Land Crabs and Fission Products — Held 
small that they probably do not constitute a 
significant proportion of the naturally occurring 
isotopes. If, for example, a tissue contained 
10 7 d/m /g wet of Sr 90 , or 5,000 times the maxi- 
mum level found in the hermit crab, this would 
represent only 0.02 mg. of strontium, or about 
10~ 5 per cent of the ash weight. The presence 
of strontium has been reported qualitatively in 
Crustacea and a quantitative estimate of about 
1 per cent strontium has been given for the ash 
of Eupagurus hernhardus (Vinogradov, 1953). 
RESULTS 
Exoskeleton 
The carapace was taken as the sample of 
exoskeleton. It is easily removed, separated from 
other tissues, and washed free of possible ex- 
ternal contamination. 
The radioactivity in the carapace due to long- 
lived isotopes remained approximately constant 
throughout the period of 537 days during which 
collections were made. This was determined by 
recounting all of the samples approximately 600 
days after the Nectar test (Figs. 2, 3). 
Radiochemical analysis of 18 samples taken 
21 
at various times during the collecting period 
(Table 1), and three samples taken 35 days be- 
fore Nectar, demonstrated that virtually all of 
the long-lived activity was 28-year Sr 90 and its 
Y 90 daughter. 4 
The nearly constant level in the carapace 
(ecological half life = physical decay) indicates 
that this tissue quickly reaches and maintains 
equilibrium with the available strontium. Gross, 
Taylor, and Watson (1954) report a plateau 
of retention of Sr 90 in rats during continuous 
feeding at the same rate, and apparent shifting 
of the plateau with change in daily dose. 
It would be expected that this relationship 
also applies to available calcium, which is meta- 
bolically similar to strontium, and to 54-day 
Sr 89 , and possibly to Ba 140 , which at the time 
the radiochemical analyses were made was pres- 
ent in amounts too small ( < 0.2 per cent of 
total activity, according to Hunter and Ballou, 
1951) to be determined by the method used. 
4 Results of calcium analyses of 74 samples of cara- 
pace made since submission of the MS give a mean of 
158 ±30 mg. Ca/g wet, and 1 5,000±4900 strontium 
units. 
100 200 300 400 500 600 700 
DAYS AFTER MAY 14, 1954 
Fig. 2. Radioactivity in Coenobita carapace on successive collection dates compared with the decay of 
radiostrontium. Values in disintegrations per minute per gram wet weight. 
