Environ. Sci. Technol. 1999, 33, 3451-3457
Pharmacokinetics and Distribution of Dietary Tributyltin and Methylmercury in the Snow Crab (Chionoecetes opilio) C L A U D E R O U L E A U , * ,† C H A R L E S G O B E I L , † A N D H A N S T J A¨ L V E ‡ Department of Fisheries and Oceans, Maurice Lamontagne Institute, P.O. Box 1000, Mont-Joli, Quebec, Canada G5H 3Z4, and Faculty of Veterinary Medicine, Department of Pharmacology and Toxicology, Swedish University of Agricultural Sciences, Biomedicum Box 573, S-751 23 Uppsala, Sweden
The pharmacokinetics and distribution of a single 5-µg dietary dose of radiolabeled [113Sn]tributyltin (TBT) and [203Hg]methylmercury (MeHg) were studied over 154 days in the snow crab, using in vivo gamma counting and wholebody autoradiography. Experiment was done under conditions typical of those encountered in the cold natural habitat of this crustacean. Retention efficiency was high for both compounds (80-100%), and two kinetic pools could be distinguished. Elimination of the first pool proceeded within 20-80 days, but it accounted for 27-62% of the assimilated TBT, compared to 8-11% for MeHg. Biological half-life of the second pool was 33-187 days for TBT and 520650 days for MeHg. Autoradiographic and dissection data revealed a less homogeneous distribution of the radiolabel and much higher radioactivity in gut lumen for TBT compared to MeHg. This suggests that the larger size of the first pool in the case of TBT resulted from metabolization in the hepatopancreas and fecal elimination of the metabolites. The whole-body biomagnification factor (BMF) that would result from the long-term chronic exposure of snow crab to TBT-contaminated food was estimated as 0.1-0.6. Although these BMF values were an order of magnitude lower than those estimated for MeHg (1.8-2.4), they are not negligible and indicate that uptake of TBT via food may be an important accumulation route.
Introduction The presence of the antifouling biocide tributyltin (TBT) in marine coastal ecosystems is a major environmental concern. Though TBT persists for years once buried into the sediments (1, 2) and is efficiently accumulated by deposit-feeders and sediment-dwellers (3), little is known about its uptake via food in larger benthic organisms of higher trophic levels. Recently, we reported the results of a laboratory experiment on the pharmacokinetics and tissue distribution of dietary TBT in the American plaice (Hippoglossoides platessoides) (4), which showed that dietary uptake may be a significant accumulation route in such a benthic predator. * Corresponding author phone: (418)775-0725; fax: (418)775-0718; e-mail:
[email protected]. New address as of September 1st: National Water Research Institute, 867 Lakeshore Road, P.O. Box 5050, Burlington, Ontario, Canada L7R 4A6. phone: (905)336-4928; e-mail:
[email protected]. † Maurice Lamontagne Institute. ‡ Swedish University of Agricultural Sciences. 10.1021/es990250j CCC: $18.00 Published on Web 08/24/1999
1999 American Chemical Society
Decapod crustaceans constitute another important group of benthic predators. The snow crab, Chionoecetes opilio (Brachyura, Majidae), is a common predator on mud and sand-mud substrates in cold waters along the Canadian coast (5). As its diet mainly consists of benthic crustaceans, echinoderms, polychaetes, molluscs (70-80%), and demersal fish (10-15%) (6, 7), TBT accumulated from the sediments in these prey may be taken up by the snow crab. Elevated levels of butyltin compounds have been reported in the crabs Paralomis multispina and Tachypleus tridentatus (8, 9). Though evidence provided was indirect, authors of these studies suggested that uptake via food may be an important accumulation route, because butyltin levels measured in crabs were higher than those measured in organisms of lower trophic levels (8) and exceeded those expected to result through accumulation from water only (9). Earlier results obtained from short-term (4 days) experiments with the mud crab Rhithropanopeus harrisii also showed that uptake through food was a more important accumulation route than direct uptake from water (10). However, there is a lack of data on the long-term fate of dietary TBT uptake in decapod crustaceans, which is needed to assess its environmental significance. The present study was conducted to determine the pharmacokinetics and tissue distribution of a single dietary dose of radiolabeled [113Sn]tributyltin in the snow crab over a 5-month period, under environmental conditions similar to those encountered in the cold coastal waters this species inhabits. To investigate mechanisms governing the fate of dietary TBT in the body of snow crab, we compared its behavior with that of another radiolabeled organometallic compound, [203Hg]methylmercury (MeHg). Biomagnification of MeHg in food chains is a well-known phenomenon (11), and the mechanisms of its uptake, distribution, and elimination in living organisms have been extensively studied (1215).
