Fiddler Crab Austruca Research Articles

The effect of anthropogenic substrate-borne vibrations on locomotion of the fiddler crab Austruca lactea

The anthropogenic construction activities on the coasts, such as pile-driving, generate vibrations that propagate through the substrate. Such substrate-borne vibrations could potentially affect marine organisms inhabiting the benthic environments. However, there is a lack of documented studies on the effects of vibrations on benthic animals. To investigate whether anthropogenic substrate-borne vibrations such as pile-driving operation influence the fiddler crab, Austruca lactea, we measured their locomotion response under vibrations of 35, 120, 250, 500, and 750 Hz generated by a vibrator. We compared the locomotion of crabs between control and vibration-treatment groups using videography. The duration of movements was significantly lower under 120 Hz vibrations compared to the control. Moreover, crab velocity was significantly higher under vibrations of 120 Hz and 250 Hz compared to the control group. Our result suggests that A. lactea can detect low-frequency substrate-borne vibrations and experience stress, leading to increased energy consumption.

Males signal their breeding burrow characteristics to females in the fiddler crab Austruca perplexa

Abstract Females often choose mates based on their courtship signals. Males may signal their heritable genetic quality, defended resources, or parental care efforts; however, the reasons why females choose males based on their signals are often not clear. Here, we show that, in the fiddler crab Austruca perplexa, male signals (major-claw waving rates) were correlated with important characteristics of their defended resources (width and depth of breeding burrows). By using the male signals, females may be able to roughly predict the burrow quality and decide whether to enter and check the burrow characteristics. The signals are predicted to be honest because the female’s final decision is based on burrow quality. Since females can reject males if their burrow quality is insufficient for breeding, the courtship efforts of deceptive males will be dismissed. The honesty of the signals is beneficial for both sexes and thus easily evolved in their signalling system.

Larval Development of the Mangrove Fiddler Crab Austruca albimana (Kossmann, 1877) (Crustacea: Brachyura: Ocypodidae) Under Laboratory Conditions.

Larvae of the mangrove fiddler crab Austruca albimana (Kossmann, 1877), hatched from an ovigerous female collected from the mangroves of Sumariat, Jazan Province, Saudi Arabia in the southern Red Sea, were reared in the laboratory. Four zoeal and a megalopal stages were recorded, and their morphological features are described herein for the first time. The setations of the cephalothoracic appendages of the zoeas of A. albimana and their congeners exhibit several variations that help differentiate larvae of this genus easily from other meroplankton. However, a character of phylogenetic significance -minute spines on the forks of the telson of pleon -is common to larvae of this genus. These minute spines were studied with the aid of scanning electron microscope images. There were five common morphological features between A. albimana and other fiddler crab megalope, including Minuca burgersi, Leptuca uruguayensis and Leptuca thayeri. These features were a deflexed front, rounded to obtuse frontal margin, seven-segmented antennal flagellum, unsegmented endopod of maxilla and three cincinnuli on the endopods of pleopods. Two zoeal morphological features described in this study and other studies (i.e., the absence of lateral spines on carapace [vs. their presence in species of Uca, Afruca and Ocypode in the Ocypodinae] and the presence of a maximum of four pairs of inner setae on the telson of pleon [vs. presence of more than four pairs of setae in species of Uca, Afruca and Ocypode]) support the taxonomic amendment of transferring Uca spp. and Afruca spp. crabs from Gelasiminae to Ocypodinae.

Eat Better to Look Better: The Relationship between Food Availability, UV Brightness of the Major Claw, and Mating Success of the Fiddler Crab Austruca lactea.

Many factors affect male fiddler crab courting and female choice during underground mating, including claw-waving patterns, claw-waving frequency, hood structures, burrow quality, and visible and ultraviolet signals of the major claw. Under food-limited conditions, fiddler crabs decrease their investments in reproduction-e.g., lower their claw-waving frequency. However, the effect of food availability on the visible or ultraviolet signals important for courtship success is unknown, not only for fiddler crabs, but for crustaceans in general. This is the first study to explore the influence of food availability on the early stages of female choice in Austruca lactea. In the present study, we tested (1) the effect of food availability on the UV brightness of A. lactea's major claw and burrow quality and (2) the correlation between A. lactea courtship success and the UV brightness of its major claw. Our results showed that higher food availability increased the UV brightness of the major claw and enhanced the male's burrow quality. The UV component to the signal is important for courtship success. The males with a UV component of signal on their major claw had a higher courtship success. This is an indication that food availability may affect the attractiveness of male fiddler crabs.

