Uca Pugnax Research Articles

Importance of the fiddler crab Uca pugnax to salt marsh soil organic matter accumulation

The potential influence of fiddler crab burrow density on the processes controlling organic matter (OM) accumulation was examined in Virginia salt marshes. As burrows may affect important chemical and biological processes that influence belowground plant production and decomposition, experimental manipulations were designed to modify fiddler crab burrow density by either increasing them (artificially constructed holes) or decreasing them (exclosures) and comparing them to areas naturally with and without fiddler crab burrows. The only significant difference among treatments was associated with the presence of holes without regard for whether they were artificial or natural. Higher burrow density resulted in an increase in soil redox potential, which most likely caused higher decomposition, even though sulfate-reduction rates were not different among treat- ments. Belowground production decreased with increased burrow density resulting in less OM addi- tion to the soil. Higher decomposition and lower belowground production resulted in a net loss of 3 g C m -2 yr -1 in marshes with higher burrow densities, while areas with few to no burrows accumulated up to 245 g C m -2 yr -1 , equaling a surface accretion rate of 4 mm yr -1 . Examination of 6 other salt marshes in the region revealed a negative correlation between fiddler crab density and soil OM con- tent, lending strength to the argument that bioturbation is a potentially important explanatory factor of OM accumulation in salt marshes and may influence how sea level rise impacts coastal marshes.

Fiddler crab control of cordgrass primary production in sandy sediments

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 399:253-259 (2010) - DOI: https://doi.org/10.3354/meps08331 Fiddler crab control of cordgrass primary production in sandy sediments Christine Holdredge*, Mark D. Bertness, Nicholas C. Herrmann, Keryn B. Gedan Department of Ecology and Evolutionary Biology, Brown University, Providence, Rhode Island 02912, USA *Email: choldredge@ufl.edu ABSTRACT: The cordgrass Spartina alterniflora Loisel is a foundation species critical to the establishment and maintenance of western Atlantic salt marshes. Although the factors regulating cordgrass growth along sheltered, fine-sediment shorelines have been exhaustively studied, less is known about the mechanisms that maintain cordgrass production in high-energy marshes characterized by sandy substrates. We investigated whether deposit-feeding fiddler crabs Uca pugilator Bosc and U. pugnax Smith can mediate local physical conditions and nutrient availability and stimulate cordgrass primary production on sandy marsh sediments. We experimentally removed fiddler crabs from 3 × 3 m plots in an exposed, sandy marsh in Wellfleet, Massachusetts, USA, and found that above- and belowground cordgrass biomass decreased by >53% and 50%, respectively, and above- and belowground nitrogen in cordgrass (g N m–2) decreased by >47% and 45% in fiddler crab removal plots compared to controls. Fiddler crab removal did not measurably influence sediment edaphic parameters, such as redox potential or waterlogging, suggesting that fiddler crabs enhance cordgrass productivity through nutrient deposition on sandy sediments and thus may be a critical component in the successful establishment and maintenance of salt marshes that develop along high-energy shores. KEY WORDS: High energy marsh · Positive interactions · Nutrient deposition · Nutrient limitation · Spartina alterniflora · Uca pugilator · Uca pugnax Full text in pdf format Supplementary material PreviousNextCite this article as: Holdredge C, Bertness MD, Herrmann NC, Gedan KB (2010) Fiddler crab control of cordgrass primary production in sandy sediments. Mar Ecol Prog Ser 399:253-259. https://doi.org/10.3354/meps08331Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 399. Online publication date: January 28, 2010 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2010 Inter-Research.

