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Fighting
Both crabs defend their temporary or permanent possession of a burrow against other individuals. When a wandering crab draws near another's burrow, the occupier of the hole rushes out, or if feeding outside, runs to take up a position at the burrow entrance. This may be enough to frighten away the intruder but, if it continues to advance, the defending crab adopts a characteristic “threat” attitude. Bethe (1895) describes this in Carcienus maenas as the “Aufbaumreflex”. In both species the threat attitude presents to an animal standing in front of the crab the most obvious and—one cannot resist the temptation to say—frightening aspect of its form. The chelipeds are raised and held with the “fingers” open and reveal a colour pattern which accentuates their definition and size. At the same time the animal spreads or extends its legs to add further to the impression of size (Text-fig. 1, Figs. A and B). The adoption of this posture by the defending crab may be enough to send the rival crab scuttling away, but frequently the rival also adopts a threat attitude. The two crabs face each other and draw together until their chelae touch. Helice crassa individuals extend their legs so that the body is raised as high as possible, and raise and flex their chelipeds so that the large pincers are held a little out from the body with their external surfaces directed towards and abutting against those of the rival (Text-fig. 1, Fig. C).
Hemiplax hirtipes individuals at first hold their chelae open but flexed in such a way as to reveal to best advantage their startling distribution of white and red. As they move closer together the combatants extend their chelipeds outwards to the maximum extent. Thus they meet with the inside surfaces of their chelipeds turned toward and abutting against those of their opponent (Text-fig. 1, Fig. D). Hemiplax hirtipes does not rise on “tip-toe” but spreads its legs sideways as far as possible bringing the ventral surface of the body close to the ground and emphasizing the impression of width.
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Text-fig. 1.— A—Helice crassa, the threat or “Aufbaumreflex” attitude. Filg. B—Hemiplax hirtipes, the threat or “Aufbaumreflex” attitude. Fig. C—Helice crassa, ritualised fighting. Fig. D—Hemplax hirtipes, ritualised fighting.
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In this position the mouthparts of the two animals are brought quite close together. It seemed from the Helice crassa fights observed on the open ground, that its breathing water is often “blown” or “bubbled” into the opponent's face. Probably the breathing currents of Hemiplax hirtipes, clinching below the water level, also oppose one another. Opposition of breathing currents may play a part in this ritualised fighting.
Several Hemiplax hirtipes individuals timed in this “mutual threat” held it for over five minutes. Helice crassa individuals tended to take less time to decide the issue, and this is not surprising since these crabs, exposed on the open ground, are more vulnerable to predator attack. Such fights usually terminate in the retreat of one of the combatants with occasionally the dominant crab pursuing his opponent for a short distance. One contest observed between Hemiplax hirtipes individuals lasted an hour and ten minutes and consisted of a series of mutual threatenings and pursuits in which the same crab was dominant throughout. With Helice crassa individuals, which are more tied to their burrows, the chance of success in a fight seems to be an inverse function of the distance from the burrow mouth.
In both crabs real fights, involving beating and tearing with pincers, are reduced to a minimum. A whole crab is more likely to survive to breed than an injured one, so that reduction of real fighting has survival value for the species. Real fights were observed between Helice crassa individuals on occasions when the intimidation phase tended to be lengthened. One Helice crassa male was seen to seize another by a pincer, lift it bodily in the air, and thrust it onto the ground on its back. In other contests limbs were lost or thrown off. Physical struggles seemed to be inevitable when several Helice crassa individuals were put together in a laboratory aquarium with little room for escape, and many such crabs were killed or lost all their limbs. Crane (1941) makes the following observation on Uca: “Several instances have been observed where crabs with badly damaged chelae, or with chelipeds missing, were definitely bullied by other crabs”. In Helice crassa under similar circumstances, sick or weak individuals seemed to be the first to succumb.
It appears that Helice crassa individuals, male and female, behave in the same way defending a burrow against an intruder of either sex. The males seemed to be readier to fight than the females.
With Hemiplax hirtipes fights were observed to take place only between males. No female was observed in the “Aufbaum” attitude. A strange point about defence of property in Hemiplax hirtipes is that often a crab which has vigorously and victoriously defended a burrow may almost immediately wander away from it and apparently never return.
At times fighting takes on a rather different character. On May 23 and 24, 1955, both fine clear days, the following field notes were taken:
“8.15 a.m. Hemiplax hirtipes males engaged in fighting. Apparently no teiritorial boundaries are recognized, crabs entering a whole succession of burrows and following each other into the same burrow.
“9.31 a.m. Five crabs contesting the possession of one hole. One crab chases another into the burrow and, after much rushing to and fro with flourshings and measuring of pincers, the defending crab is chased deep into the burrow and forced to vacate by bursting through the roof of a side tunnel. The evicted tenant, after one unsuccessful attempt to regain his property, departs. Three smaller crabs now advance on the burrow and systematically worry the new owner. While two of the new intruders attract the large crab's attention at the tunnel mouth, the third crab enters by the side hole made when the original occupant escaped. The new tenant is forced to defend his prize by rushing backwards and forwards from one opening to the other and brandishing his chelae”. I watched this epic struggle for thirty minutes and it was still in progress when I left.
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Pearse (1914) states that crabs frequently “dart about without a serious purpose and are sometimes downright mischievous”. This description seems to fit similar observations of Hemiplax hirtipes.
Activities of a similar nature were observed in Helice crassa. As with Hemiplax hirtipes the warmest and sunniest days provided the most examples, and peak periods of fighting and burrowing seemed to coincide with peak mating periods. During these times some Helice crassa males wander far from their burrows and may engage in a series of fights with other wandering males or make raids on other burrows, even going so far as to dig the end-plug out of blocked-up burrows to provoke the owner into defending his territory.
Not only is this fighting without the obvious purpose of more usual territorial defence; it tends to be longer drawn out and more ritualised—i.e., real blows follow threat to a lesser extent.
Most fights, whether on a territory or not, are punctuated by short pauses in which the crabs may perform feeding movements, but in such a rapid and agitated manner that very little mud or sand is conveyed to the mouthparts and no rejection pellets formed. Gordon (1955) described such movements in corresponding circumstances for Uca species and labelled them “displacement activities”—i.e., irrelevant movements performed as a result of a conflict of two drives, the tendency to attack the rival and the tendency to flee from him.