Dynamic Selenium Assimilation, Distribution, Efflux, and MaternalTransfer in Japanese Medaka Fed a Diet of Se-enriched MayfliesJustin M. Conley,† AtLee T. D. Watson,† Lingtian Xie,‡ and David B. Buchwalter*,†

†Environmental and Molecular Toxicology Program, Department of Biological Sciences, North Carolina State University, Raleigh,North Carolina 27695, United States‡Key Laboratory of Pollution Ecology and Environmental Engineering, Institute of Applied Ecology, Chinese Academy of Sciences,Shenyang 110016, China

ABSTRACT: Selenium (Se) trafficking in oviparous speciesremains understudied and a major source of uncertainty indeveloping sound Se regulations. Here, we utilized 75Se tofollow Se through a simulated natural food chain (water,periphyton, mayflies (Centroptilum triangulifer), fish (Japanesemedaka)). We specifically examined Se assimilation efficiency,tissue distribution, efflux rate, and maternal transfer in medaka.Selenium assimilation efficiency (AE) averaged 63.2 ± 8.8%from mayfly diets and was not affected by mayfly [Se] across adietary range of 5.6−38.7 μg g−1 (dry wt). However, AEdecreased significantly as mayfly larva size increased. Efflux rateconstants (ke) were consistent between reproductively inactive(0.066 d−1) and spawning females (0.069 d−1). Total Se lossrate constant (ke+egg; efflux and egg deposition) was 0.17 d

−1 inspawning females. Interestingly, medaka appeared to rapidly shuttle Se to their eggs directly from their diet via the ovary, asopposed to mobilization from surrounding tissues, resulting in dynamic egg [Se] that was more attributable to recent dietary Seingestion than female whole body [Se] in this asynchronous spawning fish. Spawning strategy likely plays a large role in theprocess of fish egg Se deposition and requires further attention to understand risk and toxicity of Se to fish.

■ INTRODUCTION

Selenium (Se) is an essential element that can be particularlytoxic to oviparous species via dietary exposures.1−3 A widevariety of human activities introduce elevated Se into aquaticecosystems including mining (e.g., coal, phosphate, uranium),inappropriate storage and/or spills of coal combustion wastes,irrigation of Se-rich soils, and fossil fuel refining.4,5 Fish andbirds, which are either permanent or transient residents in thesesystems, are at risk of ingesting Se-enriched prey and may sufferreproductive failure and/or teratogenic defects in offspring.4,6

Understanding the dietary incorporation and maternal transferof Se across a range of species is critical to properly assessingthe risk to fish and wildlife populations from Se exposure.Regulatory entities struggle with Se, in part because the

determination of acceptable exposure levels is complicated byhighly variable and dynamic processes.7 Environmental Secycling, foodweb incorporation, trophic transfer, and maternaltransfer each play important roles in determining Se risk.1,8 Theavailable scientific literature that has addressed these processesremains surprisingly limited given the prevalence of elevated Sein aquatic systems and is skewed toward observational fieldstudies (as opposed to controlled laboratory studies). Inparticular, we currently know very little about the biologicalprocesses that govern tissue-specific Se distribution andtrafficking in fish (and its variation across species), including

the deposition into eggs (i.e., are tissue Se stores mobilized tothe ovary for egg deposition or is Se more directly shuttledfrom the diet?). Further, the influence of fish reproductivestrategies (synchronous, group synchronous, asynchronous) onSe partitioning into eggs remains poorly understood. Theseprocesses are extremely important from a regulatoryperspective as entities attempt to create defensible fish tissuebased criteria (e.g., refs 9 and 10).In order to begin addressing these questions, the goal of the

current study was to investigate the biodynamic nature ofdietary Se in female fish via a simulated natural food chain(dissolved 75Se to periphyton to mayfly larvae to fish to eggs).While we previously reported on the speciation and dynamicsof Se at the base of our laboratory food chain,11,12 here wefocused on the kinetics and partitioning of Se within the highestlevel consumer, Japanese medaka. We utilized environmentallyrelevant dissolved Se exposures (3−10 μg L−1)7,13−15 togenerate Se-enriched periphyton diets, which were then fed tonewly hatched Centroptilum triangulifer mayfly larvae. Oncelarvae were large enough to handle, we fed them to both female

Received: November 5, 2013Revised: January 27, 2014Accepted: February 4, 2014

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(non-egg-bearing and egg-bearing) and male medaka. Wereport on the assimilation efficiency (AE), tissue-specificdistribution (digestive tract, liver, ovary/testis, and carcass),efflux rate constants, and maternal transfer of Se in medaka.

