Food limitation in salt marsh fiddler crabs Uca rapax (Smith) (Decapoda : Ocypodidae)

The Asian shore crab, Hemigrapsus sanguineus, was observed occupying fiddler crab burrows (Uca pugnax) at low tide in a Spartina salt marsh at Sherwood Island, Westport, Connecticut. Forty-seven percent (48/103) of the fiddler crab burrows censused were occupied by crabs. Of those, 81% held fiddler crabs while the remainder held Asian crabs. Fiddler crabs and Asian crabs were never found in the same burrow. Unlike the Asian crab, fiddler crabs preferred areas of the “marsh edge” where rocks and small stones were not present. Hemigrapsus sanguineus, which can be found under the shelter of rocks, shells, and other debris on tidal flats along the fringes of the marsh, probably searches the marsh edge as the tide recedes for unused burrows to occupy. Field caging experiments used to investigate possible competitive interactions between these two species indicated that the presence of the Asian crab had no effect on burrow utilization by the fiddler crab. It is unlikely that patterns of habitat use by the east coast salt marsh fiddler crab, Uca pugnax, will be significantly affected by the recent introduction of the Asian crab, H. sanguineus, to this area.

Fiddler crabs are one of the principal agents of bioturbation in intertidal salt marshes. The physical, chemical, and biological properties of sediments can be modified by fiddler crab burrowing activity. This study examined the effect of fiddler crab burrowing on sediment reworking and the distributions of 210 Pb and 137 Cs in salt marsh sediments at North Inlet Estuary, South Carolina. Fiddler crab burrow density, turnover, and volume were measured along a transect from the forest to the creek bank. Burrow density ranged between 40 and 300 burrows m -2 with highest densities at the creek bank. Sediment reworking is related to burrow turnover, density and size, Sediment reworking rates ranged between 4.4 X 10 3 and 5.7 X 10 4 cm 3 m -2 y -1 . Excess 210 Pb and 137 Cs profiles indicated that fiddler crab burrowing mixed the top 8 to 15 cm of sediment. Direct field measurements of burrow density, turnover, and size were used as input to a modified version of the regeneration model of Gardner et al. (1987) to assess the effect of fiddler crab bioturbation on 210 Pb profiles. The modification takes into account the filling of abandoned fiddler crab burrows from both the infilling of surface sediment and the collapse of burrow walls. Model results were in good agreement with the observed 210 Pb distributions in the sediments. Overall the results of this study suggest that fiddler crabs directly influence sediment composition and biogeochemical cycles in salt marsh systems.

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.

Increased burrowing by fiddler crabs Uca rapax (Smith) (Decapoda : Ocypodidae) in response to low food supply

Evidence of the passive deposition of meiofauna into fiddler crab burrows

A survey was conducted to examine spatial variations in the population density of major meiofaunal taxa and the assemblage structure of free-living marine nematodes within 5 mangrove areas on the west and east coast of Zanzibar. Meiofauna densities in surface sediments (0–5 cm) ranged from 205 to 5263 ind. 10 cm2, being on average 1493 ind. 10 cm2. Of the 17 major taxa recorded, nematodes dominated (64–99%) in all samples while harpacticoid copepods were usually second most abundant. Within all areas the numbers of meiofauna were very variable and significant differences among areas were only detected for oligochaetes and turbellarians. Densities of nematodes, harpacticoids, polychaetes and turbellarians were, however, significantly (P<0.001) higher at low water stations compared with mid and high water stations. Harpacticoids were negatively correlated with the numbers of fiddler crab (Uca spp.) burrows. Other correlations between environmental factors (grain size, temperature, salinity, oxygen tension, prop root density, fiddler crab burrows) and major meiofaunal taxa were non-significant. A total of 94 nematode genera were recorded from four mangrove areas. The most abundant and frequent genera were Microlaimus and Spirinia, followed by Desmodora and Metachromadora. Representatives of the genera most common in current study are found all over the globe. There was a high variation in nematode assemblage structure within and between sampling areas indicating the absence of a well defined nematode assemblage confined to mangrove areas. In a hypersaline area diversity was much reduced and where salinity was over 100%. the fauna was restricted to 3 nematode genera, Microlaimus, Theristus and Bathylaimus. Multidimensional scaling ordination (MDS) of the nematode genera separated samples taken from low water stations from other stations, the assemblage structure being significantly different at the low water stations. Numbers of selective deposit feeders were negatively correlated with average grain size and positively correlated with silt content.

