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.     

Predation by red-jointed fiddler crabs on congeners: interaction between body size and positive allometry of the sexually selected claw

The enlarged (major) claw of male fiddler crabs is used in contestsover breeding burrows and is waved to attract females. We recentlydiscovered that males of the red-jointed fiddler crab, Uca minax,also use the claw to kill smaller-sized fiddler crabs, U. pugnaxand U. pugilator, with which they co-occur in Atlantic coastsalt marshes. Large U. minax males use walking legs or the enlargedclaw to capture prey feeding on moist sand flats. On sand flats,small U. minax males and females are much less common than largemales, suggesting that large males move onto sand flats to seekprey. Males of prey species use the major claw against attackingpredators and, consequently, are more likely than females toescape. In laboratory experiments, large U. minax males weremore likely to attack and kill small-clawed males and femalesthan large-clawed males, consistent with a preference for morevulnerable, less threatening prey. The size of the major clawis a positive allometric function of body size. The allometricfunction varies little among species. Also, the mechanical advantageand indices of closing speed and closing force of the majorclaw, when corrected for body size, are not consistently greaterin U. minax relative to prey species. Thus, predation by U.minax males may reflect the opportunity afforded by larger bodysize and positive allometric growth, which result in a majorclaw that is more massive than the prey it is directed against.     

Stimulation of feeding behavior in three species of fiddler crabs by hexose sugars

Feeding responses to five hexoses were examined in three closelyrelated species of fiddler crabs, Uca minax, U.pugnax and U.pugilator.Hexoses tested were glucose, galactose, sorbose, fructose andmannose. Intact crabs and eyestalk-ablated crabs were tested.Responses to sugars were species specific. Eyestalks are directlyinvolved in vision and overall neural integration as well aswith chemosensory and metabolic pathways associated with feeding.Overall, eyestalk-ablated crabs were more sensitive uian intactcrabs. Studies of responses of individuals within a populationto hexoses showed tiiere are individual U.pugnax that respondto galactose and others that do not. Similarly, there were U.pugilatorindividuals that were mannose responsive and others that didnot respond to mannose. An additional study of differences inpopulation responses to hexoses would provide valuable toolsin studying geographic relationships between fiddler crab populations.     

The SW Atlantic burrowing crab Chasmagnathus granulatus Dana affects the distribution and survival of the fiddler crab Uca uruguayensis Nobili

In this paper, we address the question of whether the presence of the burrowing crab Chasmagnathus granulatus affects the habitat use of the fiddler crab Uca uruguayensis. Field samples showed that the species have a disjoint spatial distribution. Male fiddler crab density decreased in zones with C. granulatus, however, female density increased. Male fiddler crabs avoided feeding on sediment affected by C. granulatus and were more preyed. Predation was higher during the fiddler crab reproductive season and, probably due to predation risk, males showed lower reproductive display in shared zones. Field experiments shows that when C. granulatus were excluded, densities of U. uruguayensis increased mainly due to an increase in density of males. Habitat differentiation of these species may be because C. granulatus affects U. uruguayensis in several ways, including direct predation, disturbance and behavioural changes associated to predation risk. Males and females are affected differentially probably because of the extreme sexual dimorphism of this crab species. Coloration on enlarged claw and waving activities are all factors that increase predation risk for male and the presence of only one feeding claw may increase sediment-mediated effects.     

Certain organism-substrate relationships affecting the distribution of Uca minax (Crustacea: Decapoda)

The involvement of substrate in the ecology of the fiddler crab Uca minax was investigated by means of both field and laboratory studies. These included determination of the oxygen-holding capacities of five types of substrates on which this organism lives, as well as experiments to determine if there is substrate selection. The effect of population density on burrowing was also studied to determine the optimum number of individuals needed in the selection experiments, and to delineate the mechanisms these organisms use for avoiding dispersion onto less favorable substrates. Results indicate that Uca minax prefers substrates with high organic content, although these were shown to contain the lowest substrate oxygen. This evidence suggests that Uca minax prefers substrates of high energy value, and explains the significance of this species' adaptation in withstanding low oxygen tensions. Population density experiments indicate that at high population densities these crabs reduce intraspecific encounter and competition by burrowing and subsequently covering their burrows. Seasonal monitoring of burrow temperatures indicate the stability of this microenvironment as compared with the surrounding air temperature. These data also demonstrate the significance of the adjacent water in the habitat ecology of Uca minax.     