Molecular ecology of the fiddler crab Austruca perplexa (H. Milne Edwards, 1852): genetic divergence along a major biogeographical barrier, Wallace’s Line

Abstract Genetic diversity and population structure in the fiddler crab Austruca perplexa were investigated to acquire a better understanding of the evolutionary history of the species. Nucleotide sequence analysis was performed from a polymerase chain reaction-amplified fragment of the mitochondrial DNA control region. A 691 bp nucleotide sequence was obtained from 618 specimens collected from 13 sites across Japan, Taiwan, the Philippines, Indonesia and Australia. Haplotype diversity ranged from 0.8 to 0.99, and nucleotide diversity values were lower (range, 0.30–1.9%) than those reported previously for other crustacean taxa. Gene flow was evident within populations in Japan and western Indonesia, but absent among all other populations, including eastern Indonesia. This pattern conforms to the one observed in many other marine taxa across the major biogeographical region referred to as Wallacea. The population pairwise fixation index (FST) and FSTP-values were high and significant among many sites, implying that gene flow is restricted among most of the geographical regions sampled here. We hypothesize that physical oceanic barriers coupled with a short pelagic larval duration are responsible for creating the patterns we found. Strong evidence for population structure in a species that has relatively high dispersal potential, resulting in among-population differentiation, is a potential driver of evolutionary novelty. Our results provide a foundation for developing better conservation strategies for this widespread intertidal species.

Human trampling decreases surface activity and disturbs behavioral rhythm of an endangered intertidal crab

Human recreational visits to tidal flats increase trampling pressure, which has a significant impact on the coastal benthic ecosystem. The fiddler crab Austruca lactea, which inhabits the upper intertidal region, is endangered in temperate regions such as Japan and South Korea. The vulnerability of their habitat to human activities along with their limited summer reproduction period may have contributed to the decrease in fiddler crab populations in these regions. We conducted experiments to elucidate the effects of trampling on the surface activities and courtship behaviors of A. lactea, which represent ecological indicators for tidal flat ecosystems. During the 46-day trampling period, we observed the number of active crabs on the surface of the tidal flat under three different trampling intensities (heavy trampling = 60 steps/m2; moderate trampling = 20 steps/m2; and no trampling). The density of individuals of both sexes active on the surface and that of courting males significantly decreased under both heavy and moderate trampling intensities. In addition, the feeding and courtship rhythms of male A. lactea collapsed under continuous trampling events. We monitored the site once per week, and surface activity did not recover fully until the sixth week after the trampling period. Our results demonstrated that trampling on the tidal flat disrupted the intensity and timing of crab surface behavior and male reproductive surface behavior, and thus may, ultimately, have an impact on the population dynamics of A. lactea.

Small males of the fiddler crab Austruca perplexa court more in the off-peak breeding period when large males court less

Small males of the fiddler crab Austruca perplexa court more in the off-peak breeding period when large males court less

Balancing risk and reward: mating opportunity influences thermal refuge use in fiddler crabs

Behavioural thermoregulation can ameliorate thermal stress but is costly. For species that court in a thermally stressful microhabitat, sexual selection via endurance rivalry favours individuals that are able and willing to endure harsh conditions in the courtship habitat, as retreats to a thermal refuge will reduce mating opportunities. The relative costs and benefits of refuge use versus continued courtship in the face of thermal risk, which vary across abiotic and biotic contexts, determine the optimal behavioural strategy. We examined the social and abiotic factors driving behavioural decisions related to thermoregulatory retreat in the fiddler crab Austruca mjoebergi. Male fiddler crabs perform a courtship display on the thermally stressful intertidal sediment surface. Time on the surface, and thus time available for display, was limited by high temperatures; as temperature increased, surface time decreased. Yet when presented with a stimulus female, males were more likely to perform the courtship display, displayed at a higher rate and increased time spent on the surface. These results demonstrate that behavioural decisions related to thermal retreat depend both on the abiotic conditions that influence the degree of thermal stress and on the social conditions that influence the reproductive prospects of the individual.