Multiple stressor effects of methoprene, permethrin, and salinity on limb regeneration and molting in the mud fiddler crab (UCA pugnax)

Exposure to multiple stressors from natural and anthropogenic sources poses risk to sensitive crustacean growth and developmental processes. Applications of synthetic pyrethroids and insect growth regulators near shallow coastal waters may result in harmful mixture effects depending on the salinity regime. The potential for nonadditive effects of a permethrin (0.01 2 gg/L), methoprene (0.03-10 tg/L ), and salinity (10-40 ppt) exposure on male and female Uca pugnax limb regeneration and molting processes was evaluated by employing a central composite rotatable design with multifactorial regression. Crabs underwent single-limb autotomy followed by a molting challenge under I of 16 different mixture treatments. During the exposure (21-66 d), individual limb growth, major molt stage duration, abnormal limb regeneration, and respiration were monitored. At 6 d postmolt, changes in body mass, carapace width, and body condition factor were evaluated. Dorsal carapace tissue was collected, and protein and chitin were extracted to determine the composition of newly synthesized exoskeleton. The present results suggest chronic, low-dose exposures to multiple pesticide stressors cause less-than-additive effects on U. pugnax growth processes. Under increasing concentrations of methoprene and permethrin, males had more protein in their exoskeletons and less gain in body mass, carapace width, and body condition compared to females. Females exhibited less gain in carapace width than controls in response to methoprene and permethrin. Females also displayed elevated respiration rates at all stages of molt, suggesting a high metabolic rate. Divergent growth and fitness between the sexes over the long term could influence crustacean population resilience.

Impacts of biochemical cues on horizontal swimming behavior of individual crab larvae

To determine if larval crabs alter their horizontal swimming behavior in response to biochemical cues in the water, fiddler crab (Uca pugnax) and blue crab (Callinectes sapidus) megalopae were video recorded in a laminar-flow flume while putative cues were introduced and allowed to establish a gradient in the flow field. Changes in horizontal orientation, distribution, and swimming velocity were observed for six different cue treatments. Overall results show that crab megalopae can detect biochemical cues in ambient water and that they adjust their horizontal swimming behavior in response to these cues. The dominant response was a change in swimming velocity in the presence of a cue. While this response may affect the dynamics of larval patches, it is not clear if swimming in the horizontal plane has a substantial effect on the spatial aggregation of the larvae in the field.

Effects of Salinity Stress on Survival, Metabolism, Limb Regeneration, and Ecdysis in Uca Pugnax

Abstract Estuarine intertidal zones are characterized by variable salinity regimes due to seasonal and daily fluctuations. Salt-marsh inhabitants, such as the mud fiddler crab Uca pugnax, are physiologically adapted to this habitat. Crustacean growth and development are energy demanding and sensitive hormonal processes which are susceptible to natural stressors. This study investigated physiological and metabolic changes in the molt cycle of U. pugnax in response to different salinity levels by assessing the survival, respiration, and post-molt tissue protein and carbohydrate content. Crabs were subjected to a limb regeneration and ecdysis challenge under four different salinity regimes. For the study, a limb was removed and its regenerative growth was photographed every two days. After ecdysis, crabs were dissected, and the tissues collected were analyzed for their protein and carbohydrate contents. Crabs held at 60 and 75 ppt seawater showed 100% mortality. Crabs at 60 ppt did not undergo basal limb gro...

Relating Disparity in Competitive Foraging Behavior Between Two Populations of Fiddler Crabs to the Subcellular Partitioning of Metals

