Crab Burrows Research Articles

Differences in microphytobenthos and macrofaunal abundances associated with groundwater discharge in the intertidal zone

We investigated the impact of intertidal groundwater seepage on benthic microalgae and macrofauna in 4 study sites located in 2 large tidal flat ecosystems along the western coast of Korea by comparing the chemical, physical, and biological characteristics of 'glossy' seepage sites with those of nearby areas without visually distinct groundwater discharge (dry sediment surface). At 3 of the 4 sites, sediment properties as well as pore water chemistry were similar in groundwater seepage and dry areas. At the 4th study site, the groundwater seepage areas were more coarse- grained compared to the dry areas. Here, the groundwater seepage also had lower salinity and higher nutrient concentrations than the pore water of the dry area and the seawater in a nearby tide pool. Although diatoms were the dominant algal class in seepage and dry areas alike, the seepage areas in 3 of the sites had elevated contributions of other marker pigments such as chlorophyll (chl) b compared to the dry areas. Chl a concentrations were higher in all seepage areas compared to dry areas, and all dry areas had high pheophytin a:chla ratios, indicating a substantial amount of degraded algal material. In the seepage areas of 3 of the sites, we found large numbers of the snail Batillaria cumingi, while crab burrows of Scopimera sp. were only present in the neighboring dry areas. Correlations of sediment chl a concentrations with physicochemical properties of the ambient pore water indicated that microphytobenthos responded specifically to groundwater seepage, which may provide shelter from desiccation and salt stress during emersion of the tidal flat. Our results sug- gest that globally common groundwater seepage significantly impacts the ecosystem structures and microphytobenthos production of the tidal flats.

Large striated burrows from fluvial deposits of the Neogene Vinchina Formation, La Rioja, Argentina: A crab origin suggested by neoichnology and sedimentology

This study includes three aspects: 1) the ichnotaxonomic treatment of large and dominantly vertical burrows ornamented with sharp bioglyphs from the fluvial Neogene Vinchina and Toro Negro formations (northwest Argentina); 2) the description and interpretation of the sedimentary facies where these burrows occur; and 3) neoichnologic observations on large ornamented burrows from seasonal wetlands of the Río Pilcomayo National Park (Argentina) as possible modern analogues of the trace fossils and the sedimentary environment. A new ichnogenus and ichnospecies, Capayanichnus vinchinensis, is proposed to include the large striated burrows. The new ichnotaxon is distinguished by a combination of a predominantly vertical orientation, overall “L” shape (when fully developed), absence of lining and burrow bifurcation, a distinct surface texture, and lack of burrow enlargements. The hosting sedimentary facies of the Vinchina Formation can be grouped into fluvial channel and splay (floodplain) facies associations. The rivers of that unit were of two main types: single channel rivers with frequent channel avulsion and overbank flow and multichannelised rivers with poorly defined margins. At least some of these rivers were ephemeral or with intermittent discharge. The climate was probably seasonal and semiarid, as suggested by sedimentologic evidence. C. vinchinensis was dominantly recorded from the top of fluvial channels (interpreted as abandoned channels) and proximal, intermediate and distal splay facies. The large ornamented burrows found in an extant comparable example were constructed by freshwater crabs. These burrows can be distinguished by the overall architecture, the taxonomy and sex of its occupants, and the environmental setting where they were found. The neoichnologic signatures used in the interpretation of the fossil example include the common oval burrow cross-section and the surface texture. In particular, the surface texture in the modern crab burrows includes abundant comma-shaped marks, sets of grooves oblique to burrow axis, and long segmented grooves. The predominantly oval cross-section, absence of lining, and the strongly ornamented burrow surface suggest that C. vinchinensis was produced by freshwater crabs. The key features of the surface ornament are long sets of three–four ridges whose width is in average one third of the burrow major axis. Other features that also point to a crab origin are a massive filling, common burrow ends in a casing mudstone, chevron pattern in sets of surface ridges, and possible cheliped marks. The new ichnotaxon is restricted, in the Vinchina and Toro Negro formations, to fluvial facies and is absent in shallow lacustrine facies.