Material and Methods In November 1996, 1 week before the beginning of experiments, six adult female snow crabs (60-85 g) from the St. Lawrence Estuary (Que´bec, Canada) were placed in three 60-L aquariums provided with running seawater (T ) 3-3.5 °C). Synthesis of radioactive [113Sn]tributyltin chloride and [203Hg]methylmercury hydroxide from commercially available 113 Sn(IV) (33.3 MBq‚mg-1, New England Nuclear) and inorganic 203Hg(II) (8.2 MBq‚mg-1, Amersham) was carried out as described previously (16, 17). Chemical purity of both radioactive organometals was >98%, as assayed by thinlayer chromatography. One day before the beginning of the experiment, contaminated food was prepared by adding both radioactive organometals to 4-5 g of supplemented fish food to achieve final concentrations of 15 µg Sn‚g-1 as TBT and 15 µg Hg‚g-1 as MeHg. After a 5-min manual homogenization with a stainless steel spatula, the contaminated food was molded into small balls weighing 0.3 g and kept at 4 °C overnight to allow for the binding of TBT and MeHg to food components, such as proteins. On day 0, crabs were given one ball of contaminated food each, which was almost entirely eaten within 10 min. Water in the aquarium was then immediately changed to avoid its contamination by radiolabeled organometals leaching from food particles. The radioactivity of the animals was monitored 1 h after feeding, and they were then fed twice a week with chopped capelin, from day 3 until the end of experiments. In a separate experiment, six additional snow crabs received inorganic 203Hg(II) with their food, accordingly to the above protocol. VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. (A) Typical evolution of 113Sn (O) and 203Hg (b) activity over 154 days in a female snow crab that received 113Sn-TBT and Me203Hg with food (animal no. 13b). Data standardized as percent of activity at t ) 0. Solid lines show the results of nonlinear regression analysis with eq 2. Shaded area represents range of 203Hg activity versus time observed for crabs fed with inorganic 203Hg(II) (n ) 6). (B) Percentage of 113Sn and 203Hg body burden and concentration index, IC, in tissues at day 154 (mean ( SD, n ) 5). Legend: Hep ) hepatopancreas, L ) legs, R ) rest of body. The rate of water renewal in the aquaria was kept high enough (1.5-2.0 L‚min-1) to approximate an open system. Three-milliliter water samples were collected at least twice a week throughout the experiment (daily during the first 10 days), and their radioactivity was monitored for 10 min with a LKB Clinigamma counter. Radioactivity in all water samples was below the detection limits of 0.15 Bq‚mL-1 for 113Sn and 0.11 Bq‚mL-1 for 203Hg. We repeatedly monitored 113Sn and 203Hg activities in each crab over 154 days by in vivo gamma counting, as described elsewhere (4). This technique takes advantage of the penetrating nature of γ-rays emitted by these radioisotopes to precisely and repetitively measure their level in a living animal. It allows for quantification with more accuracy of eventual changes in distribution and elimination rates, e.g., distinguishing between mono- and biexponential kinetics. Since in vivo gamma counting is nondestructive, its use reduces by 80-90% the number of experimental animals needed for such a kinetic study compared to classical dissection techniques (4). Snow crabs were placed upside down, motionless, under the detector and precisely positioned on a target drawn on a plastic plate. The γ-ray emissions of 113Sn and 203Hg (392 and 273 keV, respectively) were monitored for 1-3 min, daily during the first 10 days and twice a week thereafter. Measured 113Sn and 203Hg activities were standardized by expressing them as a percentage of the activity measured at the beginning of the experiment. No behavioral changes were observed despite the repeated manipulations needed for gamma counting. One crab died on day 75, and data from this animal were not used. On day 154, all animals were sacrificed and dissected, and radioactivity in hepatopancreas, legs, and the rest of the body was measured. Data obtained were used to calculate the percentage of the 113Sn and 203Hg body burden contained in tissues. The concentration index, IC, was then calculated with