Behavioral changes in aquatic organisms such as reduced prey capture and decreased mobility have been linked to exposure to contaminants in the field. The purpose of this study was to compare competitive foraging and dominance behaviors of two populations of the fiddler crab, Uca pugnax, and to examine the relationship between tissue metal residues and observed differences in behavior. Foraging behavior (number of total scoops and scoops on a protein-rich patch) and dominance behavior (percentage of successful attacks) of fiddler crabs from an impacted site (Meredith Creek, New York) and a reference site (Tuckerton, New Jersey) were compared in the laboratory. Tuckerton (Tk) crabs were found to have twice the number of total scoops (70 vs. 38 scoops, p < 0.05) and three times the number of scoops on patch (34 vs. 10 scoops, p < 0.05) than Meredith (Me) crabs. No difference was observed between crab populations in the number of successful attacks (i.e., fights over the protein-rich patch). Analyses of total metal body burdens and metals associated with various subcellular fractions showed that, when compared with Tk crabs, Me crabs had higher levels of Ag, Cd, Cu, Ni, and Se in the heat-denatured proteins (HdeP) (i.e., enzymes). Metal bioaccumulation can have inhibitory effects on enzymes, which play an essential role in the regulation of various biochemical, metabolic, and physiological activities in crustaceans. This study suggests that there is a relationship between the accumulation of metals in HdeP (i.e., enzymes) and impairment of competitive foraging behavior in fiddler crabs. Additionally, this study shows that when compared with dominance behavior, foraging behavior is a more sensitive indicator of metal exposure and might be used as an end point in ecotoxicology studies.

Different crab species influence feeding of the snail Nucella lapillus through trait‐mediated indirect interactions

AbstractIndirect interactions are among the many important factors that influence the community structure of the rocky intertidal zone. Trait‐mediated indirect interactions, in which the presence of a predator or competitor can influence the relationship between two other species, have emerged as vital for understanding community dynamics. This study examined the effect of different crab species on the feeding habits of an intertidal snail, Nucella lapillus. Crab species were defined as being sympatric predatory (Carcinus maenas and Cancer irroratus), sympatric non‐predatory (Uca pugnax and Pagurus longicarpus), or allopatric predatory (Mithrax sculptus and Percnon gibbesi). Nucella lapillus were potentially exposed to risk cues from each of the crab species. Crabs were kept in perforated boxes, which allowed any chemical cues to be emitted but prevented direct contact. Nucella lapillus had significantly lower feeding rates in the presence of sympatric predatory crab species than N. lapillus exposed to either sympatric non‐predatory crabs or allopatric predatory crabs. There was no difference in feeding rate between N. lapillus exposed to the sympatric non‐predatory crabs and to the allopatric predatory crabs. Nucella lapillus in the presence of sympatric predatory crabs had a feeding rate of only 0.07 barnacles per snail per day, whereas N. lapillus housed with non‐predatory crabs and allopatric predatory crabs had rates of 0.11 and 0.12, respectively, suggesting that N. lapillus alter their behavior in response to chemical risk cues from local predators. These results suggest that the ability to detect and respond to risk cues is a selectively evolved trait.

Aspects of population ecology in two populations of fiddler crabs, Uca pugnax

Fiddler crabs, Uca pugnax, were collected from a highly contaminated site and a relatively clean site, both in New Jersey to determine if and how environments with varying levels of pollutants may impact aspects of population biology including individual size, morphology (major cheliped size), population density, fecundity, recruitment and survivorship of early benthic phases. Crabs from the highly contaminated site were significantly larger in size, but had lower population density, lower recruitment, reduced reproductive season and lower survivorship of early benthic phases. Our study suggests that contamination may play a role in population ecology of U. pugnax. This study also determined that the reproductive season for U. pugnax in New Jersey is much longer than reported in the literature and could potentially be impacted by global climate change.

An Animal Physiology lab project that promotes undergraduate student interest and responsibility

This lab accompanies a lecture about comparative animal cell and systems physiology. Fiddler crabs (Uca pugnax) are collected during a field trip to mudflats of Arthur Kill between Staten Island and New Jersey. However, other euryhaline animals could replace the fiddler crabs. Students are responsible for the animal care throughout the semester. The crabs are acclimated to waters of 3 different salinities (30, 20 and 10 ‰). Students use simple approaches to determine the osmotic concentrations in the external media and in the animals' hemolymph to reveal the osmotic gradients maintained by the crabs. Passive salt loss is estimated from the conductivity increase of a volume of distilled water after inserting 1‐2 crabs, allowing to estimate the energy needed to maintain the osmotic gradient. Finally, the metabolic rate of the crabs in the different waters is measured and compared to the theoretical expectation on the basis of different assumptions about the metabolism of the crabs. At the end of the course, the animals are returned to their natural habitat.

