Some Aspects of the Behaviour of the Soldier ,

ANN M. CAMERON1

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

224 Behaviour of the Soldier Crab-CAMERON 225

BEHAVIOUR Numerous environmental factors such as , temperature, solar radiation, precipitation, and are generally considered to influence the activities of mob ile intertidal anima ls and of shore crabs in particular (Crane, 1958). Soldier crabs are usually active for several hours before and after low water, although they are more active on warm sunny days than in cold over­ cast conditions. They were observed to be ac­ tive at night, but their nocturnal behaviour was not studied. In daylight hours, cessation of ac­ tivity and retreat below the surface were fre­ FIG. 2. Section of an army of M. longicarpus. (Photograph by B. Morgan. ) quently observed in response to apparently unfavourable conditions, such as the onset of rain. Such periods of retreat ranged from a few rivity become obvious some time before the minutes to the remainder of an intertidal period. crabs make their appearance. On the surface of A pattern of activities is followed by undi s­ the previously smooth sand small hummocks turbed populations of M . longicarpus during appear and increase in dimensions ( Fig. 4 ) . the intertidal period. This pattern will be de­ These hummocks are always made prior to the scribed as a series of phases of activity, each of crabs' emergence but their formation does not which merges with the one following but is necessarily imply that the crabs will emerge. nevertheless generally well defined. Variations Hummock building lasts from 10-30 minutes. in the sequence and in the duration of each Just prior to the second phase of activity the phase occur, and the pattern may be interrupted summits of the hummocks are broken open or terminated by unfavourable environmental (Fig. 5). conditions. 2. Emergence phase 1. Subterranean Phase A population may emerge in less than five Activi ties performed below the surface were minutes, or the period may be pr otracted for as not studied, but indications of subterra nean ac- long as one hour. Generally adults emerge be­ fore the juveniles. Takahasi's (1935 ) statement that "Mictyris never comes out to the surface" is inexplicable. It was ascertained by digging that many individuals remain beneath the sur­ face for an entire intertidal period. Thus, the emerged population does not represent the total population of the area. Further, the proportion of the population which does emerge varies on subsequent days. At any locality the emerged population may be nearly all adult males on one day, and a mixture of all sizes and both sexes on the following day.McNeill (1926) thought that female M. longicarpus do not congregate on the surface to the same extent as do the males. This was verified in the present study. Upon reaching the surface individuals per ­ form a routine of body care, in which they re­ FIG. 1. Adult male M. longicarpus. (Photograp h move adhering sand grains from the eyes, by S. Breeden.) mouthparts, and carapace. They clean the eye- 226 PAOFIC SOENCE, Vol. XX, April 1966

