in the symbiotic forms (31). Conversely the tridacnids, especially T'ridacna, sceral mass “tivity | I 0 0 Tridacna kidney o, of activity 2.2 0.% ) ' 0 0 0 \ 11.6 1.0 ] 85.8 0 0 raction tivity 6 LO 7 0 Heavy fraction % of activity | 0.3 0.04 16.4 83.0 sactivity was from Co5?. 58, 60, suunted for the major part of ons of the kidney was studied ind a heavy fraction through vy fraction contained more ry fraction, Mn54 contributed ve anions Ru!®8-Rh?°, Zr?5— vity as they did in the light organ and radioactive anions olism and retention of stable shate, bromide, and chloride “als probably also exerts an its. tend to parallel those of the ‘y but not in that of Tridacna. vel 17 times that in Tridacna e of the total radioactivity did in the light fraction, and tion at a level six times that in the kidney fractionsparalily stored in the concretions ch symbiosis does not occur are lost to the environment are able to reject almost completely at least one ion, zinc, which is taken up in large amounts by other marine organisms. Summary Two physical factors which control the distribution and the availability in time of radioactive contamination in the sea are the distribution on fallout particles of the individual radioelements, and the half-lives of the radionuclides. Two principal factors control the geographical distribution of radioactive contamination in the sea: oceanographic effects and gravity. The dominant oceanographic influence is that of ocean currents, and the degree to which these affect the body of contamination depends upon surface winds, magnitude of subsurface currents, vertical and horizontal density gradients, and the size of the contaminated area. The horizontal dispersion of the radioactive contamination is much greater than the vertical dispersion. The rate of drift of radioactivity is about equal to that of the ocean currents, although the vertical migration of plankton may reduce the rate of movement with respect to that of the surface currents. Near the Eniwetok Test Site most of the contamination was carried by the surface currents at depths less than 25 m in the direction of the wind. Contamination in the deeper water moved in the direction of the north equatorial current. In an area of contamination near Eniwetok Atoll the radioactive material associated with the soluble-colloidal and the particulate fractions moved down through the mixed layer at a rate of 2.5 m per hour, and at 48 hours most of the radioactivity was concentrated at the upper edge of the thermocline. At depths of 100 and 300 m the percentage of total activity asso- ciated with particulate material increased at an almost exponential rate during a period of 48 hours following contamination. In the area of the Marshall Islands and west to Guam, homogeneous vertical dispersion of the radioactive contamination throughout the mixed layer did not occur within a period of six to eight weeks following the introduction of contamination into the sea. The uptake of radioisotopes by plankton appears to depend upon the physical form of the individual radioelements in sea water. Radioactive iodine, strontium and caesium are deposited in the soluble form and are not concentrated by planktonic organisms. Radiolements in the particulate form are concentrated by plankton. Factors that determine whether or not a radioelement is present in the particulate form include solubility products, coprecipitation, and adsorption to inorganic and organic detritus or to micro-organisms. The uptake of a given radioisotope by plankton depends upon the amount of isotope dilution and competition by chemically similar elements. Also, organisms may actively discriminate against certain radicelements. In the sea, discrimination against the uptake of Sr°°—Y®occurs in most organisms. Several biological factors affect the uptake and retention of radioisotopes by marine plants and animals. The biomass at each trophic level determines the total amount of radioactive material contained within that level. Because the efficiency of conversion is low between ascending trophic levels, the 135 a ay D BY DIFFERENT ISOTOPES IN ) IN THE LIGHT AND HEAVY DNEY