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CHAPTER 8 LABORATORY EXPERIMENTS ON THE UPTAKE, ACCUMULATION, AND LOSS OF RADIONUCLIDES BY MARINE ORGANISMS1 HOWARD BOROUGHS, Hawaii Marine Laboratory, University of Hawaii, Honolulu, Hawaii WALTER A. CHIPMAN, Fishery Radiobiological Laboratory, U. S. Fish and Wildlife Service, Beaufort, North Carolina THEODORE R. RICE, Fishery Radiobiological Laboratory, U. S. Fish and Wildlife Service, Beaufort, North Carolina WHAT happens to radioactive materials that are introduced into the oceans may be studied by a marine biologist from at least two points of view. As a physiologist, he will be interested in the uptake, accumulation, and loss of radioele- ments as a function of the element, and its con- centration; in the physical factors of tempera- ture, light, and salinity; and in differences between species of organisms, as well as their age and sex, to mention some of the most im- portant parameters. As an ecologist, he will be interested in these same parameters under a steady-state condition. The physiologist would profit most by exposure of the organism to a single dose of radioactive material, while the ecologist must concern himself with the results of chronic exposure. Both types of biologists may be interested in tracing the history of an element through the food webs of the various trophic levels. Un- fortunately, the experimental data involving the metabolism of radionuclides by marine organ- isms is extremely meager. In this section some experiments will be described on the uptake, accumulation, and loss of radionuclides by vari- ous marine organisms in the three trophic levels. It must be emphasized that the results of these few experiments must be extrapolated with ex- treme caution in predicting what may happen to radioactive materials introduced into the oceans from nuclear reactor plants, bomb deto- nations, or from any other sources. 1 Work performed at the Fishery Radiobiological Laboratory of the U. S. Fish and Wildlife Service and the Hawaii Marine Laboratory (Drs. H. Boroughs, S. J. Townsley, and R. W. Hiatt). Contribution No. 95, Hawaii Marine Laboratory. In discussing the uptake of radionuclides by marine organisms, it is sometimes difficult to state exactly what constitutes a single or a chronic exposure. For a unicellular alga, a few hours may represent chronic exposure, while a few weeks may be insufficient for a fish to reach a steady-state condition. No long-term repeti- tive feeding experiments have been done, so for the purpose of this report, we will discuss the metabolism of the various radionuclides solely on the basis of the trophic level concerned. The term uptake implies passage through a membrane. Radioactive material may be pres- ent in the gut of an organism, but until it enters the organism through a membrane, it can play no role in the metabolism of that organism ex- cept by producing radiation effects or by inter- fering with a chemical reaction occurring within the gut. In some of the experiments to be de- scribed, particularly those involving phytoplank- ton, it was not established whether or not the radioisotope was actually incorporated into the organism, or merely adsorbed to the surface. For simplicity, we will therefore discuss uptake in the sense that the radioisotope is associated with the organ or organism in question. Isotopes o? a given element usually have similar chemical behavior, so that in tracing the path of most elements in biological systems, it can be assumed that a radioactive atom will be- have in the same way as a non-radioactive atom of the same species. The only parameters to be considered in the discussion to follow will be the species and age of the organism, the ele- ment, the concentration of the element, the tem- perature, and the duration of exposure or treat- ment. No work using radioisotopes has been 80