Burrowing,root mat density,and the distribution of fiddler crabs in the eastern United States

This paper discusses the distribution of two species of fiddler crabs across an estuary. The possibility that sediment size, physical factors (or micro-climate), or food limitation could lead to the observed distributions is discussed, and evidence is presented to show that these factors probably play a minor rôle in controlling the distribution of Uca pugnax (Smith) and U. minax (Le Conte) across an estuary. The results show that the best correlate of the dominant species of Uca in a particular habitat is the density of the root mat of that area. A series of experiments showed that variations in the abundance of large U. pugnax at two tide levels in a marsh very closely parallel the ability of U. pugnax to burrow in substrata taken from those same two areas of the marsh. These two areas have significant differences in density of the root mat.     

Shell use by juvenile fiddler crabs Uca pugnax and U. pugilator

The present study documents for the first time shell use by juvenile fiddler crabs in the salt marsh. Twenty visits were made to six salt marsh sites at Tybee Island, Georgia between 2007 and 2009. One hundred empty Littorina irrorata shells were collected at each site on each field trip. Juvenile carapace width was measured, crabs sexed, and species identification completed using RFLP analysis. Shell use of up to 79% was observed. Two species of fiddler crabs were found in empty shells, Uca pugnax and U. pugilator. U. pugnax was the dominant species at all sites representing 62-84% of the juvenile fiddler crab population. Juvenile sex ratios were female-biased (1.7:1) at all six sites. Juvenile size did not vary significantly between species but males of both species were significantly larger than females. Size frequency distribution of carapace width revealed that shell use varied with size and sex. In the 3 to 4 mm size class, juvenile females outnumbered juvenile males in empty L. irrorata shells while in the 5 to 6 mm size class and greater, juvenile males outnumbered juvenile females in shells. Significantly more juvenile fiddler crabs were found in empty shells during flood than ebb tide at 3 of the sites. This discovery illuminates the resourcefulness of juvenile fiddler crabs and provides another mechanism that might enhance survival.     

Effect of cadmium exposure on lactate dehydrogenase activity in the hepatopancreas and abdominal muscle of the fiddler crab,Uca pugilator

1. Lactate dehydrogenase (LDH) activity in the hepatopancreas and abdominal muscle of fiddler crabs, Uca pugilator, was determined after 24 and 48 hr of exposure to 2 ppm cadmium chloride.2. For the cadmium exposed crabs, LDH activity in the hepatopancreas decreased, whereas that in the abdominal muscle increased.3. The increased LDH activity in the abdominal muscle may reflect increased dependence on anaerobic carbohydrate metabolism in fiddler crabs exposed to cadmium in their environment.     

Carapace color change in Uca pugilator as a response to temperature

Brachyurid species have various coloration patterns that fluctuate among individuals due to behavioral interactions or a changing environment. In the fiddler crab Uca pugilator, the historical notion is that color change is regulated by circadian rhythms. In this study, color patterns of U. pugilator were affected by changes in temperature; crab carapaces become light in warm temperatures and dark in colder temperatures. The response to temperature occurred within 15 min (often within 5 min of exposure to the treatment), suggesting that temperature may play a key role in color change, and it is activated not through a hormonal pathway, but rather a direct response of the chromatophores. Color change occurred differently between day and night for each sex, which implies that endogenous rhythms are not the sole mechanism for color change in U. pugilator. Both sexes became darker in the cold treatment, however there were differences between the sexes with high temperatures. Females did not show significant coloration changes, while males became significantly lighter and more speckled. Results from this study suggest sex-specific constraints in response to temperature that could be associated to morphological body mass to surface area ratios, as males have large cheliped, which may be involved in thermoregulation. Constraints could also be associated to different diel activities that each sex performs or visual cues associated with mating behaviors.     

Determination of Key Environmental Factors Responsible for Distribution Patterns of Fiddler Crabs in a Tropical Mangrove Ecosystem

