Land Crabs and Fission Products — Held 
Samples of carapace (exoskeleton), muscle, 
hepatopancreas ("liver”), gut with its content, 
and gill were removed, either from the fresh or 
frozen specimens, at the Eniwetok Marine Bio- 
logical Laboratory. The tissues were weighed at 
the time of dissection and then dried. The pack- 
aged dried samples, together with data cards, 
were sent by air mail to the Applied Fisheries 
Laboratory, University of Washington, for fur- 
ther processing. 
The dried samples were ashed at temperatures 
up to 550° C. on stainless steel counting plates 
and were counted in an internal gas-flow count- 
ing chamber. The beta counts per plate were 
converted to total disintegrations per minute 
per gram (d/m/g) of wet tissue, as of the date 
of collection, by correcting for sample weight, 
geometry, backscatter, self-absorption, coinci- 
dence, and decay. (See WT-616 (UWFL-33) 
for a more complete discussion of these pro- 
cedures. ) 
The decay corrections for all tissues except 
carapace were based on the decay rate of a soil 
sample collected at Belle Island the day after 
the Nectar shot. (This decay rate was deter- 
mined by beta counting. ) Decay corrections for 
the carapace were based on the decay rate of 
Sr 90 + Y 90 and Sr 89 , which constituted vir- 
tually 100 per cent of its activity at the time 
the chemical determinations were made. The 
decay correction factors ranged from 1.09 to 
12.7. 
The variation in amount of radioactivity for 
each tissue at each collection date, although 
great, was not great enough to obscure general 
trends in changes of radioactivity with time or 
differences in levels of radioactivity between 
tissues. 
The term "activity” as used here means radio- 
activity per unit weight. 
"Rate of decline” refers to the rate at which 
radioactivity is decreasing in a given tissue, or- 
gan, or organism in its native environment. 
Levels of activity in the crab tissues three 
days after the Nectar test ranged from 5 X 10 6 
d/m/g in the gut to 7 X 10 4 d/m/g in the 
muscle (Figs. 1, 2). The rate of decline of 
activity decreased with time and was different 
for each tissue, but in general followed the same 
trend as the decay of mixed fission products dur- 
19 
ing the first 200 days. Thereafter the rate of 
decline for each of the crab tissues approached 
a constant value with a half life in excess of 
20 years. 
This half life is dependent on factors which 
include relative abundance and availability of 
radionuclides in the food and/or environment, 
rate of decay of radionuclides absorbed, bio- 
logical half life, and selective uptake of radio- 
nuclides. Each of these, except the rate of 
physical decay, is in turn dependent on varying 
environmental and physiological conditions. The 
terms "ecological half life of radioactivity,” or 
more briefly, "ecological half life” and "rate of 
decline” will be used to include these factors. 
Ecological half life will be used as the time re- 
quired for an organism, or its tissues or organs, 
in its native environment to lose 50 per cent 
of its radioactivity. When the ecological half 
life and physical half life are equivalent (rate 
of decline = rate of decay), the tissue in ques- 
tion must be at equilibrium with respect to the 
radioisotopes it contains. For single isotopes an 
ecological half life greater than the physical 
half life (rate of decline < rate of decay) in- 
dicates accumulation of the isotope. In the con- 
verse situation where the ecological half life is 
less than the physical half life, a net loss of the 
isotope is indicated. This condition could re- 
sult from loss of the isotope by the environment, 
or eco-system, or from a physiological change 
in the organism or its primary food source. Such 
physiological changes may be transitory or 
seasonal. 
The increase in radioactivity over preshot 
levels during the first few days after the Nectar 
test was less in muscle and carapace than in the 
three other tissues by a factor of 5 to 10. Maxi- 
mum post-Nectar levels of activity were 100 
to 250 times greater than pre-Nectar levels in 
gut, liver, and gill, but only 22 and 26 times 
greater in muscle and carapace respectively. The 
lower rate of accumulation in muscle and cara- 
pace would be expected since the material must 
be absorbed from the gut and hepatopancreas 
where some selection takes place. The specific 
patterns of changing radioactive content of the 
tissues with time, the rate of decline, will be 
presented individually for each tissue. 
The amounts of radioisotopes involved are so 
