Effects of Exposure of Pink Shrimp, Farfantepenaeus Duorarum, Larvae to Macondo Canyon 252 Crude Oil and the Corexit Dispersant

The release of oil into the Gulf of Mexico (GOM) during the Deepwater Horizon event coincided with the white and pink shrimp spawning season. To determine the potential impact on shrimp larvae a series of static acute (24–96 h) toxicity studies with water accommodated fractions (WAFs) of Macondo Canyon (MC) 252 crude oil, the Corexit 9500A dispersant, and chemically enhanced WAFS (CEWAFs) were conducted with nauplii, zoea, mysid, and postlarval Farfantepenaeus duorarum. Median lethal concentrations (LC 50) were calculated and behavior responses (swimming, molting, light sensitivity) evaluated. Impacts were life stage dependent with zoea being the most sensitive. Behavioral responses for all stages, except postlarvae, occurred at below LC 50 values. Dispersants had the greatest negative impact while WAFs had the least. No short-term effects (survival, growth) were noted for nauplii exposed to sub-lethal CEWAFs 39 days post-exposure. This study points to the importance of evaluating multiple life stages to assess population effects following contaminant exposure and further, that the use of dispersants as a method of oil removal increases oil toxicity.


Introduction
The Gulf of Mexico (GOM) has some of the most productive coastal bodies of water in the world, making it a major source for the U.S. seafood industry and the most economically important of all domestic commercial seafood harvesting sectors [1].One of the most important GOM fisheries is the shrimp industry, extending from Brownsville, Texas to Key West, Florida.In 2010 the GOM provided 68% of U.S.-harvested shrimp with a total dockside value of $281 million [2].The fishery consists of three major species: brown shrimp (Farfantepenaeus aztecus), pink shrimp (Farfantepenaeus duorarum), and white shrimp (Litopenaeus setiferus) [3].Both the pink and white shrimp began migrating offshore to spawn in the spring, with continued spawning migration throughout the summer (pink shrimp) and fall (white shrimp), while the spawning season for brown shrimp is less defined in terms of season [4][5][6].Fertilized eggs pass through nauplii, zoea, and mysis stages in offshore waters before migrating back to coastal estuaries as postlarvae within three to four weeks, throughout the spring and summer, dependent on species.On April 20, 2010 the Deepwater Horizon (DWH) oil platform exploded resulting in 200 million gallons of oil being released into the GOM until the well was capped on July 15 [7,8].It is estimated that 100,000 km 2 of the GOM was affected by the spill, which coincided with the spring spawning season of a number of key GOM species, including shrimp [3][4][5][6]8].In an effort to contain the spill and prevent the oil from reaching the shoreline, booms, skimmers, burning, direct recovery, and dispersants were used [7].It has been calculated that 1.9 million gallons of dispersant (Corexit 9527 and Corexit 9500A) were used [9].Dispersants do not remove oil from water but act to break the oil into smaller droplets that are more readily dispersed into the water column [10].While dispersant use decreases the amount of surface oil lessening the amount of oil that reaches shorelines, the small dispersed droplets that remain in the water column are now made available to pelagic organisms that inhabit the water column [7].
Most studies concentrate on one stage of development.Early life stages are typically more sensitive to pollutants than juveniles or adults and may be impacted at concentrations that, at least on the surface, do not cause acute mortality in juveniles or adults.Yet, in the long term survival may be impacted by behavioral modifications such as reduced activity that may affect predator avoidance and food intake [27][28][29][30][31].The aim of our study was to determine what concentration of MC252 oil, Corexit dispersant and chemically dispersed MC252 would adversely affect the survival, development, and behavioral responses of the four major larval stages of shrimp (nauplii, zoea, mysis, and postlarvae).Behavioral responses included swimming activity, light response, feeding, and molting.

