Some Aspects of the Behaviour of the Soldier Crab, 
Mictyris longicarpus 
Ann M. Cameron 1 
The CRAB, Mictyris longicarpus Latreille 1806, 
belongs to the brachyuran family Mictyridae 
Dana 1856. According to McNeill (1926) the 
species ranges northward from southern New 
South Wales and from Perth, Western Austra- 
lia, to New Caledonia in the east, to Singapore 
in the north, and to the Bay of Bengal in the 
west. 
In Australia Mictyris is known as the "soldier 
crab.” This appellation is appropriate in view of 
the habits of the genus. Immense numbers of 
M. longicarpus (Figs. 1 and 2) congregate in 
dense masses and wander over tidal flats in ap- 
parent formation. Their activities exert a par- 
ticular fascination not only because of these 
huge "armies,” but also because soldier crabs 
walk forward and not sideways, as do most 
crabs. Several authors have commented on the 
habit of Mictyris of wandering in armies (Mc- 
Neill, 1926; Lazarus, 1945; Balss, 1955-56; Al- 
tevogt, 1957; Schone, 1961; Stephenson, 1961). 
However, little is known of the organisation of 
this behaviour and I decided to study M. longi- 
carpus with a view to describing it. Because of 
the immediate flight and burrowing reactions 
of soldier crabs to the disturbing influence of 
observers, the pattern of behaviour to be de- 
scribed has been previously overlooked. 
METHODS 
Field observations were made from within a 
pentagonal plywood hide, 3 ft high, in the sides 
of which uncovered windows, 11 X 6 inches, 
were cut. A tripod was used for mounting bi- 
noculars and a 16 -mm movie camera. On each 
occasion a place was selected prior to the ap- 
pearance of the crabs, and observations were 
made from there for the duration of that par- 
ticular intertidal period. (Transference of the 
1 Department of Zoology, University of Queens- 
land, Brisbane, Australia. Manuscript received No- 
vember 9, 1964. 
hide to another area of the beach after the ap- 
pearance of the crabs invariably disturbed them, 
and was attempted only a few times.) Apart 
from the displacement activity to be described 
later, the crabs did not appear to be disturbed 
by the presence of the hide. 
Field observations were made at least twice 
a month, and frequently as often as six times a 
month, throughout 1961 and the first half of 
1962. Most of the observations were made at 
Dunwich, Stradbroke Island and at the mouth 
of the Pine River (Fig. 3) - Localities at which 
additional observations were made are shown in 
Figure 3 also. 
No period of observation at any one locality 
was of sufficient duration to allow study of any 
lunar rhythm in soldier crab activity. 
HABITAT 
Although Lazarus (1945) stated that Mic- 
tyris is found "where the mixture of silt and 
sand is fairly coarse and free from matting man- 
grove roots,” it was observed that the crabs 
burrow in a wide range of substratum types 
from silty sand to shell grit. However, material 
collected from the top 2 inches of a typical M. 
longicarpus habitat at Dunwich (Fig. 3) con- 
tained 95-98% sand of particle size 1.96-0.05 
mm (F. C. Vohra, personal communication). 
The method of analysis consisted of removal of 
gravel, calcium carbonate, and organic matter, 
followed by sieving the coarse fraction and sedi- 
mentation of the fine fraction. 
Stephenson (1961) has described the habitat 
of M. longicarpus at Dunwich, where the crabs 
are found in both clean sand and in areas of 
muddy sand covered with the eel grass, Zoster a 
capricorni. The beach at Dunwich is very wide 
and flat. No Zoster a grew in the study area at 
Pine River, and the beach there has extensive 
drainage channels in some parts and secondary 
sand bars in others. 
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