Environ. Sci. Technol. 2009, 43, 7916–7923
Importance of Prey and Predator Feeding Behaviors for Trophic Transfer and Secondary Poisoning AMY C. BROOKS, PAUL N. GASKELL, AND LORRAINE L. MALTBY* Department of Animal & Plant Sciences, The University of Sheffield, Western Bank, Sheffield S10 2TN, United Kingdom
Received March 10, 2009. Revised manuscript received August 12, 2009. Accepted August 20, 2009.
Hydrophobic contaminants accumulate within aquatic sediments, hence pelagic predators may have limited direct contact with such compounds, but can be exposed via their benthic prey (i.e., via dietary exposure). Here we examine the importance of feeding behaviors of both prey (sediment ingesters or noningesters) and predators (piercers or engulfers) in determining the extent of dietary exposure and toxic effects. A freshwater macroinvertebrate system was used, consisting of two predator species, a piercer (Notonecta glauca) and an engulfer (Ischnura elegans), and three prey species, a sediment noningester (Cloe¨on dipterum) and two sediment ingesters (Asellus aquaticus, Chironomus riparius). Predators were fed prey previously exposed to artificial sediment dosed with 30 µg/g of 14C benzophenone. The piercer predator accumulated more benzophenone from sediment ingester compared to sediment noningester prey, whereas the engulfer predator accumulated a similar concentration for all three prey species. Toxic effects, in terms of reduced feeding rate, were only observed with the engulfer feeding on sediment noningesters, probably due to the interaction between the narcotic mode of action of benzophenone and predator hunting strategy. The importance of dietary exposure in risk assessments may therefore depend on exposure pathways of prey, feeding behaviors of predators, and the contaminant’s toxic mode of action.
Introduction Accumulation of contaminants can occur directly from aqueous exposure, by both adsorption to external surfaces and uptake via respiratory structures, e.g., gills (1). Uptake of contaminants through the diet has also been found to be an important exposure pathway in both aquatic (2) and terrestrial (3) systems. Dietary exposure may be a particular issue for contaminants that are persistent, bioaccumulative, and toxic (PBT), highlighted as a high risk group in the new EU chemicals strategy, REACH (4). Although bioaccumulation in itself is not a toxic effect, it does represent an exposure pathway for subsequent trophic levels and may result in secondary poisoning in predators (5). Risk assessment assumes that the body burden achieved by a prey animal represents the exposure dose to their predators (5). However, the extent of predator exposure may not only depend on * Corresponding author phone: +44 (0)114 222 4827; fax: +44 (0)114 222 0002; e-mail:
[email protected]. 7916
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how the contaminant is distributed within the tissues of their prey, but also depend on which parts of the prey the predator eats. A scenario in which dietary exposure to PBTs may be particularly important is sediment-associated aquatic food chains. Due to their hydrophobic nature, PBTs will move out of the water phase and become associated with sediments (6). Animals that reside in or on these sediments are therefore at risk of bioaccumulating PBTs, acting as vectors for transfer to predators that may themselves have limited direct contact with contaminated sediments (7, 8). For example, the consumption of Tubifex tubifex worms previously exposed to contaminated sediment resulted in significantly higher accumulation of hexachlorobenzene in three-spined sticklebacks (Gasterosteus aculeatus) compared to sedimentexposure alone (8). Benthic species vary in their ability to bioaccumulate contaminants from sediment (9, 10), due to differences in feeding rates, gut physiology, biotransformation, and excretion rates (9). As species that process large volumes of sediment generally achieve higher body burdens (10, 11), ingesters of sediment would generally be expected to have higher body burdens than benthic species that inhabit sediment but do not ingest it. Such differences in exposure pathway may also affect the distribution of accumulated contaminants within the tissues. Dietary exposure, e.g., sediment ingestion, results in mainly internal accumulation whereas