Display Specificity and Reproductive Isolation in the Fiddler Crabs,Uca panacea and U. pugilator

The factors responsible for reproductive isolation between two sibling species of fiddler crabs (Uca pugilator and U. panacea) were studied under both laboratory and field conditions. ♂♂ showed species differences in their visual and acoustical displays. These differences were exaggerated in the overlap zone, where U. pugilator showed character displacement of its acoustic signals. ♀♀ of U. pugilator confined with ♂♂ of U. panacea produced fewer clutches of young. Occasionally, forced matings took place in the laboratory, resulting in hybrids suffering greater mortality through development. The data indicate that both premating (behavioral) as well as postmating (higher larval mortality) barriers act to prevent interbreeding.     

Female Choice in the Synchronously Waving Fiddler Crab Uca annulipes

In the fiddler crab, Uca annulipes, males attract receptive females into their burrows by waving their greatly enlarged major claw. We have previously shown that males clustered around a female wave in close synchrony. Females may have a preference for leading signals and synchronised waving may arise as an epiphenomenon of competition between males to signal first. Indeed, the males in clusters that females approach and visit in their burrows are more likely to produce leading waves than are their neighbours. Here we document two other differences in the waving behaviour of visited males and their neighbours. First, visited males complete the downward component of the wave more rapidly than their neighbours. Second, the interval between the end of one wave and the start of the next is shorter for visited males. How can waving be synchronous if visited males wave faster than their neighbours? While only 9% (40/431) of waves by neighbours did not overlap those of the visited male, 22% (110/501) of visited male waves did not overlap the wave of a focal neighbour (111 visited male-neighbour dyads). Hence, while overlapping waves are nearly synchronous, visited males produce additional, ‘nonoverlapping’ waves that result in a higher wave rate than that of their neighbours.     

Variation in the Boldness of Courting Sand Fiddler Crabs (Uca pugilator)

Individuals can express boldness in their readiness to resume courtship signaling following a perceived threat. The degree of boldness that is selectively favored depends on the magnitude of costs and benefits that may vary across time and space. We examined within‐ and between‐individual variation in the boldness of courting male sand fiddler crabs, Uca pugilator, across an entire breeding season at a South Carolina (USA) salt marsh where courtship is restricted to supratidal embankments. Boldness was assessed by the time to re‐emergence and the number of re‐emergences of males who were purposely startled into their breeding burrows once every 3 min for a total of five times. The two measures of boldness were significantly positively correlated. Courting males are on average bolder when their density is high and when tidal conditions correspond to peaks in the number of females moving over the embankment surface. Time to re‐emergence increases with successive startles although some males consistently re‐emerge faster than others. Large males are not bolder than small males. When male density is high, nearest neighbors frequently re‐emerge at the same time, suggesting that males cue on the responses of other nearby males, perhaps by assessing substrate vibration. This may reduce the chance of losing a potential mate to a local competitor.     

Visual and Acoustical Signalling during Courtship by Fiddler Crabs (Genus Uca)

Male fiddler crabs (Genus Uca) employ both visual and acousticalsignals to attract females for mating. In U. pugilator and severalother American species, the males attract females during theday first by waving, then by producing sounds just within theirburrows. At night, the males produce sounds at low rates, butwhen touched by a female, they increase their rate of soundproduction. In the European species, U. tangeri, many elements of courtshipare similar to those in U. pugilator, but two types of soundsare produced. One of these, the short drumwhirl, appears tosubstitute for waving when the male is temporarily obscuredfrom the female during his diurnal courtship activities. Thelong drumwhirl is used under different circumstances. The acoustical responses of a male to a female influence thecourtship behavior of other males in the area. When sounds fromstimulated males are played back to test males during the day,their lates of waving increase. At night, the playbacks elicitincreases in rates of sound production. The influence of tidal oscillations, temperature, and lightcycles on the behavior of males is discussed. Courtship activities of aquatic crabs are compared to thoseof terrestrial Brachyura. In aquatic forms, courtship may beabsent or, if present, does not involve elaborate signallingby the male. Chemical or visual cues at close range are themost important stimuli. In several genera of terrestrial crabs,visual signalling for prolonged periods is common, and soundsare often emitted by males to "call" females from their burrowsto the surface for mating. Some of the factors that may accountfor differences in courtship activities in aquatic and terrestrialspecies are discussed.     

