenced by the significantly increased percent tail DNA, tail moment, and tail length as compared to reference shrimp. However, no significant difference was observed in total antioxidant potential. Our findings suggest that genotoxicity may be an important mode of toxicity of CCR, and that DNA SSB may serve as a useful biomarker of exposure and effect of this very common, complex waste stream.

Acute Toxicity and Acetylcholinesterase Inhibition in Grass Shrimp (Palaemonetes pugio) and Oysters (Crassostrea virginica) Exposed to the Organophosphate Dichlorvos: Laboratory and Field Studies

The use of various organophosphates to control mosquito populations is a common practice across the globe. We review the literature (LC50s) on dichlorvos, the primary breakdown product of Dibrom, and use laboratory and field experiments to determine the lethal and sublethal (bioassays) effects of dichlorvos on two widely distributed and ecologically important estuarine invertebrate species, the marsh grass shrimp, Palaemonetes pugio and the Eastern oyster, Crassostrea virginica. Laboratory results based on LC50s and sublethal acetylcholinesterase (AChE) inhibition activity bioassays indicate that adult grass shrimp are more sensitive (approximately 500 x ) to dichlorvos than juvenile oysters. Although potentially an important factor for intertidal or shallow-dwelling estuarine organisms, the toxicity of dichlorvos was not enhanced in the presence of simulated sunlight for adult P. pugio. The most notable decreases in AChE activity were for grass shrimp and oysters exposed to dichlorvos concentrations above those considered ecologically relevant. In field experiments, both species were deployed in cages in unsprayed (n = 2) and sprayed (n = 3) sites and water samples collected pre- and post-spraying. Quantifiable dichlorvos levels were measured at the two narrowest creek treatment sites following mosquito spraying, suggesting that overspray can occur and there was evidence of a sublethal AChE response at these same sites. However, experiments at the widest creek revealed no measurable dichlorvos or sublethal responses. Results from this research suggest that adult grass shrimp are more sensitive to dichlorvos than juvenile oysters. Spraying near small tidal creeks may have measurable impacts on resident species, while larger (wider) creeks appear to be capable of buffering organisms from transient fluxes of mosquito control agents that may enter the system.

Toxicity of the Mosquito Control Pesticide Scourge® to Adult and Larval Grass Shrimp (Palaemonetes pugio)

This study investigated the toxicity of various concentrations of technical resmethrin and Scourge® on adult and larval Palaemonetes pugio, a common grass shrimp species. Two types of tests were conducted for each of the resmethrin formulations using adult and larval grass shrimp life stages, a 96-h static renewal aqueous test without sediment, and a 24-h static nonrenewal aqueous test with sediment. For resmethrin, the 96-h aqueous LC50 value for adult shrimp was 0.53 μg/L (95% confidence interval (CI): 0.46–0.60 μg/L), and for larval shrimp was 0.35 μg/L (95% CI: 0.28–0.42 μg/L). In the presence of sediment, technical resmethrin produced a 24-h LC50 value for adult shrimp of 5.44 μg/L (95% CI: 4.52–6.55 μg/L), and for larval shrimp of 2.15 μg/L (95% CI: 1.35–3.43 μg/L). For Scourge®, the 96-h aqueous LC50 for adult shrimp was 2.08 μg/L (95% CI: 1.70–2.54 μg/L), and for larval shrimp was 0.36 μg/L (95% CI: 0.24–0.55 μg/L). The 24-h sediment test yielded an LC50 value of 16.12 μg/L (95% CI: 14.79–17.57 μg/L) for adult shrimp, and 14.16 μg/L (95% CI: 12.21–16.43 μg/L) for larvae. Adjusted LC50 values to reflect the 18% resmethrin concentration in Scourge® are 0.37 μg/L (adult), 0.07 μg/L (larvae) for the 96-h aqueous test, and 2.90 μg/L (adult), 2.6 μg/L (larvae) for the 24-h sediment test. Larval grass shrimp were more sensitive to technical resmethrin and Scourge® than the adult life stage. The results also demonstrate that synergized resmethrin is more toxic to P. pugio than the nonsynergized form, and that the presence of sediment decreases the toxicity of both resmethrin and Scourge®

Reproductive alterations in adult grass shrimp, Palaemonetes pugio, following sublethal, chronic endosulfan exposure