The role of claw color in species recognition and mate choice in a fiddler crab

Many animal signals are brightly colored and convey information about species identity as well as information about individual conspecifics. Colorful bird and lizard signals have received much attention, and many studies have related specific spectral properties of these signals to variation in mating success and territory defense. Far less attention has been given to invertebrates even though there are spectacularly colorful species. The enlarged claw of the male banana fiddler crab Austruca mjoebergi, for example, is bright yellow and contrasts vividly against the mudflat substrate. It is used in waving displays to attract females and in male territory defense and combat. Claw color varies among males in the degree of “yellowness,” ranging very pale yellow to orange. In this study, we examined female responses to claw color variation in two-choice tests using robotic crabs. We found that although females strongly discriminate against colors that fall outside the natural range of intensity, hue, and chroma, they show no consistent preferences for different claw colors within the natural range, and no single component of claw color (hue, chroma, or intensity) independently affected female choices. Using three-choice tests, we also showed that female preferences induce stabilizing selection on male claw color. We conclude that, although claw color is sufficient to facilitate species recognition, it is unlikely to be used in intraspecific mate choice to provide information about male quality.Significance statementFiddler crabs are often brightly colored, are visually orientated animals, and have a highly complex social system. Despite this, there are few studies that have looked at the role of color in species recognition and mate choice in these animals. In this study, we use robotic crabs with painted claws to determine the role of claw color in species recognition and mate choice in the banana fiddler crab, Austruca mjoebergi. We found that color is important in conspecific mate recognition but the variation among males in claw “yellowness” is unlikely to be used by females in intraspecific mate choice decisions.

Burrowing-site selection by the soldier crab Mictyris guinotae Davie, Shih & Chan, 2010 (Decapoda: Brachyura: Mictyridae)

AbstractThe soldier crab Mictyris guinotae Davie, Shih & Chan, 2010 inhabiting sandy tidal flats in the Ryukyu Is., Japan, are deposit feeders in two regions: its upper habitat where it burrows and the shoreline. The crabs usually migrate between these two regions during the period of emergence at the daytime and night-time low tide. The aim of this study was to determine which kind of sediments are chosen by the crabs when burrowing. Experiments designed to investigate burrowing-site selection revealed that the crabs preferably burrowed in sediment that had been conditioned through feeding by conspecific crabs, but not in sediment that had been conditioned by conspecific crabs walking on its surface. Moreover, the crabs never showed any preference for burrowing in sediment that had been conditioned through feeding by the competitor fiddler crab Austruca perplexa (H. Milne Edwards, 1852); both soldier (prey) and fiddler (predator) crabs sympatrically inhabit the tidal flat, but segregate their habitats with a transition area. These results suggest that chemical cues in sediment conditioned through feeding by conspecific soldier crabs affect where crabs burrow.

Courtship interference by neighboring males potentially prevents pairing in fiddler crab Austruca lactea

Courtship interference occurs when dominant males hinder female assessment of prospective males during female mate choice, leading to a more complex distribution of mating success. In this study, we describe and evaluate courtship interference in underground mating of the fiddler crab Austruca lactea. When a mate-searching female enters the burrow of a courting male, neighboring males frequently interfere in the female-male interaction. We thus identified mate-searching females and observed their interactions until pairing. The duration until females’ final decision for pairing increased with the number of interferences by neighboring males. The females reappeared more frequently from the burrow of the finally selected male when neighboring males interfered. These results suggest that courtship interference by neighboring males delays pairing between the mate-searching female and the finally selected male in this species. The number of interferences by neighboring males increased with female size, implying that large females with high fecundity potential induce interference by neighboring males. Moreover, in approximately half of the cases in which interference occurred at the burrow of the immediate last male before the finally selected male, the finally selected male was the interfering one. The distribution of mating success was therefore biased toward males that combined attractiveness (according to female preference) and dominance (which is associated with courtship interference) in this species. Courtship interference is a type of male-male competition and it may hamper female mate choice. In Austruca lactea, neighboring males often interfere in the interaction between a courting male and a mate-searching female. We demonstrated the effect of courtship interference by neighboring males in these interactions. The interferences prolonged the duration of female decision-making for pairing and temporarily expelled the female from the burrow of the finally selected male. These results imply that these interferences may potentially prevent pairing. Moreover, the finally selected males had often succeeded in interfering in the courtship interaction between the female and another male. Therefore, males with high mating success were not only attractive to the females of this species but also dominant in courtship interference.