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Chapter 8 81 Laboratory Experiments on Uptake done on the mineral metabolism of marine or- ganisms relative to sex. The data that will be presented were collected either at the Fishery Radiobiological Laboratory of the United States Fish and Wildlife Service (R.L.F.W.S.) or the Hawaii Marine Laboratory, University of Hawaii (HML). First trophic level Experiments performed at the R.L.F.W.S. very clearly show that different species of planktonic algae have remarkably different abili- ties to concentrate a particular element from the sea water medium. Algae were grown in the presence of radiostrontium obtained from Oak TABLE 1 THE DIFFERENTIAL UPTAKE OF RADIO- ACTIVE STRONTIUM AND YTTRIUM BY ALGAE Percentage Percentage activity activity from from Species strontium yttrium Carteria sp 100.0 0.0 Thoracomonas sp 50.4 49.6 Amphora sp 10.0 90.0 Navicula sp 8.5 91.5 Chromolina sp 8.2 91.8 Chlamydomonas sp 6.5 93.5 Nitzschia closterium 6.0 94.0 Nannochloris atomus 5.7 94.3 Chlorella sp 5.3 94.7 Porphyridium curentum . .. 4.4 95.6 Gymnodinium splendins ... 4.1 95.9 Gyrodinium sp 2.3 97.7 Ridge. The material used contained both Sr99 and Sr90; the latter decays to form Y90. By counting the algal samples immediately after they were removed from the culture medium, and again after several weeks, in order to allow the secular equilibrium of the Sr90-Y90 pair to be reached, it was possible to determine what percentage of the original radioactivity was due to strontium. Table 1 shows that Carteria sp. accumulated strontium 89 and 90 from the iso- topic mixture, and that Gyrodinium sp. removed almost no strontium 89 or 90, but instead ac- cumulated yttrium 90. It was found that Nitz- schia closterium under an apparent steady state condition concentrated strontium 17 times over its concentration in sea water (weight of algae/ weight of water). The concentration factor for strontium Carteria sp. was found to vary with condition of culture but was much greater than for Nitzschia closterium. Experiments using cesium127 show that while different species concentrate cesium to different degrees (Table 2) none of the nine species TABLE 2 CONCENTRATION OF CESIUM BY MARINE ALGAE Concentration Algae factor1 Bacillariaceae Nitzschia closterium 1.2 Amphora sp 1.5 Nitzschia sp 1.7 Chlorophyceae Chlamydomonas sp 1.3 Carteria sp 1.3 Chlorella sp 2.4 Pyramimonas sp 2.6 Nannochloris atomus 3.1 Rhodophyceae Porphyridium curentum 1.3 1 The concentration factor is reported as the ratio of Csm in the algae (wet weight) to that in an equivalent weight of sea water at an apparent steady- state condition. tested from three families showed any marked concentration of this element from sea water. The effect of the concentration of an element on its uptake by Nitzschia cells is shown in Figure 1. Nitzschia cells were grown in sea water to which had been added labelled zinc at three different concentrations. From the graph 100 e o.img./f. • 5 mg./t. 40 60 HOURS FIGURE 1. Uptake of Zinc" by Nitzschia Cells from Culture Medium Containing Different Concen- trations of Zinc. © 0.1 mg./l O 1 mg./l • '5 mg./l

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82 Atomic Radiation and Oceanography and Fisheries it is evident that at low concentrations all the zinc was removed after about four days. The lowest concentration used was still ten times higher than the average zinc concentration of sea water. The rate of uptake of zinc65 by Nitzschia cells is shown in Figure 2. At the normal con- 100 456 HOURS FIGURE 2. Uptake of Zinc" by Nitzschia Cells from Culture Medium Containing 10 Micrograms of Zinc/Liter. centration of zinc in sea water, a dividing cul- ture of Nitzschia depleted the zinc65 in a closed system in less than one day. Apparently phyto- plankton cells concentrate zinc relative to sea water and any radioactive zinc present in the water will be quickly taken up in large amounts. The radioisotopes so far discussed are very likely always ionic in sea water. Ruthenium solution, however, forms colloids and particles when put into sea water. Ruthenium100 ob- tained as an acid solution from Oak Ridge was added to a sea water culture of Nitzschia cells. Figure 3 shows that the cells continued to take up the ruthenium for the 12 days of the experi- ment. The amount of ruthenium per cell de- creased, however, since the cells of the culture were dividing