Solution Preparation
Oil and dispersant solutions for all experiments were prepared with MC252 oil (British Petroleum Company, BP PLC, London, UK) or Corexit 9500A dispersant (Nalco/Exxon Energy Chemicals, Sugarland, TX, USA).Solutions were prepared following CROSERF procedures [32,33].Prior to solution preparation, crude oil was physically weathered in the lab for 24 h by placement of oil in a beaker on a stir plate and mixing with a magnetic stir bar in the dark in a chemical fume hood.Stock solutions of water accommodated fractions (WAFs) of crude oil (2 g L ´1), dispersant (2 g L ´1) and chemically enhanced WAFs (CEWAFs) (1:10 ratio) were prepared in 2 L flasks of filtered, UV treated seawater (28 ppt), covered, and mixed at moderate intensity (25% vortex) for 24 h.Stock solutions were allowed to settle for 3 h prior to preparation of working solutions.

PAH Analysis
Samples of oil and dispersed oil stocks (2 g L ´1) used in the acute toxicity experiments were preserved in glass jars with dichloromethane (1:10 v/v) and extracted using modified EPA method 3510C (Mote Marine Laboratory, Sarasota, FL).Polycyclic aromatic hydrocarbons (PAHs, parent compounds, and homologues) were analyzed using GC/MS (Agilent 7890A/5975C), modified EPA method 8260.Total petroleum hydrocarbons (TPH) n-C9 to n-C42 were analyzed using a GC with a flame ionization detector (FID, Agilent 7890A, Agilent Technologies Inc., Santa Clara, CA, USA).

Behavioral Responses
Several experiments were conducted to evaluate behavioral responses.Activity level (swimming behavior) and molting frequency were evaluated for M 1 and Pl 6 stages for both WAF (0, 100, 200, 400, 800, and 1200 mg L ´1) and CEWAF (0, 6.25, 12.5, 25, 50, and 100 mg L ´1) exposures, with five replicates per treatment group, and 12 shrimp per replicate.Activity level was scored on a scale of 1-4: 1 = actively moving, 2 = moderately active, 3 = lethargic/moving appendages only, 4 = dead.Molting frequency was calculated as the percent of shrimp that molted compared to the total number of shrimp.
Subsequent behavioral response experiments were conducted for CEWAF exposures only for N 5 , Z 1 , Z 3 , and M 2 stages.Concentrations used varied based on life stage evaluated, with five replicates per treatment group, 15 shrimp per replicate.CEWAF concentrations of 0, 6.25, 12.5, 25, 50, and 100 mg L

´1
were used for nauplii and mysis stages, while concentrations were adjusted to 0, 3.125, 6.25, 12.5, and 25 mg L ´1 for the more sensitive proto-zoeal stages as determined by the LC 50 experiments.Behavioral parameters assessed included activity and molting as defined above, and feeding and photo-taxic response.Feeding was scored on a 1-4 scale: 1 = actively feeding (food in gut, fecal strands), 2 = 50% or less feeding, 3 = 25% or less feeding, 4 = 0% feeding.Photo-taxic response was evaluated by placing a light source to one side of the container and noting the proportion of shrimp that were attracted to the light (N 5 , Z 1 ) or avoided the light (Z 3 , M 2 ).Photo-taxic response was scored on a 1-3 scale: for N 5 , Z 1 -1 = actively moving towards light, 2 = sluggish response, 3 = no response; for Z 3 , M 2 -1 = actively moving away from light, 2 = slow avoidance response, 3 = no response.The proportion of shrimp that underwent metamorphosis to the next stage was also noted in these experiments.

