Feeding ecology of the semi-terrestrial crab Ucides cordatus cordatus (Decapoda: Brachyura) in a mangrove forest in northern Brazil

The objective of this thesis was to investigate the feeding ecology of the intensively exploited semi-terrestrial crab Ucides cordatus cordatus, and to contribute to the understanding of its influence on the flow of organic matter, nutrients, and energy in a mangrove ecosystem in northern Brazil. The mangrove crab U. cordatus is the most conspicuous species of the benthos, contributing to about 84% of its biomass. Stomach content analyses showed that the crabs´ diet is mainly composed of mangrove leaves, unidentified plant material, roots, and sediment. The gut evacuation rates were used in conjunction with the mean daily gastrointestinal contents to calculate the daily food intake of U. cordatus. The daily food intake was 1.0 g dw in small males (cw 3.0-3.5 cm), corresponding to 19.8 % of the crabs´ body dry weight. Large males (cw 7.0-7.5 cm) consumed 3.3 g dw daily, corresponding to 6.0 % of their body dry weight. The overall daily food intake of the U. cordatus population at an Rhizophora mangle dominated forest stand was estimated as 4.1 g dw m-2, corresponding to 81.3 % of the daily litter and propagule production. This indicates that litter processing by U. cordatus highly influences the flux of organic matter, leading to the retention of nutrients and energy inside the mangrove forest. High litter removal rates of the crabs, a low quantity of litter material in most investigated burrows, and high consumption rates during field experiments indicate that the U. cordatus population is food-limited in the investigated areas.Crabs maintained on a pure R. mangle diet showed higher assimilation efficiencies than those fed on Avicennia germinans leaves. Energy assimilation by the U. cordatus population was 10291 and 2870 kJ m-2 y-1 in an R. mangle and A. germinans dominated forest, respectively. The role of microorganisms for the nutrition of U. cordatus was investigated by using fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes

Artificial Crab Burrows Facilitate Desalting of Rooted Mangrove Sediment in a Microcosm Study.

Water uptake by mangrove trees can result in salt accumulation in sedimentaround roots, negatively influencing growth. Tidal pumping facilitates salt release and canbe enhanced by crab burrows. Similarly, flushing of burrows by incoming tidal waterdecreases sediment salinity. In contrast to burrows with multiple entrances, the role of burrowswith one opening for salinity reduction is largely unknown. In a microcosm experiment westudied the effect of artificial, burrow-like macro-pores with one opening on the desalting ofmangrove sediment and growth of Rhizophora mangle L. seedlings. Sediment salinity,seedling leaf area and seedling growth were monitored over six months. Artificial burrowsfacilitated salt release from the sediment after six weeks, but seedling growth was notinfluenced. To test whether crab burrows with one opening facilitate salt release in mangroveforests, sediment salinities were measured in areas with and without R. mangle stilt roots inNorth Brazil at the beginning and end of the wet season. In addition, burrows of Ucidescordatus were counted. High crab burrow densities and sediment salinities were associated with stilt root occurrence. Precipitation and salt accumulation by tree roots seem to have alarger effect on sediment salinity than desalting by U. cordatus burrows

Burrows of the semi-terrestrial crab Ucides cordatus enhance CO2 release in a North Brazilian mangrove forest