Oxygen microgradients in the rhizosphere of the mangrove Avicennia marina

The constant increase in human populations and its associated activities has become a stress factor for ecological communities of the aquatic ecosystem/environment. The aquatic environment/ecosystem particularly, the intertidal zones, mangrove forests and wetland ecosystems serve as a refuge for a diversity of organisms including fiddler crabs (Uca tangeri), with habitation and protection often coming in the form of extensive mudflats/sandy beaches and dense vegetations. This study evaluated the effect of anthropogenic activities on fiddler crabs (Uca tangeri) population particularly, on the fragmentation/loss of their habitats due to impact from solid wastes. Habitat fragmentation and loss due to solid wastes along the intertidal zone could eventually lead to biodiversity loss. The decapods (fiddler crabs) are among the common burrowers found in the intertidal and wetland zones of the coastal environment in the Niger Delta, Africa and other parts of the world. They are in the family Ocypodidae and have a number of features (size and shape of eyestalk, antenna, carapace, pairs of walking legs with chelipeds, body colour and sexual dimorphism) that could easily distinguish them from other crabs. They play very important ecological roles and interactions that sustain their ecosystems, serving as prey and predators in the ecological food chain/web. Fiddler crabs actively participate in bioturbation and organic matter degradation which has significant effect on ecosystem health/processes and so contributes to ecological diversity. Therefore, the impact of solid wastes due to anthropogenic activities is a menace of concern to the ecology, activity and life of fiddler crabs. The domination of solid wastes on the intertidal zones of our estuaries and coastal system had caused serious habitat degradation and loss for fiddler crabs and other organisms. Review of studies within the Niger Delta (Iwofe, Eagle Island and Ibeto) in Africa indicated serious threats to the habitats and existence of fiddler crabs due to the menace of solid wastes, indicating they are endangered. Significant variation in fiddler crab holes between different sites suggested population density variation across the sites. This was attributable to the influence of solid wastes that had taken over and fragmented the habitats of these intertidal dwelling crabs. This also suggests the level of anthropogenic activities (municipal wastes from run-off and direct discharges into the aquatic environment) impacting fiddler crab habitats by blocking fiddler crab burrows and occupation of available shoreline space, dislodging the crabs from their habitats. Increasing population and change in consumption pattern are responsible for increased waste generation in surrounding towns and cities. Other factors related to anthropogenic activities such as dredging/land reclamation, urban development, waste from urban run-offs and direct disposal, exploitation of mangrove resources and commercial boat activities further fragmented and degenerated the ecology of this very important species. These anthropogenic impacts could lead to a decrease in available refuge for such intertidal dwellers (fiddler crabs) and further endanger them, which could expose the crabs to increased predation, mortality rate and an overall reduction in population and consequent biodiversity loss. Sequel to the important ecological roles of fiddler crabs, alteration of their population and behaviour may have an adverse effect on the habitat and ecosystem. This may lead to biodiversity loss and eventual loss of important ecosystems such as intertidal environments, salt marshes, mudflats and mangrove forests.KeywordsFiddler crabs-Uca tangeriAnthropogenic activitiesSolid wastesImpacts Conservation

Bioaccumulation and biomagnification of potential toxic elements (PTEs): An Avicennia germinans–Uca rapax trophic transfer story from Jobos Bay, Puerto Rico

The intensive drilling and extraction of fossil fuels in the Gulf of Mexico result in a considerable risk of oil spills impacting its coastal ecosystems. Impacts are more likely to be far-reaching if the oil affects ecosystem engineers like fiddler crabs, whose activities modify biogeochemical processes in the sediment. The present study investigated effects of oil on the fiddler crabs Uca longisignalis and Uca panacea, which are important as ecosystem engineers and as prey for a wide variety of species. The present study used mesocosms and microcosms to investigate the effects of crude oil on fiddler crab burrowing and to assess cellular and tissue damage by the oil. Fiddler crabs were exposed for periods of 5 or 10 d to oil concentrations up to 55 mg/cm2 on the sediment surface. Their burrowing was delayed, their burrows were smaller, and they transported less sediment in the presence of oil. The hepatopancreas had elevated levels of oxidative stress and a higher abundance of blister cells, which play a role in secretory processes. Interspecific differences were observed; most effects were strongest in U. panacea, though burrowing was more strongly affected in U. longisignalis. The present study demonstrates that crude oil is likely to impact fiddler crabs and many species that depend on them for their diet or for the ecological changes that result from their burrowing. Environ Toxicol Chem 2018;37:491-500. © 2017 SETAC.

Bioturbation, which includes burrowing and foraging behaviors, is an important component of the functional role of fiddler crabs (Brachyura, Ocypodidae) within mangrove forests because it modifies sediment properties and composition of mangrove substrates. In this study, fiddler-crab population density and burrow architecture were measured to evaluate the influence of bioturbation by the fiddler crab Leptuca speciosa (Ives, 1891) on mangrove peat from Barnes Sound, Florida, USA. Measurements of burrow architecture were used to estimate the contribution of fiddler-crab burrowing to the bioturbation of mangrove peat. Comparisons were made between the total organic matter in bioturbated sediments, including feeding and burrowing pellets. A population density of 12 burrows m–2 was measured with no significant trends in the spatial distribution of fiddler crabs within the site. Although the deepest burrow depth was 18 cm, results show the upper 5 cm of the peat surface was consistently burrowed by crabs. Fiddler crabs were estimated to increase the total below-ground air-peat surface area m–2 by 5% and accounted for 22% of the excavated volume of mangrove peat per year. Fiddler crabs will thus rework the entire peat substrate within five years. Because data on carapace geometric mean size suggested that a juvenile population of L. speciosa was sampled, the bioturbation rate of the peat substrate will accelerate once this population matures. Feeding pellets had significantly lower percentages of total organic matter (P &amp;lt; 0.01) than other bioturbated peat samples, suggesting that fiddler-crab foraging behaviors significantly (P &amp;lt; 0.01) decrease the organic composition of surficial peats. These results imply that fiddler crab burrowing extends the depth of the taphonomically active zone thus enhancing mangrove peat decomposition and changing the bioavailability and distribution of organic matter in mangroves.