Sub-Lethal Toxicity Bioassays
Approximately 10,000 L. duorarum nauplii were evenly divided between one of six 13 L buckets containing either filtered, UV treated HBOI salt well water (N = 3) or 23 mg L ´1 CEWAF (N = 3).During the 24 h exposure, shrimp were fed Isochrysis galbana during experimental exposure at a rate of 15,000 (or ˆ10 3 ) cells/mL.Surviving shrimp from both control buckets and treatment buckets were sieved, combined and then redistributed into one of four 400-L larval rearing tanks (two control, two treatment) containing filtered, UV treated HBOI salt well water.On day 1 (24 h exposure) and on alternate days, seven to eight shrimp were randomly removed from each of the four tanks (15 control, 15 exposed) for 39 days, collected and placed in vials containing 10% NBT formalin.After a 24 h fixation period, shrimp were placed in 70% ethanol, examined microscopically, and photographed (Infinity 2 digital camera, Luminera Co., Sachse, TX, USA).Developmental stage was recorded and length measurements averaged for each data point using Infinity Analyze (Luminera Co.).

PAH Analysis
The total PAH level in the CEWAF stock solution (1429 µg L ´1) was three times greater than that of the WAF stock solution (452 µg L ´1) while the TPH level (62,613 µg L ´1) in the CEWAF solution was 25 times greater than that of the WAF stock solution (2467 µg L ´1) (Table 1).The predominant compound was napthalene, which made up 83.5% of the compounds in the WAF and 65% of the compounds in the CEWAF stock solution.Compounds containing three and four carbon rings (e.g., anthracene, fluorene, pyrene, chrysene, and phenanthrene) were approximately two times greater in the CEWAF compared to the WAF solution.Dispersants had the greatest impact on survival of all larval stages while WAFs had the least, with the proto-zoeal (Z 1 ) stage exhibiting the greatest sensitivity and the postlarval (Pl 6 ) stage the least sensitivity to all three contaminants (Table 2).The dispersant had the greatest impact on Z 1 shrimp (3.1 mg L ´1, LC 50 , 24 h; 2.5 mg L ´1 LC 50 , 48 h; 100% mortality, 72 h), with all other stages having similar LC 50 values at 24 h (21-33 mg L ´1) (Table 2).LC 50 values continued to decrease for nauplii (N 2 ) and mysis (M 1 ) over time, but not for Pl 6 (22-28 mg L ´1).CEWAFs, likewise, had the greatest impact on Z 1 shrimp (15.4 mg L ´1, LC 50 , 24 h; 100% mortality, 48 h), with all other stages having similar LC 50 values at 24 h (81.5-100 mg L ´1) (Table 2).LC 50 values continued to decrease for all stages over time, but less for Pl 6 (44 mg L ´1, LC 50 , 96 h) than for M 1 (8.5 mg L ´1, LC 50 , 96 h).WAFs had the least impact on all life stages, with Z 1 being the most sensitive (67.4 mg L ´1, LC 50 , 24 h; 25.5 mg L ´1 LC 50 , 48 h; 100% mortality, 72 h) and Pl 6 the least, with no LC 50 value determined for concentrations tested (>400 mg L ´1, 96 h) (Table 2).PAH and TPH values for nominal CEWAFs and WAFs could only be compared at 24 h for Z 1 and 96 h for M 1 (Table 2).Toxicity of CEWAFs and WAFs were similarly toxic when PAH concentrations were compared, however WAFs were more toxic than CEWAFs when TPH concentrations were compared.

Survival
No difference in survival was seen between control and treatment groups.Survival was approximately 25% for both groups at day 39 post exposure.

Growth
No significant difference was seen in growth.Growth, as defined by total body length, was not significantly different between control and exposed groups from N 5 to Pl 28 (F 1,12 = 0.42, p = 0.5302) (Figure 6).

Developmental Stages
A slight developmental delay was seen between the control and exposed treatments on day five.Development from Z3 to M1 proceeded at a slower pace in the exposed groups resulting in a delayed development from M2 to M3 from day seven to nine.By day 11, development was similar and both groups reached Pl1 (Figure 7).

Developmental Stages
A slight developmental delay was seen between the control and exposed treatments on day five.Development from Z 3 to M 1 proceeded at a slower pace in the exposed groups resulting in a delayed development from M 2 to M 3 from day seven to nine.By day 11, development was similar and both groups reached Pl 1 (Figure 7).

