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PACIFIC SCIENCE, Vol. XXII, October 1968 
the characteristic fauna of amphinomid and poly- 
noid worms, alpheid shrimp, and xanthid crabs 
was greatly reduced or entirely absent. The only 
exception to this reduction in numbers was 
Gonodactylus falcatus. In 20 liters of coral, 5 
G. falcatus were found, which is about the nor- 
mal number for samples of similar size from the 
reef. On July 17 all the loose coral was collected 
from an area 25 m 2 on the same reef. The fauna 
in these coral heads appeared to be returning to 
normal, though it was still noticeably reduced. 
From this coral, however, 69 G. falcatus were 
collected. Of the 42 females in the collection, 
9 were carrying eggs. 
Though qualitative, these observations sug- 
gest that Gonodactylus falcatus has a wider range 
of salinity tolerance than does Pseudosquilla 
ciliata . To test this hypothesis a series of tests 
on the differential salinity tolerance of these two 
species was performed. Two procedures were 
used. In the first, the animal was removed from 
seawater of normal salinity and placed directly 
into the water of the test dilution. In the second 
series, the animal was acclimatized for specified 
time intervals to increasing dilutions until the 
maximum dilution was reached. The second 
method probably more closely approximated the 
natural conditions where the amount of runoff, 
and hence dilution, increases gradually. 
The results of these tests are shown in Table 
1. In both series of experiments Gonodactylus 
falcatus exhibited a greater degree of salinity 
tolerance than did Pseudosquilla ciliata . 
in the area of the Philippines and the South 
China Sea. The towed speed of these barges was 
about three knots, an uncommonly slow speed 
for sea-going vessels, but one that would not 
dislodge any organisms living in fissures or 
holes in the hull. The almost simultaneous ap- 
pearance of Gonodactylus falcatus and G. hen- 
dersoni , soon after the arrival of these slow- 
moving barges from the area where they co- 
occur, is strong circumstantial evidence for 
their introduction by this means. 
ECOLOGY OF REPLACEMENT 
In investigating the mechanisms of replace- 
ment of Pseudosquilla ciliata by Gonodactylus 
falcatus, three parameters of the environment 
and biology of these species were compared: 
salinity tolerance, larval ecology, and behavior. 
Salinity Tolerance 
Both Pseudosquilla ciliata and Gonodactylus 
falcatus live on the shallow reef flat where fluc- 
tuations in salinity can be substantial. In . an area 
such as Kaneohe Bay (Fig. 2) where the re- 
placement of Pseudosquilla ciliata in coral heads 
by Gonodactylus falcatus is complete, the effect 
of differential salinity tolerance might have been 
important in effecting the replacement. 
In May 1965, exceptionally low (O' to -0.04') 
tides occurred in the mornings during the first 
days of the month. During the night of May 
2-3 there was a very heavy rainfall. Coconut 
Island in Kaneohe Bay reported 10.52 inches of 
rain during the night and morning, and a 
nearby weather station reported 21.61 inches. 
On May 7 surface salinities in the southern part 
of the bay ranged from 7.8 o/oo to 30.3 o/oo 
with an average surface salinity of about 20 
o/oo. These lowered salinities showed a gradient 
from the surface to about 1.5 meters. Water in 
the southern part of the bay showed consider- 
able dilution for at least two weeks. On May 5 
a collection was made of dead animals that had 
washed up on a mud flat in the bay (Fig. 2, 1 ). 
In this collection were 49 dead stomatopods of 
which 42 were Pseudosquilla ciliata , 5 were 
Gonodactylus falcatus , and 2 were Lysiosquilla 
mac ul at a. On May 23 collections were made on 
a reef near Coconut Island (Fig. 2, 2). All the 
coral on the surface of the reef was dead, and 
Larval Ecology 
In order to obtain information on the breed- 
ing periods of Gonodactylus falcatus and Pseu- 
dosquilla ciliata , as well as the relative repro- 
ductive activity of these two species, investiga- 
tions were carried out on the planktonic larvae 
of both species for a period of one year. Plank- 
ton tows were made at three areas in Kaneohe 
Bay from October 1964 to October 1965 (Fig. 
2). Run A, close behind Coconut Island, was 
chosen to sample water from as close as possible 
to a reef where both species were known to be 
abundant. Run B was chosen to give a sample of 
the more inshore water of the reef system in the 
bay. Run C sampled water that had less contact 
with the inshore reefs and was more open to 
the waters of the ocean. 
