22 
PACIFIC SCIENCE, Vol. XIV, January I960 
Fig. 3. Radioactivity in Coenobita carapace on successive collection dates compared with the decay of 
radiostrontium. Values in disintegrations per minute per gram of ash. 
The amount of Sr 89 present in the carapace 
immediately after Nectar was calculated from 
the yields given by Sullivan ( 1949) on the basis 
of the average amount of Sr 90 in the 44 speci- 
mens collected during the first 50 days following 
Nectar, less the amount present before Nectar. 
The relative radioactivity of the two isotopes 
was calculated from their specific activities. A 
theoretical decay curve was then calculated for 
the combined Sr 89 , Sr 90 + Y 90 contributed by 
the Nectar test and the Sr 90 + Y 90 residual 
from prior tests. Figures 4 and 5 show the actual 
values superimposed on this theoretical curve. 
Although there were no specific radiochemical 
determinations early in the period following 
Nectar, it is reasonable to assume that the exo- 
skeleton has a high degree of selectivity for 
strontium and that equilibrium must be reached 
within a few days at most. The assumptions are 
further supported by decay curves which ap- 
proach the theoretical curve (Fig. 4). 
The relatively low levels of activity at 145 
days post-Nectar are a reflection of a change in 
ratio of ash weight to wet weight; Figure 4 
represents the data on an ash weight basis. The 
change in ratio may be associated with molting, 
but observations were not made at frequent 
enough intervals to confirm or deny such an 
association. 
Contributions of radiostrontium to the crab 
skeleton at Belle Island from past tests at Eni- 
wetok and Bikini are represented in Figure 5. 
The pre-Mike level is an approximation since 
it is based on a single specimen and there was, 
unfortunately, no biological survey during the 
1950 tests. The pre-Nectar curves were derived 
by the method outlined above. The Mike test 
contributed about twice as much activity as the 
Nectar test; fallout from the pre-Mike tests and 
the Bikini tests of 1954 together contributed 
about 5 per cent of the total Sr 90 activity. 
Sr 90 on the island is being maintained at an 
essentially constant level (decreasing only with 
physical decay), if the omnivorous hermit crab 
can be considered an accurate index of biolog- 
ically available strontium. However, the ratio 
of the strontium in the crab skeleton to that in 
food items is not known. Judging from the 
meager data presently available, the radiostron- 
tium content of the crab skeleton is more than 