Developmental Stages
A slight developmental delay was seen between the control and exposed treatments on day five.Development from Z3 to M1 proceeded at a slower pace in the exposed groups resulting in a delayed development from M2 to M3 from day seven to nine.By day 11, development was similar and both groups reached Pl1 (Figure 7).

Discussion
Exposure of various stages (nauplii, zoea, mysis, and postlarvae) of F. duorarum shrimp larvae to MC252 surrogate oil from the Deepwater Horizon well, and the primary dispersant, Corexit 9500A, used during the spill, adversely affected survival and behavior.Zoea (Z 1 ) were more sensitive to contaminant effects than other life stages.Dispersant exposure had a more pronounced affect, than did water accommodated fractions of crude oil (WAFs) or chemically enhanced WAFs (CEWAFs), and affected all larval stages equally and negatively.CEWAFs generally had a more negative impact than WAFs.Effects were dose and exposure dependent, with short-term sublethal effects resulting in slight developmental delays, with no longer term consequences to growth or survival seen in the laboratory.
The water column is the most likely route of contaminant uptake following an oil spill and therefore the toxicity of oil within the water column are commonly measured by analyzing the amount of oil contaminants within the WAFs and CEWAFs.Oil used was artificially weathered, to decrease the amount of volatile organic compounds (benzene, toluene, ethylbenzene, xylene) that tend to evaporate readily, in order to more closely mimic the type of oil that most organisms in the water column would likely encounter.
Concentrations of WAFs and CEWAFs used in this study represent moderate to maximum environmentally relevant levels based on reported literature.Oil levels ranging from 20 to 600 mg L ´1 and dispersed oil levels ranging from 25 to 75 mg L ´1 have been reported in the water column 24 h after a spill and may be a magnitude greater immediately following a spill [34].Dispersants are used in relatively few spill events due in part to unfavorable conditions that are necessary for dispersants to work effectively [35].Due to the magnitude of the Deepwater Horizon event large amounts of dispersant were used.Although the majority of water column organisms were likely exposed to either oil or dispersant and oil mixtures during the event it is possible that some organisms were inadvertently exposed to dispersants alone.Levels of dispersant used in these experiments were considered relevant based on reported literature.Dispersant concentrations of 0.1-15 mg L ´1 have been reported in the field [10,31,36], although initial dispersant concentrations are generally below 10 mg L ´1 (maximum range 5-15 mg L ´1), dropping to less than 1 mg L ´1 in a few hours [24,36].