continually. One may conclude from this experiment, that since the ruthenium concentration in sea water is low, dividing planktonic algae would take up large amounts of any radioactive ruthenium present. Second trophic level The work reported in this section was also done at the R.L.F.W.S. Larvae of the brine shrimp Artemia were put into filtered sea water containing radiostrontium and the daughter FIGURE 3. Uptake of Ru"* by Nitzjcbia Cultures in the Light. yttrium00. These larvae rapidly took up the SR59-Sr90Y90 and reached an apparent steady- state in a few hours. After exposure of the or- ganisms to the isotopes for one day, it was found that the amount of radioactivity/g of Artemia was only 70 per cent of that of an equal weight of the sea water. A count of the samples 30 days after their preparation indi- cated that a considerable amount of Y90 was taken up. Other crustaceans used were the shrimp Penaeus setiferus and the edible blue crab Callinectes sapidus. The molluscan shell- fish studied included oysters (Crassostrea vir- ginica), clams (Venus mercenaria), and scal- lops (Pecten irradians). All of these organisms accumulated stron- tium rapidly from sea water. The internal dis- tribution of strontium in oysters is shown in Table 3. This table indicates that the bulk of the radioactivity accumulates in the shell. When TABLE 3 DISTRIBUTION OF RADIOACTIVITY IN OYSTERS FOLLOWING EXPOSURE TO SEA WATER CONTAINING SR* Per cent of total Tissues weight Mantle 2.5 Gills 1.7 Adductor muscle 1.9 Other 3.8 Total soft tissues 9.9 Shell . .90.1 Per cent of Per cent Per cent activity of total of soft of soft activity tissues tissues 4.1 25.0 27.7 5.1 17.5 21.2 2.4 19.2 16.2 5.1 38.3 34.9 14.7 — «• 85.3 — —

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Chapter 8 83 Laboratory Experiments on Uptake the radioactive shellfish were returned to a nor- mal sea water environment, the radioactivity present in the soft tissues declined within one day to 10 per cent or less of the maximum con- centration. This residual amount was held by the tissues for several days. The uptake of radiostrontium by oysters from food was studied by growing Carteria cells in sea water to which Sr29 was added. Oysters in Sr89 sea water served as the controls; the treated oysters were kept in Sr29 sea water to which the labelled Carteria cells were added. Fresh sea water and plankton suspensions were prepared each day. The curves in Figure 4 show that an OYSTERS FED WITH ALGAC ACTIVITY V SEA WATER t AL4AE UNfCD OYSTERS ACTIVITY OF SEA WATER FIGURE 4. The Increased Accumulation of Sr* by Oysters Feeding on Sr*-Fed Algae. apparent steady-state is reached in eight days. In the unfed oysters the concentration of Sr59 in the soft parts is approximately the same as the concentration in the sea water. The oysters which fed on the radioactive algae, however, concentrated the Sr29 by a factor slightly greater than two, based on the radioactivity of the sus- pension per unit of weight. These filter-feeding organisms removed the algal cells from many volumes of water. The uptake of cesium157 by clams, Venus mercenaria I*., is shown in Figure 5. At the end of 20 days the soft parts of clams had con- centrated the cesium by a factor of six over the cesium concentration of sea water. Obviously a steady state had not occurred, so that it is not possible to say what the final concentration fac- tor of clams might be for cesium in solution. Similar experiments using the bay scallop, Pec- ten trradians L., show that the concentration factor of cesium is greater than eight, since the uptake was still increasing at the end of 10 days. SIA WATER ACTIVITY FIGURE 5. The Accumulation of Cesium1*7 by Clams as a Function of Time. Bay scallops immersed for two hours in sea water containing Znos very rapidly accumulated this isotope. Table 4 lists the internal distribu- tion of zinc05 in the various tissues. The con- centration factor for each organ is readily cal- culated since the activity of the sea water was 10 nfytc/g. This means that the figures given in the second column divided by 10 equal the con- centration factors. The over-all