Impact of fiddler crab activity on microphytobenthic communities in a South African mangrove forest

Microphytobenthos (MPB) are important primary producers in mangroves, where they are an important source of food for fiddler crabs. The St Lucia mangroves are currently disconnected from the sea and thus considered a non-tidal mangrove ecosystem. Fiddler crabs occur close by, along with sympatric sesarmid crabs. This study examined the foraging effects of the fiddler crab Austruca occidentalis on MPB biomass and composition. The effects of two dominant sesarmids, Chiromantes eulimene and Neosarmatium africanum, on A. occidentalis were also investigated as sesarmids are known to interfere with fiddler crab surface activities such as foraging and burrow maintenance.In situ experimental plots contained fiddler crabs at three different densities, combinations of fiddler crabs and sesarmids, and controls with no crabs. The biomass of three main microalgal groups were measured (i.e. diatoms, cyanobacteria and green algae) using a fluoroprobe and overall MPB biomass was measured using acetone extraction analysis.Austruca occidentalis affected cyanobacteria and diatoms, allowing green algae to increase in biomass during the experimental period. The presence of sesarmids was shown to affect cyanobacteria biomass more than either of the other two algal groups. Overall, MPB biomass was unaffected by fiddler crab density or sesarmid presence, suggesting that abiotic factors played a more important role in controlling biomass while biotic grazing and interference only affected microalgal group composition.

Thermal and desiccation constraints drive territory preference in fiddler crabs

Territory quality is often critical for reproductive success, survival and ultimately fitness. Holding a good quality territory can make a male more attractive to females but can also have several other advantages. It can decrease the effects of environmental stressors, for example, by reducing the need for behavioural thermoregulation or decreasing the risk of predation. We found that, in the fiddler crab Austruca mjoebergi, male territories differed in the shaded mangrove and unshaded open areas of the habitat. The lower temperatures and wetter sediments of the mangrove areas resulted in lower desiccation rates allowing males to spend longer periods of time on the surface than they did in the sun-exposed open areas. Males lived at higher densities in the mangrove areas, and preferentially selected shaded territories when given a choice. Male fights to attain/retain territories in the mangrove areas were longer than those in the open areas. Because females do not preferentially mate with males with territories in shaded mangrove habitats, the advantages to males in maintaining these territories is likely a reduction in thermal and/or desiccation stress.

Multimodal signaling in fiddler crab: waving to attract mates is condition-dependent but other sexual signals are not

Many animals utilize multimodal signals for mate attraction and courtship. Each one of these signals could possibly provide information to receivers about aspects of the sender’s quality. Demonstrating condition-dependence of each of the multiple signals may shed light on which are related to the sender’s quality. This is because condition-dependence can reliably convey the sender’s quality due to the costs associated with signal production; only males in good condition are able to pay the costs of producing the signal. Male fiddler crabs use multimodal signals to attract females from a distance (they build semidomes and produce attraction waves) and for courting the females once they have approached (they produce courtship waves and substrate vibrations). Here, we investigate condition-dependence of the multimodal signals produced by the fiddler crab Austruca lactea. We manipulated the food levels of focal crabs (added food, control, removed food) for approximately 2 weeks in the field. We then compared the intensity of the male’s signals and the male’s blood lactate level. We found that the number of attraction waves significantly differed among treatments; the number of the waves increased with food availability. Blood lactate concentration also increased in the food-addition treatment. However, other signal traits were not significantly affected by the level of food available. These results suggest that adding food facilitated an increase in the male’s ability to produce attraction waves, and this would impose higher energy costs. In this species, only attraction waving reflects male nutritional condition and so could potentially be used by females to distinguish between low- and high-quality males. Animals often employ multimodal signals for mating. Here, we examine whether each of the multimodal signals is dependent on male condition in Austruca lactea. We compared the intensity of their multimodal signals, i.e., semidome, attraction waving, courtship waving, and vibration, among three nutritional treatments (food-addition, control, food-removal). Energy costs were additionally estimated by measuring blood lactate level. The number of attraction waves and the blood lactate level increased in food-adding treatment, while the frequency of semidome construction, rate of courtship waving, and pulse interval in vibrations were not influenced by nutritional condition. This suggests that additional food improved male condition and increased the number of attraction waving. Only attraction waving potentially conveys information about the male nutritional quality. Attraction waves could potentially be used by females to distinguish between low- and high-quality males.