aqueous exposure is mainly externally accumulated (12-15). Such nonuniform distributions of contaminants within prey may have implications for trophic transfer to their predators. Although this has been addressed for metals (16, 17), no such studies exist for organic contaminants. Contaminant distribution in prey is relevant when considering the differences in feeding behavior of aquatic predators, which can be broadly categorized as piercers or engulfers (18). Piercers have stabbing mouthparts adapted for sucking out the internal tissues and fluids from their prey whereas engulfers consume their prey whole. Consequently, piercers will be exposed to contaminants only via the internal tissues of prey whereas engulfers will be exposed to contaminants both via the exoskeleton and internal tissues of prey. Thus the combination of predator type (engulfer or piercer) and prey type (sediment ingester or noningester) may influence the extent of accumulation by predators. The influence of such factors on trophic transfer has not been studied previously, and may be important in determining the extent of secondary poisoning in predators. Toxic effects resulting from dietary exposure to contaminants has been documented in various predators, including mammals (19), fish (20), and invertebrates (21). These toxic effects have included reduced fitness (19), impaired growth (22), and reduced food intake (21). It is hypothesized that accumulation results in toxic effects when a critical body residue (CBR) is reached in an organism (23, 24). Therefore if the combination of predator and prey type affects accumulation by predators, it may also determine the magnitude of toxic response (25). The aim of this study was to investigate whether the exposure pathway to prey and the feeding behavior of predators determines the extent of trophic transfer to, and thus secondary poisoning of, predators. The model hydrophobic contaminant used was 14C benzophenone, with a Log Kow of 3.2 (26). Benzophenone has a variety of uses and is a constituent of agricultural chemicals, pharmaceuticals, and paints (27, 28). It has been detected at environmental concentrations of 99% purity) used in this study was purchased from ARC (American Radiolabeled Chemicals, St. Louis, MO) and had a specific activity of 11.2 MBq/mg, verified using high-performance liquid chromatography (HPLC) at Unilever Research (Unilever, Bedfordshire, UK). Test Species. Four of the five test species were collected from field populations in Sheffield, South Yorkshire, UK. Asellus aquaticus (5 mm long) were collected from Havelock Dam in the Rivelin Valley (National Grid Reference (NGR) SK 324 887) or the River Don (NGR SK 316 921), Cloe¨on dipterum (10 mm long) were collected from Lower Crabtree Pond (NGR SK 361 899), Ischnura elegans (20-25 mm long) from Arbourthorne Pond (NGR SK 371 850) and adult Notonecta glauca (15-20 mm long) from both Lower Crabtree Pond (NGR SK 332 828) and Millhouses Boating Pond (NGR SK 361 899). All species were maintained in aquaria filled with artificial pond water (APW; (33)) at 15 °C with a light/ dark period of 16:8 h. Asellus aquaticus were fed with detritus (predominantly alder leaves, Alnus sp.) and C. dipterum were fed with detritus and fresh plant material (predominantly Elodea sp. and Ceratophyllum sp.), all collected from the source ponds. Ischnura elegans and Notonecta glauca were fed a mixture of Asellus aquaticus, Cloe¨on dipterum, and Chironomus riparius ad libitum. Chironomus riparius was cultured according to a method adapted from Credland (34). Briefly, small plastic containers (approximately 10 cm diameter) with play pit sand (1 cm deep) and overlying APW (2 cm deep) were used. Larvae were fed three times a week on powdered Tetramin tropical fish food flakes and were maintained at 20 °C. All animals were subject to a photoperiod of 16 h light to 8 h dark. Sediment Dosing. Artificial rather than field-collected sediment was used for these experiments to allow more consistent replication of exposure conditions. The composition of the sediment was 75% sand, 20% kaolin, and 5% cellulose, based on dry mass. Sixty-milliliter glass test vessels were used for prey exposures, each containing 2.5 g of dry mass of artificial sediment. The LC50 for benzophenone was taken to be 280 µg/L (35), and therefore a nominal concentration of approximately 10% of LC50 (30 µg/g) was used for prey exposures to minimize mortality, but maximize bioaccumulation in prey. Half a milliliter of a 150 mg/L stock solution of benzophenone dissolved in HPLC-grade acetone was added to each test vessel and mixed thoroughly. Benzophenone stock solution was produced by diluting radiolabeled benzophenone with unlabeled benzophenone (Sigma-Aldrich, Gillingham, United Kingdom), using a dilution ratio of 1:266 (labeled/unlabeled) for N. glauca and 1:133 for I. elegans, to account for lower feeding rates, and hence