Non-destructive sampling techniques for the rapid assessment of population parameters in estuarine shore crabs

Retrieving burrowing shore crabs to obtain information regarding their morphological attributes is often difficult and results in significant disturbance and habitat modification. A simple, rapid and non-destructive sampling method was proposed for indirectly estimating crab abundance, morphology and mass through the counting of burrow numbers and their diameter. Measurement of the number of crab burrows was found to correlate with crab abundance via both excavation and video observation techniques. Burrow diameters of individuals were also correlated with carapace width and body mass. Measuring burrow opening diameters along with the number of holes allows an indirect estimate of morphology and mass, which increases the information gained through such rapid sampling techniques and allows assessment of both structural and functional processes at the population level which may be affected through anthropogenic disturbance.

The role of biogenic structures on the biogeochemical functioning of mangrove constructed wetlands sediments – A mesocosm approach

Benthic metabolism (measured as CO2 production) and carbon oxidation pathways were evaluated in 4 mangrove mesocosms subjected daily to seawater or 60% sewage in the absence or presence of mangrove trees and biogenic structures (pneumatophores and crab burrows). Total CO2 emission from darkened sediments devoid of biogenic structures at pristine conditions was comparable during inundation (immersion) and air exposure (emersion), although increased 2–7 times in sewage contaminated mesocosms. Biogenic structures increased low tide carbon gas emissions at contaminated (30%) and particularly pristine conditions (60%). When sewage was loaded into the mesocosms under unvegetated and planted conditions, iron reduction was substituted by sulfate reduction and contribution of aerobic respiration to total metabolism remained above 50%. Our results clearly show impacts of sewage on the partitioning of electron acceptors in mangrove sediment and confirm the importance of biogenic structures for biogeochemical functioning but also on greenhouse gases emission.

First field record of mangrove crab Ucides cordatus (Crustacea: Decapoda: Ucididae) recruits co-inhabiting burrows of conspecific crabs

Recruits of the mangrove crab Ucides cordatus (Linnaeus, 1763), rarely encountered in the field were found co-inhabiting burrows of larger male and female conspecifics in the mangrove forest. They were located in the sediment of the inner walls and burrow plugs. Average carapace width (CW) of the hosting and co-inhabiting crabs was 3.8 ± 0.20 and 0.9 ± 0.03, respectively. As shown by the size-frequency distribution, while most recruits leave the conspecific burrows after reaching 1.0 cm CW, some stay until they reach a size of 2.5 cm CW. The results of this study contribute to a better understanding of recruitment patterns in this ecologically and economically important mangrove crab species. Follow-up studies are however needed to fully determine the role of conspecific burrows for juvenile habitat choice and survivorship in U. cordatus.

Fine-scale environmental heterogeneities of tidal creeks affect distribution of crab burrows in a Chinese salt marsh

Tidal creeks are an important structure of salt marshes in estuarine ecosystems, providing valuable ecosystem services to wildlife in the estuary. To determine the effects of environmental heterogeneities within tidal creeks on the features of crab burrows, we divided a typical creek section into four parts (i.e., microhabitats): bottom, slope, edge and flat, investigated the distribution of crab burrows and sediment properties on creek sections in the Yangtze River estuary, and compared the burrow distribution in tidal creeks with that in non-creek areas. Our results showed that from the creek bottom to flat soil water content declined ( F 3, 60 = 93.8, p < 0.001), and the variations of other sediment physical and chemical properties associated with the change of soil water content were significant among the microhabitats on the creek sections ( p < 0.001 for pH, conductivity, and grain size). No crab burrows were found at the creek bottom. The burrows on the slope were smaller in size ( p < 0.001 for burrow opening diameter) while the density was higher than that at the edge and on the flat ( F 2, 45 = 31.2, p < 0.001). Moreover, although the correlations between burrow distribution and sediment properties varied among the microhabitats on the creek sections, crabs generally selected relatively solid sediments to build their burrows. On the slope, there was a significantly negative relationship between burrow density and soil water content ( r 2 = 0.53, p < 0.001). At the edge, the correlation between total burrow opening area and soil water content was significantly negative ( r 2 = 0.44, p < 0.002). The density of small crab burrows (<10 mm) was greater, but that of large burrows (>10 mm) was lower in tidal creeks than in non-creek habitats. Therefore, sediment properties showed a gradual transition from hydrophytic to terrestrial environments on the creek section, which caused significant differences of burrow distribution among the microhabitats. The creeks of tidal salt marshes could affect ecological processes and functioning through affecting crab burrows.

