TRANSURANIC RADIONUCLIDES IN MARINE ENVIRONMENT 527 



voided as feces. Direct uptake of Pu(+4) by mussels from filtered seawater was not 

 investigated. 



For shrimp direct uptake from Pu(+6)-labeled seawater was slow and was strongly 

 influenced by molting. A single individual that did not molt during a 25-day exposure 

 period reached a concentration factor of only 19. Three individuals that molted during 

 the first 18 days of exposure lost between 92 and 100% of their total body content of 

 plutonium, a value that may be artificially high owing to the capacity of such material to 

 further adsorb plutonium from labeled solutions once they have been cast. Animals that 

 molted twice during the loss period showed virtually no plutonium in these second 

 exuviae, which indicated that their whole-body content was indeed the result of 

 systemically deposited plutonium. Excretion in L. seticaudata following a single feeding 

 of Pu(+6)-labeled brine shrimp (Artemiaj was rapid during the first 3 days but then 

 decreased sharply to an exponential rate for 1 month until only 1% of the initial burden 

 remained. Shrimp fed daily rations of labeled ^rrem/a for 15 days did not accumulate 

 higher levels of plutonium than those fed a single ration of labeled food. Although shrimp 

 that were starved after a single feeding of labeled Artemia retained a significant fraction 

 of the initial dose up to day 8 (40%), they quickly eliminated this material once feeding 

 was resumed. It is therefore likely that accumulation in tissues other than the exoskeleton 

 in the shrimp would be a slow process. 



Interestingly, the marked decrease in the rate of excretion after a single feeding of 

 labeled food and the gut clearance of this material suggested that the assimilation 

 efficiency in the shrimp greatly exceeded the tenths to hundredths of 1% assimilation 

 efficiencies reported for terrestrial mammals (Thompson, 1967). This appears to be the 

 case whether the plutonium is derived from food or directly from water. This was perhaps 

 one of the first indications that marine invertebrates were capable of retaining a 

 substantial portion of the plutonium they derived from these two major routes. 



For worms direct uptake of plutonium from water for either the +4 or +6 valence 

 state was both rapid and efficient since, after 15 days of exposure, concentration factors 

 approached 200 for both valences. Eight days following uptake [Pu(+6)] , worms placed 

 in unlabeled seawater rapidly lost some 30% of their plutonium in 4 days; thereafter the 

 rate of loss slowed dramatically, giving a T^yj of 79 days (computed between days 4 and 

 35). Once again a surprisingly high percentage of the initial plutonium body burden 

 appeared to have been retained by the organism, but it was not determined whether the 

 plutonium had been systematically incorporated into tissue or sequestered by the 

 external mucus. Moreover, it was clearly shown that the exometabolites excreted by 

 worms into seawater can render the plutonium less available to fresh worms introduced 

 into this conditioned water. 



Not only do these experiments give interesting insight into the rates of accumulation 

 and loss of the plutonium in the animals used but they also confirm the general tissue 

 distributions found in similar species that accumulate plutonium from fallout and in 

 those at Thule, Greenland (Aarkrog, 1971; 1977). Crustacea contain large amounts of 

 plutonium in their exoskeletons, molluscs retain the majority of their plutonium in the 

 shell and byssus threads, and polychaetes efficiently accumulate plutonium and are 

 expected to evidence higli levels of the element when exposed to contaminated water. 

 Finally, if one were to assess the relative importance of the pathways by which the 

 element is accumulated by marine organisms, direct uptake from water may be significant 

 and in some cases more significant than uptake from labeled food. By contrast, for 



