PACIFIC SCIENCE, Vol. XVIII, January 1964 
10 
for a near relative, Pachygrapsus eras sipes. 
Hiatt’s description, the first for the Grapsidae, 
clearly shows that this species is strikingly dif- 
ferent from other members of the Brachyura 
in its method of copulation, in that the male 
assumes an inferior position to the female. Our 
research attempted to learn the time of copu- 
lation in the total reproductive cycle of Hemi- 
grapsus, and also to determine if the account 
given by Hiatt applied to Hemigrapsus as well. 
The discovery that both H. nudus and H. ore- 
gonensis are similar to Pachygrapsus in this re- 
spect suggests that the Grapsidae in general may 
differ from most other families of Brachyura. 
In December, soon after the shortest day of 
the year, H. nudus was seen on numerous oc- 
casions in the act of copulation. Hiatt’s notation 
of the extreme speed with which a pair of 
Pachygrapsus will begin copulation applies also 
to Hemigrapsus, as frequently pairs of the latter 
genus would be found in copulo with no ob- 
servation of the onset of this act. 
H. nudus does not precede copulation with a 
period of courtship, as is the case with many 
Cancroid crabs as cited by Williamson (1903: 
101), Hay (1904:405), Churchill (1918:105), 
Knudsen (1960:7), and in this paper under 
the genus Cancer. A male initiates copulation 
by approaching the female and grasping her by 
the chelae or carapace. After a few seconds to 
a minute of "wrestling” with variable gestures 
and shifts in position, the male begins to posi- 
tion the female. The first uniform act occurs 
when the male lifts the female by her chelae 
and places her in a vertical position with her 
anterior end upward and the posterior part of 
the carapace down on the substrate. Simultane- 
ously the male tilts his body upward in a similar 
manner so that both male and female come to 
rest upon their last two pairs of walking legs 
with their ventral thoracic surfaces in contact 
( Fig. 2 ) . From this position the male rolls over 
on his dorsal side with the female held firmly 
above him. The female is held with her dorsal 
side upward and her ventral thorax in contact 
with that of the male (Fig. 3). The male uses 
the chelae to hold the female by the carapace 
and uses pereiopod pairs two and three, and on 
occasion four, to maintain his grasp. The act of 
penetration of the vulvae by the male’s pleopods 
could not be seen due to our vantage point. 
9 
FIGS. 2, 3. Copulation postures of Hemigrapsus 
nudus. Fig. 2 (above): The male has grasped the 
female by her chelae and is positioning her with his 
pereiopods, prior to rolling backward. Fig. 3 (below) : 
The male has rolled backward, the female abdomen 
has been opened by that of the male, and the male 
pleopods have been inserted in the vulvae. (The legs 
are not shown for the sake of clarity.) 
However, by carefully capturing a pair in copulo 
in such a way as to hold the two firmly in the 
mating position, the relationship of the male 
and female abdomens was observed. Such ob- 
servation, together with accounts in the litera- 
ture (Hiatt, 1948:199; Knudsen, 1960:7), sug- 
gests that the following activity occurred. As 
the male proceeded to roll from a vertical posi- 
tion to its "back,” he simultaneously raised his 
abdomen in such a way as to hook the abdomen 
of the female. With a posterior thrust of his 
abdomen the male then forced the female’s 
abdomen down and thus exposed the vulvae. 
Mechanically, as the male’s abdomen is moved 
backward, the pleopods or intromittent organs 
are brought into position for penetration and 
sperm transfer. When interrupted, or when cop- 