lat . 27 0 S

IN

Magnetic. North

M O RET O N BA Y

PACIFIC OCEAN

o

Scale in miles

FIG. 3. Map of Moreton Bay, Queensland, showing localiti es at which field observations were made. stalks and mouthparts by vigorously rubbing less than a second. Th e crab falls backwards, the chelae over these regions. deansing is facili­ lies with the carapace on the substratum, and tated by vibratory movements of the third max­ then flips back to the normal resting position. illipeds and by alterna te raising and lowering of 3. Preliminary Feeding Phase the eyestalks. Sand on the carapace is removed as a result of a "back-flip" manoeuvre. Th is is Subsequent to emergence the crabs feed and essentially a half somersault and is executed in walk alternately, the feeding rate being much Behaviour of the Soldier Crab-CAMERON 227 slower than that of normal feeding (phase 5). accumulates at the bases of the third maxilli­ This phase occupies 10-15 minutes. Those crabs peds, and is discarded in the form of pellets which are first to emerge do not immediately (Fig. 6) which either drop off, or are wiped feed in this slow fashion, but run about, stop, away from the ventral surface of the body by a remain motionl ess, run further, and so only lateral movement of one of the chelae. A similar gradually begin to feed. The slightest disturb­ deposition of discarded material is performed ance at this stage, such as the flight of a bird by Uca spp. (Miller, 1961) and D otilla mic­ overhead, results in their immediate retreat be­ tyroides (Tweedie, 1950). Feeding pellets de­ low the surface, from where they emerge again posited by crabs in the van are generally left after about 3 minutes. undisturbed by following crabs. As the feeding phase progresses, the initial 4. Tr ekking Phase rapid rates of walking and of deposition of . Within 15 minutes of emergence the crabs pellets slow down. Aggregation into groups of simultaneously begin walking toward the wa­ about 100 crabs takes place, and the individuals ter. This spectacular migration is termed the continue to feed, although the groups dissolve "trek." Usually the movement is down the and reform continuously. After about Y:! hr, the beach, but if surface water is present in chan­ formation of larger aggregations, called armies, nels or run-offs the direction of the trek is across takes place. Such groups are distinguished from the beach toward these channels. Walking of the former feeding groups in that the indi­ individuals is interrupted by frequent short viduals of armies tend to remain in company, ,stops to feed, but the movement of the popula­ whereas previously they lacked this cohesion. tion as a whole does not stop unti l the water is Feeding continues. The armies increase in size reached. Trekking occupies 15 minutes at the by the addition of recruits either singly or in most, and is nearly always performed in a vir­ groups, until aggregations of many hundreds tually straight line from the area of emergence. are formed. The duration of the feeding phase There is no cohesion among the individuals is variable. Usually it lasts for about 1 hr, but during the trek. Each crab makes its own way often can be for as long as 2 Y:! hr. W ith dimi­ down the beach and the population appears dis­ nution of the feeding rate, the armies begin to persed at random. Generally, a distance of about move over the shore, frequently remaining in 18 inches separates the individuals, which re­ puddles and run -offs and performing no visible flects the fact that they have emerged in prox­ activity. imity, and have proceeded at the same rate. Examination of the gut contents of soldier Juveniles do not take part in the trek proper. crabs was made by dissecting the alimentary They usually move about 50 yards in the genera l canal under the microscope, and staining the or­ direction taken by the adults, and then wander ganic matter present with Toluidine Blue and about feeding at this level of the beach. Methylene Blue. Almost all the material in the 5. Feeding phase gut was organic debris.Some very fine silt was When they have trekked to areas which ap­ present, but there were no sand grains in the parently are suitably moist, the crabs increase cardia as reported by Lazarus ( 1945) . their rate of feeding until it is extremely rapid. occurred in all, though not as a major consti­ They progress as they feed, the direction of tuent of the contents, and one specimen con­ movement being more or less along the beach tained gastropod eggs and a . These parallel to the water. Frequently , they transverse data agree with those of Lazarus ( 1945) who the Zostera-covered areas on beaches where the stated that the food of Mict yris is "organic frag­ grass occurs. ments and minute organisms" extracted from McNeill (1926) and Lazarus (1945 ) have the sand. However, she remarked that, although described the mechanics of soldier crab feeding. "the'mouthparts must discard much of this use­ Portions of the substratum are lifted to the buc­ less sand the gut is usually full of it either in cal cavity and are sampled by the mouthparts. I its original state in the cardia or ground into a observed that material unsuita ble for ingestion finer consistency in the intestine." Since soldier 228 PACIFIC SCIENCE, Vol. XX, April 1966

FIG. 4. Hummocks made prior to the emergence of the crabs. (The ruler is 15 inches long.)

FIG. 5. Holes opened by emer gence of the crabs. (The ruler is 15 inches lon g.) crabs may feed for periods of the order of 2 hr, feed for such extended periods favours the ex­ it would seem impossible for the gut to accom­ planation that they need to sample a large modate a continuous intake of sand for this volume of sand, and that they efficiently select length of time. The fact that they habitually from this the small fraction suitable for inges- Behaviour of the Soldier Crab-CAMERoN 229 tion. Furthermore, the faeces of the are spooned seta, which are shown in Figure 7. devoid of sand grains. 6. Army Wandering ph ase In discussing the feeding mechanism of Uca, Miller (1961) remarks that "the thoroughness Soldier crab armies present a spectacular with which the crabs are able to sort food ma­ sight. They may cover vast areas of the beach and their tightly packed masses create a rustling terial from the mineral fraction of the sub­ noise audible at a considerable distance. The stratum corresponds to the availability of the army wandering phase begins with the cessation food material." He showed that the sandy beach­ of feeding. Sometimes the transition is sudden inhabiting U. pugilator ingests a minimal but usually it is gradual. Armies are generally amount of sand, whereas for the marsh-inhabit­ composed of similarly sized individuals, usually ing U. pugnax "food is more readily available adult males. Only rarely do females wander in because the silt with which it is generally as­ armies, though mixed armies occur. In mixed sociated is of sufficiently fine texture to be in­ armies, the largest crabs progress in the van­ gested ." Furthermore, in those ocypodid crabs guard, probably because they walk faster than which possess them, the spoon-tipped setae of do the smaller ones. In no sense does any section the mouthparts are present in greater numbers of an aggregation lead the rest. The advancing and exhibit more elaborate spooning in species front changes continuously, and the crabs often inhabiting sandy substrata than in species of walk over each other, two and three deep, in the muddy environments (Crane, 1941, 1943; AI­ general scramble. Crowding may be even worse revogr, 1957; Miller, 1961). Mictyris longicar­ than this, especially when the advancing front pus possesses large numbers of densely packed reaches standing water and stops. Armies cover spooned setae. They are present on the inner great distances during this phase. They com­ surface of the third maxillipeds, the outer and monly walk as far as 500 yards, the rate of inner surfaces of the second maxilli peds, the progression being about 10 yards per minute. basipodite of the first maxillipeds, and the outer Army wandering occupies from 30 minutes to surface of the endite of the coxopodite of 2 hr. When emerged females are present but the first maxillipeds. There are four types -of have not joined armies, their feeding activities