IC )
[113Sn or 203Hg] in tissue % of body burden ) 113 % of body weight [ Sn or 203Hg] in whole body (1)
Values of IC > 1 indicate a higher radiolabel concentration compared to the whole-body averaged concentration. Twelve additional crabs were fed as described above with food containing one of either 113Sn-TBT, Me203Hg, or inorganic 203Hg(II). Two crabs from each treatment group were collected on days 3 and 14 and were used for whole-body autoradiography (18). Immediately after removal from their aquaria, the animals were embedded in a carboxymethylcellulose gel 3452
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and quickly frozen in a slurry of hexane and dry ice. Resulting blocks were sectioned on tape (50 µm-thick sections) at -20 °C with a specially designed cryomicrotome (Jung Cryomacrocut, Leica). Tissue sections were then freeze-dried and applied to an X-ray film (3H-Hyperfilm for 113Sn and AGFA Structurix for 203Hg) for 3-6 months, at -20 °C.
Results Pharmacokinetics. In snow crabs fed with 113Sn-TBT and Me203Hg, fecal elimination of nonretained radiolabels, reflected in a sudden drop of radioactivity, occurred 2-4 days after feeding (Figure 1A). Activity of both 113Sn and 203Hg then decreased biexponentially as a function of time, indicating the presence of two kinetically distinct pools. Such a trend can be modeled with the following mathematical expression
Q(t) ) Ae-Rt + Be-βt
(2)
where Q(t) is the relative activity at time t, and R and β are first-order rate constants characterizing the fast and slow eliminating pools, respectively. A and B are the initial amounts of radioactivity in each pool. Their sum is equal to the retention efficiency (0% e RE0 e 100%). Values of A, B, R, and β were calculated from experimental data by nonlinear regression analysis with the software Statgraph. In 113Sn-TBT-fed snow crabs, values of RE0 ranged from 79 to 106% (Table 1). On average, the relative sizes of the slow and fast pools were similar, though quite variable from one individual to another, with A > B for two crabs and B > A for the three others. Values of β were 3-12 times lower than those of R. The elimination rates were highly variable, with 95% of pool A being eliminated in 19-70 days and the half-life of pool B ranging from 33-187 days. Most of the 203Hg given as MeHg was also retained by the crabs (RE0 ) 89 to 101%). However, the slow eliminating pool B was 8-12 times larger than the fast eliminating pool A (Table 1). Values of R were similar to those measured for the TBT group. Values of β were an order of magnitude smaller, and the biological half-life values measured for pool B ranged from 520 to 650 days. In snow crabs fed with food containing inorganic 203Hg(II), values of RE0 were relatively high (range 60-90%), but elimination was monoexponential and 10-20 times faster (t0.5 ) 31-66 days) than for the Me203Hg-fed group (Figure 1A). Tissue Distribution. In crabs collected 3 days after feeding with radioactive TBT, a high labeling of midgut, hindgut, and hepatopancreas was observed, whereas that of the foregut was comparatively lower (Figure 2A,B). The labeling of all other tissues was very low. On day 14 (Figure 2C,D), the
TABLE 1. Pharmacokinetic Parameters of Dietary A (%) animal no. 12a animal no. 13a animal no. 13b animal no. 14a animal no. 14b animal no. 12a animal no. 13a animal no. 13b animal no. 14a animal no. 14b
49 ( 2 29 ( 4 26 ( 5 24 ( 5 64 ( 2 b
8(1 10 ( 2 7(1 9(4
B (%)
113Sn-TBT
RE0 (%)
and Me203Hg in the Snow Crab Chionoecetes opilioa
