298 
PACIFIC SCIENCE, Vol. XVIII, July 1964 
< — 4M — * 
1 
2 
«■ 
A 
B 
C 
D 
H 
G 
F 
E 
1 
J 
K 
L 
P 
0 
N 
M 
Fig. 1. Sampling grid, Eniwetok Atoll. 
A to P as depicted in Figure 1. Three people 
participated in each of the four collections of 
these quadrats. One person started at Square A 
and collected in alphabetical order areas A to P 
while a second started at Square P and collected 
in reverse order P to A. The third person, after 
distributing labeled jars to each square, searched 
the area a third time. The majority of the 
pagurids were obtained the first time the square 
was examined, although two or three individuals 
were usually found in the second examination. 
Hermit crabs were rarely obtained by the third 
collector. Using double-block aqua-lung tanks, 
it was feasible to collect continuously and min- 
imize pagurid movement into the area during 
the collecting process. The entire procedure re- 
quired 1.5-2 hr to clear the 256 m 2 area of 
essentially all pagurids, gastropods, and empty 
gastropod shells. Collections were made, starting 
around noon, on July 10, 1961 (collection no. 
F-2), July 22, 1961 (F-3), July 29, 1961 (F-4), 
and August 7, 1961 (F-5). Three species of 
hermit crabs occurred consistently in most 
squares of each sample at Eniwetok: Diogenes 
gardineri, Dardanus scutellatus , and Calcinus 
latens. Their abundances (expressed as mean 
number/square) are presented in Table 1. 
Appreciation is expressed to Anthony J. Pro- 
venzano, Jr., who identified the Eniwetok pa- 
gurids, and to David Au, David Fellows, John 
Shoup, and Stanley Swerdloff for their aid in 
making the Eniwetok collections. Ernst Reese 
and R. T. Paine offered many valuable com- 
ments during the preparation of the manuscript. 
INVASION RATES 
At Horseshoe Cove all of the crabs of the 
most abundant species were removed at weekly 
intervals from two pools in which P. samuelis 
was the most abundant pagurid and from one 
pool in which the most abundant species was 
P. hirsutius cuius. The results are indicated in 
Figure 2D. Whereas populations of the most 
abundant crab were definitely reduced by the 
removals, the influx was of surprising magni- 
tude, resulting in a total removal from the pools 
in a 3 -week period of over twice the number 
of crabs initially present in one case. Crabs from 
the pool in which P. hirsutius cuius was most 
abundant were removed only once due to tidal 
conditions, but in this pool the final population 
of P. hirsutius cuius was larger than the initial 
one. 
Since the time interval between collections 
was not constant, it is difficult to compare the 
rates of invasion after successive samples. A 
TABLE 1 
Mean Number of Pagurids per Square for Each Species in all Eniwetok Samples 
SAMPLE 
DATE 
MEAN NUMBER PAGURIDS/ SQUARE 
Diogenes Dardanus Calcinus 
gardineri scutellatus latens 
TOTALS 
F-2 
10 July 
11.2 
11.4 
5.5 
28.1 
F-3 
22 July 
4.9 
4.4 
2.3 
11.6 
F-4 
29 July 
7.1 
2.3 
0.8 
10.2 
F-5 
7 Aug. 
4.9 
1.9 
1.2 
8.0 