FIG. 6. Feeding pellets deposited by soldier crabs. (The ruler is 15 inches long. ) 230 PACIFIC SCIENCE, Vol. XX, April 1966

(phase 5) merge with the return phase (7) de­ scribed below. Juveniles do not participate in army wandering. 7. Return phase The return phase begins when the armies cease to wander about the beach in a seemingly undirected fashion, and orient their progression in the direction of the upper levels of the shore. As they proceed, individuals tend to dissociate themselves from their fellows so that the armies lose their cohesion. The crabs continue walking Q· l m m until they reach the level of the beach at which they emerged. This return may be protracted for as long as 1Y2 hr, bur it may be as short as 15 minutes. 8. Aggressive Wandering phase On completion of the return phase, the crabs dig holes in typical corkscrew fashion (Mc­ Neill, 1926 ) . Although McNeill believed that these burrowing actions "are probably unique among the ," Tweedie ( 1950) has since noted that Dotilla mictyroides employs a similar method. Soldier crabs do not remain in these newly dug holes bur re-emerge , dig more ·· 0 · 0 5 .,, 11') holes, abandon these in turn, and continue thus FIG . 7. Camera Iucida drawings of the four types for th e duration of the phase. At the same time of spoon-tipped seta on the mouthparts of -M. longi­ they respond to encounters with one another. carpus. Threat displays between pairs of adult males result from these encounters. In each case, one Those walking legs thus freed are extended of the pair of crabs involved was temporarily laterally, as are the chelipeds. Attainment of the associated with a hole. Females were not ob­ posture of maximum threat occurs when the served to participate in such displays. Four types plane of the thoracic sterna is vertical. Thus, of encounter were observed: (i) walking over the ventral surfaces of the crabs are brought into an occupied hole, followed by emergence of the proximity. However, they do not touch, and no occupant and a threat display between him and pushing occurs. The eventual winner may re­ the intruder; (ii) physical collision of two crabs main poised in the position of maximum threat in the vicinity of a hole, followed by a threat for up to a second after the loser has fallen display; (iii) threat display by an occupant at away. Throughout the encounter the free walk­ the entrance of a hole, to which another male ing legs are vibrated rapidly, and the chelipeds in the vicinity responded with a threat dis­ are gradually raised vertically from their posi­ play; (iv) intrusion by a male into an occupied tion of horizontal extension. Immediately after hole, or digging by a male so close to an occu­ the display the winner exhibits the following pied hole that its occupant emerged , followed characteristics: (i) maximum vertical extension again by a threat display. of the chelipeds, (ii ) maximum extension of Soldier crab threat display is performed as the free walking legs, (iii) maximum exten­ follows: Rival males meet . Each raises the an­ sion of the legs on which he is balanced, (iv) terior legs off the substratum so that he is bal­ maintenance of the eyestalks in the raised posi­ anced on the third and fourth pairs of walking tion, and (v) maintenance of position with legs, or even on the most posterior pair alone. respect to the hole. Behaviour of the Soldier Crab-CAMERoN 231