B/A
r (10-3 d-1)
t0.95r (d)
TBT 43 ( 4 161 ( 37 150 ( 50 53 ( 13 45 ( 2
70 19 20 57 67
30 ( 3 73 ( 2 69 ( 3 56 ( 6 42 ( 1
79 102 95 80 106
0.6 2.5 2.7 2.3 0.7
99 ( 1 82 ( 1 82 ( 1 82 ( 1 92 ( 1
99 90 92 89 101
b
MeHg 10.3 8.2 11.7 10.2
b
b
44 ( 12 47 ( 11 37 ( 10 190 ( 79
68 64 81 16
β (10-3 d-1) 3.7 ( 0.7 21.1 ( 0.6 20.3 ( 0.8 14.5 ( 1.0 15.6 ( 0.6 1.13 ( 0.06 1.07 ( 0.07 1.34 ( 0.08 1.10 ( 0.08 1.23 ( 0.04
t0.5β (d)
r2
β/r
187 33 34 48 44
0.99 0.99 0.99 0.99 0.99
0.086 0.131 0.135 0.274 0.347
610 650 520 630 560
0.88 0.92 0.93 0.91 0.96
0.024 0.027 0.030 0.006
a Values ((SE) of A, B, R, and β were calculated by nonlinear regression analysis with eq 2. RE is the retention efficiency (A + B). Data from 0 day 0 to 3 were not used to account for gut residence time. Time needed to eliminate 95% of the fast pool, t0.95R, and biological half-life of the slow pool, t0.5β, were calculated with 2.996/R and 0.693/β, respectively. bMeHg kinetics for this animal were monoexponential, i.e., Ae-Rt ) 0.
FIGURE 2. Autoradiograms of female snow crab 3 days (A) and 14 days (C) after feeding with a single dietary dose of 113Sn-TBT. (B) and (D) are corresponding tissue sections. Contrast has been enhanced in areas besides the central band in (C). Legend: Exo ) exoskeleton, For ) foregut, Gi ) gills, Go ) gonads, Hep ) hepatopancreas, Hin ) hindgut, Hyp ) hypodermis, Mid ) midgut, Mus ) muscle, Sper ) spermatheca. Bars ) 1 cm. concentration of the radiolabel was still high in the digestive system. However, labeling of gut lumen was higher than in digestive diverticulae of the hepatopancreas. The labeling of other tissues was higher compared to day 3, especially in the gonads, but remained low compared to the digestive system. At the end of the experiment, 49% of the 113Sn body burden was found in the hepatopancreas, 5% in the legs, and 46% in the rest of the body (gonads, gills, exoskeleton, and muscle) (Figure 1B). As the hepatopancreas accounts for only 7% of the total body weight, compared to 64% for the rest of body, the value of IC for this organ, 6.9 ( 2.1, was higher than for the rest of the body, 0.7 ( 0.1. The legs, with an IC of 0.2 ( 0.2, had the lowest value.
In the case of the Me203Hg-fed group, most of the radiolabel was found in the hepatopancreas at day 3, but all the other internal tissues were also uniformly labeled (Figure 3A,B). Fourteen days after feeding, the labeling of muscle was higher compared to tissues such as gills, gonads, spermathecae, and hypodermis (Figure 3C,D). The radiolabel also reached the nervous system (Figure 3E,F). The hepatopancreas was still highly labeled, but it is noteworthy that lumen of gut and hepatopancreas ducts was not. On day 154 (Figure 1B), 97% of the body burden was contained in tissues other than the hepatopancreas. Values of IC for the legs and the rest of body, 1.4 ( 0.1 and 0.9 ( 0.1, respectively, were higher than for the hepatopancreas (0.4 ( 0.4). This clearly indicates that VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Autoradiograms of female snow crab 3 days (A) and 14 days (C and E) after feeding with Me203Hg. (B), (D), and (F) are corresponding tissue sections. See Figure 2 for legend. Bars ) 1 cm. the transfer of 203Hg activity from the hepatopancreas toward the legs and the rest of body continued after day 14. In the case of crabs that ate inorganic 203Hg-contaminated food, the hepatopancreas was the only anatomical structure labeled in autoradiograms (Figure 4). However, in contrast to MeHg-fed crabs, the gut lumen and the hepatopancreatic ducts were labeled. Crabs that received inorganic 203Hg with their food were not dissected. Due to the short radiological half-life of 203Hg (46 days) and the short biological half-life of this mercury species, radioactivity became too low to be measured 70-100 days after the beginning of the experiment.