Beyond competition: the stress‐gradient hypothesis tested in plant–herbivore interactions

The stress-gradient hypothesis predicts that interactions among plants are context dependent, shifting from facilitation to competition as environmental stress decreases. Although restricted to facilitation/competition, the mechanistic model behind the hypothesis is easily modified to include other negative interactions that are as important as competition in structuring natural communities, e.g., herbivory. To evaluate this hypothesis we experimentally tested if the balance between the facilitative and trophic effect of an intertidal, burrowing, herbivorous crab in marsh plants is context dependent and shifts from positive to negative as stress decreases. By sampling salt marshes differing in sediment size characteristics, we show that sites with larger sediment particle size had less stressful oxygen levels than sites with fine sediment particles, and that the level of stress was reduced by the presence of crab burrows. We then conducted a factorial experiment manipulating sediment size and crab presence. Results show that, by decreasing soil anoxic stress, crabs increase plant growth in stressful zones, but their ecological importance as herbivores increases in more benign zones. Our findings suggest that the balance between positive and negative interactions along stress gradients is more important than previously perceived and also applies to facilitation and herbivory between animals and plants.

Fiddler crab burrowing affects growth and production of the white mangrove (Laguncularia racemosa) in a restored Florida coastal marsh

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.

The burrowing crab Neohelice granulata affects the root strategies of the cordgrass Spartina densiflora in SW Atlantic salt marshes

Salt marshes are among the most productive systems of the world, with plant primary production limited by soil oxygen deficiency and nutrient availability. Nevertheless, root adaptations to anoxia and nutrient acquisition are different and often incompatible. The SW Atlantic salt marshes are characterized by high densities of the deep (up to 1 m) burrowing crab Neohelice granulata (Dana, 1851) that may change soil physical and chemical characteristics by burrow construction. In this work, we experimentally evaluated the hypothesis that crab burrowing can enhance soil oxygenation, causing changes in Spartina densiflora Brongniart below ground tissues from structures adapted to anoxia to systems adapted for nutrient acquisition. This response, in turn, would enhance plant productivity. Results from field observations show that oxygen availability is higher in zones with high burrow densities. As burrow densities increased, the plant root distribution changed from shallow (associated to low oxygen availability) to deeper and vertically homogeneous, with a positive correlation between burrow density and plant aboveground biomass. Experimental exclusion of crabs shows that they induce changes of root strategies from anoxia toleration to nutrient efficient acquisition, with increasing plant productivity. The invasive success that this plant shows in other parts of the world is likely to be due to their ability to tolerate harsh environmental conditions. Our results suggest that the morphological plasticity of S. densiflora is also important in their native zone given the characteristics of their specific habitat.

Effects of crab burrows on pore water flows in salt marshes

Macro-pores such as crab burrows are found commonly distributed in salt marsh sediments. Their disturbance on the soil structure is likely to influence both pore water flows and solute transport in salt marshes; however, the effects of crab burrows are not well understood. Here, a three-dimensional model simulated tidally driven pore water flows subject to the influence of crab burrows in a marsh system. The model, based on Richards’ equation, considered variably saturated flow in the marsh with a two-layer soil configuration, as observed at the Chongming Dongtan wetland (Shanghai, China). The simulation results showed that crab burrows distributed in the upper low-permeability soil layer, acting as preferential flow paths, affected pore water flows in the marsh particularly when the contrast of hydraulic conductivity between the lower high-permeability soil layer and the overlying low-permeability soils was high. The burrows were found to increase the volume of tidally driven water exchange between the marsh soil and the tidal creek. The simulations also showed improvement of soil aeration conditions in the presence of crab burrows. These effects may lead to increased productivity of the marsh ecosystem and enhancement of its material exchange with coastal waters.

Human disturbance as a cause of bias in ecological indicators for sandy beaches: Experimental evidence for the effects of human trampling on ghost crabs (Ocypode spp.)