No acute toxicity to Uca pugnax, the mud fiddler crab, following a 96‐h exposure to sediment‐bound permethrin

In coastal areas, the application of pyrethroid insecticides and the resulting sediment residues pose a potential threat to marine benthic ecosystems. Pyrethroids cause acute toxicity and exhibit a wide range of sublethal effects on fish and crustaceans when exposure is aqueous. Fiddler crabs that inhabit salt marsh sediment are sensitive to sediment-associated pollutants and serve as a sentinel species for xenobiotic exposure. We exposed adult U. pugnax to salt marsh sediment spiked with different 60% trans/40% cis permethrin concentrations for 96 h, and evaluated changes in oxygen consumption rate, hemolymph osmolarity, and glutathione S-transferase activity (GST) following exposure. Marsh sediment was not lethal to U. pugnax at permethrin concentrations of 100-10,000 microg/kg. Sediment-bound permethrin had no significant effect on respiration and osmoregulation. Exposure caused an induction of hepatopancreas GST in a dose-dependent manner. Gill and midgut tissues showed induction at permethrin concentrations at 10,000 microg/kg. We conclude that short term exposure to permethrin-contaminated sediment does not pose a significant threat to this species or impact respiration and osmoregulation. Furthermore, increased GST activity allows us to evaluate this enzyme's induction as a generalist biomarker for sediment-bound pyrethroid exposures.

Limb regeneration and molting processes under chronic methoprene exposure in the mud fiddler crab, Uca pugnax

Insect growth regulator application for wetland mosquito control remains controversial due to the potential for disruption of normal development and growth processes in non-target crustaceans and beneficial arthropods, e.g. Apis mellifera. Concerns include slow-release methoprene formulations and its environmental breakdown products which mimic an endogenous crustacean hormone and retinoids, respectively. Our primary objective was to evaluate the effect that a chronic methoprene exposure would have on male and female Uca pugnax limb regeneration and molting. After single limb autonomy, limb growth and molt stage were monitored every two days while eyestalk ablation was used to induce proecdysis. Dorsal carapace was collected 6 days post-molt to determine protein and chitin content. In post-molt crabs, methoprene-exposed individuals displayed lower percent gain in body weight. Male crabs lost more weight per body volume than females, took significantly longer to proceed through proecdysis than females exposed to 0.1 µg/L methoprene and exhibited significantly elevated frequency for abnormal limb formation at 1.0 µg/L while females displayed no such trend. Methoprene did not significantly alter extractable exoskeleton protein or chitin content. However, variable water-soluble protein expression increased with exposure at 1.0 µg/L (1 ppb) which contributed to overall variability in total protein content. Our findings suggest that adult male U. pugnax possess greater sensitivity to chronic methoprene exposure during limb regeneration and molting, potentially affecting their post-molt fitness. Furthermore, methoprene has the potential to impact post-molt biomass and exocuticle quality.

Effect of summer storms on early life stages of Uca minax, U. pugnax and U. pugilator in North Inlet Estuary, South Carolina, USA