Ucides cordatus is an abundant mangrove crab in Brazil constructing burrows of up to 2 m depth. Sediment around burrows may oxidize during low tides. This increase in sediment-air contact area may enhance carbon degradation processes. We hypothesized that 1) the sediment CO2 efflux rate is greater with burrows than without and 2) the reduction potential in radial profiles in the sediment surrounding the burrows decreases gradually, until approximating non-bioturbated conditions. Sampling was conducted during the North Brazilian wet season at neap tides. CO2 efflux rates of inhabited burrows and plain sediment were measured with a CO2/H2O gas analyzer connected to a respiration chamber. Sediment redox potential, pH and temperature were measured in the sediment surrounding the burrows at horizontal distances of 2, 5, 8 and 15 cm at four sediment depths (1, 10, 30 and 50 cm) and rH values were calculated. Sediment cores (50 cm length) were taken to measure the same parameters for plain sediment. CO2 efflux rates of plain sediment and individual crab burrows with entrance diameters of 7 cm were 0.7–1.3 µmol m−2s−1 and 0.2–0.4 µmol burrows−1s−1, respectively. CO2 released from a Rhizophora mangle dominated forest with an average of 1.7 U. cordatus burrows−1m−2 yielded 1.0–1.7 µmol m−2s−1, depending on the month and burrow entrance diameter. Laboratory experiments revealed that 20–60% of the CO2 released by burrows originated from crab respiration. Temporal changes in the reduction potential in the sediment surrounding the burrows did not influence the CO2 release from burrows. More oxidized conditions of plain sediment over time may explain the increase in CO2 release until the end of the wet season. CO2 released by U. cordatus and their burrows may be a significant pathway of CO2 export from mangrove sediments and should be considered in mangrove carbon budget estimates

The influence of crab burrows on sediment salinity in a Rhizophora-dominated mangrove forest in North Brazil during the dry season

Many ecological processes are influenced by salinity. Burrowing crabs, abundant fauna of mangrove forests around the world, can facilitate sediment water fluxes, which may decrease the salinity in mangrove sediments. We investigated whether and how crab burrow density and secondary fine root biomass interact to drive sediment salinity during the dry season in a northern Brazilian mangrove forest. Areas with high density of Rhizophora mangle prop roots and areas free of such roots were compared. We found no correlation between burrow density and sediment salinity in areas with dense prop and fine roots, while crab density correlated negatively with sediment salinity in areas without prop roots, where fine root density was low. Hence, the strength of sediment desalination effects of crabs seems to be context dependent, and high root density of a salt-excluding mangrove species (R. mangle) seems to counteract the crabs’ effect. Our results complement those of a former study conducted in the same area during the rainy season, highlighting that the findings are independent from seasonality and should be considered when evaluating the overall ecological effects of crabs in mangrove ecosystems

Mangrove Crab Ucides cordatus Removal Does Not Affect Sediment Parameters and Stipule Production in a One Year Experiment in Northern Brazil

Mangrove crabs influence ecosystem processes through bioturbation and/or litter feeding. In Brazilian mangroves, the abundant and commercially important crab Ucides cordatus is the main faunal modifier of microtopography establishing up to 2 m deep burrows. They process more than 70% of the leaf litter and propagule production, thus promoting microbial degradation of detritus and benefiting microbe-feeding fiddler crabs. The accelerated nutrient turn-over and increased sediment oxygenation mediated by U. cordatus may enhance mangrove tree growth. Such positive feed-back loop was tested in North Brazil through a one year crab removal experiment simulating increased harvesting rates in a mature Rhizophora mangle forest. Investigated response parameters were sediment salinity, organic matter content, CO2 efflux rates of the surface sediment, and reduction potential. We also determined stipule fall of the mangrove tree R. mangle as a proxy for tree growth. Three treatments were applied to twelve experimental plots (13 m × 13 m each): crab removal, disturbance control and control. Within one year, the number of U. cordatus burrows inside the four removal plots decreased on average to 52% of the initial number. Despite this distinct reduction in burrow density of this large bioturbator, none of the measured parameters differed between treatments. Instead, most parameters were clearly influenced by seasonal changes in precipitation. Hence, in the studied R. mangle forest, abiotic factors seem to be more important drivers of ecosystem processes than factors mediated by U. cordatus, at least within the studied timespan of one year

Life-history, movement, and habitat use of Scylla serrata (Decapoda, Portunidae): current knowledge and future challenges.