Behavior of Four Species of Fiddler Crabs,Genus Uca,in Southeast Sulawesi,Indonesia

We studied the behaviors of four species of sympatric fiddler crabs on Kaledupa Island, Indonesia. Species differences in activity level, grooming, burrowing and feeding were related to their habitat and food. Uca chlorophthalmus, living in muddy mangrove areas, were inactive and spent most of the time feeding in place. Females fed 50% faster than males and spent more time feeding. U. vocans was the dominant species at the beach in silty sand and was very active. Its feeding rate was about twice that of the former species, females fed more rapidly than males, and many crabs of both sexes fed in droves at the water’s edge during ebb tides. During ebb tides, they spent most of their time feeding, while at flood tide they engaged in a greater variety of activities, including burrow maintenance. They frequently walked while feeding and interacted aggressively. U. tetragonon lived in a pebbly band along one edge of the beach, by a quay. Their feeding rate was comparable in both sexes and slower than that of U. vocans; they fed largely on filamentous algae growing on the quay, which provides better food, and fed faster during flood tide than ebb tide. They spent more time in waving and other sex-related activities, and were seldom aggressive, except during the week of the full moon. Burrowing activities included placing excavated mud balls some distance away from their burrows and re-arranging them. U. dussumieri inhabited the other end of the beach in muddier substrate. They did not have sex differences in feeding rates and their rate of scooping food into their mouths was slow, but feeding claws made multiple pinches of the substrate, thus accumulating more material in each clawful of food.     

Contextual Cross-Referencing of Species Names for Fiddler Crabs (Genus Uca): An Experiment in Cyber-Taxonomy

Cyber-taxonomy of name usage has focused primarily on producing authoritative lists of names or cross-linking names and data across disparate databases. A feature missing from much of this work is the recording and analysis of the context in which a name was used—context which can be critical for understanding not only what name an author used, but to which currently recognized species they actually refer. An experiment on recording contextual information associated with name usage was conducted for the fiddler crabs (genus Uca). Data from approximately one quarter of all publications that mention fiddler crabs, including 95% of those published prior to 1924 and 67% of those published prior to 1976, have currently been recorded in a database. Approaches and difficulties in recording and analyzing the context of name use are discussed. These results are not meant to be a full solution, rather to highlight problems which have not been previously investigated and may act as a springboard for broader approaches and discussion. Some data on the accessibility of the literature, including in particular electronic forms of publication, are also presented. The resulting data has been integrated for general browsing into the website http://www.fiddlercrab.info; the raw data and code used to construct the website is available at https://github.com/msrosenberg/fiddlercrab.info.     

Locomotor Rhythms in the Fiddler Crab, Uca subcylindrica

The locomotor activities of individual specimens of Uca subcylindrica (Stimpson) collected from semi-arid, supratidal habitats in south Texas and northeastern Mexico were studied in the laboratory using periodogram analysis. When crabs were placed under constant darkness (DD) or constant illumination (LL), free-running circadian rhythms were observed in the activity recordings. The locomotor activity of strongly rhythmic crabs in LL has an average period length of 24.4 h. Crabs held in DD express motor rhythms with periods of approximately 24.0 h. In LL the most common wave form for activity is unimodal, while under DD it is bimodal. Recordings under natural illumination (NL) revealed that both period length and the time of maximum activity (phasing) varied through the year. During winter months, the crabs are primarily diurnal with peaks in activity occurring between 0900 and 2100 h and possess a circadian rhythm with a 23.9 h period. During summer, crabs were nocturnal with maximal activity between 1300 and 0600 and a circadian period closer to 24.0 h. In these experiments, the rhythmic locomotor activities of U. subcylindrica are best described as “circadian”. This is unusual for a genus known for its expression of circatidal and circalunidian rhythms.     

Short-range Homing in Fiddler Crabs (Ocypodidae, Genus Uca): A Homing Mechanism not Based on Local Visual Landmarks

This field study investigated the spatial strategies and homing ability of two East African fiddler crabs, Uca lactea annulipes and Uca vocans hesperiae, using various experimental procedures. A clear-cut spatial segregation between the two species was observed, with U. l. annulipes occupying areas even a few centimetres above U. v. hesperiae . Females of both species were extremely faithful to their holes while the males exhibited behavioural differences. U. l. annulipes males were quite faithful to their holes; in contrast, U. v. hesperiae males, during a single low tide, visited several holes which they did not defend. Two displacement experiments were conducted in order to identify the stimuli that fiddler crabs use in their homing. In the 'dislocation experiment', the crabs were moved (by hand) 1 or 2 m away from their refuge and released. None of these crabs returned to its burrow and, moreover, the initial directions they followed were not homeward orientated. In the 'translation experiment', the crabs were passively translated while actively feeding. When the crabs attempted to go 'home' after the translation, they darted along a straight path that led them to points that would have corresponded to their homes if the translation had not taken place. After this first response crabs then commenced searching strategies that led half of them home. Short-range homing seems to depend strongly on information actively gathered during the outward journey, other than local visual landmarks.     