Abstract Human pressures on coastlines are increasing globally, particularly on urban beaches where maintenance of sand budgets and erosion control are the main focus of current shoreline management. By contrast, biological attributes are rarely considered and few, if any, ecological indicators are routinely monitored on beaches. Abundance of ghost crabs (genus Ocypode ) generally responds predictably to human stressors, and is thus a potentially suitable ecological indicator for beaches. The crabs construct burrows with a single, prominent opening at the surface, and population sizes are commonly estimated by counting the number of these burrow openings. While such ‘burrow counts’ are attractive as a low-cost and simple monitoring technique, they may violate a key performance criterion of indicators: not to be overly sensitive to expected sources of interference. On urban beaches such interference is human trampling and, consequently, we evaluated its influence on the performance of burrow counts. The effects of short-term, intense human trampling on numbers and sizes of crab burrows were measured in a series of impact experiments, in which pedestrian trampling was repeatedly applied over 5 h on 4 consecutive days. Burrow counts were highly sensitive to interference from short-term trampling disturbance, which can substantially bias population estimates inferred from such counts. Importantly, burrow densities recovered overnight and apparent shifts in entrance size structures recorded immediately after the trampling impacts were also no longer evident on the following day. Thus, short-term trampling shifted parameter estimates without significant biological effects underpinning such changes—a clear case of bias. Although crab density and size structure are susceptible to artefacts caused be human trampling, they remain valuable indicators for sandy beaches, if interference by pedestrians is small in field measurements or can be accommodated in numerical analyses.

Emission of CO2 and CH4 to the atmosphere by sediments and open waters in two Tanzanian mangrove forests

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 370:53-67 (2008) - DOI: https://doi.org/10.3354/meps07642 Emission of CO2 and CH4 to the atmosphere by sediments and open waters in two Tanzanian mangrove forests Erik Kristensen1,*, Mogens R. Flindt1, Shadrack Ulomi1,2, Alberto V. Borges3, Gwenaël Abril4, Steven Bouillon5,6 1Institute of Biology, University of Southern Denmark, 5230 Odense M, Denmark 2Faculty of Aquatic Sciences and Technology, University of Dar es Salaam, PO Box 35064, Tanzania 3Unité d’Océanographie Chimique, Interfacultary Center for Marine Research, Université de Liège, Institut de Physique (B5), 4000 Liège, Belgium 4Laboratoire Environnement et Paléoenvironnement OCéaniques (EPOC), Université Bordeaux 1, CNRS-UMR 5805, Avenue des Facultés, 33405 Talence, France 5Department of Analytical and Environmental Chemistry, Vrije Universiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium 6Netherlands Institute of Ecology, Centre for Estuarine and Marine Ecology (NIOO-KNAW), Korringaweg 7, 4401 Yerseke, The Netherlands *Email: ebk@biology.sdu.dk ABSTRACT: Carbon gas balance was evaluated in an anthropogenically impacted (Mtoni) and a pristine (Ras Dege) mangrove forest in Tanzania. Exchange of carbon dioxide (CO2) was measured for inundated and air-exposed sediments during day and night using in situ and laboratory incubations. In situ methane (CH4) emissions were measured in the dark during air exposure only. Emission of CO2 and CH4 from open waters (e.g. creeks) was estimated from diurnal measurements of CO2, partial pressure (pCO2) and CH4 concentrations. CO2 emission from darkened sediments devoid of biogenic structures was comparable during inundation and air exposure (28 to 115 mmol m–2 d–1) with no differences between mangrove forests. Benthic primary production was low with only occasional net uptake of CO2 by the sediments. Emissions of CH4 from air-exposed sediment were generally 3 orders of magnitude lower than for CO2. Presence of pneumatophores and crab burrows increased low tide emissions several fold. Emissions from open waters were dependent on tidal level and wind speed. Lowest emission occurred during high tide (1 to 6 mmol CO2 m–2 d–1; 10 to 80 µmol CH4 m–2 d–1) and highest during low tide (30 to 80 mmol CO2 m–2 d–1; 100 to 350 µmol CH4 m–2 d–1) when supersaturated runoff from the forest floor and porewater seepage reached the creek water. Based on global average primary production and measured gas emissions, the carbon gas balance of the 2 mangrove forests was estimated. The densely vegetated Ras Dege forest appears to be an efficient sink of greenhouse carbon gases, while extensive clear-cutting at the Mtoni forest apparently has reduced its capacity to absorb CO2, although it is seemingly still a net sink for atmospheric CO2. KEY WORDS: Mangrove · Pneumatophore · Burrow · Carbon dioxide · Methane · Emission Full text in pdf format PreviousNextCite this article as: Kristensen E, Flindt MR, Ulomi S, Borges AV, Abril G, Bouillon S (2008) Emission of CO2 and CH4 to the atmosphere by sediments and open waters in two Tanzanian mangrove forests. Mar Ecol Prog Ser 370:53-67. https://doi.org/10.3354/meps07642 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 370. Online publication date: October 28, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Leaf removal by sesarmid crabs in Bangrong mangrove forest, Phuket, Thailand; with emphasis on the feeding ecology of Neoepisesarma versicolor