The mud crab Scylla serrata is a highly exploited species, associated to mangrove ecosystems in the Indo-West-Pacific. It has a complex life cycle with a dispersing larvae phase, and benthic juveniles and adults. The former are stenohaline depending on high-salinity conditions to survive, whereas the latter are physiologically well adapted to changing temperatures and salinities, conditions that typically occur in mangrove habitats. Movement and habitat use of large juveniles and adults are well studied, and these life stages are known to utilize and move between various habitats within the mangrove ecosystem: intertidal flats as well as subtidal channels and flats. Females undertake long movements from brackish inshore waters to waters with oceanic conditions for spawning.Sensory abilities—of early stages and adult stages— have hardly been studied, and little is known about larval and early benthic stages in the wild. Summarizing, the literature revealed substantial gaps in the understanding of the spatiotemporal dynamics of thedifferent life stages and of the clues that trigger recruitment, movement, and other behavior. This is the first comprehensive review on the life history, movement patterns, habitat use, and systemic role of S. serrata with emphasis on the respective life stages and geographic differences. We emphasize the need forfurther research into these processes as a basis for the sustainable management and conservation of this species

A functional analysis reveals extremely low redundancy in global mangrove invertebrate fauna

Deforestation results in habitat fragmentation, decreasing diversity, and functional degradation. For mangroves, no data are available on the impact of deforestation on the diversity and functionality of the specialized invertebrate fauna, critical for their functioning. We compiled a global dataset of mangrove invertebrate fauna comprising 364 species from 16 locations, classified into 64 functional entities (FEs). For each location, we calculated taxonomic distinctness (Δ+), functional richness (FRi), functional redundancy (FRe), and functional vulnerability (FVu) to assess functional integrity. Δ+ and FRi were significantly related to air temperature but not to geomorphic characteristics, mirroring the global biodiversity anomaly of mangrove trees. Neither of those two indices was linked to forest area, but both sharply decreased in human-impacted mangroves. About 60% of the locations showed an average FRe < 2, indicating that most of the FEs comprised one species only. Notable exceptions were the Eastern Indian Ocean and west Pacific Ocean locations, but also in this region, 57% of the FEs had no redundancy, placing mangroves among the most vulnerable ecosystems on the planet. Our study shows that despite low redundancy, even small mangrove patches host truly multifunctional faunal assemblages, ultimately underpinning their services. However, our analyses also suggest that even a modest local loss of invertebrate diversity could have significant negative consequences for many mangroves and cascading effects for adjacent ecosystems. This pattern of faunal-mediated ecosystem functionality is crucial for assessing the vulnerability of mangrove forests to anthropogenic impact and provides an approach to planning their effective conservation and restoration

Ernährungsökologie der semi-terrestrischen Krabbe Ucides cordatus cordatus (Decapoda: Brachyura) in einem Mangrovenwald in Nordbrasilien

The objective of this thesis was to investigate the feeding ecology of the intensively exploited semi-terrestrial crab Ucides cordatus cordatus, and to contribute to the understanding of its influence on the flow of organic matter, nutrients, and energy in a mangrove ecosystem in northern Brazil. The mangrove crab U. cordatus is the most conspicuous species of the benthos, contributing to about 84% of its biomass. Stomach content analyses showed that the crabs´ diet is mainly composed of mangrove leaves, unidentified plant material, roots, and sediment. The gut evacuation rates were used in conjunction with the mean daily gastrointestinal contents to calculate the daily food intake of U. cordatus. The daily food intake was 1.0 g dw in small males (cw 3.0-3.5 cm), corresponding to 19.8 % of the crabs´ body dry weight. Large males (cw 7.0-7.5 cm) consumed 3.3 g dw daily, corresponding to 6.0 % of their body dry weight. The overall daily food intake of the U. cordatus population at an Rhizophora mangle dominated forest stand was estimated as 4.1 g dw m-2, corresponding to 81.3 % of the daily litter and propagule production. This indicates that litter processing by U. cordatus highly influences the flux of organic matter, leading to the retention of nutrients and energy inside the mangrove forest. High litter removal rates of the crabs, a low quantity of litter material in most investigated burrows, and high consumption rates during field experiments indicate that the U. cordatus population is food-limited in the investigated areas.Crabs maintained on a pure R. mangle diet showed higher assimilation efficiencies than those fed on Avicennia germinans leaves. Energy assimilation by the U. cordatus population was 10291 and 2870 kJ m-2 y-1 in an R. mangle and A. germinans dominated forest, respectively. The role of microorganisms for the nutrition of U. cordatus was investigated by using fluorescence in situ hybridization (FISH) with rRNA-targeted oligonucleotide probes