Discussion As TBT is known to be metabolized in crustaceans, fish, and mammals (19), its fate within the snow crab body will be discussed while bearing in mind the probable occurrence of other butyltin species, referred to as BTs when necessary. However, our data clearly showed that dietary inorganic 203Hg(II) was rapidly eliminated and that its distribution was restricted to the hepatopancreas. Thus, the very slow elimination rate and the progressive transfer of the radiolabel to all tissues in Me203Hg-fed snow crabs are probably representative of the actual behavior of MeHg. The fate of radiolabeled organometals observed in this work can be divided into three main steps. The first was an initial assimilation process, lasting for the first 2-4 days and ending with the elimination of nonassimilated radiolabels with the feces. It was followed by the R-phase, corresponding to the distribution of the assimilated radiolabels in the body, and the β-phase, which occurs once distribution has reached equilibrium (20). The high RE0 values of snow crabs fed with radioactive organometals reflect the affinity of TBT and MeHg for biological ligands (21, 22). The low labeling of foregut in autoradiograms taken at day 3, i.e., after or shortly before fecal elimination, shows that both radiolabels were efficiently transferred to and initially stored in the hepatopancreas during the first assimilation step. However, BTs pharma3454
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cokinetics were characterized by a larger size of pool A and a more rapid elimination of pool B, and tissue distribution was less uniform compared to MeHg. As the two organometals were administered simultaneously to a given crab, they were subjected to identical physiological conditions. Thus, factors that could explain the experimental differences observed are likely related to differing physical and chemical characteristics of the organometals, of which some are listed in Table 2. The bigger size of pool A compared to Me203Hg indicates that more 113Sn-BTs were lost during the R-phase. Tributyltin is metabolized in the crustacean hepatopancreas, and the resulting metabolites (monobutyltin, dibutyltin, polar conjugates) have been found in both gut and hepatopancreas (23, 24). A remarkable feature of autoradiograms from day 14 is the labeling of gut lumen, which was very high for TBTfed crabs and very low for MeHg-fed ones. This strongly suggests that, unlike the situation for MeHg, some of the 113Sn-butyltin compounds initially stored in the hepatopancreas were transferred back into the lumen of the hepatopancreatic diverticulae, transported to the gut with the digestive juices (25), and subsequently excreted. This would result in a larger size of pool A for TBT. The slower elimination of MeHg compared to TBT during the β-phase is probably related to the stability of its covalent Hg-C bond (15). The less homogeneous tissue distribution seen in TBTfed snow crabs may be due to different processes, though their relative importance cannot be inferred from the present set of data. A first possibility is that the distribution pattern observed in autoradiograms is related to the distribution of 113Sn-metabolites. For instance, total butyltins concentrations measured in the hepatopancreas of the crab Paralomis multispina were 4-5 times higher than those measured in muscle and gills, whereas tri-, di-, and monobutyltin were found in the hepatopancreas and only TBT was detected in muscle and gills (8). The capacity of chemicals to cross
TABLE 3. Predicted Values of Equilibrium Whole-Body Biomagnification Factor (BMF) and Relative Size of Compartments (C1:C2) in Snow Crab upon Chronic Dietary Exposure to TBT and MeHg, According to Models I and II model I BMF [(k1 + k2) R/rβ]
model II
C1:C2 [C1/C2 ) k2/k1]
BMF [R(1 + (k1/β))/r]
C1:C2 [C1/C2 ) k e2/k1]
0.28 0.11 0.11 0.13 0.13
1:4.1 1:4.7 1:4.6 1:1.9 1.3:1
2.2 1.8 2.2 2.2
1:36 1:32 1:31 1:140
BTs
FIGURE 4. Autoradiogram (A) of female snow crab 14 days after feeding with inorganic 203Hg(II). (B) is the corresponding tissue section. See Figure 2 for legend. Bar ) 1 cm.