■ MATERIALS AND METHODSTest Animals. The mayfly Centroptilum triangulifer

(Ephemeroptera: Baetidae; WCC-2 clone) was obtained fromculture at Stroud Water Research Center (SWRC, Avondale,PA) and maintained at North Carolina State University(NCSU, Raleigh, NC) in culture as described previ-ously.11,12,16−18 Japanese medaka (Oryzias latipes) wereobtained from a stock population maintained under recirculat-ing freshwater aquaculture conditions at NCSU and fed a dietnewly hatched Artemia f ranciscana nauplii (INVE Aquaculture,Inc., Salt Lake City, UT) as described previously.19 Animal careand use were in conformity with protocols approved by theNCSU Institutional Animal Care and Use Committee inaccordance with the National Academy of Sciences Guide forthe Care and Use of Laboratory Animals.Background [Se] Measurement. Background [Se] was

analyzed in newly hatched Artemia nauplii, male and femalemedaka, and fish eggs. Artemia were collected on a 47-mm glassfiber filter via vacuum filtration and dried overnight at 80 °C.Medaka and eggs were freeze-dried at −50 °C overnight. Allsamples were weighed (dry) and digested whole in a MarsXpress microwave digester (CEM Corporation, Matthews, NC)in 40-mL PTFE bombs according to USEPA method 1052using ultrapure HNO3 (OmniTrace Ultra High Purity,Darmstadt, Germany). Digests were then transferred to 10-mL PTFE tubes and analyzed by ICP-MS at the Environmentaland Agricultural Testing Services Lab (Department of SoilSciences, NCSU). Quality control was monitored via sampleblanks (n = 11, no detection of Se above method detectionlimit of 0.3 μg L−1) and standard reference material (n = 5,NIST 2976 freeze-dried mussel tissue; analytical determination1.78 ± 0.06 μg g−1 (dry wt), expected 1.80 ± 0.15 μg g−1 (drywt)).Radioactivity Measurement. 75Selenium was obtained

from the Missouri University Research Reactor as selenous acid(H2

75SeO3; i.e., Se(IV)). All measurements of radioactivity inwater, periphyton, mayflies, fish, and eggs were performed usinga Wallac Wizard 1480 automatic gamma counter (Perkin-Elmer, Shelton, CT). All radioactive samples were counted for3 min, and all counting errors were ≤5%. All water sampleswere collected and analyzed in duplicate, and all periphytonsamples were collected and analyzed in triplicate. Seleniumconcentrations herein are reported incorporating appropriatecorrection for background (stable) [Se] in periphyton asreceived from SWRC and reported on a wet weight (wet wt) ordry weight (dry wt) basis.Mayfly Se Enrichment and Radiolabeling. All experi-

ments involved feeding Se-enriched, radiolabeled C. trianguliferlarvae to medaka. Mayflies were raised on a natural periphytondiet grown at SWRC and shipped to NCSU as previouslydescribed.11,12,16 Upon arrival each batch of periphyton plateswas randomly sampled (n = 3) and analyzed for background[Se] using ICP-MS. This analysis allowed us to account for thebackground Se naturally found in periphyton in our calculationsof the movement of Se through the simulated food chain usingthe radiotracer.Two different sets of periphyton plates (and mayflies grown

on those plates) were utilized. One set of periphyton plates

(grown in October 2011) was used to rear mayfly prey formeasuring medaka Se AE, tissue distribution, and efflux rateconstants, while the other (grown in November 2011) was usedfor medaka Se maternal transfer experiments. In both instancesperiphyton was loaded with Se by exposing it to dissolved Sefor 7 days in 2.0 L glass jars containing 1.8 L of synthetic softwater (48 mg L−1 NaHCO3, 30 mg L

−1 CaSO4·H2O, 30 mg L−1

MgSO4, and 2 mg L−1 KCl; pH 7.6 ± 0.4).20 Periphytonexposure solutions were spiked with both H2

75SeO3 andNa2SeO3, each dissolved in 0.1 N HNO3 (specific volumes andconcentrations below). Water samples were collected bothprior to adding periphyton to the solutions and at the end ofloading to characterize dissolved [Se]. Periphyton subsampleswere collected from each plate at the end of loading tocharacterize periphyton [Se].After loading and sampling, periphyton was moved to clean

water and newly hatched C. triangulifer larvae were reared untilthey were large enough to collect and feed to medaka (∼15−25d). Before feeding to medaka, mayfly larvae were rinsed in cleanwater, and a subsample was weighed wet and assayed forradioactivity to determine mayfly [Se].