Reduced genetic diversity and alteration of gene flow in a fiddler crab due to mangrove degradation.

The fiddler crab Austruca occidentalis is a dominant species in mangrove forests along the East African coast. It enhances soil aeration and, through its engineering activities, makes otherwise-inaccessible food available for other marine organisms. Despite its importance, the habitat of A. occidentalis is threatened by human activities. Clearing the mangroves for salt farming and selective logging of mangroves trees continue to jeopardise mangrove ecosystems in the Western Indian Ocean. This study aims to use partial mitochondrial COI gene sequences and nuclear microsatellites to determine whether salt farming activities in mangroves have a negative impact on the genetic diversity and gene flow of A. occidentalis collected along the Tanzania coast. The level of genetic diversity for both mitochondrial DNA and nuclear microsatellites are relatively lower in samples from salt ponds compared to natural mangrove sites. Analysis of molecular variance (AMOVA) among all populations showed low but significant differentiation (COI: Fst = 0.022, P < 0.05; microsatellites: Fst = 0.022, P < 0.001). A hierarchical AMOVA indicates lower but significant genetic differentiation among populations from salt ponds and natural mangroves sites (COI: Fct = 0.033, P < 0.05; microsatellites: Fct = 0.018, P = < 0.01). These results indicate that salt farming has a significant negative impact on the genetic diversity of A. occidentalis. Since higher genetic diversity contributes to a stable population, restoring the cleared habitats might be the most effective measures for the conservation of genetic diversity and hence adaptive potential to environmental change in this species.

Elevated predation risk changes mating behaviour and courtship in a fiddler crab

The fiddler crab, Uca beebei, lives in individually defended burrows, in mixed-sex colonies on intertidal mud flats. Avian predation is common, especially of crabs unable to escape into burrows. Mating pairs form in two ways. Females either mate on the surface at their burrow entrance ('surface mating') or leave their own burrow and sequentially enter and leave ('sample') courting males' burrows, before staying in one to mate underground ('burrow mating'). We tested whether perceived predation risk affects the relative frequency of these mating modes. We first observed mating under natural levels of predation during one biweekly, semi-lunar cycle. We then experimentally increased the perceived predation risk by attracting grackles (Quiscalus mexicanus) to each half of the study site in two successive biweekly cycles. In each experimental cycle, crabs were significantly less likely to mate on the side with more birds. Moreover, on the side with elevated predation risk, the number of females leaving burrows to sample was greatly reduced relative to the number of females that surface-mated. Males waved less and built fewer mud pillars, which attract females, when birds were present. We discuss several plausible proximate explanations for these results and the effect of changes in predation regime on sexual selection.

Peahens lay more eggs for peacocks with larger trains

Reproduction is costly and individual decisions about when and how much to invest in reproduction will relate to potential benefits in terms of offspring survivorship or mating success. For those organisms that can reproduce more than once, individuals should invest relatively more in reproduction when the potential benefits are high. If females gain directly or indirectly from choosing attractive, highly ornamented males, then it could be predicted that females mated to these males should invest more in reproduction. Here we report the first test of this prediction for a lekking species, peafowl (Pavo cristatus), in which males do not provide resources for offspring, and females prefer to mate with those males that possess the most elaborate trains. We find that peahens randomly mated to males that vary in the degree of ornament produce more eggs for those peacocks with more elaborate trains. We could find no evidence to support the possibility that this difference arises as a result of difference between males in their ability to fertilize eggs.