Field measurements on leaf removal by populations of sesarmid crabs at different locations in the Bangrong mangrove forest, Phuket, Thailand, indicated that crabs on average can remove 87% of the daily leaf litter fall by ingestion or burial. The removal rate is correlated positively with the number of crab burrows and negatively with tidal inundation time. The results from the field were supplemented with observations on the behavior of Neoepisesarma versicolor in laboratory microcosms and a mangrove mesocosm. N. versicolor feeds primarily at night and total time spent feeding was up to an order of magnitude higher in the artificial microcosms than under simulated in situ conditions in the mesocosm. Most of the time during both day and night was spent resting near the entrance or inside burrows. N. versicolor mainly feeds on mangrove leaves and scraps of food material from the sediment surface. This is supported by examinations of stomach content, which showed that 62% is composed of higher plant material and 38% of detritus and mineral particles from the sediment. The nutritive value of leaves and detritus is insufficient to maintain crab growth. Sesarmid crabs may instead obtain the needed nutrients by occasional consumption of nitrogen-rich animal tissues, such as carcasses of fish and crustaceans, as indicated by the presence of animal remains in the stomach and the willingness of crabs to consume fish meat. Laboratory experiments on leaf consumption and leaf preferences of N. versicolor indicate that they preferentially feed on brown leaves, if available, followed by green and yellow leaves. If all species of sesarmid crabs in the Bangrong mangrove forest consume leaves at the same rate as N. versicolor, they could potentially ingest 52% of the total litter fall.

Sediment dynamics modulated by burrowing crab activities in contrasting SW Atlantic intertidal habitats

Biogenic bottom features, animal burrows and biological activities interact with the hydrodynamics of the sediment–water interface to produce altered patterns of sediment erosion, transport and deposition which have consequences for large-scale geomorphologic features. It has been suggested that depending on the hydrodynamic status of the habitat, the biological activity on the bottom may have a variety of effects. In some cases, different bioturbation activities by the same organism can result in different consequences. The burrowing crab Neohelice granulata is the most important bioturbator at SW Atlantic saltmarshes and tidal plains. Because of the great variety of habitats that this species may inhabit, it is possible to compare its bioturbation effects between zones dominated by different hydrodynamic conditions. Internal marsh microhabitats, tidal creeks bottoms and basins, and open mudflats were selected as contrasting zones for the comparison on a large saltmarsh at Bahía Blanca Estuary (Argentina). Crab burrows act as passive traps of sediment in all zones, because their entrances remain open during inundation periods at high tide. Mounds are generated when crabs remove sediments from the burrows to the surface and become distinctive features in all the zones. Two different mechanisms of sediment transport utilizing mounds as sediment sources were registered. In the first one, parts of fresh mound sediments were transported when exposed to water flow during flooding and ebbing tide, with higher mound erosion where currents were higher as compared to internal marsh habitats and open mudflats. In the second mechanism, mounds exposed to atmospheric influence during low tide became desiccated and cracked forming ellipsoidal blocks, which were then transported by currents in zones of intense water flow in the saltmarsh edge. Sedimentary dynamics varied between zones; crabs were promoting trapping of sediments in the internal saltmarsh (380 g m −2 day −1) and open mudflats (1.2 kg m −2 day −1), but were enhancing sediment removal in the saltmarsh edge (between 10 and 500 g m −2 day −1 in summer). The implication is that biologically mediated sedimentological changes could be different among microhabitats, potentially leading to contrasting geomorphologic effects within a particular ecosystem.