concentration factor of the soft tissues of the bay scallop was 20 for this short interval. Other observations showed that these scallops contained close to 35,000y of zinc per gram (wet weight) and thus had a concentration factor for this element of about 3500. Oysters that were kept in sea water with added Zn65 also quickly accumulated the iso- tope to very high levels. The zinc content of fresh oyster tissue measured almost 170,000y per gram. This represents a concentration fac- tor of 17,000, since the zinc concentration of the sea water in which the oysters lived was about 10 mcgm/1. Ruthenium100 was one of the separated fis- sion products used to study the uptake of par- ticulate radioisotopes by organisms in the sec- ond trophic level. Ruthenium was co-precipi- tated with calcium carbonate, dried, and ground TABLE 4 DISTRIBUTION OF ZN™ IN THE ORGANS OF THE BAY SCALLOP AFTER A Two HOUR IMMERSION Tissue rape Zn"/g. Total m/ic Kidney 1384 824 liver 243 507 Gills 218 857 Testes and ovaries 138 193 Foot 131 25 Rectum 120 8 Heart 105 13 Adductor muscle 100 375 Mantle 92 321

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84 Atomic Radiation and Oceanography and Fisheries to a very fine state. Plutei of the sea urchin, Arbacia punctulata were put into sea water con- taining the radioruthenium which was kept in suspension by aerating the culture flask. After 18 hours, the larvae were rinsed and resus- pended in fresh sea water. Aliquots of larvae were then removed at intervals and tested for radioactivity (Table 5). A microscopic ex- TABLE 5 THE DECREASE OF Ru10° IN SEA URCHIN LARVAE AS A FUNCTION OF TIME IN NON-RADIOACTIVE WATER Hours 1 .. 4 .. 8 .. Radioactivity in 500 larvae (counts/minute) 1413 179 148 amination of the larvae at zero time showed that the intestines were filled with the radioactive particulate material, but at 8 hours, very little material was left in the gut. Apparently little ruthenium was actually absorbed through the digestive tract. The ingestion of the particulate (co-precipi- tated) ruthenium by the bay scallop, Pecten irradians, also indicated that the radioactivity was mostly associated with the digestive tract. The crystalline style was highly radioactive, al- though the radioactivity in it decreased during the five days the scallops were kept in running water. The hepatopancreas, on the other hand, showed an increase in radioactivity during this time. No radioactivity was found associated with the internal organs other than those in the digestive tract. Third trophic level The uptake, accumulation, and loss of radio- nuclides has been studied in many fishes by both the R.L.F.W.S. and the H.M.L. These fishes include the skipjack tuna (Euthynnus yaito), yellowfin tuna (Neothunnus macrop- terus), dolphin (Coryphaena hippurus), papio (Carangoides ajax), aholehole (Kuhlia sand- vicensis), Tilapia Mozambique, menhaden (Bre- voortia tyrannus), bluefish (Pomatomus salta- trix), little tuna (Euthynnus allitteratus), croak- ers (Micro pogon undulatus), and king whiting (Menticirrhus sp.). At the H.M.L., strontium59 in gelatine cap- sules was fed to skipjack, dolphin, and yellowfin tuna. These are all fast-swimming pelagic fish. Figure 6 shows that the excretion of strontium is very rapid. In 24 hours, only about two per cent of the dose remains in the fish. Similar ex- periments with Tilapia, a small, sluggish bottom feeder, indicate that the strontium is also mainly excreted, but that the time required to reach a minimum level of about five per cent of the dose requires at least four days. This informa- tion is consistent with the idea that the meta- bolic rates of these fishes are very much dif- ferent, and the sluggish fish might be expected to retain the strontium for longer periods. HOURS AFTCn BOSC FIGURE 6. The Percentage Accumulation by Tuna Fish of Sr* Given Orally. The internal distribution of the total radio- activity recovered is shown in Table 6. By plotting the radioactivity of each organ against time, it is apparent that the soft, visceral tissues rapidly excrete the strontium, but that the bony structures, gills, integument, and muscles re- tain the strontium for a long period. Tilapia show the same behavior. The data are presented in Table 7. The direct uptake of strontium29 in solution by Tilapia was also studied at the H.M.L. Fig- ure 7 shows that after about two weeks, the FIGURE 7. The Uptake of Sr* in Solution by Tilapia Mozambique.

