- —— um levels in eleven samples Pacific. The samples were vel of 0.00226 d/m/g of wet ted near the Eniwetok Test wu survey during June 1956, 5 contained approximately '6 d/m/g wet), and one con: (about 0.6 d/m/g wet). Of most highly contaminated 1uscle. m 9 samples of fish tissues 954 and 12 February 1955, , to 0.9% of the total radio(0.9°4) was found in shark (Table VII). Hiyama (28) for fish collected in the open sea in the vicinity of the Marshall tATION RATE PER GRAMME (FWETOK DURING 1954—1955 Percentage Sr__ye Total d/m/g wet weight 0.08 0.02 0.03 ! 3 0.02 0.1 0.9 0.01 0.01 0.02 trate many radioelements from relatively low levels of environmental contamination, radicelements may be concentrated in the organisms to a greater degree from a long-term than from an acute exposure. Weaponstests are of use in the study of radioisotopes in the marine environment in that fairly large amounts of radiomaterials may be introduced into a given area in a relatively short time and at levels so high that they may be detected in the water and organisms for an appreciable time after contamination. Because large areas are contaminated from the fallout, most of the animals collected in the contaminated area may be considered to have been exposed to a chronic rather than an acute exposure of radioactive material. However, the marine organisms analysed by Lowman, PaLumMBo and Soutn (9), WELANDER (30), and SoutH (24) had been subjected to relatively long-term exposures of Sr®, and the observed levels were not significantly different from those reported by NaGcasawa ef al (25) nor 64 | | 1,500** 4,900 ** 500 ** _ 3,700 160,000 | 83 1,400 Islands. Thus Sr®is probably not concentrated to any extent by most marine organisms, even under conditions of chronic exposure. In laboratory experiments utilizing radioactive strontium under conditions approaching chronic exposure, CHIPMAN (15) observed that Sr® in Artemia larvae reached a steady state at about 0.7 that of the sea-water, and Borovuaus, TownsLtey and Hiatt (14) reported a value of about 0.3 in Tilapia. In fish bone and scales, however, the concentration factors may be higher. In the croaker (Micropogon undulatus), concentration factors for strontium in vertebrae and scales were 2.5 and 2.1 respectively those of sea-water. Little is known about the uptake and retention of radioisotopes by symbiotic organisms. On the reefs at the Eniwetok Test Site the corals and their associated algae are symbiotic and comprise the major part of the biotic mass. The effect of the symbiotic relationship between coral and algae on the uptake and retention of radioisotopes by these two organisms is not known. Studies on uptake of radioisotopes by symbiotic organisms have been -exchange separations on id Sr, although the total et weight. e uptake of Sr® in aquatic 0 much more readily than ie per cent of the amount e in sea water to salinity. yn and dilution by stable xduced uptake. ely one to two per cent vas retained after a period ained on the average only 3. These animals were subwould probably differ from ‘anisms are able to concen- was from cations, 85° from Co57, 58.60, and 12% from Mn5*. A total of 2.59% of the activity was contributed by the anions Ru!6—Rh! and Zr*>—Nb®*. The radioisotopic content of the kidney of Hippopus was similar to that of Tridacna except that small amounts of Zn*> were present. 133 eee ne eetee mee nagging vere made, however, on two related genera of tropical clams. Both Tridacna, the giant clam, and Hippopus, the horse clam, contain symbiotic algae, zooxanthellae, within their phagocytic blood cells. The algae are ‘‘farmed”’ in great numbers in a mantle that is well developed in both species but more highly in Tridacna. Both genera of clams are further modified for their specialized mode of feeding by having the abnormally large kidneys required to get rid of the waste material from the digested zooxanthellae. Because the digestive tract contains a constriction which blocks passage of all except microscopic particles and there is thus no faecal excretion, at least part of the waste materials are stored in the kidneys, mostly in the form of spherical concretions. Tridacna “is the supreme example of exploitation of associated algae by animals’ (30). Tridacna and Hippopus are the only animals known to the author in which radioactive cations are deposited in one organ and radioactive anions in another. In Tridacna (Table VIII) 95%of the total radioactivity in the kidney