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.

Zoos increasingly transform their exhibitions from traditional one‐species enclosures to more natural exhibits, that is, environments that capture part of an ecosystem including a selection of animals and plants that occur there. Thus, enhancing the experience of its human visitors while also allowing its residents to possibly show more natural behavior. In 2017 Royal Burger's Zoo (Arnhem, The Netherlands) created and opened a mangrove‐like environment containing fiddler crabs. Fiddler crabs display a broad range of behaviors, and this research examines which wild‐type behavior and behavioral patterns can be observed on a seminatural mudflat. The behavior shown by Uca rapax and Uca tangeri on the mudflat was counted each hour between 07:00 and 17:00. An asymmetric tidal regime was present in the enclosure including two high water periods. Various known fiddler crab behaviors, including waving and combat, were observed but no copulation. A clear pattern in exposed crabs on the mudflat was found, with low numbers visible in the early morning and the highest numbers present in the early afternoon, while number of visitors did not have a significant effect on this pattern. Interestingly, the highest abundances were not observed around the ebbing tide (07:00–09:00), as observed in the wild, but somewhat later, possibly due to the asymmetric tidal scheme or the interaction of tidal and daily rhythms. This study shows that in captivity, fiddler crabs indeed show a range of natural behaviors which is linked to the tidal and possibly daily rhythm as well.

In the literature, it is commonly stated that Uca spp. (fiddler crab) burrows aerate marsh sediment. However, to date few studies have attempted to determine the pore water oxygen (O2) concentrations associated with Uca spp. burrows; instead most studies have looked at redox potential, a measure of oxidation and not of O2. In an in situ study conducted in salt marshes on the Atlantic coast of Virginia, USA, O2 concentrations surrounding individual burrows were measured to assess the potential of burrows to aerate bulk sediment. Using an O2 microelectrode, concentration profiles were measured at depth intervals of 500μm to a depth of 10mm. The O2 profiles were measured at distances of 2, 4, 6, 8, and 10mm from the burrow wall. Measurements were made surrounding natural and artificial burrows as well as where burrows were absent as a control.Contrary to convictions in the literature, this study determined that, within poorly drained muddy sediments, oxygen penetration depth only had a significant increase out to a distance of 2mm from the burrow wall, and the increase in oxygen depth penetration was only 0.5mm compared to areas not affected by the presence of the burrow. In typical muddy sediments of salt marshes along the mid-Atlantic coast, USA, the presence of Uca pugnax burrows increased the oxic zone little, but may have had an effect on the extent of the suboxic zone. Therefore, generalizations should not be made concerning the ability of burrows to aerate surrounding bulk sediments. In addition, the capacity of sediments to drain is an important factor in determining the potential for increased sediment aeration due to the presence of Uca spp. burrows.

Positive plant–animal interactions are important in community ecology, but relatively little attention has been paid to their effect on the production of mangroves, dominant halophytic trees in tropical coastal marshes. Here, the role of fiddler crab (Uca spp.) burrowing on the growth and production of the white mangrove, Laguncularia racemosa (<2 years old), was examined in a restored marsh in Tampa Bay, Florida (27°41.65 N, 82°30.34 W) with manipulative experiments from June 2006 to May 2007. Fiddler crab burrowing significantly increased mangrove height by 27%, trunk diameter by 25%, and leaf production by 15%, compared to mangroves in crab exclusion enclosures. Additionally, the exclusion of fiddler crabs significantly increased interstitial water salinity from 32.4 to 44.2, and decreased the oxidation–reduction potential of the low organic sediments, but did not affect soil pH or sulfide concentration. Mangrove height, trunk diameter, and leaf production along a transect that varied in crab burrow density were positively associated with the number of crab burrows. Further, the density of sympatric Spartina alterniflora shoots was positively correlated with crab burrow density along the transect. As in temperate marshes, fiddler crabs can have significant ecological effects on mangrove communities, serving as ecological engineers by modulating the amount of resources available to marsh plants, and by altering the physical, chemical, and biological state of these soft sediment communities. In restored coastal systems that typically have very poor sediment quality, techniques such as soil amendment could be used to facilitate a more natural interaction between crabs and mangroves in ecosystem development.