The Intertidal Burrowing Crab Neohelice (=Chasmagnathus) granulata Positively Affects Foraging of Rodents in South Western Atlantic Salt Marshes

The role of positive and indirect interactions is often crucial in communities with intense abiotic stress such as salt marshes. The burrowing crab, Neohelice (=Chasmagnathus) granulata, is the dominant benthic macroinvertebrate of southwest Atlantic marshes (southern Brazil to Northern Argentinean Patagonia), having strong direct and indirect effects on marsh soil and, in consequence, on marsh vegetation and primary consumers. In this work, we investigate if this crab indirectly modifies habitat use by the granivorous rodents, Akodon azarae and Oligoryzomys flavescens, by increasing nutrient availability and thus enhancing seed production by the marsh plant Spartina densiflora. The study was conducted at the Mar Chiquita Coastal Lagoon, Argentina (37°32′ S). Rodent frequencies in S. densiflora were positively correlated with crab densities throughout the low and middle marsh. Additionally, the highest quality of S. densiflora and inflorescence density was recorded at the highest crab densities. Experimental manipulation of crab densities shows that N. granulata indirectly enhances the performance of S. densiflora (e.g., decreased fiber content and C/N ratios) and increases density of seeds. Moreover, N. granulata also facilitates S. densiflora seed availability to rodents by concentrating them in sediment mound at their burrows entrances. Experimental rodent exclusions showed that rodent species used S. densiflora seeds, a variable positively related to crab burrow density. Thus, our results show that N. granulata drives the granivorous rodent distribution and the intensity of seeds–rodent interaction trough facilitative and indirect interactions in marsh community.

Circannual rhythm in spatial distribution of burrows of freshwater crab, Barytelphusa cunicularis (Westwood, 1836)

Ecologists use a number of techniques to quantify and statistically analyse patterns of spatial distribution. The present study deals with the temporal distribution and nature of the burrows of the crab Barytelphusa cunicularis, found at Maroda Sector, Bhilai, India. All the burrows present in an area of 112 m2 were examined, during a span of 24 consecutive months. The number and pattern of burrows were observed every month in the study area. The data were subjected to nearest neighbour analysis. Rn values were computed to ascertain whether the distributions are clustered, random or regular. The burrows are considered more randomly spaced when Rn gets closer to 1.0. Time series data on Rn values, number of the total, single- and double-mouthed burrows were analysed by cosinor rhythmometry. The pattern of burrow distributions becomes more random in the early September with a spread between the beginnings of July and November. The timing of the peak in the burrows follows the peak in Rn values suggesting a post-reproductive increase in burrowing activities. The chronobiological analyses of Rn values add a new dimension to the interpretation of spatial distribution of crab burrows. This paper documents rhythm as an ecological phenomenon.

Implications of intraguild predation for sea turtle nest protection

In the United States, raccoons Procyon lotor are often removed from sea turtle nesting beaches to decrease egg mortality. However, raccoons also consume ghost crabs Ocypode quadrata, another common egg predator. Reducing predator populations can benefit secondary predators, inflating total predation pressure and leading to a decline in prey species. We used track and burrow counts to compare raccoon and ghost crab abundance at four beaches in Florida, USA, that differ in management activity and determined predation rates on loggerhead Caretta caretta nests by each predator. Mean raccoon abundance (range 0.12–0.46 tracks plot −1 night −1) and ghost crab density (0.09–0.19 burrows m −2) were inversely correlated. Ghost crabs were largest at the site with the fewest raccoons. The stable nitrogen isotope ratios of ghost crabs (mean 9.8‰) were positively correlated with body mass, indicating larger ghost crabs feed at a higher trophic level and suggesting large ghost crabs may consume more loggerhead eggs. The highest rates of egg predation by both predators (31%) occurred where raccoon abundance was lowest and ghost crab abundance was highest, suggesting ghost crab burrows may facilitate predation by raccoons. Our data suggest that predation by raccoons limits ghost crabs and that removing raccoons can increase ghost crab abundance and sea turtle egg mortality. Although predator removal can be effective when nest predation rates are quite high, maintaining moderate raccoon densities may be important for controlling ghost crabs. These results highlight the importance of understanding food web connectivity in developing management strategies to achieve conservation goals, especially when the species of concern are threatened or facing extinction.