Experimental determination of stable carbon and nitrogen isotope fractionation between mangrove leaves and crabs

The analysis of benthic food webs in mangrove forests is complicated by the lack of experimental investigations of consumer-diet discrimination factors (Δ15N and Δ13C) for most mangrove invertebrates. This study aimed to determine the fractionation of δ13C and δ15N between mangrove leaves and Episesarma singaporense and E. versicolor (Sesarmidae) for 90 d in Java, Indonesia. The consumption rates of both species and stable isotope fractionation of leaves from 5 plant species (Acanthus ilicifolius, Aegiceras corniculatum, Avicennia alba, Derris trifoliata, Rhizophora apiculata) were compared. Δ15N between R. apiculata leaves and muscle tissue was 5.0‰ for E. singaporense and 5.4‰ for E. versicolor after 50 d. δ15N of muscle tissue increased significantly after 50 d, most likely due to the internal recycling of nitrogen. δ13C did not change during the experiment, and Δ13C was 5.1‰ for E. singaporense and 4.1‰ for E. versicolor after 90 d. Leaves of D. trifoliata and R. apiculata were preferentially consumed, indicating their higher nutritive value. We concluded that (1) the discrimination values for mangrove crabs feeding on a leaf diet are much higher than previously assumed, which can most likely be explained by the selective assimilation of isotopic heavy carbon compounds and by metabolic cycling of non-essential amino acids; (2) the frequently used average discrimination factors are inappropriate for the study of benthic food webs in mangrove forests with a high biomass of leaf-eating crabs; and (3) E. singaporense and E. versicolor can meet their nitrogen demand by assimilating nitrogen from R. apiculata leaves and by using internal reserves for at least 50 d

Activity patterns, feeding and burrowing behaviour of the crab Ucides cordatus (Ucididae) in a high intertidal mangrove forest in North Brazil

Activity patterns, feeding and burrowing behaviour of the economically important semi-terrestrial mangrove crab Ucides cordatus (Ucididae, L. 1763) was studied in a high intertidal Rhizophora mangle forest stand in Bragança, North Brazil. Video observations in the rainy and dry season were conducted over 24 h cycles at different lunar phases to investigate the behaviour of these litter-feeding crabs outside their burrows. During the rainy season, crabs stayed inside their burrows for 79% and 92% of the time during day and night, respectively. Time spent for feeding, burrowing and other activities outside their burrows was significantly longer during the day with 9.9% (night: 1.7%) and at waning and waxing moon with 9% (full and new moon: 0.9%). At neap tides (no tidal inundation) foraging and feeding activities outside burrows were clearly light-dependent, increasing at dawn and decreasing at dusk. Highest activities during daytime relate to the visual localisation of food. During the dry season, crabs spent less time inside burrows at neap tides than during the rainy season (80% and 91%, respectively). However, time spent for feeding activities was similar during both seasons. During almost all observation periods crabs collected leaf litter, but rarely fed on it outside burrows. At neap tides nearly all available litter was collected, suggesting that the U. cordatus population is litter-limited during these times. At spring tides (regular tidal inundation) the surface activity of U. cordatus was tide-dependent. Crabs closed their burrow entrances 2–3 h before flooding and re-emerged as soon as the tide retreated. During the day, burrow maintenance was the second most frequent behaviour after feeding. Agonistic interactions were regularly observed and were mainly related to burrow defence. The mean foraging radius of the crabs was only 19 cm (max: 1 m) underneath high Rhizophora mangle trees where crab densities were high. The results point to a high competition for burrows and show that U. cordatus is territorial. It is concluded that several exogenous factors, in particular light, leaf litter availability, flooding of burrows and the presence of conspecifics are important in controlling the crabs' activity patterns