exposures, in I. elegans. The acetone was allowed to evaporate for at least 2 h. One milliliter of APW was added to sediment in each vessel and mixed to form a sediment paste. Overlying APW (25 mL) was then added to each vessel; a plastic disk was placed over the sediment to minimize sediment disturbance. Control test vessels were prepared in a similar way to those containing benzophenone, except that 0.5 mL of acetone was added instead of 0.5 mL of benzophenone stock solution. Prey Exposure. To each test vessel (control or benzophenone-dosed) was added 12 animals of a single prey species (either A. aquaticus, C. riparius, or C. dipterum). Prey animals were exposed for 5 days at 15 °C in a light/dark regime of 16:8 h. Following exposure, 10 prey animals were removed per test vessel and rinsed with distilled water to minimize the transfer of sediment particles into predator feeding vessels. Prey were not depurated before feeding to predators as described in the following section. A random sample of 5 control prey and 5 spiked prey was taken on each feeding day to determine whether predators were being exposed to similar levels of benzophenone in their prey. Prey were removed from the sediment exposure vessels, rinsed in distilled water, blotted dry, weighed, and frozen until required for analysis. Uptake and Depuration of Benzophenone by Predators. Pairs of predators (either I. elegans (engulfer) or N. glauca (piercer)) were placed into plastic cups containing 100 mL of APW and a plastic rod to act as a perch. For each predator, groups of 12 A. aquaticus (sediment ingester) were added to each of 135 sediment test vessels, 90 of which contained benzophenone-dosed sediment and 45 were control vessels. To continually feed 42 pairs of predators dosed or control prey over a 5-day period, the exposure of prey was staggered over the preceding 5 days. This allowed each predator to be fed daily with a group of 10 prey that had been previously exposed to dosed or control sediment for 5 days. On Day 0, 3 pairs of predators were sampled to determine the background 14C in animals. Each pair was blotted dry, weighed, and frozen at -30 °C until required for analysis. The remaining pairs of predators were each fed 10 A. aquaticus from a single dosed or control sediment exposure vessel. On Days 1-5, 3 pairs of predators fed dosed prey and 3 pairs fed control prey were removed, blotted dry, weighed, and frozen as before. Remaining predators were again fed 10 A. aquaticus from a single dosed or control sediment exposure vessel. All prey from the previous day’s feeding were removed, and the number remaining alive were counted. Feeding was terminated on Day 5 and the remaining 9 pairs of predators were transferred to clean plastic cups containing 100 mL of APW. During this depuration phase, 3 pairs of predators were removed, blotted dry, weighed, and frozen on Day 6, 7, and 10. This allowed the generation of a 5-day uptake curve and a 5-day depuration curve for each predator species. Predators were not fed during this 5-day depuration phase. At the end of the experiment, animal samples were defrosted and added to individual rigid Combusto-Cones (PerkinElmer, The Netherlands), topped with an approximately 2-mm layer of cellulose powder to aid complete combustion and capped with compacted tissue paper. Animals were then combusted in a sample oxidizer (model 307, Packard, Meriden, CT) until complete combustion was observed and collected in a scintillation cocktail consisting of 10 mL of Permafluor and 10 mL of Carbosorb. The disintegrations per minute (dpm) in the resulting samples were counted for 10 min per sample using a liquid scintillation analyzer (Tricarb 3100TR, Packard, Meriden, CT). Combustocones containing only cellulose powder and tissue paper were also oxidized and analyzed to obtain a background level VOL. 43, NO. 20, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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of activity. The total µg benzophenone/g fresh mass was calculated using eq 1: D(A - B) MX