Survival
Survival is often used as the endpoint to determine oil toxicity effects.Previous studies have reported lethal concentration (LC 50 ) values for crustaceans, including shrimp, following acute exposure to oil, however, researchers typically focus on only one life stage and one contaminant (oil, dispersed oil, or dispersant).Early life stages tend to be more susceptible to toxic compounds than adults, which may negatively affect populations in affected areas, especially in the short-term.This study is unique in studying the effects of various shrimp larval stages simultaneously, to oil, dispersed oil, and a dispersant, and reporting behavioral responses along with LC 50 values.We found that larval shrimp mortality varied dependent on developmental stage, and was not age dependent as zoea were more sensitive than nauplii.This is likely the result of differing feeding modalities at these two larval stages.Nauplii have undeveloped mouth parts and rely on their yolk sac for nutrition, while zoea are indiscriminate feeders and consume anything large enough to enter their mouth, and mysis seek out and capture their food [37].Feed sources provided during this study varied: nauplii and zoea were provided with algae, mysis with rotifers, and postlarvae with commercial pellets.Exposure of WAFs or CEWAFs through either the water column or exposed feed resulted in similar alterations of metabolic enzymes in fish [38].The addition of small quantities of feed may have resulted in larvae being exposed to oil contaminants through both the water column via the gills and the digestive tract via ingestion.We believed that administration of some feed was necessary to eliminate the likelihood of starvation as the cause of death as larvae, unlike postlarvae and juvenile shrimp, need to eat continuously.Preliminary experiments conducted with untreated and unfed larvae resulted in notable lethargy at 24 h and 50%-90% mortality of nauplii and zoea, respectively, at 48 h.
We noted that dispersant exposure negatively impacted all four larval stages at similar concentrations, although zoea were the most adversely affected, with all Z 1 shrimp dead by 48 h.Our reported values for 96 h exposures for F. duorarum M 1 and Pl 6 for Corexit 9500A, were similar to those previously reported for Corexit 9527 for L. setiferus postlarvae (96 h LC 50 , 12-31 mg L ´1) [11,13].Similar LC 50 values (3.5-83 mg L ´1) have been reported following 48 to 96 h of exposure of other postlaval and juvenile crustaceans to Corexit 9500 [10,26,32,39,40].
Exposure to nominal concentrations of CEWAFs resulted in increased mortality compared to WAFs in our study.The majority of researchers have concluded that chemically dispersed oil is more toxic than physically dispersed oil [35].However, reporting methods (nominal, PAH, TPH), may impact researchers conclusions [41].In our study, when PAH, rather than nominal values were compared, the toxicity of WAFs and CEWAFs were equivalent, although WAFs appeared to be more toxic when TPHs were compared, similar to that seen for L. setiferus juveniles [13].The increase in toxicity of CEWAFs is attributed to increased availability of PAHs in the water column through the creation of a large number of small oil droplets [35].The PAH levels in the prepared CEWAF stocks in our study were three times greater and the TPH levels 25 times greater than in the WAF stocks.Oil droplets were observed in fecal strands and the digestive tract and fecal strands of some zoea and mysis larvae exposed to CEWAF concentrations ě 25 mg L ´1 indicating ingestion of oiled particulates in larvae that were still feeding.At 50-100 mg L ´1 oil was noted on appendages and molts of some shrimp larvae, implicating narcosis (PAH toxicity) and perhaps restricted motility as likely causes of mortality.
Although previous researchers have compared survival of crustaceans exposed to WAFs and CEWAFs by reporting LC 50 values, there is little consistency in the reporting method (nominal, TPH, PAH) which makes comparison difficult.In this study, we attempted to make cross-comparison easier by listing LC 50 values for all three parameters.The majority of early crustacean studies were conducted with larval mysid shrimp, Americanus (Mysiopsis) bahia, where TPH values were reported, and 96 h LC 50 values ranged from 0.15-83 mg L ´1 WAFs and 0.5-120 mg L ´1 CEWAFs [26,42,43].That these results differ somewhat may be explained by variation in exposure methods used (constant, spiked, static renewal), however, each study reported a similar toxicity for WAFs and CEWAFs based on comparison of TPHs.Similar results have been reported for Americamysis (Holmesimysis) costata (1-35 mg L ´1 WAF, 8-33 mg L ´1 CEWAF, 96 h) and L. setiferus juveniles (6.5 mg L ´1 WAFs, 5-7.5 mg L ´1 CEWAFs, 96 h) [13,25,44].In contrast, we report a 96 h TPH toxicity with larval F. duorarum (0.029-0.037 mg L ´1 WAFs, 0.27-1.38mg L ´1 CEWAFs), indicating that WAFs were more toxic.This is in contrast to 96 h LC 50 PAH values, in which little difference in toxicity was seen for WAFs and CEWAFs.In the wake of the Deepwater Horizon event, reported concentrations of TPAHs in May 2010 varied greatly dependent on site and sampling depth and ranged from 0 to 146 mg L ´1 at the wellhead, 0 to 0.9 mg L ´1 in field collected WAFs, 0 to 18 mg L ´1 in field collected CEWAFs, and 0 to 0.17 µg L ´1 in shoreline samples [45][46][47].Following capping of the well in July concentrations in collected sample were significantly lower at all sites and depths.