Medaka Se Assimilation Efficiency, Tissue SpecificDistribution, and Efflux Rate. Periphyton was loaded withSe at three treatment levels: low (background), medium, andhigh. All three periphyton treatments received 1000 μL ofH2

75SeO3 (2.64 μCi mL−1; 2.77 μg Se L−1). The medium (3 μg

Se L−1) and high (10 μg Se L−1) treatments also received 27and 90 μL of a 200 μg Se mL−1 solution of Na2SeO3,respectively, whereas the low treatment received no stable Se inaddition to the radiotracer. In the low treatment the Seradiotracer was used to follow the movement of background Se.At the end of the loading phase, and after accounting forbackground [Se], the resulting periphyton Se concentrationswere 2.1 ± 0.3 (low), 4.9 ± 0.7 (medium), and 15.5 ± 4.4(high) μg g−1 (dry wt).Medaka Se AE was measured by feeding each of 22 fish a

single radiolabeled, Se-enriched C. triangulifer larva and assayingeach fish for radioactivity both after engulfing the mayfly andagain after defecation of the remaining undigested mayflycarcass. The use of γ-emitting isotopes allowed for the in vivomeasurement of Se mass in medaka. Assimilation efficiency wastaken as the % of retained Se following defecation by dividingthe total mass of Se (μg) in the fish after defecation by the totalmass of Se (μg) in the fish immediately after consuming themayfly prey. Mayfly larval masses ranged from 0.9 to 5.4 mg(wet wt) and average mayfly whole body [Se] was 1.6 ± 0.1 μgg−1 (wet wt; low), 4.2 ± 0.4 μg g−1 (wet wt; medium), and 8.4± 0.8 μg g−1 (wet wt; high). Assimilation efficiency wasassessed in six female fish per treatment level. Two male fishwere assessed at the medium and high treatment levels for basiccomparative purposes only as Se is known to be a femalereproductive toxicant, and therefore females were the primaryfocus of our experiments.Following AE measurement, fish were fed 2 additional

radiolabeled C. triangulifer larvae (from their respectivetreatment level) per day for 4 days. Fish were then fed newlyhatched Artemia nauplii for one day to evacuate gut contents ofradioactive food. Fish were then euthanized with MS-222(0.015% w/v), assayed for radioactivity, blotted dry, weighed,and individually dissected for collection of digestive tract, liver,ovary/testes, and remaining carcass tissue. For ovary dis-sections, no attempt was made to remove previtellogenic orearly vitellogenic follicles. Each tissue was weighed wet and

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assayed individually for radioactivity to determine distributionof newly acquired dietary Se. We further report the percentmass of each tissue out of the fish whole body mass (wet wtbasis) for comparison to the Se burden of those tissues.Se efflux rate constants (ke) of both male and female medaka

were determined by feeding a separate cohort of medakaradiolabeled mayfly larvae and following the loss of assimilatedradiotracer. For this experiment only mayfly larvae from thehigh treatment level (8.4 ± 0.8 μg g−1 (wet wt)) were utilizedas prey items. Six female fish and four male fish were each fed asingle radiolabeled mayfly larva. Fish were individually assayedfor radioactivity following defecation of the undigested mayflycarcass, returned to clean water, and fed a control diet ofArtemia nauplii twice daily. Fish were serially assayed daily for 7days to follow the loss of 75Se with daily water refreshment toprevent unintended absorption of excreted Se.Medaka Se Maternal Transfer. Medaka Se maternal

transfer was measured by feeding female fish radiolabeledmayfly larvae and following the movement of tracer into eggs.Medaka spawn asynchronously, typically producing a dailyclutch of eggs for several days. Periphyton was loaded with Seat a single treatment level using 489 μL of H2

75SeO3 (specificactivity, 2.22 μCi mL−1; [Se], 2.7 μg Se L−1) and 27 μL of 200μg mL−1 Na2SeO3 (final dissolved [Se], 3 μg L−1). Followingloading, periphyton [Se] was 4.7 ± 1.7 μg g−1 (dry wt). Newlyhatched C. triangulifer larvae were exposed to treatment dietsfor ∼25 d then removed and rinsed for fish feeding. Theaverage mayfly whole body [Se] was 4.1 ± 1.0 μg g−1 (wet wt).Twelve female fish were individually housed and fed radio-labeled mayflies ad libidum for 4 days. In general, three mayflieswere continuously available for consumption throughout theday. Fish consumed 18−30 larvae (mean larval mass, 1.5 ± 0.6mg wet wt) over the 4-day period with an average of 6.2 larvaeconsumed per fish per day. Fish were given ∼20 h to purgetheir guts of radiotracer by feeding Artemia nauplii, and thentwo male fish were introduced to each female tank to stimulateegg production and fertilize spawned eggs. During egg layingfish were fed a control diet of Artemia nauplii twice daily. Eachmorning tanks were checked, and if a given female produced aclutch of eggs the female was removed, eggs were collected, andboth were assayed for radioactivity. Only complete egg clutchesthat were adherent to the females were collected and assayed.Following radioactivity determination females were returned totheir tanks for a total of 6 days of possible egg laying. Aftercollecting eggs on day six, female fish were euthanized in MS-222 (0.015% w/v), weighed, and assayed for radioactivity.Ovaries were dissected (as above), weighed, and assayed forradioactivity.Fish [Se] and egg [Se] were calculated based on the mass of