animal no. 12a animal no. 13a animal no. 13b animal no. 14a animal no. 14b
0.62 0.15 0.14 0.16 0.18
animal no. 13a animal no. 13b animal no. 14a animal no. 14b
2.4 1.9 2.4 2.4
2.6:1 7.6:1 8.2:1 9.8:1 5.6:1 MeHg 22:1 18:1 26:1 21:1
Modeling of TBT and MeHg Accumulation via Food. The magnitude of β values for BTs leads one to expect that longterm accumulation would result in lower tissue levels of these compounds, compared to MeHg. To verify this, we modeled accumulation resulting from the chronic exposure of snow crab to TBT- and MeHg-contaminated food, using twocompartment models to account for the occurrence of two kinetically distinct pools. There are several types of twocompartment systems that fit a biexponential curve (20), and we considered two different ones. Model I is the most commonly used open two-compartment model (20), featuring exchange between compartments and elimination from compartment 1 only:
TABLE 2. Comparison of Some of the Physical and Chemical Properties of TBT and MeHga TBT volumeb
molecular octanol/water partition coefficient (seawater) binding sites in biological media binding geometry relative stability in biological media
nm3
0.26 5000-6300
MeHg 0.04 nm3 1.5-1.9
-SH (covalent) -NH, -SH (covalent) -OH (coordination) trigonal bipyramidal linear lowerc higherd
C1 and C2 represent the amount of chemical in central and peripheral compartments, whereas k1, k2, and ke are rate constants characterizing exchange and elimination. Model II exhibits unidirectional exchange between compartments and elimination from both:
a From refs 15, 21, 22, 24, and 39-42. b Calculated for the cation according to Bondi (43). c Easily metabolized by many organisms, shorter biological half-life. d Stability of the Hg-C bond in biological media, longer biological half-life.
biological membranes also affects their distribution in organisms. For a given chemical, this process has been shown to be dependent upon chemical properties such as lipophilicity, sterical hindrance (26), and molecular size (27). Methylmercury easily diffuses through biological membranes while bound to glutathione and other small mobile thiol ligands (28, 29), whereas transport of the larger TBT molecules (Table 2) is related to the passive diffusion of neutral species (TBT-Cl, TBT-OH) (30). In the sea star Leptasterias polaris, diffusion of dietary TBT through the epithelium of digestive glands toward the general body cavity was six times slower than for MeHg (31). In the present work, the different distribution patterns seen for TBT and MeHg in the body of snow crab may thus reflect differences in their respective rates of diffusion across biological membranes (the hepatopancreatic and midgut epithelia).