Grass shrimp ( Palaemonetes pugio) populations exposed to anthropogenic contaminant sources in South Carolina (SC) have reduced densities when compared with populations at SC-reference sites. This laboratory study examined the effects of a commonly used agricultural insecticide, endosulfan, on grass shrimp reproduction. Reproductively active grass shrimp were chronically exposed to sublethal concentrations of endosulfan (200 or 400 ng/l) for 43 days. The cumulative number of females that became gravid and the rate at which they became gravid were measured. Endosulfan exposure reduced the cumulative number of gravid females by 31% in the 200 ng/l exposure and 39% in the 400 ng/l exposure. The first appearance of gravid females in the population was significantly delayed in treated populations compared with the control treatment in a dose dependent manner. Clutch size in these gravid females was not significantly different among the treatments. Additionally, there was no difference in the onset of reproduction in the treated populations. These results implicate a population reduction due to a decrease in the overall number of females becoming gravid in a population over time, not a reduction in clutch size per individual. While the mechanisms of action have yet to be defined, these results indicate that sublethal endosulfan concentrations may have a negative effect on grass shrimp reproductive biology.

Characterization of cholinesterase activity in tissues of the grass shrimp ( Palaemonetes pugio)

Inhibition of cholinesterases in estuarine organisms is a useful indicator of organophosphorus insecticide exposure. Recent research has indicated that more than one cholinesterase may be present in tissues of estuarine organisms and that these cholinesterases vary in their sensitivity to organophosphorus insecticides. Baseline cholinesterase activity for adult grass shrimp ( Palaemonetes pugio) was determined with four substrates (acetylthiocholine iodide, acetyl-β-methylthiocholine iodide, propionylthiocholine iodide, S-butyrylthiocholine iodide) and three cholinesterase inhibitors (eserine sulfate, tetraisopropyl pyrophosphoramide, 1,5-bis(4-allyldimethyl-ammoniumphenyl) pentan-3-one dibromide). Cholinesterase activity was the highest with acetylthiocholine iodide at 57.42 nmol/mg P/min and the lowest with acetyl-β-methylthiocholine iodide at 6.17 nmol/mg P/min. The four substrates tested were not inhibited by tetraisopropyl pyrophosphoramide. Eserine sulfate significantly inhibited cholinesterase activity against all substrates except S-butyrylthiocholine iodide. The results of substrate specificity and cholinesterase inhibition in grass shrimp indicate that acetylthiocholine iodide is the most appropriate substrate for assessing cholinesterase inhibition in grass shrimp.

Determination of acute mortality in adults and sublethal embryo responses of Palaemonetes pugio to endosulfan and methoprene exposure

Adult grass shrimp ( Palaemonetes pugio) were exposed to endosulfan or methoprene for 96 h and LC 50 values were calculated. Male and female P. pugio cohorts were also exposed to endosulfan for 96 h in an attempt to determine potential differences in sensitivity between the sexes. Results from the methoprene exposure indicated that this pesticide was not acutely toxic to adult grass shrimp at 1 mg l −1. Due to the lack of sensitivity, sex specific tests with methoprene were not performed. The calculated LC 50 for a population of grass shrimp, including both males and females exposed to endosulfan, was 0.62 μg l −1. The LC 50 determinations for the sex specific tests were 0.92 μg l −1 for males and 1.99 μg l −1 for females. Following these acute exposures, reproductively active grass shrimp were chronically exposed to 200 ng l −1 endosulfan or 1 mg l −1 methoprene and were allowed to produce embryos. The resulting embryos were assessed for potential sublethal toxicity. There were no observed differences in the percent successfully hatching or larval mortality 3-days post hatch among the treatments. However, endosulfan treated embryos had a significantly increased hatching time (9.76 days compared to 8.72 days in controls). Methoprene treated embryos also took longer to hatch (9.55 days), but this delay was not significantly different from controls. These findings suggest that endosulfan may preferentially affect male grass shrimp and exposed female grass shrimp may produce embryos with delayed hatching times.

The sensitivity of grass shrimp, Palaemonetespugio, embryos to organophosphate pesticide induced acetylcholinesterase inhibition