The loser is characterized by: (i) lowering the V1CIU1ty of a hole. In each case the larger of the chelipeds, (ii) flexion of the free walk­ individual won. ing legs: (iii) flexion of the legs on which he is A considerable degree of social facilitation balanced and subsequent dropping to the sub- _ was evident in dense populations of soldier stratum, ( iv) lowering of the eyestalks, (v) crabs during this aggressive wandering phase. submissive posture in front of the still threat­ On many occasions threat displays between pairs ening winner, and (vi) retreat from the vicinity of crabs were joined by a third, fourth, and of the hole. even a fifth individual. Each individual partici­ Although the winner may remain in the vi­ pating in these multiple displays assumed the cinity of the hole for a few seconds, he generally typical threatening posture as far as the crowded moves away shortly after the loser has departed. space occupied by the several crabs allowed. A stopwatch was used to time 84 threat dis­ Threat display occurred throughout the year. plays. The longest encounter lasted for 30 sec Breeding also occurred throughout the year. In and the shortest for one sec. From the data col­ no month from February 1961 to October 1962 Iecred, the significance of occupancy of a .hole, were ovigerous females absent in the popula­ and of disparity in the sizes of the participants tions studied. were estimated. Occupancy was indicated by a The aggressive wandering phase may con­ crab's position either within a hole or in its tinue for as long as 1Y2 hr but generally oc­ immediate vicinity. Size was subjectively deter­ cupies about half that time. Threat displays are mined with categories of "bigger," "smaller," frequent throughout the phase. As many as 12 and "apparently equal." The error in allotting a such displays were observed simultaneously in pair of individuals to the last c3!tegory may be an area 20 fr square. Aggressive wandering considerable, but not so great as to obscure the diminishes as the individuals remain in holes general picture revealed by the data. That this instead of re-emerging, and the whole popula­ error in categorisation was consistent is indi­ tion gradually disappears beneath the surface. cated by the coefficients of variability calculated For each of these phases of activity during for 'data of threat displays between apparently the intertidal period, the approximate duration equal-sized crabs on the one hand, and between has been given. It is emphasised that these times unequal-sized crabs on the other. These are are highly variable , and that the entire sequence 57% and 54%, respectively. is not always completed. Although the initiation Of 42 threat displays between unequal-sized of each phase is generally synchronised for all crabs, 38 were won by the larger of the pair. the individuals of the emerged population, oc­ Less than 10% of these encounters were won casions were noted when feeding, army wan­ by individuals perceptibly smaller than their op­ dering, and returning were being performed ponents. Of 40 threat displays between appar­ simultaneously in different areas of a large ently equal-sized crabs, 35 were won by the beach. occupant of the hole. Clearly, occupancy is an advantage. REACTIONS TO OTHER ORGANISMS The mean time of threat displays between apparently equal-sized crabs (9.8 sec) was com­ During this study seven species were ob­ pared with that of threat displays between served to prey on M. longicarpus. They are unequal-sized crabs (7 .8 sec) by use of a "t" Threskiornis spinicollis (Jameson), the straw­ test. The means are probably significantly dif­ necked ibis; Halcyon sordidus (Gould), the ferent (p between 0.05 and 0.1), indicating mangrove kingfisher; Egretta alba (Linne), the that disparity in size of the participants shortens white crane; Sphaeroides hamiltoni (Richard­ the time of threat display, compared with that son) , the toad fish; ceratophthalmus elapsing during threat display between appar­ (Pallas ), the ; Metopo grapsus mes­ ently equal-sized crabs. sor ( Forskal ) , a grapsid crab; and Conuber Only four instances of threat display were sordida Swainson, a sand snail. The ghost crab observed in which neither participant was in and the sand snail preyed on juvenile soldier 232 PACIFIC SCIENCE, Vol. XX, April 1966 crabs. Toad fish