Locomotor Rhythms in the Fiddler Crab,Uca subcylindrica

The locomotor activities of individual specimens of Uca subcylindrica (Stimpson) collected from semi-arid, supratidal habitats in south Texas and northeastern Mexico were studied in the laboratory using periodogram analysis. When crabs were placed under constant darkness (DD) or constant illumination (LL), free-running circadian rhythms were observed in the activity recordings. The locomotor activity of strongly rhythmic crabs in LL has an average period length of 24.4 h. Crabs held in DD express motor rhythms with periods of approximately 24.0 h. In LL the most common wave form for activity is unimodal, while under DD it is bimodal. Recordings under natural illumination (NL) revealed that both period length and the time of maximum activity (phasing) varied through the year. During winter months, the crabs are primarily diurnal with peaks in activity occurring between 0900 and 2100 h and possess a circadian rhythm with a 23.9 h period. During summer, crabs were nocturnal with maximal activity between 1300 and 0600 and a circadian period closer to 24.0 h. In these experiments, the rhythmic locomotor activities of U. subcylindrica are best described as “circadian”. This is unusual for a genus known for its expression of circatidal and circalunidian rhythms.     

Comparative Studies of Reproductive Behavior in Mantis Shrimps and Fiddler Crabs

Comparative ethological studies of closely related species canplace into clear perspective the functions of behavioral traitsas species diverge and adapt to different environments. In thispaper we contrast the reproductive behavior of species withintwo crustacean groups: the mantis shrimps (stomatopods) andthe fiddler crabs (genus Uca). For the stomatopods, we identifyprobable selective relationships between features of species'ecology and their mating systems. Population density, the kindand availability of shelters in which these shrimps live, theintensity of predation and spawning cycles all play importantroles in molding reproductive behavior in this group. Associationsbetween the ecology and mating systems of fiddler crabs havebeen discussed recently elsewhere. Here we focus on a comparativeanalysis of sexual communication in these crabs. A study ofcompetitive courtship signaling and mate choice in the fiddlercrab Uca beebei has shown that certain male courtship signalsare highly attractive probably because they exploit female sensory-responsesystems that have been molded by selection for escape from predators.Interspecific comparison of male courtship displays and theresponses of females to these displays suggest that sensoryexploitation may play an important role in the evolution ofsexual signals in the genus. Comparative studies have advancedour understanding of how natural and sexual selection affectthe reproductive behavior of both stomatopods and fiddler crabs.     

Regeneration of Walking Legs in the Fiddler Crab Uca pugilator

SYNOPSIS. Regeneration of walking legs in the fiddler crab Ucapugilator is most efficient when it follows autotomy (the reflexiveloss of a limb). Closure of the wound and would healing occurimmediately following autotomy and visible regeneration beginswithin a few days. Regeneration of the walking leg occurs intwo distinct stages: The first stage, called Basal Growth, involvesmitosis and differentiation. The second stage involves primarilyprotein synthesis and water uptake and is called ProecdysialGrowth. Proecdysial Growth is, in part, under direct hormonalstimulation.     

A Spatially Explicit Model of Synchronization in Fiddler Crab Waving Displays

Fiddler crabs (Uca spp., Decapoda: Ocypodidae) are commonly found forming large aggregations in intertidal zones, where they perform rhythmic waving displays with their greatly enlarged claws. While performing these displays, fiddler crabs often synchronize their behavior with neighboring males, forming the only known synchronized visual courtship displays involving reflected light and moving body parts. Despite being one of the most conspicuous aspects of fiddler crab behavior, little is known about the mechanisms underlying synchronization of male displays. In this study we develop a spatially explicit model of fiddler crab waving displays using coupled logistic map equations. We explored two alternative models in which males either direct their attention at random angles or preferentially toward neighbors. Our results indicate that synchronization is possible over a fairly large region of parameter space. Moreover, our model was capable of generating local synchronization neighborhoods, as commonly observed in fiddler crabs under natural conditions.     