Estuarine fiddler crabs have a complex life cycle that spans vastly different salinity regimes. Larvae are spawned in estuaries and travel to the coastal ocean where development takes place; after metamorphosing to the megalopal stage, fiddler crabs reinvade estuaries where they set- tle and metamorphose. In the coastal ocean, larvae experience relatively stable physical conditions; however, upon reinvading estuaries, they are exposed to great fluxes in salinity and temperature. In this study, we measured shifts in the salinity regime caused by storm events, and determined the impact of these changes on species frequencies of megalopae from 3 Uca species in the water column and of their recently settled juvenile crabs on the benthos along a salt marsh creek in South Carolina. Adults of the 3 species examined are found in different salinity regimes: adult Uca pugnax generally occupy areas with relatively high salinity and plant cover, while U. minax are found in low salinity or freshwater areas; U. pugilator are found in open sandy areas over a broad range of salinities, but gen- erally occupy higher salinity environments than U. minax. For megalopal stages in the water column, we found that the relative proportions of the 3 species changed significantly along salinity gradients and in response to storm events. When the salinity regime normalized, species frequencies for mega- lopae returned to pre-storm proportions. Only U. minax megalopae were present in the water column during a storm event in July, whereas U. pugnax had been the most abundant species in the water column prior to the storm. Although all 3 species were able to survive storm events as juveniles, U. pugnax juveniles survived low salinity conditions significantly better than the other 2 species during the July storm. During a less severe storm in August, which lasted 5 d and altered the existing profile for a week, the benthic juvenile stages did not exhibit significant changes in species frequencies. Our results show that the duration and severity of salinity change are important variables that impact on the presence of these Uca species in the water column, and suggest that the physiology and behav- ior of the megalopal stage strongly influence where populations occur within estuaries.

Molting as a mechanism of depuration of metals in the fiddler crab, Uca pugnax

Metal distributions in the exoskeleton and soft tissues of the fiddler crab, Uca pugnax, were examined during intermolt and immediate postmolt to determine if distribution of the metals changed prior to molting and to determine if molting is a feasible mechanism to depurate metals. Fiddler crabs were collected from two locations in New Jersey, a highly contaminated site and a relatively clean environment. The crabs from the contaminated site had higher concentrations of metals in their soft tissues for Cu, significantly higher concentrations of Pb in their soft tissues and carapace, but did not have any significant differences in concentrations of Zn in comparison to their conspecifics from the relatively clean site during intermolt. Crabs from the contaminated site has significantly higher levels of Cu, Pb, and Zn in both their soft tissues and exuvia immediately after molting. Crabs from both sites shifted copper and zinc from the carapace into the soft tissues prior to molting. Lead distribution shifted from the soft tissues to the exoskeleton prior to molting in the population from the contaminated site but shifted from the exoskeleton into the soft tissues for the relatively clean site. Average percent of the total body burden eliminated during the molting process for the highly contaminated site varied with each metal, 12% Cu, 76% Pb, and 22% Zn. Average percent of the total body burden eliminated during molting process for the relatively clean site also varied with each metal and was significantly lower than the conspecifics from the contaminated site, 3% Cu, 56% Pb, and 8% Zn. Molting can reduce overall body burdens significantly and is a feasible mechanism to depurate lead.

Long-term biological effects of petroleum residues on fiddler crabs in salt marshes

In September 1969, the Florida barge spilled 700,000 L of No. 2 fuel oil into the salt marsh sediments of Wild Harbor (Buzzards Bay, MA). Today the aboveground environment appears unaffected, but a substantial amount of moderately degraded petroleum still remains 8–20 cm below the surface. The salt marsh fiddler crabs, Uca pugnax, burrow into the sediments at depths of 5–25 cm, and are chronically exposed to the spilled oil. Behavioral studies conducted with U. pugnax from Wild Harbor and a control site, Great Sippewissett marsh, found that crabs exposed to the oil avoided burrowing into oiled layers, suffered delayed escape responses, lowered feeding rates, and achieved lower densities. The oil residues are therefore biologically active and affect U. pugnax populations. Our results add new knowledge about long-term consequences of spilled oil, a dimension that should be included when assessing oil-impacted areas and developing management plans designed to restore, rehabilitate, or replace impacted areas.

