Locomotor Activity of the Fiddler Crab, Uca subcylindrica (Stimpson), under Artificial Illumination

Specimens of the fiddler crab Uca subcylindrica (Stimpson) were captured in south Texas (USA) for locomotor rhythm studies. Actographic data were analyzed using Tau™ sofware. Under constant illumination (LL) and darkness (DD), the semiterrestrial crabs express a circadian rhythm of locomotion. When exposed to illumination/darkness cycles (LD_12:12 or LD_14:10), their bouts of activity are entrained to the photoperiod. In LD, activity is generally bimodal with an initial burst about 0.5 h after illumination. A second burst of activity begins 1 to 2 h before the end of illumination. When transferred from LD to LL, a locomotor rhythm with an average period of 24.6 ± 1.0 h (n = 19) is expressed in 89 percent of the crabs. On the other hand, when placed in DD after LD (n = 8), the crabs are either arrythmic or weakly rhythmic (period = 23.8 ± 0.2 h; n = 2). If the onset of illumination is advanced by 6 h, a period of less than 24.0 h is detected in the actogram. If the onset of illumination is delayed by 6 h, a locomotor rhythm with a period greater than 24.0 h appears. The locomotor behavior of this species of fiddler crab, Uca subcylindrica, is not related to the tidal rhythmicities seen in other members of the genus Uca. Rather, it has strong circadian components.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Circadian Locomotor Activity Under Artificial Light in the Freshwater Crab Pseudothelphusa americana

Long-term recordings of locomotor activity were obtained from intact freshwater crabs, Pseudothelphusa americana in constant darkness (DD), constant light (LL) and different light-dark (LD) protocols. Bimodal rhythms were typically observed in this crab when subjected to DD or LD, with bouts of activity anticipating lights-on and lights-off, respectively. Freerunning circadian rhythms were expressed in both DD and LL for longer than 30 days. In DD, we observed that some animals presented different period lengths for each activity component. During LL, activity was primarily unimodal, however spontaneous splitting of the rhythms were observed in some animals. When activity was recorded under artificial long days, the morning bouts maintained their phase relationship but the evening bouts changed their phase relationship with the Zeitgeber. Our results indicate that, bimodal locomotor activity rhythm in the crab Pseudothelphusa americana is variable among organisms. The characteristics of phase relationship with LD and responses to LL for morning and evening bouts, suggest that, locomotor activity could be driven by multiple oscillators, and that coupling between these oscillators may be regulated by light.     

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     

Circadian Locomotor Activity Under Artificial Light in the Freshwater Crab Pseudothelphusa americana

Long-term recordings of locomotor activity were obtained from intact freshwater crabs, Pseudothelphusa americana in constant darkness (DD), constant light (LL) and different light-dark (LD) protocols. Bimodal rhythms were typically observed in this crab when subjected to DD or LD, with bouts of activity anticipating lights-on and lights-off, respectively. Freerunning circadian rhythms were expressed in both DD and LL for longer than 30 days. In DD, we observed that some animals presented different period lengths for each activity component. During LL, activity was primarily unimodal, however spontaneous splitting of the rhythms were observed in some animals. When activity was recorded under artificial long days, the morning bouts maintained their phase relationship but the evening bouts changed their phase relationship with the Zeitgeber. Our results indicate that, bimodal locomotor activity rhythm in the crab Pseudothelphusa americana is variable among organisms. The characteristics of phase relationship with LD and responses to LL for morning and evening bouts, suggest that, locomotor activity could be driven by multiple oscillators, and that coupling between these oscillators may be regulated by light.     

氨氮对不同生长阶段的弧边招潮急性毒性研究

在pH8.09、水温28℃±1℃、盐度5‰条件下,进行了氨氮对不同生长阶段的弧边招潮急性毒性试验。结果显示:氨氮对全甲宽小于1.5cm、1.5cm、1.5cm至2.5cm之间3个生长阶段的弧边招潮96h的半数致死浓度(LC50)分别为47.28mg·L-1、71.49mg·L-1和90.57mg·L-1,三者之间差异极显著(P<0.01),对应的非离子氨浓度为3.71mg·L-1、5.61mg·L-1和7.11mg·L-1;氨氮的安全浓度分别为4.73mg·L-1、7.15mg·L-1和9.06mg·L-1,对应的非离子氨浓度为0.37mg·L-1、0.56mg·L-1和0.71mg·L-1。结果表明,氨氮对弧边招潮的毒性与氨氮浓度和中毒时间呈正相关。3个生长阶段弧边招潮对氨氮的耐受性大小次序为:全甲宽1.5cm至2.5cm之间>全甲宽1.5cm>全甲宽小于1.5cm。     

Pillar Function in the Fiddler Crab Uca beebei (I): Effects on Male Spacing and Aggression