capt ured adults in run-off chan­ to drop it immediately. The chelae continue nels. M . messor seized soldier crabs near a rocky their upward movement but do not reach the point at Dunwich (W. Stephenson, persona l buccal opening. Furthermore, material not con­ communication ) . The three birds preyed on veyed to the mouthparts during normal feeding adults, usually during the army wandering phase is picked up in this displacement feeding, for of activity. It is noteworthy that, except for a example, small pebbles and pieces of Zostera slight increase in the walking rate of the crabs two inches long. at the approach of a predator, they exhibited no It is evident that these two types of displace­ escape reactions. This contrasts strongly with the ment feeding are essentially similar, differing in behaviour of soldier crabs in the presence of their degree of disorientation to the environ­ human observers. Their reactions in such cir­ ment, rather than ' in their organisa tion. Such cumsta nces are as follows: correlation between intensity of thwarting or 1. Burrowing. On being chased by humans, conflict and the degree of disorientation of dis­ soldier crabs burrow in typical corkscrew placement activity is well documented ( Arm­ fashion. strong, 1950 ) . 2. "Fright reflex." M. longicarpus occasionally Circumstances eliciting displacement feeding reacts to the presence of an observer by assum­ in M. longicarpus are: ing a "frozen" posture in which the limbs are (i) Simultaneous activation of the drives extended rigid ly. The presence of normal preda­ to retreat and to stay. Crabs were disturbed by tors was never observed to elicit this response, the observer unti l they retreated, after which common among brachyurans (Schone, 1961: the observer remained motionless at a distance 474.) of approximately 10 ft. After a few seconds the 3. Crouching. A crab in this posture lowers crabs halted and, while either standing station­ the body to the substratum and folds the flexed' ary or approaching the position of the ob­ legs as closely as possible to the body. The eye­ server, began displacement feeding . As they stalks are lowered. approached, the rate of movement of the chelae 4. Interspecific threat. Occasionally, crabs increased. which were being chased by the observer, raised When the observer's hide was placed in such the chelipeds and ran away from the observer. a position that the crabs trekked around or This ambivalent behaviour is apparently an at­ past it, they began displacement feeding as they tempt to. flee and to perform a threat display approached to within about 4 ft. Some indi­ simultaneously. viduals spent as long as 3 minutes at a distance 5. Displacement feeding. Displacement feed­ of less than 4 fr from the hide, displacement­ ing differs from the autochthonous activity in feeding throughout that interval. Then they that any material that is raised to the vicinity of made their way past the hide and ceased dis­ the buccal opening is never introduced into it, placement feeding at about 4 ft from it. and consequently no feeding pellets are formed . (ii ) Simultaneous activation and thwart­ Two types of displacement feeding occur. ing of the escape drive. Soldier crabs were a. A crab remains stationary, continuously chased until they burr owed. When the excava­ raising and lowering the chelae which mayor tion was as deep as the body of the crab, the may not touch the substratum. No material is crab was removed from it and placed on the picked up, and the chelae are not lifted as high surface beside it. The crab burrowed again, and vertically as they are in normal feeding. was again interrupted. After about five repeoi­ b. Displacement feeding of the second type tions of this interruption, some of the crabs so has a much more "nervous" appearance (in the treated began displacement feeding. Many could sense in which this term is used by Gordon, not be induced to do so, but assumed the 1955). Walking with unusually rapid steps, a crouching posture described above. crab lowers the chelae and picks up material All the instances of displacement activity de­ from the substratum in such quantity and with scribed above were induced by the presence of such lack of normal manipulative dexteri ty as the observer or of the hide, or by interference. Behaviour of the Soldier Crab-CAMERoN 233