Spectral sensitivity of four species of fiddler crabs (Uca pugnax,Uca pugilator,Uca vomerisandUca tangeri) measured byin situmicrospectrophotometry

Fiddler crabs have compound eyes that are structurally fairly well understood. However, there has been much debate regarding their spectral sensitivity and capacity to enable colour discrimination. We examined the visual pigments of two North-American species (Uca pugnax and U. pugilator), one species from the Indo-West Pacific (U. vomeris) and the only Eastern-Atlantic species (U. tangeri) of fiddler crabs using in situ microspectrophotometry of frozen sections of dark-adapted eyes. Only one spectral class of visual receptor was found in the larger (R1-7) retinular cells of each species, with maximum absorption peaking between 508 nm and 530 nm (depending upon species). The R8 retinular cell, that might contain a short-wavelength sensitive photopigment and provide a basis for colour vision, was too small to analyze by these methods. Rhabdoms were lined with screening pigment which strongly influenced each species' spectral sensitivity, sharpening the peak and shifting the maximum towards longer wavelengths, on occasion to as far as the 600 nm region. We hypothesize that sensitivity to longer wavelengths enhances contrast between background (blue sky or tall vegetation) and the male major claw during the waving display.

LARVAL TOLERANCE, GENE FLOW, AND THE NORTHERN GEOGRAPHIC RANGE LIMIT OF FIDDLER CRABS

Despite growing interest in species' range shifts, little is known about the ecological and evolutionary factors that control geographic range boundaries. We investigated the processes that maintain the northern range limit of the mud fiddler crab (Uca pugnax) at North Scituate, Massachusetts, USA (42 degrees 14' N), located approximately 60 km north of Cape Cod. Larvae from five populations in Massachusetts were reared under controlled temperatures to test whether cooler water near the edge of this species' range inhibits planktonic development. Few larvae completed development at temperatures < 18 degrees C, a threshold that larvae would regularly encounter north of Cape Cod. Extensive salt marshes are present north of the current range boundary, and a transplant experiment using field enclosures confirmed that benthic fiddler crabs can survive severe winter conditions in this northern habitat. Taken with oceanographic data, these results suggest that the range boundary of fiddler crabs is likely maintained by the influence of cooler water temperatures on the larval phase. Analyses of mitochondrial DNA sequences from a neutral marker (COI) indicate high gene flow among U. pugnax populations in Massachusetts with little differentiation across Cape Cod. Consistent with predictions regarding the homogenizing influence of gene flow, larvae from source populations north and south of Cape Cod shared a common lower threshold for development. However, larvae produced near the range edge had faster growth rates than those from the south side of Cape Cod (typically reaching the final megalopal stage 1.0-5.5 d faster at 18 degrees C). Additional studies are needed to determine the mechanism underlying this counter-gradient variation in development time. We hypothesize that dispersal into cooler water on the north side of Cape Cod may act as a selection filter that sieves out slow developers from the larval pool by increasing planktonic duration and exposure to associated sources of mortality. Thus while high gene flow may prevent the evolution of greater cold tolerance in northern populations, recurrent selection on existing variation may lead to an unexpected concentration of favorable adaptations at the edge of the range. Such a pattern could permit edge populations to play a dominant and unrecognized role in future range extensions.

Adult fiddler crabs Uca pugnax (Smith) enhance sediment-associated cues for molting of conspecific megalopae

Adult-associated chemical cues can stimulate settlement and metamorphosis of invertebrate larvae into habitats with an enhanced likelihood of juvenile and adult survival. For example, sediments from adult fiddler crab habitat stimulate fiddler crab megalopae to metamorphose (molt) sooner than sediments without adult cues. A similar stimulation of molting occurs after exposure to waterborne chemical cues from adult habitats and to exudates and extracts of adult crabs. We tested whether sediments from habitats without adult Uca pugnax (Smith), which do not stimulate molting of their megalopae, could become stimulatory through brief exposure to adult crabs. Sediments were collected from tidal flats at several distances (∼ 1 m, ∼ 50 m, and ∼ 5.4 km) from adult habitats, and incubated for 24 h with or without adult crabs. Molting rates of laboratory-reared megalopae exposed for 48 h to adult-conditioned sediments were compared to those for untreated controls. Sediments collected in or within 1 m of adult habitat elicited the highest molting rates, and natural sediments from 50 m and 5.4 km had little or no effect on molting. However, incubating sediments collected away from adult habitat with adult crabs produced a higher molting response, and the magnitude of the enhancement increased with distance from adult habitat. Results suggest that the chemical cues that adult crabs release are retained by sediments and consequently stimulate molting of megalopae, regardless of the nature of the sediments themselves. Lack of chemical cues may retard colonization of newly created or heavily disturbed habitats that are otherwise suitable settlement and adult habitat.