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Chapter 8 85 Laboratory Experiments on Uptake TABLE 6 ACCUMULATION OF SR* IN THE VARIOUS ORGANS AND TISSUES OF TUNA AFTER INGESTION Percentage of total activity Dose: 5.55/ic Dose: 480/ic Dose: Dose: Dose: 51.0/ic 240/ic Dose: Dose: Dose: Dose: Dose: 464/ic 464/ic 464/ic 371/ic 371/ic Tissue Ihr 2j hr 6hr 7hr 11ihr 24 hr 96 hr 264 hr 480 hr 648 hr Heart 0.04 0.01 0.03 0.049 0.11 0.05 0.028 0.01 0.014 0.007 Gall bladder 0.05 0.04 0.07 0.10 0.08 0.03 0.0001 0.01 0.004 0.002 Blood 4.21 6.68 15.00 0.85 8.07 2.73 1.14 0.35 0.51 0.12 Gill flesh \ [0.18 0.91\ fl,, / 5.06\ f 2.42 1.42 2.21 Gill bone f12-44 \1.34 6.47 f 8'56 \26.391 30,61 25'72 \16.80 19.48 22.76 Caecum 37.01 7.67 7.84 2.70 2.64 0.34 0.15 0.05 0.04 0.029 Foregut 0.89 9.32 1.12 0.74 1.03 0.20 0.24 0.04 0.04 0.018 Midgut 2.28 14.16 16.50 1.08 1.48 0.65 0.25 0.05 0.036 0.003 Hindgut 11.78 3-98 21.26 2.26 0.11 0.15 0.024 0.03 0.015 0.016 Gut contents — 48.32 12.73 0.056 19.65 — 0.10 0.013 — 0.0008 Head, operculum .. — 0.41 1.09 24.99 6.28 18.33 24.58 28.18 29.91 24.58 Appendicular skel. . 3.60 0.40 1.19 36.21 8.45 23.69 30.32 29.15 30.47 31.43 Liver 3.34 1.48 3.04 0.39 2.46 0.15 0.04 0.03 0.04 0.027 Spleen 0.20 0.32 1.39 0.08 0.60 0.03 0.008 0.03 0.010 0.003 Tail — 0.42 0.15 — 0.00 — — — — — Brain, spinal cord..\ t , fo.00 0.01\ , . , f0.05\ . „ Eyes.. } 0.2} J0.04 0.06/ J.24 |o.60J IM l™ Integument 5.28 1.69 0.86 10.20 5.89 7.69 11.37 Integument flesh .. — 0.01 0.01 — 0.05 — — (aliquot) Integument scales . — 0.02 0.02 — 0.11 (aliquot) Gonad \ , 4Q /0.09 0.47\ Q,, f0.08\ 0 Q& J0.004 0.03 0.023 0.020 Kidney f "'' \0.08 0.16J \0.09J * ' \0.027 0.07 0.035 0.022 Light muscle \\<7* J3.23 8.74 4.19 10.01 12.84 3.94 5.26 5.69 5.79 Dark muscle [1***' -i 0 10 0 86 5 25 0 70 0 72 o4g 047 063 0 95 f1.33 0.030 0.004 \2.02 1.34 1.34 13.73 10.25 10.51 — — 0.065 — — — — 0.091 TABLE 7 THE INTERNAL DISTRIBUTION AND PER- CENTAGE RECOVERY OF A DOSE OF 75 inc. OF SRW BY Tilapia Percentage Days after of total dose Tissue recovered 1 Skin 25.27 Eyes 0.35 Visceral organs 1.82 Gills 15.62 Muscle 5.65 Skeleton 51.30 TABLE 7—Continued Total .Skin Eyes Visceral organs Gills Muscle Skeleton Total .Skin Eyes Visceral organs Gills Muscle Skeleton Total 100.01 24.44 0.18 1.08 8.13 3.10 63.07 100.00 24.56 0.25 1.01 6.42 8.40 59.35 99.99 Days after dose Tissue .Skin Eyes Visceral organs Gills Muscle Skeleton Total Percentage of total recovered 22.79 0.33 1.14 10.11 3.01 62.62 100.01 uptake apparently levels off at a value which corresponds to a concentration factor of about 0.3. This means that these fish can to some extent exclude the strontium ion in solution. Even the skeleton had not yet come to equi- librium with the radioactivity in the sea water. This may mean that only about 70 per cent of the strontium in the bone is readily exchangea- ble. The remainder may be firmly bound in a lattice or to an organic matrix which has a slow rate of turnover. It should be emphasized that these fish were mature. Experiments done at the R.L.F.W.S. with SrR9 on post-larval flounders indicate that age

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Atomic Radiation and Oceanography and Fisheries and temperature influence the uptake of ele- ments in solution. One group was kept at 20-22° C in sea water containing Sr20, and another group at 8-12° C. The fish in both groups averaged 0.02 g. each. Figure 8 shows TABLE 8 ZINC" DISTRIBUTION IN CROAKERS (Micropogon undulatus) 12 HOURS AFTER ORAL ADMINISTRATION I » 10 3 z 3 'o FIGURE 8. Uptake of Sr™ by Larval Flatfish. that strontium was taken up more rapidly at the higher temperature. Thus at one day, the fish at the lower temperature had less than one third of the radioactivity of the fish at the higher temperature. The graph also shows that very young fish continue to take up strontium from