C)
(1)
where C is the total µg benzophenone/g fresh mass, A is the activity of the sample (dpm), B is the background activity (dpm), M is the mass of the sample (g), X is the activity per microgram of benzophenone (674586 dpm/µg), and D is the dilution factor of cold to radiolabeled benzophenone (266 for N. glauca and 133 for I. elegans). This experiment was repeated for both predators using C. dipterum (sediment noningester) and C. riparius (sediment ingester) as prey, but there was no depuration phase for these two prey species. Therefore only 33 pairs of predators were required and 90 sediment test vessels prepared per predator-prey combination. The experiment involving I. elegans and C. riparius was run for 4 days rather than 5, with 6 predators instead of 3 being sampled for each treatment on Day 4. Kinetic Models. First-order two-compartment bioaccumulation models were fitted to the uptake and depuration data generated for each predator-prey combination. Dietary uptake over time can be modeled by eq 2 (simplified from (36)) dCpredator ) k1Cprey - k2Cpredator dt
(2)
where Cprey is the concentration of benzophenone in the prey (µg benzophenone/g prey animal), t is time (d), k1 is the uptake rate constant for benzophenone from the prey (g prey/g predator/d), Cpredator is the concentration in the predator (µg benzophenone/g predator), and k2 is the rate constant for chemical elimination (d-1). Uptake from water has been removed from the model as predators are fed contaminated prey in clean APW. The integrated form of eq 1 was used to fit the uptake curves to the data and estimate k1 (eq 3) (1): Cpredator )
k1 C [1 - e-k2t] k2 prey
(3)
The elimination rate constant (k2) was estimated using eq 4 on the 5-day depuration phase data (1): dCpredator ) -k2Cpredator dt
(4)
All parameter estimates were generated using the statistical software package R (37). Transfer Efficiency from Prey to Predator. The transfer efficiency (TE) for each predator-prey combination was calculated as the percentage of benzophenone transferred from prey to predator (eq 5): TE )
p × 100 nb
(5)
where p is the body burden of a pair of predators on Day 1 (µg), n is the number of prey animals eaten by Day 1 per pair of predators, and b is the average body burden of a single prey animal (µg). Feeding Rates of Predators. Forty-three individual I. elegans or N. glauca were fed dosed or control prey using the method previously described for the uptake and depuration experiment. However, prey were exposed in groups of 6 rather than 12, to allow predators to be fed 5 prey per day for 5 days. Three predators were sampled on Day 0 to measure the background 14C in animals. The remaining predators were 7918
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fed control or dosed prey for the whole 5-day exposure period. The proportion of prey eaten each day was calculated by dividing the number of prey animals remaining alive by 5. Proportional data were transformed using the arcsine square root transformation to comply with the assumptions of parametric statistics. A random sample of 5 control prey and 5 spiked prey was taken on each feeding day as described in the previous experiment. At the end of the 5-day exposure period, 5 control-fed predators and 5 benzophenone-fed predators were sampled to determine the concentration of benzophenone in predators at the start of a feeding trial, either for 2 h with N. glauca or 4 h with I. elegans to allow for their slower feeding rate. Ten clean A. aquaticus not previously exposed to dosed or control sediment were offered to each predator. The number of prey remaining alive after the 2 or 4 h feeding period was recorded. Predators were then blotted dry, weighed, and frozen to be stored for analysis. Animal samples were defrosted and oxidized, and the activity within them was counted using liquid scintillation counting as in the previous experiment. This experiment was repeated for both predators using C. dipterum (sediment noningester) as prey and with C. riparius (sediment ingester) for N. glauca.
