Pseudosquilla Replaced by Gonodactylus — Kinzie 
473 
Pseudosquilla ciliata was observed in burrows, 
only the eyes and most anterior part of the body 
were visible. When disturbed, an animal would 
rapidly retreat into the burrow, and reappear 
after a few minutes. Since P. ciliata can be 
found in burrows in muddy bottoms and walk- 
ing freely on the bottom, and since, prior to 
1953, it was taken from coral heads, this spe- 
cies seems to have a wider range of possible 
habitats than does Gonodactylus falcatus. 
The fact that coral heads were occupied by 
Pseudosquilla ciliata before 1953 suggested ex- 
perimental comparison of agonistic behavior in 
relation to a "burrow.” Ten trials were made to 
study the behavior of P. ciliata and Gonodactylus 
falcatus in relation to a glass vial placed in the 
sand in the bottom of an aquarium to simulate 
a burrow. Individuals of either species would 
readily enter the vial. However, if Pseudosquilla 
ciliata had entered the vial, it was invariably 
driven out by Gonodactylus falcatus when the 
second animal was placed in the aquarium 
(Table 2). G. falcatus , on the other hand, was 
never driven from a vial by Pseudosquilla ciliata. 
When fighting, Gonodactylus falcatus curls 
its telson up under the ventral surface of the 
body in the same attitude as when it is in a coral 
head. While this is a very effective method of 
fighting when the animal is in a small crevice 
or hole, it leaves the animal unsupported when 
fighting in the open. When in the glass vial, G. 
falcatus blocks the opening with the telson, 
but its eyes are able to see out and the raptorial 
TABLE 2 
Sizes of Gonodactylus falcatus and Pseudosquilla 
ciliata Used in Behavior Experiments 
(In All Cases G. falcatus Drove P. ciliata 
Out of the Vial and Went in Itself) 
EXPT. NO. 
Pseudosquilla 
ciliata 
Gonodactylus 
falcatus 
1 
5.0-cm male 
4.5-cm male 
2 
6.3-cm female 
4.0-cm female 
3 
5. 5 -cm male 
4.5-cm female 
4 
5.5-cm male 
4.0-cm female 
5 
5.3-cm male 
4.3-cm female 
6 
5.5-cm male 
4.3-cm female 
7 
6.0-cm male 
6.0-cm female 
8 
5.5-cm male 
5.0-cm male 
9 
7.0-cm male 
5.0-cm male 
10 
6.8-cm male 
5.0-cm female 
claws can be snapped out the opening. This 
position and method of defense is similar to 
that exhibited by G. falcatus living in coral 
heads. 
Pseudosquilla ciliata , on the other hand, does 
not exhibit any such specialized fighting be- 
havior, and it always fled from the vial when 
attacked by Gonodactylus falcatus. In their bur- 
rows in mud flats, the animals either retreated 
deeper inside or left the burrows if they were 
disturbed. 
discussion: The introduction of Gonodacty- 
lus falcatus into an area where Pseudosquilla 
ciliata was originally the dominant stomatopod 
affords an opportunity to study the effects of 
competition and competitive exclusion in a nat- 
ural situation. Gonodactylus falcatus is a species 
specialized for living within the spaces between 
the branches of coral heads and similar habitats. 
This is borne out by the uniformity of situations 
in which it was found, and by its behavioral spe- 
cializations. In the terminology of MacArthur 
and Levins (1964) it is a "coarse-grained” 
species, differentiating between this type of 
habitat and others less suitable. Pseudosquilla 
ciliata would be termed a "fine-grained” species 
in regard to habitat selection, because it will 
occupy a greater variety of habitats. 
Both species are generally carnivorous, cap- 
turing prey by raptorial feeding. There appears 
to be ample food on the reef flats for animals of 
this trophic level within each of the habitat 
areas, and competition seems to be restricted 
primarily to obtaining a place to live. However, 
it must be emphasized that this conclusion was 
reached on a rather intuitive level and that, until 
more is known about the trophic structure of 
coral reef systems (see Kohn, 1959, and Hiatt 
and Strasburg, I960), generalizing statements 
should be considered suspect. 
The rapid replacement of Pseudosquilla cili- 
ata by Gonodactylus falcatus verifies the pre- 
diction that the specialized species will exclude 
the generalist from the habitat for which the 
former is specialized (MacArthur and Levins, 
1964). The wider salinity tolerance and greater 
reproductive activity of G. falcatus may have 
hastened the replacement, but probably the 
competition was primarily for living space, and 
behavioral differences were the deciding factor. 