Behavior
Factors in addition to mortality need to be considered when assessing contaminant effects, as behavioral responses, such as swimming ability, and response to stimuli affect the ability to locate prey or escape predation.Some researchers have reported behavioral inhibitory (IC 50 ) effects following exposure to lower levels of oil contaminants than at which mortality (LC 50 ) occurs.Changes in behavior due to sublethal exposures are considered to be the most sensitive indicators of environmental disturbance, and yet are among the least studied effects with regards to toxicity [48].A variety of behavioral responses of marine organisms to pollutants, including oil, such as motivation (e.g., feeding response), sensory responses (e.g., phototaxis), and motor activity (e.g., swimming performance) are given in a summary of early work [49].Examples of depressed feeding responses associated with PAHs have been shown in a variety of invertebrates including rotifers, crabs, and shrimp [50][51][52].Examples of differential phototaxic responses associated with invertebrates have been reported with crabs and barnacles [27,53].Invertebrates, such as crabs, shrimp, and barnacles, have also been shown to exhibit erratic swimming behavior in response to oil contaminants [27,53,54] and it has been postulated that differential sensory and motor responses that resulted in differential depth distribution might affect larval distribution and recruitment via directional current activity [53].
Some researchers have reported behavioral effects, such as reduction of swimming ability, at lower than LC 50 concentrations [15,28].Others have reported similar LC 50 and EC 50 values following exposure to oil contaminants, including swimming ability, settlement behavior and burying behavior [27,31,39].Decreased swimming behavior is likely a result of narcosis typically seen in acute toxicity of high short-term exposures to naphthalene [55].Narcotic chemicals affect the lipid bilayer in membranes reducing activity and the ability to react to stimuli, which may ultimately lead to mortality [56].However narcosis does not account for other reported toxic effects such as deformities, edema, and cardiovascular effects [57] Regardless of whether this is the result of narcosis, or some other phenomenon the end result is that decreased swimming ability results in decreased ability to find food or escape predation, either of which will likely reduce survival.
In this study, swimming ability was significantly decreased for M 1 larvae exposed to both CEWAF and WAF at all exposure times.Exposure to CEWAFs caused an initial decrease in activity at concentrations that were two times less than LC 50 values at 24 h and three to four less than LC 50 values at 48 and 72 h.Similar results were seen with N 5 , Z 1 , Z 3, and M 2 larvae exposed to CEWAFs.However, results were stage dependent, in that Z 1 larvae were the most sensitive, whereas, older shrimp (Pl 6 ) did not exhibit reduced swimming ability at sublethal concentrations of either CEWAFs or WAFs.Although significant differences in activity in Pl 6 shrimp exposed to CEWAFs at 48 and 96 h and to WAFs at 48 h occurred, they were not dose dependent and thought to be due to water quality issues.Phototaxic response, whether attraction to light (N 5 , Z 1 ) or avoidance (Z 3 , M 2 ) followed a similar pattern to that seen with swimming ability, and this manifested itself in reduced feeding response for N 5 , Z 1 stages.

Molting
Molting frequency increased in response to CEWAFs at 24 or 48 h post-exposure.Response was stage dependent with Z 1 and Z 3 larvae responding at lower concentrations than M 1 or M 2 larvae.It is postulated that this behavior is a stress response to compounds present in CEWAFs or an attempt by the shrimp to rid itself of oil adhering to the carapace or appendanges.Molting was not associated with metamorphosis to the next stage, as, except for the Z 3 stage, metamorphosis occurred at the same rate as the controls or was somewhat delayed, and control shrimp molted less frequently.An unwanted side effect of this response might be increased susceptibility to oil contaminants.Crustaceans are more susceptible to environmental stressors, including oil pollutants during molting, and may experience increased mortality [58,59].In the present study, molting frequency was not followed after 72-96 h post-exposure.PAHs has been shown to increase the length of the intermolt period, resulting in decreased molting in a variety of invertebrates [58,60,61].The use of additional measurements, such as biochemical endpoints, provide researchers with another set of tools for evaluating oil toxicity, allowing additional means of assessing the potential consequences of oil exposure for marine organisms, such as shrimp.