Results Uptake and Depuration of Benzophenone by Predators. Both predator species accumulated benzophenone from their prey (Figure 1). There was significant accumulation of benzophenone in N. glauca and I. elegans (ANOVA: F g 25.3, df g 1,24, p < 0.05) compared to the controls for all three prey species. Accumulation of benzophenone by the piercer predator (N. glauca) depended on the prey species consumed (Figure 1a-c), whereas the engulfer predator (I. elegans) accumulated similar benzophenone concentrations for all three prey species by the end of the exposure period (Figure 1d-f). Both predator species were able to eliminate benzophenone, with concentrations decreasing more rapidly in N. glauca (Figure 1a) than I. elegans (Figure 1d). Kinetic Models. Uptake rate constants ranged between 0.007 and 0.064 g prey/g predator/d (Table 1). The order of uptake rate constants varied between the two predator species, being A. aquaticus > C. riparius > C. dipterum in N. glauca and C. dipterum > C. riparius > A. aquaticus in I. elegans. Elimination rate constants were generated for the depuration phases of experiments involving N. glauca (Figure 1a) and I. elegans (Figure 1d) feeding on A. aquaticus. During depuration, N. glauca had a significantly higher elimination rate constant (0.54 ( 0.085 d-1) compared to I. elegans (0.13 ( 0.065 d-1) (t ) 2.7, df ) 16, p < 0.05), though the estimate for I. elegans was not significantly different from zero (t ) 2.0, df ) 10, p > 0.05). Transfer Efficiency from Prey to Predator. The transfer efficiencies (TEs) calculated for each predator-prey combination are presented in Table 2. The TEs for the piercer predator feeding on sediment ingesters (both 13%) were much higher than for the sediment noningester prey (2%). The lowest TE in the engulfer (5%) was calculated for Asellus aquaticus, which had the highest total µg benzophenone in prey (nb). The remaining two prey species both had 0.5 total µg benzophenone and similar TEs (20-30%). Feeding Rates of Predators. There was no significant effect of benzophenone exposure on the proportion of prey animals eaten each day by N. glauca (F e 0.1, df ) 1,190, p > 0.05) or I. elegans (F e 0.6, df e1,182, p > 0.05) over the 5-day feeding period for any of the prey species. The variability in feeding rates differed between the predator-prey combinations. The coefficients of variation for feeding on C. riparius, C. dipterum, and A. aquaticus were 0.05, 0.10, and 0.41 for N. glauca, and 0.53, 0.37, and 0.61 for I. elegans, respectively.
FIGURE 1. Mean uptake of benzophenone ((1 SE) over 5 days by a piercer predator (Notonecta glauca) feeding on (a) Asellus aquaticus, (b) Chironomus riparius, or (c) Cloe¨on dipterum; or on engulfer predator (Ischnura elegans) feeding on (d) A. aquaticus, (e) C. riparius, or (f) C. dipterum. Depuration over five days is shown for (a) N. glauca and (d) I. elegans when previously fed A. aquaticus (dotted vertical line indicates start of depuration phase). Solid curves are two-compartment bioaccumulation models fitted using eqs 3 (uptake) and 4 (depuration). Prey were classified as either sediment ingesters (I) or sediment noningesters (NI).
There was no significant reduction in the proportion of prey animals eaten by dosed-fed compared to control-fed N. glauca within the 2-h feeding period for A. aquaticus, C. dipterum, or C. riparius (Figure 2a-c, one-tailed t test: t e 0.4, df e 27, p > 0.05). Neither was there a significant reduction in the proportion of A. aquaticus eaten by I. elegans within 2 or 4 h (Figure 2d) (t e 0.5, df e 24, p > 0.05). However there was a significant reduction in the proportion of C. dipterum eaten by I. elegans within 2 h, and more so within 4 h (Figure 2e, t g 2.3, df g 13, p < 0.05). There was no significant depuration in N. glauca during the 2-h period feeding on uncontaminated C. dipterum, A. aquaticus, or C. riparius (t e 1.6, df e 8, p > 0.05). However, there were significant reductions in benzophenone concentrations in I. elegans by the end of the 4-h feeding period when feeding on either uncontaminated C. dipterum or uncontaminated A. aquaticus (F g 3.8, df g 2,16, p < 0.05),
TABLE 1. Uptake Constants Generated for Each Predator-Prey Combination during the 5-Day Uptake Phase for Benzophenonea predator
prey
uptake constant, k1 (g prey/g predator/d)
Notonecta glauca Asellus aquaticus Chironomus riparius Cloe¨on dipterum
0.026 (0.004) 0.010 (0.001) 0.007 (0.001)
Ischnura elegans
0.009 (0.001) 0.021 (0.003) 0.064 (0.008)
Asellus aquaticus Chironomus riparius Cloe¨on dipterum
a Numbers in parentheses denote standard errors associated with parameter estimates. All parameter estimates were significantly different from zero (T-test: t g 7.2, df ) 17, p < 0.001).