newly acquired Se from days 0, 1, 2, and 3 only because eggradioactivity counts decreased below the level of acceptablecounting error (i.e., >5%) by day 4 of the experiment. In orderto estimate egg dry mass we subsequently collected eggs fromstock medaka females (n = 10) and determined the relationshipbetween clutch size and total egg dry mass:

= −Y x0.2719 0.0129

where x is the number of eggs in a clutch and Y is the totalestimated dry mass (mg) of those eggs (r2 = 0.997, p < 0.0001).Similarly, fish whole body dry mass was estimated bydetermining the whole body moisture content of stock females(72.5 ± 1.3%, n = 6). We assayed for radioactivity daily;however, we only measured the wet weight of the females at the

end of the experiment. Therefore, we used the female bodymass on day 6 to calculate the whole body [Se] on days 0, 1, 2,and 3. We suspect that female body mass did not changeappreciably over the spawning period; however, we acknowl-edge that it may have slightly decreased from days 0−6 due toegg release, in which case our estimates of whole body [Se] inrelation to egg [Se] are conservative.We were also interested in the pattern of Se movement from

the mayfly diet to the eggs. We estimated the average total mass(ng) of Se consumed by females by multiplying the number oflarva consumed each day by the average mass of larvae (1.5 mgwet wt) and the average [Se] concentration of the larvae (4.1 ±1.0 μg g−1 (wet wt)). Egg [Se] was calculated as the mean egg[Se] of all egg clutches oviposited on a given day.

Statistical Analyses. All data reported represent mean ±standard deviation unless otherwise noted. Further, all dataanalyses were performed using Prism (v6.02, GraphPadSoftware, Inc., La Jolla, CA) at α = 0.05. Background medaka[Se] in females and males were compared using t test.Assimilation efficiency was compared in females and malesusing t test and assessed as a function of mayfly [Se] and mayflymass using simple linear regression.Efflux rate constants (ke) were calculated as the slope of the

natural log of the proportion of metal retained in the fish tissuerelative to the initial Se burden as a function of depuration timeusing the following equation:

= −C C etk t

ie

where Ct is the mass of Se in the fish at time t (μg), Ci is theinitial mass of Se (μg), and t is the time (d). The efflux rateconstant (ke) only considers physiological excretion, as definedby Luoma and Rainbow,21 and does not include egg deposition.To calculate ke in reproductively active females we added the Semass (μg) in daily egg clutches back to the Se mass in thefemale (μg) and found the slope of the proportion of Seretained using the above equation. We further calculated thetotal Se loss rate constant (ke+egg) in reproductively activefemales, which combined the Se loss due to physiologicalexcretion and egg deposition. To calculate the total loss rateconstant (ke+egg) we simply took the slope of the proportion ofSe retained using only the Se mass in the female (μg) each dayassayed after oviposition (identical equation to ke above).Fish whole body [Se] and ovary [Se] (both wet wt basis)

were compared using simple linear regression for bothreproductively active females (post egg spawning, from thematernal transfer experiment) and reproductively inactivefemales (from the AE and tissue distribution experiment).Comparing slopes of these regressions allowed us to determineif the ovary becomes depleted in newly acquired Se relative tothe whole body due to the process of Se deposition into eggs.Fish whole body [Se] and egg [Se] (both dry wt basis) were

compared based on egg:whole body [Se] ratios for each day (0,1, 2, and 3 of egg production). Egg:whole body [Se] wascalculated for each individual fish by dividing the egg [Se] bythe fish whole body [Se] and calculating the mean ratio acrossreplicate fish.

■ RESULTS

Background Se Concentrations. Selenium concentra-tions in medaka males and females, Artemia nauplii, andmedaka eggs were measured in order to determine backgroundlevels and provide a reference for the radiotracer experiments.