Males of 14 species of fiddler crabs (genus Uca) are known to build structures out of mud or sand at the entrances to burrows they court from and defend. A study of spacing, space use and aggression between courting male Uca musica (Zucker 1974, 1981) suggested that the hoods males build reduce territorial overlap and rates of aggression between neighboring males. Thus, each male may have more time to court females during limited lunar, diurnal and tidal mating periods. I studied the courtship and aggressive behavior of male Uca beebei in the field to determine if the pillars males of this species build affect male behavior as do the hoods of U. musica. U. beebei occurs sympatrically with U. musica on the Pacific coast of Panama and is broadly similar in its ecology and mating behavior. Unlike the hoods of U. musica, pillars did not focus a male's activity space away from its closest neighbor nor did they reduce either overlap with neighbors' activity spaces or rates of aggressive interaction among neighbors. Pillar builders courted more but also fought their neighbors more than did males that did not build pillars. The pillars of U. beebei and the hoods of U. musica affect male behavior differently and probably have different functions.     

Distal Retinal Pigment of the Fiddler Crab,Uca pugilator: Release of the Dark-Adapting Hormone by Methionine Enkephalin and FMRFamide

The neuropeptides methionine enkephalin and FMRFamide, when injected into intact fiddler crabs, Uca pugilator, produce dark adaptation of the distal retinal pigment. Furthermore, both neuropeptides stimulate release of distal retinal pigment dark-adapting hormone activity from the isolated eyestalk neuroendocrine complex. It is hypothesized that both neuropeptides, when injected into intact fiddler crabs, act only indirectly on the distal retinal pigment, by stimulating release of this dark-adapting hormone.     

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.     

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.     

Circadian Locomotor Activity Rhythm in the Freshwater Crab Pseudothelphusa americana (De Saussure, 1857): Effect of Eyestalk Ablation

The circadian rhythm of locomotor activity in the freshwater crab, Pseudothelphusa americana , was studied in aquaria using infrared crossing sensors. Individuals with ablated eyestalks were compared with intact individuals in constant darkness (DD) and in light-dark cycles (LD). Our results showed that intact animals in DD displayed bimodal rhythms. In LD conditions the two peaks were associated with lights on and lights off, respectively. A significant difference in the free running periods before and after LD was observed in all intact animals. After eyestalk ablation (ES-X), the circadian rhythm of locomotor activity disappeared immediately, but reappeared several days later. Diurnal activity was seen in some ES-X animals when exposed to LD. Our results indicate that locomotor activity rhythm in P. americana is driven primarily by oscillators located outside the eyestalks, and that extraretinal photoreceptors mediate either entrainment or masking effects.     

Circadian Locomotor Activity Rhythm in the Freshwater Crab Pseudothelphusa americana (De Saussure, 1857): Effect of Eyestalk Ablation

The circadian rhythm of locomotor activity in the freshwater crab, Pseudothelphusa americana, was studied in aquaria using infrared crossing sensors. Individuals with ablated eyestalks were compared with intact individuals in constant darkness (DD) and in light-dark cycles (LD). Our results showed that intact animals in DD displayed bimodal rhythms. In LD conditions the two peaks were associated with lights on and lights off, respectively. A significant difference in the free running periods before and after LD was observed in all intact animals. After eyestalk ablation (ES-X), the circadian rhythm of locomotor activity disappeared immediately, but reappeared several days later. Diurnal activity was seen in some ES-X animals when exposed to LD. Our results indicate that locomotor activity rhythm in P. americana is driven primarily by oscillators located outside the eyestalks, and that extraretinal photoreceptors mediate either entrainment or masking effects.     

Reproductive Aspects of a Tropical Population of the Fiddler Crab Uca annulipes (H. Milne Edwards, 1837) (Brachyura: Ocypodidae) at Costa Do Sol Mangrove, Maputo Bay, southern Mozambique

Uca annulipes(H. Milne Edwards) is the dominant fiddler crab inhabiting mangrove forests in East Africa. However, several aspects concerning its reproduction are poorly known. Such information is necessary for a better understanding of several biological aspects occurring in this species. The aim of this study was to assess the reproductive biology of Uca annulipes as well as some factors determining its breeding season. Samplings were undertaken from January to December 2002 at Costa do Sol, a tropical mangrove forest around Maputo Bay, southern Mozambique. Crabs were randomly sampled, measured for carapace width (CW) and checked for the presence of eggs on female pleopods. Like most tropical brachyurans, Uca annulipes follows a continuous reproduction throughout the year with peaks of spawning during summer season and low frequency of egg-bearing females during winter. The frequency of ovigerous females was correlated with the air temperature (r ²= 0.652, p < 0.05) and rainfall (r ²= 0.897, p < 0.001), with the last parameter being the most important factor regulating the reproductive activity of this crab.