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TABLE 2. The Transfer Efficiency (%) of Benzophenone Residues from Prey to Predator on Day One of Exposure predator Notonecta glauca (Piercer)
mean no. prey eaten
body burden (µg) per prey animal
µg in predators
5
0.80
0.65
10
0.07
0.10
Cloe¨on dipterum (NI)
9
0.17
0.03
Asellus aquaticus (I)
3
0.80
0.10
Chironomus riparius (I)
8
0.06
0.10
Cloe¨on dipterum (NI)
3
0.17
0.13
preya Asellus aquaticus (I) Chironomus riparius (I)
Ischnura elegans (Engulfer)
a
13 (4.6) 13 (2.6) 2 (0.1) 5 (1.5) 20 (2.3) 30 (8.4)
I ) sediment ingester species; NI ) sediment noningester species. Numbers in parentheses are standard errors.
decreasing by an average of 1 µg benzophenone/g predator tissue in both cases.
Discussion Feeding on contaminated prey resulted in predator exposure to hydrophobic contaminants associated with sediments and, as observed with metals (16), the exposure pathway of prey influenced the transfer of organic contaminants to predators. This study provides the first evidence that the extent of such transfer is also influenced by the feeding behavior of predators. The piercer predator (Notonecta glauca) accumulated higher concentrations of benzophenone when feeding on sediment ingester compared to sediment noningester prey. The engulfer predator (Ischnura elegans) accumulated approximately the same benzophenone concentration for all three prey species of between 2.2 and 3.5 µg/g by the end of the exposure period. The large amount of variation in predator benzophenone concentrations for all but two of the predator-prey combinations (N. glauca feeding on C. riparius or C. dipterum) was attributable to variation in the feeding rates of predators during the uptake phase. The importance of prey exposure pathway and the feeding behavior of predators was evident in the transfer efficiencies calculated. The transfer efficiencies calculated are likely to be conservative estimates of the true values, as they do not account for incomplete feeding by predators or for potential differences in bioavailability of metabolites formed in prey animals. Dietary exposure of sediment ingester prey resulted in higher transfer efficiencies to piercer predators, due to their consumption of internally accumulated benzophenone. Recent work using cadmium also demonstrated that trophic transfer efficiency from prey (C. riparius) to predator (Danio rerio) was higher when prey had been exposed via dietary compared to aqueous exposure (16). In the engulfer predator, the transfer efficiencies did not follow the same pattern, with the sediment noningester resulting in the highest transfer. However, this may be explained by the predator accumulating a very similar benzophenone concentration for all three prey species. Therefore, lower transfer efficiencies result from feeding on prey with high body burdens and vice versa. Although other examples exist of predator body burdens being independent of food type (38), the dependence of engulfer predator body burdens on the prey species has also been demonstrated (39, 40). Therefore, although the engulfer used in this study did not demonstrate the expected patterns in accumulation, this may be specific to the species used. Further experiments using different species are required to establish how general the pattern found in this study is among engulfers. 7920
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As hydrophobic contaminants, such as benzophenone, accumulate mainly within the lipids of organisms (41, 42), differences in benzophenone concentration may be due to the relative lipid contents of the species involved. Lipid contents reported for predator species similar to those used in this study indicate little difference between them, being 11.2% for an Ischnura species and 9.4-14.2% for Notonectidae, on a dry mass basis (43). Therefore the higher maximum benzophenone concentration in N. glauca cannot be attributed to a higher lipid content. The prey species used do differ, with the percentage of lipid on a fresh mass basis being 0.69, 0.82-1.08, and 2.1% for A. aquaticus (44), Chironomus sp (45), and mayfly nymphs (46), respectively. Therefore, it might be expected for the transfer efficiency of benzophenone residues to be highest for prey with the highest lipid content, in this case C. dipterum. Although this was the case for I. elegans, A. aquaticus resulted in the highest trophic transfer efficiency in N. glauca. Therefore, lipid