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Female [Se] was nearly identical between those that wereactively producing eggs (1.85 ± 0.22 μg g−1 (dry wt)) andthose that were not (1.85 ± 0.20 μg g−1 (dry wt)). Male [Se](1.66 ± 0.06 μg g−1 (dry wt)) was marginally lower than female[Se] on average; however, neither this difference (t test, p =0.210) nor moisture content (overall mean, 72.5 ± 1.3%; t test,p = 0.553) were significantly different. Medaka cultured onArtemia nauplii with Se concentrations of 1.74 ± 0.15 μg g−1

(dry wt) had trophic transfer factors (TTF) that ranged from0.84 to 1.24. Finally, the medaka egg [Se] (1.84 ± 0.11 μg g−1

(dry wt); moisture, 79.5 ± 1.6%) was remarkably consistentwith the female whole body [Se] (and the average Artemia[Se]) producing an egg:whole body [Se] of 1.0-fold.Medaka Se Assimilation Efficiency. Medaka assimilated

63.2 ± 8.8% (range 45.6−76.7%) of the Se in their mayfly diets,on average, with no significant difference between male andfemale AE (t test, p = 0.307). There was also no significanteffect of mayfly [Se] on medaka AE (linear regression, p =0.157; Figure 1a) across the dietary range used here (1.4−9.7μg g−1 (wet wt basis) or 5.6−38.7 μg g−1 (dry wt basis)). Theonly factor that significantly affected medaka Se AE was the sizeof the larva ingested, where larger larvae yielded lower AEs (y =−3.5x + 74, where x = mayfly mass (mg wet wt) and y =medaka AE; r2 = 0.38, p = 0.0064, Figure 1b).Medaka Tissue-Specific Se Distribution. Tissue-specific

Se distribution was assessed in both female and male medakaafter consuming radiolabeled mayfly larvae followed by a chase(purging of radiolabeled gut contents) of Artemia nauplii.Overall, the mass of Se in each tissue compartment increasedwith treatment level; however, the proportional distribution wasnot significantly different across treatments. Female and malemedaka distributed nearly identical proportions of Se to thedigestive tract (18 ± 4% and 17 ± 5% in females and males,respectively) and liver (15 ± 3% and 14 ± 3% in females andmales, respectively) (Figure 2). These tissue compartments alsocomprised a similar proportion of the overall body mass (5 ±1%, 4 ± 1% for digestive tract and 3 ± 1%, 3 ± 1% for liver infemales and males, respectively).The major difference in tissue-specific Se distribution

between female and male medaka was in reproductive tissue.Female medaka partitioned 24 ± 6% of their newly acquired Seto ovary tissue, while males partitioned only 0.8 ± 0.3% to theirtestes. This was largely explained by tissue mass where theovary comprised 10 ± 3% of the body mass in females, whereasthe testes comprised only 0.3 ± 0.1% of the body mass inmales. The difference in reproductive tissue Se distribution

between females and males resulted in a large difference in Separtitioning to the remaining carcass. Females partitioned 42 ±7% of their Se to the remaining carcass, and males partitioned64 ± 7%, which was also largely explained by the difference inproportional body mass of the carcass (75 ± 4% in females and87 ± 1% in males).

Medaka Se Efflux. Selenium efflux was determined inmales and both reproductively active and reproductivelyinactive females. There was no significant difference in kebetween males (0.069 ± 0.011 d−1) and reproductively inactive

Figure 1. Medaka Se assimilation efficiency (AE) as a function of dietary (C. triangulifer larva) [Se] (a) and C. triangulifer larval size (b). Each datapoint represents a single female medaka fed a single radiolabeled mayfly larva. Assimilation efficiency did not change as a function of dietary [Se];however, there was a significant decrease in AE as larval size increased.

Figure 2. Tissue-specific Se distribution (large pie charts) and relativetissue mass recovered from (small pie charts, wet wt basis) female(upper pie charts) and male (lower pie charts) medaka. Valuesrepresent mean ± SD of the proportion of Se in each tissuecompartment and the proportional weight of each tissue out of thewhole fish Se and fish mass, respectively. Proportions do not add up to100% due to some loss of body fluid during dissections.