In tropical regions, different species of fiddler crabs coexist on the mangrove floor, which sometimes makes it difficult to define species-specific habitat by visual inspection. The aim of this study is to find key environmental parameters which affect the distribution of fiddler crabs and to determine the habitats in which each species was most abundant. Crabs were collected from 19 sites within the mudflats of Sepang-Lukut mangrove forest. Temperature, porewater salinity, organic matter, water content, carbon and nitrogen content, porosity, chlorophyll content, pH, redox potential, sediment texture and heavy metals were determined in each 1 m² quadrate. Pearson correlation indicated that all sediment properties except pH and redox potential were correlated with sediment grain size. Canonical correspondence analysis (CCA) indicated that Uca paradussumieri was negatively correlated with salinity and redox potential. Sand dwelling species, Uca perplexa and Uca annulipes, were highly dependent on the abundance of 250 μm and 150 μm grain size particles in the sediment. Canonical Discriminative Analysis (CDA) indicated that variation in sediment grain size best explained where each crab species was most abundant. Moreover, U. paradussumieri commonly occupies muddy substrates of low shore, while U. forcipata lives under the shade of mangrove trees. U. annulipes and U. perplexa with the high number of spoon tipped setae on their second maxiliped are specialized to feed on the sandy sediments. U. rosea and U. triangularis are more common on muddy sediment with high sediment density. In conclusion, sediment grain size that influences most sediment properties acts as a main factor responsible for sediment heterogeneity. In this paper, the correlation between fiddler crab species and environmental parameters, as well as the interaction between sediment characteristics, was explained in order to define the important environmental factors in fiddler crab distributions.     

Stereotypy and the effects of temperature on some spatio-temporal subcomponents of the ‘courtship wave’ in the fiddler crabs Uca minax (Le Conte) and Uca pugnax (Smith) (Brachyura,Ocypodidae)

The effects of air temperature on some spatial and temporal subcomponents of the male wave display were investigated under laboratory conditions in the fiddler crabs, Uca minax and Uca pugnax. Also investigated were the stereotypies of these subcomponents. In general, wave duration, ascending and descending wave times decreased significantly with increasing air temperatures between 13 and 26 C. These decreases were generally not significant at air temperatures greater than 26 to 30 C. Wave frequency and the number of cheliped jerks (U. minax) increased with temperature, whereas the interwave time showed no systematic change with temperature. Uca pugnax had waves with discrete cheliped jerks at temperatures less than 20 C, whereas the wave appeared relatively smooth at higher temperatures. Temperature effects on waving varied between species and among behavioural contexts. Stereotypies of the wave subcomponents, as measured by the coefficient of variation, underwent no systematic change with temperature. However, stereotypies were different within and between species. Behavioural context also had an effect on subcomponent stereotypy. Comparisons of stereotypies yielded predictions concerning communication functions of waving subcomponents.     

In vivo evaluation of 5-hydroxytryptamine stimulation of the testes in the fiddler crab,Uca pugilator: A presumed action on the neuroendocrine system

1. Male fiddler crabs, Uca pugilator, received injections of 5-hydroxytryptamine (5-HT, serotonin). The crabs showed dose-dependent testicular development.2. Like 5-HT, the 5-HT releaser fenfluramine and the 5-HT potentiator fluoxetine induced testicular maturation also, but the 5-HT receptor blocker LY-53857 did not.3. The data support the hypothesis that 5-HT exerts this stimulatory effect on the testes indirectly, by triggering release of gonad-stimulating hormone.     

Developmental shift in the selective tidal-stream transport behavior of larvae of the fiddler crab Uca minax (LeConte)

Following hatching, larvae of the fiddler crab Uca minax (La Conte) are exported from the adult habitat in estuaries to coastal and shelf waters where they undergo development prior to re-entering estuaries as postlarvae (megalopae). Studies of the spatial distribution of both newly hatched zoeae (Stage I) and megalopae indicate they undergo rhythmic vertical migrations associated with the tides for dispersal and unidirectional transport (selective tidal-stream transport) both within estuaries and between estuaries and the nearshore coastal ocean. We tested the hypothesis that U. minax zoeae possess a circatidal rhythm in vertical migration that facilitates offshore transport in ebb tidal flows, while postlarvae (megalopae) return to estuaries using a similar flood-phased endogenous rhythm. We also determined if the expression of the rhythm was influenced by the salinity conditions zoeae and megalopae experience as they transition between low-salinity regions of estuaries and high-salinity coastal waters. Stage I zoeae were collected by holding ovigerous female crabs in the lab until hatching. Megalopae were collected from the plankton and identified to species using molecular techniques (PCR-RFLP). Under constant laboratory conditions, both zoeae and megalopae exhibited endogenous circatidal rhythms in swimming that matched the principal harmonic constituent of the local tides (12.39 ± 0.07 h; X¯ ± SE). Upward swimming in Stage I zoeae occurred 2.5-4 h after high tide near the time of expected maximum ebb currents in the field. Rhythmic swimming of megalopae occurred slightly earlier in the tide (2.5 ± 0.09 h after high tide; X¯ ± SE) but was not entirely synchronized with flood currents, as expected. Salinity conditions had no apparent effect on the expression or pattern of the rhythms. Results indicate that this circatidal rhythm forms the behavioral basis of selective tidal-stream transport (STST) in early stage U. minax zoeae, but does not undergo a sufficient phase shift to account for vertical distribution patterns exhibited by megalopae in the field.     