Leaf consumption by Sesarma plicata in a mangrove forest at Jiulongjiang Estuary, China

The feeding ecology of Sesarma plicata (Grapsidae: Sesarminae), the most abundant crab species in a mangrove forest dominated by Kandelia candel at Jiulongjiang Estuary, China, was investigated through field and laboratory experiments. Feeding preference and consumption rates were determined on mature, senescent and decomposed leaves of Kandelia candel, Bruguiera gymnorrhiza and Aegiceras corniculatum. In the laboratory, S. plicata preferred leaves of K. candel over those of B. gymnorrhiza and A. corniculatum, and consumed significantly more decomposed leaves than mature and senescent ones, irrespective of crab size. Field experiments with limited power failed to reveal detectable species preferences despite more consumption of K. candel, but decomposed leaves of each species were again preferred. Leaf characteristics associated with preference changed with plant species and leaf state. Low tannins and high water content characterized the preference for decomposed state of leaves. Species preference was significantly and negatively related to crude fibers and C:N ratios for mature leaves, and crude fiber for senescent leaves, but significantly and positively related to water content for decomposed leaves. Leaf consumption rates averaged for all leaf categories from laboratory no-choice feeding experiments were 0.101, 0.055 and 0.017 gDW ind−1 d−1 for large, medium and small crabs, respectively. In this forest, mean density of S. plicata was 20.5 ind m−2 as assessed by a manual catching method. Leaf litter removal rate during neap tides by sesarmid crabs was about 1.33 gDW m−2 d−1 in April 2006. The leaves removed by crabs were grazed on the sediment surface or taken into crab burrows, shredded and stored before being eaten.

Preservation Potential of Storm Deposits in South Carolina Back-Barrier Marshes

A 5-year artificial-tracer study was used to mimic bioturbation of foraminifera in the back-barrier marshes of Folly and Rat Islands, South Carolina. Mixing rates decrease with subenvironment elevation, and decadal- to centennial-scale ecological signals and smaller hurricane deposits would not survive passage through the low-marsh mixing zone. However, the lower mixing rates in the high-marsh subenvironment allows for the preservation of such signals in the stratigraphic record. Bioturbation rates correlate poorly with plant density (primarily Spartina alterniflora). The Pearson correlation coefficient between mixing rate (Im in centimeters) and halophyte density (stems per square meter) is only 0.46, while the correlation between mixing and burrow density (apertures per square meter) is a much stronger 0.83. This corroborates earlier reports that spatiolateral variability in bioturbation is primarily a result of bioturbator (Uca spp.) preference for the low marsh and not halophyte distribution.Two factors contribute to the preservation of storm layers in the marshes behind Folly Island: (1) Hurricane deposits are laterally continuous and deposited across both low and high marsh, and (2) the coarser grain size of the washover sediments and the thickness of the larger storm deposits inhibit crab burrowing. Although the extensive paleostorm record from Folly Island appears to show a decrease in category 4 and 5 hurricanes through time, this record may reflect change in marsh paleosubenvironment. In a transgressive setting, where a formerly high marsh is submerged into low marsh, the preservation potential of storm deposits decreases through time.

The Southwest Atlantic intertidal burrowing crab Neohelice granulata modifies nutrient loads of phreatic waters entering coastal area

Along the coastal areas of the Southwest Atlantic estuaries and embayments, phreatic water often circulates through very extended areas (up to several hundred meters perpendicular to the coast), dominated by dense assemblages of deep burrows of the crab Neohelice granulata (formerly Chasmagnathus granulatus). This crab inhabits the intertidal area, from mudflats to marshes vegetated by species of Spartina, Sarcocornia and Juncus, generating extensive burrowing beds where burrow density may reach up to 60 burrows m −2. Since the lower limit of the crab burrows is usually the water table, we investigated through field experiments the effect of N. granulata and their burrows on the chemical characteristics of this phreatic water. Water analysis from experimental (1) occupied burrows (with crabs), (2) unoccupied burrows (where crabs were excluded), and (3) sediment pore water show remarkable differences. Water oxygenation, and nitrate, ammonium and sulphate concentrations inside occupied burrows were higher than in the water inside unoccupied burrows or pore waters. Moreover, directed sampling of phreatic water entering and leaving the crab bed, shows that dissolved inorganic nitrogen concentration is enhanced as the water crosses the crab bed. These results may be ascribed to the fact that in the salt marsh the crabs spend most of their time within burrows, where presumably they store food (plants) and defecate. These activities generate an area of accumulation of excrements and nutrients in different decomposition states. The present work shows a novel way by which bioturbating organisms can affect nutrients exportation from salt marshes to the open waters.