Grass shrimp, Palaemonetes pugio, are common inhabitants of salt marshes along the Atlantic and Gulf coasts of North America. Grass shrimp embryos are brooded externally on the abdomen of adult females for about 2 weeks prior to hatching. In South Carolina, the spring spawning period for grass shrimp coincides with the period of peak pesticide application on crops grown along the South Carolina coast. Thus, grass shrimp of all developmental stages are at risk of exposure to pesticides present in nonpoint source agricultural runoff. Organophosphate (OP) insecticides are commonly applied agricultural chemicals which produce toxicity by inhibiting the nervous system enzyme, acetylcholinesterase (AChE). The purpose of this study was to examine the development of AChE activity in grass shrimp embryos and to assess their sensitivity to OP-induced AChE inhibition. Embryos were exposed for 24 h to either chlorpyrifos or malathion. All exposure concentrations were nominal and ranged from 0 to 2.00 μg l −1 for chlorpyrifos and from 0 to 120.00 μg l −1 for malathion. Quantifiable levels of AChE activity first appeared at Stage V of development and increased as embryonic development progressed. AChE inhibition by the OPs was assessed in Stage VI and Stage VII embryos. Both stages of embryos were more sensitive to chlorpyrifos than malathion. The 24-h Effective Concentration (EC 50) values for chlorpyrifos were 0.49 μg l −1 (95% C.I.=0.33–0.77 μg l −1) and 0.36 μg l −1 (95% C.I.=0.33–0.38 μg l −1) for Stage VI and Stage VII embryos, respectively. In comparison, malathion 24-h EC 50 values were 55.53 μg l −1 (95% C.I.=22.08–80.73 μg l −1) for Stage VI embryos and 29.93 μg l −1 (95% C.I.=25.22–44.22 μg l −1) for Stage VII embryos. For both OPs, there were no significant differences in the EC 50 values calculated for Stage VI and Stage VII embryos; however, AChE inhibition was significantly ( P≤0.05) greater in Stage VII embryos at the two highest exposure concentrations for each insecticide. A comparison of the results of these embryo tests with those found for adult and larval toxicity tests indicated that embryos were at least as sensitive to both the OPs as larval and adult grass shrimp. Embryo bioassays provide a number of important advantages over traditional laboratory toxicity tests including reduced laboratory space requirements, large numbers of embryos from a few ovigerous females, and small volumes of waste.

CONSUMPTION OF MICROALGAE BY THE GRASS SHRIMP PALAEMONETES PUGIO

The ability of Palaemonetes pugio to consume microalgae in marshes of Spartina alterniflora was examined in field collections and laboratory experiments. Photosynthetic pigment (chlorophyll a and phaeopigments) content in P. pugio was measured after shrimp exoskeletons were removed to ensure that pigments were primarily from gut contents. Gut pigment was detectable in all 88 fieldcollected specimens examined, and was found at high levels in many individuals (much higher than shrimp starved for 24 h). Pigment content was positively correlated with shrimp size and was highly variable. Adults contained from 0.056-3.03 and subadults from 0.016-2.41 gg pigment shrimp- , 14% of which was in the form of chlorophyll a. Highest pigment levels were associated with adult grass shrimp collected at 1500 h; pigment content at 0300 h was significantly lower. The ability of P. pugio to consume microalgae from various sources was examined in the laboratory with '4C-labeled microalgae and gut-pigment analysis. Palaemonetes pugio consumed two species of cultured, '4C-labeled phytoplankton, but with low efficiency at even very high concentrations. For Thalassiosira weissflogii, 2.5%, and for Isochrysis galbana, 0.07% of the available cells were consumed in 1-h trials at high concentrations of algae. Unvegetated, mudflat sediment (with a high abundance of benthic algae) was collected and labeled with '4C bicarbonate. Incorporation of label above background was detected in a low percentage of shrimp, and average grazing rates were very low (near detection limits), suggesting that grass shrimp are not efficient grazers of the microphytobenthos. Spartina alterniflora stems with '4C-labeled epiphytic algae were also offered to grass shrimp. Grazing was highly efficient (a substantial fraction of the available epiphytic algae was consumed), and pigment content was similar to field-collected shrimp. These results suggest that microalgae may significantly contribute to the diet of grass shrimp, and that a likely source for the photosynthetic pigments in the gut contents of field-collected P. pugio is the epiphytic algae on stems of Spartina alternifiora.

Effects of Salinity and Temperature on the Bioenergetics of Adult Stages of the Grass Shrimp (Palaemonetes pugio Holthuis) from the North Inlet Estuary, South Carolina

The influence of temperature, salinity, body size, and sex on the bioenergetics and life-history parameters of grass shrimp (Palaemonetes pugio Holthuis) was determined under laboratory conditions. Sex and/or size show a highly significant effect only on energy allocation for metabolism. Temperature, salinity and their interaction are the main effects on most energetic parameters, with temperature showing the strongest influence on both life-history parameters and energetics. The adult grass shrimp's energy budget is different from that of larval or juvenile stages, in that most of the energy ingested by adults is allocated to reproduction. Under optimal conditions (25°C and 28‰), allocation of ingested energy in the adult grass shrimp is 51.7% for reproduction, 25.4% for respiration, 14.1% for somatic growth, 4.8% for exoskeletons, and 1.8% for excretion.