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females (0.066 ± 0.010 d−1) (p = 0.591). Females were furtherassessed comparatively between those that were reproductivelyactive (producing eggs; new cohort of female fish) and thosethat were not (same females from comparison with malesabove). After correcting for loss of Se to the eggs, the ke inreproductively active females was 0.069 ± 0.013 d−1, which wasnot significantly different than the ke of reproductively inactivefemales (0.066 ± 0.010 d−1; p = 0.848). Reproductively activefemales displayed a total Se loss rate constant (ke+egg, i.e.,excretion and egg deposition) of 0.17 ± 0.019 d−1, which was∼2.5-fold greater than the ke alone.Medaka Se Maternal Transfer. Because of the importance

of reproductive tissue in the assessment of fish populationsimpacted by Se contamination we examined the relationshipsbetween whole body [Se] and ovary [Se] as well as whole body[Se] and egg [Se]. Ovary [Se] increased as whole body [Se]increased in both reproductively inactive females and femalesthat had been spawning eggs for 6 d (Figure 3). Reproductively

inactive females had ovaries that were ∼2.5-fold greater in [Se](wet wt basis) than the whole body concentration, whereas theovaries of females that had been laying eggs were only ∼1.5-fold greater in [Se] (wet wt basis) than the whole body.Further, the ovary tissue comprised a similar proportion of thetotal body mass whether the female was reproductively inactive(10.3 ± 3.2%) or had been spawning eggs (10.1 ± 3.5%), yetthe ovary contributed a smaller percentage (15.8 ± 4.6%) of thetotal Se body burden in post-spawning females than inreproductively inactive females (23.8 ± 5.8%; p = 0.0004). Itappears that the ovary is a preferential site for Se distribution(along with gut and liver tissue) and that the ovary becomesdepleted in Se during the process of egg production (relative tothe whole body) presumably due to egg Se deposition.Egg [Se] was variable day-to-day, whereas fish whole body

[Se] tended to decrease for days 0−3 of egg productionresulting in a range of egg:whole body [Se]: 1.8 ± 0.3 (day 0),2.9 ± 0.7 (day 1), 2.4 ± 0.8 (day 2), 2.2 ± 0.5 (day 3).Interestingly, the pulse of Se (total mass basis) from the mayflydiet was mimicked in the pattern of egg [Se] across days(Figure 4). The greatest consumption of dietary Se frommayflies (60.8 ± 14.4 ng Se, day −2) occurred ∼72 h prior to

the greatest deposition of newly acquired Se into eggs (1.0 ±0.4 μg g−1 (dry wt), day 1), which corresponds to the period ofvitellogenesis and oocyte maturation in medaka.22−24

■ DISCUSSIONAssimilation efficiency is a key determinant in Se bioaccumu-lation. Here, we observed medaka AEs that ranged from 45.6%to 76.7% with no trend across a wide range (5.6−38.7 μg g−1

(dry wt)) of dietary [Se]. The mean AE from our study (63.2 ±8.8%) was within the range of fish Se AEs from the literature,which can be as low as 12.7 ± 2.7% (grunt (Terapon jarbua) fedbarnacles (Balanus cirratus))25 and up to 76.6 ± 15.8%(Mediterranean sea bream (Sparus auratus) fed Artemia salinanauplii).26 However, the majority of fish Se AEs tend to be>50%.27 To our knowledge, no other studies have investigatedthe effect of prey size on Se AE, which seems relevant becausefish feeding behavior is dominated by engulfing whole prey (i.e.,no mastication).28 Here, medaka Se AE decreased at a rate of∼3.5% (95% confidence interval, 5.7−1.1%) per gram increasein mayfly mass (wet wt). We speculate that the reduction ofmedaka AE as a result of increasing larval size was due toincomplete digestion of the intact (nonmasticated) larvae. It isevident from this study and the literature that fish Se AE ishighly variable and dependent on a number of factors includingfish species,29−32 prey species,33 prey [Se],34 prey size, ingestionrate,33 gut passage time,35 fish size,36 and fish age.35

There is a noticeable lack of consistency in the tissuecompartments chosen for assessing fish Se distribution.Compartments range from a variety of major internal organs(e.g., mix of liver, kidneys, gills, digestive tract, ovaries/testes)26,29,31,33,37−39 to generic body sections (e.g., head,tail).35 Ni et al.31 chose similar compartments to thoseinvestigated here in their study of the intertidal mudskipper(Periophthalmus cantonensis). Mudskippers partitioned Seroughly similar to medaka with ∼25% digestive tract, ∼7%liver, and ∼60−65% carcass. Tissue Se distributions are alsohighly dynamic, though. Mathews et al.26 displayed thevariability of fish Se tissue distribution as a function of timewhere the proportion of Se decreased in the gill and digestivetract and increased in the liver over a 2-week period.In regard to Se water quality criteria, which is focused on

protection of fish reproduction, the ovary is a primary organ of

Figure 3. Newly acquired dietary Se (i.e., radiotracer-associated) inmedaka ovary [Se] as a function of whole body [Se] in reproductivelyinactive females (gray squares) and reproductively active females post-egg laying (white circles). Each data point represents a replicate femalewith concentrations reported on a wet weight basis.