Hormonal Control of the Molt Cycle in the Fiddler Crab Uca pugilator

Comparison of the circulating steroids in the blood of crabslacking neurosecretory eyestalk centers and crabs with thosecenters intact (but lacking several walking legs) has revealeddifferences in vivo that can be attributed to eyestalk factors.It is concluded that eyestalk factors (including the putativemolt-inhibiting hormone, MIH): 1) exert control in vivo overproduction of 25-deoxyecdysone by the crab Y-organ, 2) controlcyclic steroid production throughout anecdysis and parts ofproecdysis and 3) are not solely responsible for the increasedsteroid production that occurs during late proecdysis. Usingthe data presented here and previously published data, a simplefeedback model for the control of Y-organ activity is proposed.The model suggests that the Y-organs of the crabUca pugilatorare modestly active during anecdysis but become further activatedduring late proecdysis. The increased activation requires morethan eyestalk removal and may involve additional extra-eyestalkfactors.     

Factors Affecting Mating Tactics in the Fiddler Crab, Uca vocans hesperiae

The mating strategies of male fiddler crabs are variable and highly flexible within species. In this study I examine three types of mating strategy used by individual male Uca vocans hesperaie. The most common strategy, termed a ‘standard gambit’, where males approached females at their burrow entrance and initiated courtship, accounted for 63% of mating attempts and 75% of successful matings. The rarest strategy (4% of mating attempts) was the ‘dig out’, where males attempted to mate with females whose burrows they had excavated. This strategy accounted for 19% of successful matings. ‘Herding’ behaviour which involved a male attempting to herd a female into a burrow and mate, contributed 33% of mating attempts but were generally unsuccessful, accounting for only 2.6% of successful matings. Males used more than one strategy during the study period. Smaller males used the standard gambit strategy more often than herding or dig outs while larger males used the herding strategy more often. There was no relationship between male size and mating success and males did not preferentially mate with females of a certain size. The predominant strategy adopted by males over the lunar cycle depended on female behaviour. Herding behaviour was induced by female wandering which escalated at full moon. Standard gambits were the commonest strategy adopted at and around new moon. The low success rate of male mating attempts (16%) indicates a reluctance by females to mate multiply. This may lead to conflict between the sexes because in fiddler crabs there is last male sperm precedence.     

The Function of the Four Types of Waving Display in Uca lactea: Effects of Audience,Sand Structure,and Body Size

Multiple signals that convey different messages have been reported in many taxa, but relatively few studies have been made on such signals in invertebrates. In the present study, I investigated four types of claw‐waving display used in the fiddler crab Uca lactea to test whether the displays have different functions. Three males with a sand structure beside their burrows (which can attract females) and three males without a sand structure were fenced in an opaque enclosure, and I videotaped their waving displays after releasing two burrowless males or two burrowless females to test the effects of audiences. (a) Lateral‐circular waving tended to occur in enclosures with burrowless females and was performed frequently by males that had sand structures. (b) Lateral‐flick waving was performed frequently by males without sand structures, and its frequency was positively correlated with the signaler’s body size. (c) Rapid‐vertical waving was observed frequently in enclosures with burrowless males, and its frequency was negatively correlated with the signaler’s body size. (d) Circular waving tended to occur in enclosures with burrowless females and was performed frequently by males that had sand structures, and its frequency was positively correlated with the signaler’s body size. In my previous study, lateral‐circular waving was often seen in the breeding season and was mostly performed to female audiences, lateral‐flick waving was frequently performed to neighboring resident males, rapid‐vertical waving was performed mainly to intruding burrowless males, and circular waving did not have apparent audiences in most cases. Finally, I concluded that lateral‐circular waving was used as a courtship display, lateral‐flick waving was related to border disputes, rapid‐vertical waving was used for burrow guarding, and circular waving was used to broadcast the signaler’s general quality.     

The Role of Rhythmic Systems in the Adaptation of Fiddler Crabs to the Intertidal Zone

Fiddler crabs inhabiting the intertidal zone must adapt theiractivity to both the day-night cycle and the cycle of the tides.The tidal cycle imposes on the animals a rhythmic alternationbetween terrestrial and marine periods of existence. At thesame time the crabs are influenced by the day-night cycle, sincethey exhibit specific diurnal and nocturnal habits. Moreover,the interaction of daily and tidal rhythms may give rise tosemi-monthly variations in activity. It is now well established that persistent daily and tidal rhythmsin physiological processes underlie rhythmic variations in thebehavior of crabs in the field. The present paper reviews morerecent studies, which have compared the persistent rhythms incrabs from different tidal and non-tidal regions. Rhythmic patternsrecorded in the laboratory are found to be closely correlatedwith tidal conditions in the native habitat of the crabs. Ithas also been shown that the persistent pattern can be modifiedby transplanting crabs to the intertidal zone of another coastwhere they are exposed to tidal cycles which differ in formfrom those in the original habitat.     