No displacement activity was observed in any persed food supply, the presence of the animals other circumstances. on the surface after feeding has ceased is enig­ matic. Whatever are the physiological functions DISCUSSION performed at this stage, they must be of con­ siderable significance in view of the energy ex­ Several aspects of the behaviour of M. langi­ pended in wandering about the beach, and of carpus are of interest. It would appear that the the danger from predators to which the aggre­ preliminary feeding phase may be regarded as gations are exposed. These phenomenal aggre­ appetitive feeding. Normal feeding does not gations, in which the individuals perform no take place until completion of the trek. The ob­ visible activity other than walking, may be an vious result of trekking is that the crabs move example of epideicric display (W ynne-Edwards, from the upper levels of the beach to the more 1962). moist areas. Alrevogr (1957 ) postulated that Threat display is associated with competition all ocypodid crabs possessing spooned setae on for space, the space being a hole occupied by the mouthparts select nutritive material from one crab and intruded into by another. Occu­ the sand by 'the "flotation" method he observed pancy of a hole, however temporary, appears to in Uca spp. Water is lost continuously through be the manifestation of a poorly developed ter­ ingestion, evaporation, and in the discarded pel­ ritoriality, and for this reason the threat display lets. Miller (1961) investigated this topic for must be considered reproductive fighting. ' Tin­ several species of V ca and concluded that the bergen ( 1952) remarked that displacement need for water is reflected by the moist condi­ activities occur most frequently during boun ­ tion of the material from which these crabs dary disputes. No instance was observed of prefer to feed. This appears to be the case for either of the participants in a threat display M. longicarpus. Not only do soldier crabs pos­ subsequently performing displacement activity. sesses dense spoon-tipped setae, but also they This is not unexpected, in view of the transitory frequently feed in depressions and in run-off nature of soldier crab "territory," which is based channels. on occupancy of a hole and not on construction Trekking is most obvious on wide, gently and maintenance of a permanent burrow. sloping beaches where the crabs may remain on There are similarities between the intertidal the surface for as long as 4Y2 hr. Where the behaviour pacterns of M. longicarpus and Uca beach is narrower and of steeper slope, they do spp. (Crane, 1958). The activities of both are not remain on the surface for such lengths of organised into a series of phases. There is a time, and they generally emerge earlier with temporal separation of feeding, and of other respect to the time of low water than do those activities. Although there is nothing comparable inhabiting the wider flats. On beaches of un­ with the waving display of Uca in the behaviour usual configuration, such as those at the mouth of M. longicarpus, the occurrence in the latter of the Pine River, the direction of the trek is of threat display, and its nature, are of interest. determined by the proximity of extensive drain ­ age channels. Moving down the beach would SUMMARY bring them to the elevated sand bars. From the bars to the river the beach slopes in the normal 1. Mictyris longicarpus performs a sequence way, and the crabs in this area trek down in the of phases of activity during the intertidal pe­ usual fashion. riod: subterranean activity, emergence, prelimi­ When feeding has ceased, the army wander­ nary feeding, trekking down the beach, feeding, ing phase takes place. In some respects, the be­ army wandering, return up the beach, aggressive haviour of soldier crabs in this phase resembles wandering, return to subterranean burrows. that of Uca in the "nonaggressive wandering 2. Dense, elaborate spoon-tipped setae are phase" described by Crane (1958 ). While it is present on the mouthparts of M. longicarpus. evident that the aggregations in which feeding 3. Wandering in armies occurs after feeding takes place subserve coverage of a widely dis- has ceased. Adult males comprise the majority 234 PACIFIC SCIENCE, Vol. XX, April 1966 of these aggregations. LAZARUS, M. 1945. Mictyris. The soldier crab. 4. Threat display occurs between pairs of Unpublished thesis. Zoology Department, adult males. Occupancy of a hole, and larger University of Sydney, New South Wales. size of a male, are advantageous in such displays. McNEILL, F. A. 1926. Studies in Australian 5. Displacement feeding occurs as a result of carcinology, II. A revision of the family Mic­ conflict of drives, and of thwarting of a drive. tyridae . Rec. Aust , Mus. 15: 100-128. MILLER, D. C. 1961. The feeding mechanism REFERENCES of fiddler crabs, with ecological considerations ALTEVOGT, R. 1957. Untersuchungen zur Bi­ of feeding adaptions. Zoologica 46 :89-100. ologie, Okologie, und Physiologie indischer SCHONE, H. 1961. Complex behaviour. In: Tal­ Winkerkrahben. Z. Morph. Okol, Tiere 46: bot H. Waterman, ed., The Physiology of 1-110. Crus tacea, vol. II, pp. 465-520. Academic ARMSTRONG, E. A. 1950. The nature and func­ Press, New York. tion of displacement activities. Sympos. Soc. STEPHENSON, W. 1961. Outdoor Studies on Exp. BioI. 4 :361-384. Living Animals in Queensland. 2nded. Uni­ BALSS, H. 1955-56. Decapoda. Okologie. In: versity of Queensland Press, Brisbane, Queens­ Bronn's Tierreich, Bd. 5, Abr, 1, Bch. 7, pp. land. 123 pp. 1285-1476. Akademische Verlagsges ., Leip­ TAKAHASI, S. 1935. Ecological notes on the zig. ocypodian crabs ( ) in Formosa, CRANE, J. 1941. Eastern Pacific expeditions of Japan. Annotations Zool. Japonenses 15:78­ the New York Zoological Society, XXVI. 87. Crabs of the genus Uca from the west coast TINBERGEN, N. 1952. Derived activities; their of Central America. Zoologica 26: 145-208. causation, biological significance, origin, and --- 1943. Display, breeding, and relation­ emancipation during evolution. Quart. Rev. ships of the fiddler crabs (Brachyura, genus BioI. 27 :1-32. Uca ) in the northeastern United States. .TWEEDIE, M. W . F. 1950. Notes on grapsoid Zoologica 28: 217-223. crabs from the Raffles Museum, II. On the --- 1958. Aspects of social behavior in fid­ habits of three ocypodid crabs. Bull. Raffles dler crabs, with special reference to Uca mara­ Mus. 23 :310-324. coani ( Latreille) . Zoologica 43:113-130. WYNNE-EDWARDS, V. C. 1962. disper­ GORDON, H. R. S. 1955. Displacement activities sion in relation to social behaviour. Oliver in fiddler crabs. Nature 176 :356-357. and Boyd, Edinburgh and London. 653 pp.