Complementing the Genome with an “Exposome”: The Outstanding Challenge of Environmental Exposure Measurement in Molecular Epidemiology

The sequencing and mapping of the human genome provides a foundation for the elucidation of gene expression and protein function, and the identification of the biochemical pathways implicated in the natural history of chronic diseases, including cancer, diabetes, and vascular and neurodegenerative

The paradox of the weakening combatant: trade‐off between closing force and gripping speed in a sexually selected combat structure

Summary Previous evidence demonstrates that closing force decreases proportionally as fiddler crab claw size increases. Larger crabs do have greater absolute closing force, but less than would be expected if claw proportions were isometric. In the Mud Fiddler Crab, Uca pugnax, this is explained by a decrease in mechanical advantage with increasing claw size. We here re‐measure mechanical advantage with a more direct method than used previously; the relationship of mechanical advantage to claw size was similar to the older indirect measure. Because the fiddler crab claw is a simple lever, we predicted that the observed decline in closing force should be compensated by a proportional increase in closing speed. We tested this hypothesis using high‐speed video to observe crabs closing their major claws spontaneously under controlled conditions. Closing speed scaled positively to claw length as predicted, and the value of the exponent conformed to expectation from a simple biomechanical model. Evolution of the fiddler crab claw therefore involved a trade‐off of closing force for closing speed. An adaptive explanation of relative weakening with increasing body size may lie in the increased ability to rapidly grasp an opponent, a crucial advantage in fiddler crab combat.

Distribution of juvenile Uca pugnax and U. pugilator across habitats in a South Carolina estuary, assessed by molecular techniques

Uca pugnax and U. pugilator are common fiddler crabs in salt marshes on the Atlantic coast of the United States. As adults, U. pugnax frequent muddier, vegetated (typically Spartina alter- niflora) substrate while U. pugilator usually occupy sandier, open habitats. It is unclear where juve- nile U. pugnax and U. pugilator reside because the early crab stages of these species are difficult to identify by simple gross morphology. Using a novel restriction fragment length polymorphism (RFLP) protocol to distinguish postlarval U. pugnax and U. pugilator, we studied their distribution along a horizontal gradient in the North Inlet Estuary, South Carolina. We collected juvenile crabs along tran- sects at 3 different sites that spanned S. alterniflora-covered mud and open sand habitats with adult populations of U. pugnax and U. pugilator, respectively. Over 75% of the juveniles collected were U. pugnax, showing greater recruitment by this species. U. pugnax juveniles of all sizes preferred the same muddy habitat occupied by adults, but habitat preferences of juvenile U. pugilator varied by site. Generally, U. pugilator displayed a shift in distribution from S. alterniflora cover to sandier habi- tat during early juvenile stages. The younger stages may prefer S. alterniflora-covered, muddier habitat because it provides better cover from predators, or so that they can avoid displacement by currents during high tides; alternatively, they may be able to feed better on muddy sediment. U. pugi- lator develops specialized mouthparts to scrape organic matter from larger sand grains, but these are not present in early juveniles nor in U. pugnax juveniles. Although young juvenile U. pugnax strongly favored S. alterniflora cover, older juveniles (those large enough to dig burrows for protec- tion) were occasionally found in sandier habitat with U. pugilator.