Figure 4. Time course of Se ingestion (total mass basis) from mayflydiet (left y-axis, gray bars) and subsequent change in egg [Se] (right y-axis, white bars). It appears that the pulse of Se in the diet is reflectedin the pulse of Se in egg clutches, displaying the connection betweendietary Se ingestion and Se deposition into eggs for medaka(asynchronous spawning strategy). Bars with different capital orlower case letters (separate ANOVAs for Se ingestion mass and egg[Se]) are significantly different. Bars represent mean ± SE.

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interest for assessing Se distribution. We observed a change inSe ovary distribution in response to oviposition. Ovarycontribution to the total Se body burden was significantlygreater in females before laying eggs (23.8 ± 5.8%) than post-egg laying (15.8 ± 4.6%). Alquezar et al.29 determined thefemale toadfish (Tetractenos glaber) ovary contained 8.7−11%of the total Se burden, which was lower than we found in bothnon-ovipositing and post-ovipositing medaka. The majority ofSe radiotracer studies we found did not examine ovarydistribution, however. Similar to Se AE, it is clear that tissueSe distribution is highly dynamic with variability reported inregard to dissolved versus dietary Se exposure,29 fish species,40

fish sex, and timing of tissue sampling.26 Our results indicatethat the ovary may become depleted in Se during the spawningprocess relative to the whole body as a result of Se depositioninto eggs producing variable egg:ovary [Se] ratios that dependon the timing of the female reproductive cycle as opposed to aconstant ratio for a given species.Selenium efflux was more consistent than AE, tissue

distribution, and egg deposition. Here, we found that bothmale and female medaka had similar ke’s and that females hadsimilar ke’s regardless of whether they were also activelydepositing Se into eggs. In comparison, fish Se ke’s in theliterature range from as low as 0.019 d−1 (intertidal mudskipper(Periophthalmus cantonensis))31 to as high as 0.13 d−1 (juvenileMediterranean sea bream (Sparus auratus)).26 These rateconstants can be affected by fish age35 (reduced in older fish),dietary versus dissolved Se exposure31 (reduced followingdietary Se exposure), and fish species.26 Overall though, ke’s forSe from the literature tend to fall in the range of 0.03−0.05d−1.29−31,33,36 In contrast, the total Se loss rate constant (ke+egg,0.17 d−1) we calculated for reproductively active females was∼2.5-fold greater than the ke alone. To our knowledge, no otherpublished studies have identified the discrepancy in ke versusthe total loss rate constant due to excretion and egg deposition(ke+egg). On the basis of our results, egg deposition appears toplay a major role in the trafficking and loss of newly acquireddietary Se in medaka.One potential modifying factor in fish egg Se deposition is

the variety of spawning strategies.1 Generically, fish tend to fallinto one of three spawning strategy categories: synchronous,group synchronous, and asynchronous.23 Synchronous spawn-ers ovulate all oocytes simultaneously, whereas groupsynchronous spawners have at least two populations ofmaturing oocytes to be spawned discretely. Asynchronousspawners have oocytes present in all stages of developmentduring the reproductive season. Medaka are asynchronous andspawn daily clutches of eggs (typically 20−40 eggs) for up to 3months during the reproductive season.23,24 We were able toexploit this reproductive strategy for a short time to observe themovement of radiolabeled Se from the diet to the eggs.In cultured medaka, background egg [Se] was nearly

identical to the whole body [Se] and the Artemia diet [Se](i.e., egg:whole body [Se] of 1.0-fold). However, when wetransiently increased dietary [Se] to ∼16 μg g−1 (dry wt) theegg [Se] increased (up to 2.9-fold, egg:whole body [Se]) inrelation to the mass of Se consumed. A similar pattern wasdescribed by Lacoue-Labarthe et al.41 in the cuttlefish (Sepiaof f icinalis), which is also an asynchronous spawning fish. Incomparison, Coyle et al.37 reported that bluegill fed abackground Se diet (0.8 μg g−1 (dry wt)) produced eggs thatwere ∼2.1-fold more concentrated than the whole body, butwhen the diet was increased to 16.8 μg g−1 the egg:whole body