solution very rapidly at 14 days, while at 14 days the Tilapia had reached an apparent steady state condition. The uptake of zinc85 by croakers was studied at the R.L.F.W.S. These fish were fed the iso- tope in hardened gelatine. After 12 hours only about 27 per cent of the dose remained in the fish (Table 8). The distribution of zinc is quite different from that of strontium. About 90 per cent of the strontium retained by the various fishes used at the H.M.L. was found in the gills, bones, and integument. Zinc, how- ever, is concentrated mainly in the liver and spleen. The muscle and bone, because of their bulk accounted for a large part of the total zinc05 of the body. The turnover times of the zinc-containing compounds of the skin, muscle and bone were slow, whereas those of the in- ternal organs were relatively rapid. The uptake of radiocesium by fish was studied at the R.L.F.W.S. Table 9 shows the distribu- tion of cesium157 which was fed to little tuna. It can be seen that the liver, heart, spleen, and kidney rapidly take up the cesium, but these organs also lose the cesium during the following week. Muscle, gonad, brain, and skin, on the */3 ^c G to 2 S ll "-3 £ a O \ S ° ^-Sc Tissue or 'I? 2 Organ CLi *-' isc °° N E H &,~ * Muscle 80 48.80 1^6 78.1 44.7 Bone , 11 6.71 5.5 36.9 21.1 Gills , 2 1.22 10.9 13.3 7.6 Liver 0.8 0.49 40.7 19.9 11.4 Gonads 04 0.24 17.6 4.2 2.4 Kidney 0.3 0.18 4L5 7.5 4.3 Heart 0.2 0.12 14.0 1.7 1.0 Spleen 0.1 0.06 25.3 1.5 0.9 Remainder . . 5.2 3.17 3-71 11.7 6.7 G I tract i 60.99 174.8 Blood Brain Eyes etc. 1 Based on skin and scales Dose per fish—6,100 mjtc Distribution after 12 hours Tissues 3 percent G 1 tract 24 percent Loss 73 percent (mostly excreted) other hand, continue to accumulate the cesium faster than they lose it. The accumulation of cesium in solution was demonstrated by keeping croakers in Cs1" en- riched sea water. The water was changed daily to maintain a relatively constant concentration. Figure 9 indicates that during the 29 days, the heart, spleen, liver, brain, and muscle con- tinued to accumulate the cesium. The concen- tration factor for the heart, spleen, and liver, was about 10, but this value is far below an equilibrium value. TABLE 9 THE DISTRIBUTION OF CESIUM" IN THE TISSUES OF THE LITTLE TUNA AS A FUNCTION OF TIME Cs1" content in pc/g. wet wt. Days after the dose Organ Spleen 1 3.46 3 3.29 4.71 2.58 3.15 0.40 0.26 0.67 0.70 0.35 0.46 6 3.22 3.45 1.78 2.52 0.17 0.31 0.87 1.33 0.69 0.41 8 1.57 2.59 0.95 1.57 0.26 0.30 0.79 1.34 0.66 1.01 Liver 9.07 Kidney 3.1} Heart 6 17 Bone 041 Eye . . 0 30 Muscle 0.46 Gonad 0.54 Brain 0.30 Skin . . 0.29

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Chapter 8 87 Laboratory Experiments on Uptake 12 16 20 24 DAYS OF EXPOSURE FIGURE 9. Accumulation of Csm by Croakers Kept in Sea Water Containing 5 X 10-* Microcuries/Ml. The relative concentration of cesium by the various organs is roughly the same for croakers, tuna, or bluefish. The same rank order among the organs is maintained both from ingestion, and from direct uptake. Menhaden, a filter feeder, were put into sea water with ruthenium100 that had been co-pre- cipitated with calcium carbonate. Although a considerable amount of particle settling oc- curred, the menhaden took up the ruthenium in the digestive tract, but the tissues of the fish did not become radioactive to an appreciable extent. Similar experiments using menhaden fed with Ru10« labelled Arbacia plutei, or Ru106 labelled Nannochloris cells, gave parallel results. In the latter experiment the fish were allowed to eat the labelled cells for four hours, and then they were put in running sea water. At the time of transfer about 92 per cent of the ingested dose was found in the digestive tract. The gills had 0.64 per cent of the dose, and the remainder of the fish, including the skin, had 0.76 per cent. At 128 hours, only 0.05 per cent of the ingested dose remained in the digestive tract. There was 0.25 per cent in the fish body or on the skin surface, and 0.01 per cent in or on the gills. At no time was there an appreciable increase in the radioactivity of the body of the fish.