Interspecific Communication in Fiddler Crabs: Preliminary Report of a Female Rejection Display Directed toward Courting Heterospecific ♂♂

A newly described display is apparently used by female fiddler crabs (genus Uca) in interspecific communication. The display, termed repetitive-high-rise, is directed primarily toward courting heterospecific ♂♂, which approach or are likely to approach the ♀. We postulate that the display serves to indicate the ♀'s unavailability to these ♂♂ for mating. The ♂♂ respond appropriately by ceasing their approach toward displaying ♀♀. Both reproductively receptive and unreceptive ♀♀ were observed to perform this display. ♀♀ appeared to be able to discriminate, not only between conspecific ♂♂ and heterospecific ♂♂, but also among ♂♂ of heterospecific species. The possible significance for this ability is discussed.     

Limb Regeneration in the Fiddler Crab, Uca pugilator: Hormonal and Growth Factor Control

This paper summarizes recent work on various aspects of hormonalcontrol of regeneration in the crustacean, Uca pugilator. Hormonalcontrol in this crab is effected by means of the crustacteansteroid hormones, the ecdysteroids. New evidence is presentedsupporting a role for the retinoid hormones, all-trans retinoicacid and 9-cis retinoic acid, in the control of regenerationin these animals. The possible role of fibroblast growth factorsin organization of the limb blastema is explored and the similaritiesbetween vertebrate and invertebrate control of regenerationare discussed.     

Limb Regeneration in the Fiddler Crab, Uca pugilator: Histological, Physiological and Molecular Considerations

This paper summarizes our recent work on the histological, physiologicaland molecular aspects of limb regeneration in the fiddler crabUca pugilator and new information is presented on mitotic activityin the blastema during the first days of blastemal organization.We also report for the first time the localization of vertebrategrowth factor immunoreactivity (FGF 2 and FGF 4) in the regeneratingblastema. In the first part of this paper we review recent histologicalfindings concerning the physical events that accompany autotomyof limbs and propose a new function for the autotomy membrane—thetethering of the regenerating pedal nerve to the walls of thecoxa. In the second part of the paper we review our recent findingson the identification and characterization of the Uca ecdysteroidreceptor (UpEcR, and its potential dimer partner, the retinoid-X-receptor,UpRXR). Using Uca-specific antibody probes raised in our lab,we have been able to identify specific cells in the early blastemathat express receptor proteins. The regenerating limb of thefiddler crab is responsive to both steroids and retinoids andmRNA for steroid and retinoid receptors are expressed in theregenerating limb buds during all stages of regeneration. TheDNA and deduced amino acid sequences of the ecdysteroid receptoris very similar to the sequences of insect EcRs, while the retinoidreceptor is similar to insect protein (ultraspiracle) in theDNA-binding domain, but closer to vertebrate RXRs in the ligandbinding domain     

Pillar Function in the Fiddler Crab Uca beebei (II): Competitive Courtship Signaling

Male Uca beebei court and attract females into burrows they defend on muddy sand flats in the intertidal zone on the Pacific coast of the tropical Americas. Mating, oviposition and incubation (breeding) occur underground in males' burrows. Some courting males build mud pillars (2 cm high) at the entrance of their burrow. The purpose of this field study was to assess the role of pillars in competitive courtship signaling among males. I studied the effect of pillars on female behavior by recording the responses of wandering females to courtship from males resident at burrows with and without pillars. I also caught females, released them individually in a circular arena with an equal number of empty burrows with and without pillars around its circumference, and chased the females with a simulated avian predator. Females moved to burrows of both types more often when they were courted (82%) than when they were chased (67%). Receptive females were attracted to the burrows of the males that courted them significantly more often (97%) when these burrows had pillars than when they lacked pillars (66%). However, once females entered males' burrows they were equally likely to remain, mate and breed in both types of burrows. Females also more often moved to burrows with pillars (66%) than to burrows without pillars when they ran from the simulated predator. Both male courtship displays and pillars probably provide cues females use to locate males' burrows. The visual similarity between pillars and a display courting males give immediately before they enter their burrows suggests that pillars are icons of the display. The effect of pillars on female behavior, the timing of pillar building relative to when females choose mates, and contrasts in the behavior of males that do and those that do not build pillars suggest that pillar building has evolved due to competition among males to attract females into their burrows.