Sublethal Effects
Survival following sublethal exposures of invertebrates varies based on species and life stage [21,54,62,63].In the present study, F. duorarum nauplii exposed to sub-lethal amounts (23 mg L ´1, LC 10 ) of CEWAFs for 24 h showed no difference in survival compared to controls over 39 days and was approximately 25% for both groups.Shrimp were cultured in larval tanks specific to the penaeid shrimp industry and industry operational procedures followed.Due to the sensitivity of handling the zoeal stage, tanks are not drained, water is instead added to partially filled tanks to alleviate water quality issues.Unfortunately, Artemia proliferated in the tanks, competing with shrimp for resources, and causing low survival in tanks regardless of exposure.Delayed morality has been reported by other researchers in shrimp and crab larvae and embryos exposed to low levels of WSF for short periods with zoea being more sensitive than later developmental stages [21,54,62].Other research has shown no survival effects in crab zoea following either short term exposure or continuous exposure to low concentrations of oil [63].
Developmental delays have been reported for invertebrate larvae exposed to PAHs [21,[63][64][65].Developmental delays in invertebrates are typically accompanied by an increased period of intermolt following exposure to oil [58,64].We saw a slight developmental lag in the exposed group between Z 3 and M 1 resulting in delayed development to subsequent mysis stages, but both groups developed in postlarvae at roughly the same time.Other researchers have reported similar findings.Zoeal stages of the mud crab Rhithropanopeus harrisii increased following prolonged exposure to chronic, low levels of WSF, however short term exposure had no impact [63,64].Minor differences (one day lags) in development times were also reported in Pandalus borealis larvae in early stages of development but effects decreased at later stages [21].The more time spent in pelagic larval stages, as would occur as a result of delayed development, may result in increased likelihood of predation, impact dispersion, increase time to maturity, and therefore negatively affect population growth rate [66].
In our study, despite slight developmental lags, no lasting growth effects were seen in the exposed group at day 39 (Pl 28 ).This is consistent with results reported in the literature for Cancer irroratus larvae exposed to WAFs, R. harrisii larvae exposed to low concentrations of naphthalene, and grass shrimp Palaemonetes pugio exposed to sublethal WAFs [59,67,68].Other research has indicated decreased growth following exposure to oil.Decreased growth was reported in DWH exposed juvenile brown shrimp Farfantepenaeus aztecus but not in juvenile white shrimp L. setiferus exposed to the same waters [22].

Conclusions
This study shows that the concentrations of oil released and dispersant used during the DWH event could have negatively affected penaeid shrimp in the GOM, whether through altered behavioral responses, delayed development, or mortality.Even though the spill occurred during the spring spawning season and likely affected shrimp larvae at select locations, GOM shrimp populations as a whole do not appear to have been affected long term, perhaps in part due to fishery closures that were put in place following the spill [69].

Figure 5 .
Figure 5. Proportion of F. duorarum M1 shrimp larvae (±S.D.) that molted following exposure to CEWAFs.Treatment groups consisted of five replicates with 12 shrimp each.Numerical representations indicate statistical comparisons of exposure periods.Significant differences were seen at 24 and 48 h (p ≤ 0.0006).

Figure 7 .
Figure 7. Development of F. duorarum nauplii (N = 15) exposed to sublethal concentrations of dispersed oil (23 mg L −1 ) for 24 h.Life stage, (y axis), were assigned a numerical function for graphical

Table 1 .
Individual PAH and total TPH and PAH concentrations (µg L ´1) of 2 g L ´1 stock solutions of water accommodated fractions (WAF) and chemically enhanced water accommodated fractions (CEWAF) used to prepare working solutions used in the acute toxicity experiments.