content is not the only factor important in determining accumulation patterns. Both predator species were able to eliminate benzophenone, to different extents, during the 5-day depuration phase. The calculated elimination constants for benzophenone were higher for N. glauca than I. elegans, though not statistically significant in I. elegans due to variable data. This may explain why the bioaccumulation model underestimated the extent of accumulation in I. elegans when feeding on A. aquaticus. Previous studies have demonstrated a link between elimination and the polarity of metabolites produced during biotransformation (47). Though biotransformation in predators was not tested in this study, benzophenone is known to be metabolized by rats (48) and to differing extents by all three prey species used in this study (15). Therefore, the rapid elimination of benzophenone by N. glauca may be due to more polar metabolites being produced and subsequently excreted compared to I. elegans. However, no existing studies on the biotransformation capabilities for organic compounds by either notonectids or odonates could be indentified in the literature. Accumulation of benzophenone by both predator species gave the potential for secondary poisoning effects to occur. The reported critical body residue for chronic effects of narcotic organic contaminants is 200-400 µmol/kg (49), equivalent to 36.4-72.9 µg/g for benzophenone (molecular weight of 182.22 g/mol). Although none of the five predatorprey combinations achieved this concentration, secondary poisoning effects were observed in I. elegans when feeding on C. dipterum, at a body residue over an order of magnitude lower (2-3 µg/g). The toxic response to benzophenone exposure was not solely dependent on total benzophenone concentration, as no toxic effects were observed in either N.
FIGURE 2. Effect of dietary uptake of benzophenone by predators on the mean (+ 1 SE) proportion of prey animals eaten over a 2 or 4 h feeding period by predators fed dosed (gray bars) or control (black bars) prey. Data shown for a piercer predator (Notonecta glauca) feeding on (a) Asellus aquaticus, (b) Cloe¨on dipterum, or (c) Chironomus riparius, and an engulfer predator (Ischnura elegans) feeding on (d) A. aquaticus and (e) C. dipterum. Asterisk indicates a significant difference in the proportion of prey eaten by predators fed dosed compared to control prey (p < 0.05). glauca with a higher benzophenone concentration or in I. elegans with the same benzophenone concentration achieved by feeding on sediment ingester prey. The observed differences in toxicity of benzophenone between the predator-prey combinations may be due to an interaction between contaminant mode of action and species traits. Exposure to narcotic organic contaminants, such as benzophenone, results in hypoactivity (50), which may alter the ability of predators to successfully capture prey (51). However, the susceptibility of predators to such toxic effects may depend on their hunting strategy. Ischnura elegans is an ambush hunter species, sitting and waiting for prey to come to them, whereas N. glauca is an active hunting species
that seeks out prey (52). The more pronounced response observed in the ambush predator (I. elegans) feeding on fast moving prey (C. dipterum) would thus be expected, as rapid predator response to prey encounters would be required for successful prey capture. The importance of toxicantinduced changes in prey mobility on the feeding behavior of predators is explored elsewhere (53). In conclusion, the extent of trophic transfer is influenced by the exposure pathway of prey and the feeding behavior of predators, with the piercer predator species achieving a higher benzophenone concentration from sediment ingester prey, but the engulfer predator achieving a similar benzophenone concentration from both sediment ingester and VOL. 43, NO. 20, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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sediment noningester prey. However, the subsequent occurrence and nature of toxic effects will also depend on the hunting strategy of the predator and chemical mode of action.
(20)
Acknowledgments
(21)
This work forms part of a PhD thesis undertaken by A.C.B. and funded by Unilever and The University of Sheffield. We thank Stuart Marshall, Oliver Price, and Roger van Egmond for their contributions to this work.
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