Pinching forces in crayfish and fiddler crabs,and comparisons with the closing forces of other animals

The pinching forces of crustaceans are in many respects analogous to the biting forces of vertebrates. We examined the effects of body size and chelae size and shape, on the closing forces of the fiddler crab, Uca pugilator, and the crayfish, Procambarus clarkii. We hypothesized that the allometric relationships would be similar among species, and comparable to those reported for other decapod crustaceans. We further hypothesized that the scaling of the closing forces of crustaceans, with respect to body size and with the geometry of the pinching or biting structures, would be similar to that of vertebrates. We found that pinching forces increased with body mass, claw dimensions, and claw mass in U. pugilator, but only with claw height and claw mass in P. clarkii. Contraction time increased with body mass for both species combined, whereas contraction speed decreased. Pooled data for these and 17 other species of decapod crustacean revealed a positive correlation between the pinching force and body mass with a scaling exponent of 0.71. These data are remarkably comparable to the values on closing forces of vertebrate jaws, with the pooled data having a scaling exponent of 0.58, slightly below the value of 0.67 predicted for geometric similarity. Maximum closing forces vary tremendously among both crustaceans and animals in general, with body size and food habits being among the most important determining factors.     

Interspecific assistance: fiddler crabs help heterospecific neighbours in territory defence

Theory predicts that territory owners will help established neighbours to repel intruders, when doing so is less costly than renegotiating boundaries with successful usurpers of neighbouring territories. Here, we show for the first time, to our knowledge, cooperative territory defence between heterospecific male neighbours in the fiddler crabs Uca elegans and Uca mjoebergi. We show experimentally that resident U. elegans were equally likely to help a smaller U. mjoebergi or U. elegans neighbour during simulated intrusions by intermediate sized U. elegans males (50% of cases for both). Helping was, however, significantly less likely to occur when the intruder was a U. mjoebergi male (only 15% of cases).     

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.     

A phylogeny of Pacific fiddler crabs of the subgenus Minuca (Crustacea,Brachyura, Ocypodidae: Uca) with the description of a new species from a tropical gulf in Pacific Costa Rica

The phylogeny of all Pacific fiddler crab representatives of the subgenus Minuca Bott, 1954 (sensu Beinlich and von Hagen, 2006) is reconstructed. For the molecular analysis, Cox1 mitochondrial and 28S ribosomal nuclear DNA sequences were used. According to these data, same transisthmian sister species relationships are confirmed and a new species of the genus Uca Leach, 1814, Uca osa sp. n., is described from Golfo Dulce, a tropical gulf in Pacific Costa Rica. Morphological as well as molecular data confirm distinctness of this species compared with all other members of the subgenus Minuca, to which it belongs. Distinctive morphological traits are presented to distinguish Uca osa sp. n. from its congeners in the Eastern Pacific.     

Male size and reproductive success in the communal courtship system of the fiddler crab Uca rapax

Reproductive success of male fiddler crabs Uca rapax depends on the numbers of females a male can attract, and the fecundity of these females. The latter correlates directly with fenale size. The size of a female in a mated pair is directly correlated with the size of the male. Therefore, older larger males experience greater reproductive success than smaller younger males. Females appear to be highly selective in mating, while males are not. The question of why females should select a male whose size is correlated with hers cannot be answered from the present study.     

Variability of a dynamic visual signal: the fiddler crab claw-waving display

Fiddler crabs use elaborate, species-specific claw-waving displays to communicate with rivals and mates. However, detailed comparative studies of fiddler crab signal structure and structural variations are lacking. This paper provides an analysis of the claw-waving displays of seven Australian species of fiddler crab, Uca mjoebergi, U. perplexa, U. polita, U. seismella, U. signata, U. elegans and U. vomeris. We used digital video to record and analyse the fine-scale spatiotemporal properties of these movement-based visual signals. We found that the structure and timing of the displays is species-specific, exhibiting inter-specific differences that follow phylogenetic relationships. The displays showed intra-specific variation according to individual identity, geographic location and fine-scale behavioural context. The observed differences and variations are discussed in the light of the evolutionary forces that may shape their design.