oo A comparison of the activity in different organs of crabs was made using the count found in muscle as unity. The values relative to muscle for other tissues in pre- and posttest crabs, respectively, were digestive system, 6 and ‘0; gills, 3 and 22; and exoskeleton, 6 and 16. In 40 comparisons of the shell and soft parts of pretest molluscs, the shells were more radio-- active than the soft parts for 25 per cent, und for the other 75 per cent the reverse was true. 4.4.2 Posttest Invertebrates collected after Mike ‘est are listed in Table C.2. Limitation of time made it impractical to search thoroughly for specimens that would have made possible a complete comparison of collection stations acccrding to species. Since the collections were made soon after the shot (2 to 8 days), it may be presumed that the distribution of radioactive materials was still in a state of flux in the waters of the lagoon, with consequent variability in the degree of radioactive contamination even between local areas at a given station. The specific activity of individual samples of invertebrates ranged from backgroundat Japtan to 15,000,000 d/m/g, wet (sand from sea-cucumber gut), at Engebi. One exceptional piece of coral detected from an autoradiograph (Fig. 4.18b) had a specific activity of approximately 100,000,000 d/m/g. (a) Analysis by Area. Differences in activity between organisms at various collecting stations depend upon the species and organ or tissue being considered. When compared by ranking within each of the 19 classes of items in Table 4.8, the stations, given in decreasing order of radioactivity, are: Fsogallua, Engebi, Aaraanbiru, Rigili, Runit, Igurin, and Japtan. The giant clam, Tridacna, was the only species collected at every station. Comparison of individual tissues of this clam at each station is made in Fig. 4.1. The specific activity rela- tive to Igurin is shown for gill, mantle, muscle, and digestive gland (liver). Regardless of the tissue considered, the ranking of the stations remains the same. Japtan is not included since several of the counts were background; hence the ratios are meaningless, and the relative activity was in every case less than 1. Included in Fig. 4.1 are the relative activities of beach sand or soil and bottom sand from each station. The latter has a higher relative specific activity at Engebi than at Bogallua, whereas the reverse is true with the giant-clam tissues. No landing was made at Bogallua; consequently no beach sand ia available from that station. Bottom sand was taken from sea-cucumber guts, usually Holothurta atra. This can be considéred a random sample of the bottom sand since H. afra shows no selectivity in its ingestion of bottom materials.° This difference between Engebi and Bogaliua may well be a matter of sampling. One such sample collected from each of the two stations differed by a factor of less than 3, whereas the maximum and minimum specific activities of a series of nine sea-cucumber guts taken from an area of less than 1000 sq ft at Aaraanbiru differed by a factor of more than 6. The average valuesof all invertebrate samples from each station are given in Table 4.9. The limited usefulness of these values was discussed earlier in this chapter. The values were not considered in the ranking of stations discussed in the preceding paragraph. (b) Tissue and Organ Differences. General statements can be made regarding tissue and organ differences in specific activity, although there were not sufficient specimens of any one species of invertebrates to warrant statistical analysis. The relative rankings presented here are based on a comparison of the specific activity of each tissue or organ in individual specimens. However, the samerelation can be found in Table 4.8, which is based on the average values of similar species. 1, Muscle consistently has the lowest specific activity regardless of species. 2. Liver, or digestive gland, is the only tissue sampled, other than muscle, which is not subject to external contamination. It rarely has a higher specific activity than the digestive tract and is always more radioactive than muscle. 3. The relative specific activity of the gill varies with the species. In the clamsthe gills, which are the food-gathering organ, have the highest activity, exclusive of the digestive tract with its contents. In those sn7ils having a gill, the liver is the more active. The liver is also more active than the gill in .:. crabs. _. ANGLASSIED