[Se] increased to ∼2.8-fold. Bluegill are synchronous spawners,which may explain the greater range in egg:whole body [Se] inour study (1.0−2.9-fold) relative to the range observed byCoyle et al.37 (2.1−2.8-fold). Overall, it appears medaka maydeposit Se directly from the diet to the eggs (via the ovary),producing egg [Se] that is determined by recent ingestion ofdietary Se more than the maternal whole body [Se].Spawning strategy is a potentially important process to

consider when assessing risk to fish populations in Se-contaminated systems. Depending on the home range of agiven fish species, multiple stream reaches of a watershed maybe traversed and dietary items consumed across a range ofcontamination. With asynchronous spawning fish eventransient shifts in dietary [Se] may have a large impact onegg [Se] and overall reproductive success. Further, fish speciesthat utilize synchronous (or group synchronous) spawningstrategies may also be prone to shifting egg:whole body [Se]ratios depending on changes in dietary [Se],37 albeit potentiallyless dramatically. These intraspecific shifts in egg:whole body orovary:whole body [Se] increase variability and weaken ourability to accurately predict egg [Se] based on other tissues,potentially leading to either over- or underestimation. Forexample, if we use the egg:whole body [Se] from spawning day0 (1.8-fold) to predict the average egg [Se] on the followingday, the result is a significant underestimation of the actualaverage egg [Se] (predicted, 0.7 ± 0.1; actual, 1.2 ± 0.4 μg g−1

(dry wt); p = 0.04). Conversely, a fish with an elevated wholebody [Se] transiently consuming low dietary [Se] during eggproduction may skew the prediction in the opposite direction(i.e., overestimation of egg [Se]).Mechanistically Se is known to largely be deposited in fish

eggs during the process of vitellogenesis (yolk proteindevelopment).1,7,41 Medaka are estimated to complete vitello-genesis and oocyte maturation within 72 h of oviposition.22−24

Here, medaka exhibited a bell-shaped pattern of Se acquisitionfrom the diet with a relatively similar bell-shaped pattern ofsubsequent egg [Se]. It appears that a substantial portion of Seincorporated into the eggs during vitellogenesis was derivedfrom recent dietary consumption based on this pattern and therapid appearance of Se radiotracer in eggs. Therefore, thetiming of oogenesis in relation to dietary Se consumption maydictate the deposition of Se into eggs. It is unclear whether thisobservation is unique to medaka (although a similar pattern hasbeen reported in mallards42), typical of asynchronous spawners,or typical of Se trafficking in fish overall.Our findings suggest that, within medaka, egg:whole body

[Se] is not fixed but instead linked to recent dietary Seconsumption. Thus, the application of a universal ratio forestimating egg [Se] based on other tissues (whole body,muscle, or ovary [Se]) should appropriately incorporate theinherent variability both within and among fish species. Forexample, here we found egg:whole body [Se] ranged from 1.0-to 3.8-fold (absolute range; range of means, 1.0−2.9-fold) inmedaka, whereas other researchers have found egg:muscle [Se]ranging from 1.0- to 10-fold across 8 different species.1,40 If Sewater quality criteria are to be based on fish tissue, it may beprudent to characterize the distribution of egg:whole body [Se]both within and among a wide range of species in order tocapture that variability. More fully understanding the range ofuncertainty that is inherent to this biological process could thenallow for the application of appropriate safety factors and/orthe selection of an appropriately conservative ratio based on awell characterized distribution.

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Alternatively, our results suggest that quantifying the [Se] ofcommon prey items within a given aquatic system may be areasonable approach to determining the potential for toxiceffects of Se on fish. However, it remains uncertain to whatdegree the deposition of dietary Se in eggs (via the ovary) is afunction of medaka physiology or is attributable to spawningstrategy. Overall, the aquatic biogeochemistry, food webincorporation, and physiological dynamics of Se in fish areextremely complex2,7 and variable, making it difficult to adoptgeneralizations about the movement of Se through aquaticsystems and potential for toxic effects in biota. Despite theintensity of debate over the appropriate way to regulate Se andcontroversy regarding an adequate criterion level for protection,there is still a significant shortfall in our understanding of Sedynamics in aquatic organisms and the degree to which theseprocesses vary.

■ AUTHOR INFORMATIONCorresponding Author*Phone: 919-513-1129. Fax: 919-515-7169. E-mail:dbbuchwa@ncsu.edu.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe thank Tom Augspurger (USFWS), Allison Camp (NCSU),Seth Kullman (NCSU), Gerald LeBlanc (NCSU), MonicaPoteat (NCSU), Joseph Skorupa (USFWS), and threeanonymous reviewers for revising earlier drafts of themanuscript; Seth Kullman for supplying medaka; Dave Funk(SWRC) for providing periphyton plates and initial mayflyculture material; Kim Hutchison (NCSU) for analyticaldeterminations; and Dave Hinton (Duke) for assistance inthe preliminary pilot experiments. Funding for J.M.C. wasprovided by a USEPA STAR Fellowship (GAD no. FP917322)and the SETAC/ICA Chris Lee Award for Metals Research.The USEPA has not formally reviewed this publication, and theviews expressed herein may not reflect the views of the USEPA.

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