This model represents the case where a chemical is being partially eliminated (C1) and partially metabolized (C2), the metabolite being eliminated at a slower rate. Equilibrium values of the biomagnification factor (BMF) for the whole body, (C1 + C2)/concentration in food Cf, and of the ratio C1/C2 were calculated for both models (see Appendix for mathematical equations). BMF values for BTs calculated with the two models ranged from 0.1 to 0.6 and differed by a factor of 1.2-2 for a given animal, whereas values for MeHg ranged from 1.8 to 2.4, with little difference (e5%) between the models (Table 3). The relative size of C1 and C2 was model-dependent, i.e., C1 > C2 for model I and C1 < C2 for model II. Nevertheless, in all cases the size of the fast pool was a small proportion (e3%) of the whole body MeHg content. This explains the similarity in predicted BMF values for MeHg between the VOL. 33, NO. 19, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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two models, as the long-term kinetics of MeHg in the snow crab can be well described by the slow compartment only (14). In the case of BTs, the size of the fast pool was significant, always representing g10% of the whole body content. Thus the long-term fate of butyltins derived from the ingestion of TBT-contaminated food by the snow crab must be modeled with a two-compartment model. BMF values calculated for BTs in snow crab are similar to those reported previously for various fish species (4, 8). However, BMF could be much higher in specific tissues, such as the hepatopancreas. The latter has an IC value indicating that its butyltin concentration would be seven times higher than the average whole-body concentration (BMF × IC ) 0.7-4). In comparison, the BMF calculated for the rest of body would range from 0.1 to 0.4, whereas for legs (which consist mostly of muscle) the BMF would be e 0.1. This approximately corresponds to the range of concentrations of butyltins measured in other crab species, both in the field and the laboratory (8, 10). This work provides the first reliable data on TBT pharmacokinetics for a marine benthic macroinvertebrate living in cold coastal waters. Apart from their usefulness for comparison purpose, pharmacokinetic data for MeHg and inorganic Hg are also new since previous ones for other crab species (32, 33) were not obtained at low water temperature. Our data indicate that dietary uptake of TBT in snow crab may be an important process, yet its actual significance in the field will depend on the amount of TBT accumulated from the sediments by organisms of lower trophic levels and the proportion of the diet accounted for by necrophagy (7). Refinements to our understanding of the transfer of sedimentary TBT toward the snow crab will require data on the concentrations of butyltins in its prey and in their sediment habitats and on the retention efficiency of biologically incorporated butyltins. We will also need to determine which two-compartment model actually underlies butyltins fate in the body of this crustacean. Finally, the relative importance of direct uptake from water and sediments compared to dietary uptake needs to be evaluated. Though bioconcentration factor measured for molluscs, gastropods, and fish are high (2000-11 000) (34-36), there is no data for crustaceans exposed to environmentally relevant low TBT concentration in water (1-20 ng‚L-1).
Acknowledgments This work was supported by the Toxic Chemicals Program of the Canadian Department of Fisheries and Oceans and the Swedish Foundation for Strategic Environmental Research. The authors gratefully acknowledge the comments of R. Roy and B. Sainte-Marie and the technical assistance of L. Beaudin, J. Bolduc, A. Bostro¨m, and P. Robichaud.
Appendix Mathematical Equations for Pharmacokinetic Models. For simplicity, the rate of food consumption, R, was assumed to be constant (0.003 g food‚g-1 body weight‚d-1) (37), and initial conditions (t ) 0) were set as C1 ) C2 ) 0. For complete details about derivation of equations, the reader is referred to Whicker and Schultz (38). For model I, values of C1 and C2 as a function of time are given by
[
C1 ) Cf
]
k2R R(1 - k2/β)e-βt R(1 - k2/R)e-Rt + + Rβ (R - β) (β - R) C2 ) Cf
[
]
k1R k1Re-βt k1Re-Rt + Rβ β(R - β) R(R - β)
(A-1) (A-2)
where
R ) {(k1 + k2 + ke) + [(k1 + k2 + ke)2 - (4k2ke)]1/2}/2 3456
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and
β ) {(k1 + k2 + ke) - [(k1 + k2 + ke)2 - (4k2ke)]1/2}/2 In the case of model II, the following equations are obtained
[
C2 ) Cf
R C1 ) Cf (1 - e-Rt) R
[
]
(A-3)
]
k1R(1 - e-βt) k1R(e-Rt - e-βt) Rβ R(β - R)
(A-4)
where R ) k1 + ke1 and β ) ke2. For both models, the value of rate constants (k1, k2, ke, ke1, ke2) was calculated from experimental data obtained from each animal by nonlinear regression analysis. Expressions for equilibrium values of BMF and relative size of compartments shown in Table 3 were obtained by posing t ) ∞, a condition for which the value of all exponential terms tends toward 0.
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Received for review March 3, 1999. Revised manuscript received July 12, 1999. Accepted July 14, 1999. ES990250J
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