Dominant Crab Research Articles

Do the crabs Goniopsis cruentata and Ucides cordatus compete for mangrove propagules? A field-based experimental approach

Mangroves are under threat worldwide by deforestation, overexploitation and climate change. The availability and consumption rates of propagules influence mangrove recruitment and can play a major role in their viability and restoration potential. We assess the potential trophic competition between Goniopsis cruentata and Ucides cordatus, two dominant crab species in the New World, by experimentally comparing herbivory levels between forest stands with varying crab abundance. We hypothesize that herbivory rates (HR) of G. cruentata will be lower in mangroves where it coexists with U. cordatus than in mangroves where U. cordatus is absent. The removal of Rhizophora mangle propagules was very rapid, and HR were overall high and increased through time. However, HR did not differ significantly between mangroves with and without the potential trophic competitor U. cordatus. Our study did not support previous literature indications of food competition between these two crab species, which seem to have developed strategies for competition avoidance.

Regional-scale patterns of mobile invertebrate assemblage structure on artificial habitats off Western Australia

Despite the many functions that mobile invertebrates serve in marine ecosystems, these assemblages remain relatively understudied. This study utilized artificial settling surfaces to determine how mobile invertebrate assemblages vary along a latitudinal gradient encompassing ~1100km of the Western Australian (WA) coast. We examined the structure of both ‘whole assemblages’ at a coarse taxonomic level and a subset of the assemblage (crabs) at the species level, across six locations and over two sampling periods. Assemblage structure differed significantly between locations and a moderately strong correlation with latitude was detected. More evident, was distinct partitioning between assemblages at ‘warm-water’ locations (23–24.5°S) and ‘temperate’ locations (28–33°S). This division in assemblage structure was consistent between sampling periods and taxonomic resolutions and corresponds to shifts in dominant biogenic habitat along the coast (coral vs. macrophyte). The abundances of some of the dominant crab species suggested discrete, unimodal distribution relationships with latitude/temperature, a pattern observed previously for macroalgae and fish in WA. The regional-scale patterns of mobile invertebrate assemblage structure presented here will contribute to forming a benchmark against which to detect future ecological change.

Habitat segregation of two dotillid crabs Scopimera globosa and Ilyoplax pusilla in relation to their cellulase activity on a marsh-dominated estuarine tidal flat in central Japan

This study aimed to clarify the relationship between cellulase activity and habitat segregation for two dotillid crabs – Scopimera globosa and Ilyoplax pusilla – on the marsh-dominated Tanakagawa tidal flat in central Japan. Both species are deposit feeders, and represent the dominant crab species in the study area. S. globosa was found to occur within sandy sediment habitats on the tidal flat, while I. pusilla occurred in both muddy sediment on the tidal flat and salt marsh habitats. This study was the first to detect cellulase activity in dotillid crabs. Cellulase activity was detected for both species; I. pusilla was found to exhibit significantly higher activity than S. globosa. Organic matter content, which is known to have a strong positive correlation with the cellulose content of sediments in estuarine areas, was higher in the sediments of muddy and salt marsh habitats compared with those in sandy habitats. Our findings indicate a close relationship between the habitat segregation of these two crabs and their cellulase activity. Given that cellulose is abundant in refractory organic materials and is a major component of food resources in tidal flat sediments, it is likely that these two crab species, especially I. pusilla, would play an important role in material cycling in tidal flat and estuarine areas.

Mangrove carbon sink. Do burrowing crabs contribute to sediment carbon storage? Evidence from a Kenyan mangrove system

Mangrove ecosystems are acknowledged as a significant carbon reservoir, with a potential key role as carbon sinks. Little however is known on sediment/soil capacity to store organic carbon and the impact of benthic fauna on soil organic carbon (SOC) stock in mangrove C-poor soils. This study aimed to investigate the effects of macrobenthos on SOC storage and dynamic in mangrove forest at Gazi Bay (Kenya). Although the relatively low amount of organic carbon (OC%) in these soils, they resulted in the presence of large ecosystem carbon stock comparable to other forest ecosystems. SOC at Gazi bay ranged from 3.6kgm−2 in a Desert-like belt to 29.7kgm−2 in the Rhizophora belt considering the depth soil interval from 0cm to 80cm. The high spatial heterogeneity in the distribution and amount of SOC seemed to be explained by different dominant crab species and their impact on the soil environment. A further major determinant was the presence, in the subsoil, of horizons rich in organic matter, whose dating pointed to their formation being associated with sea level rise over the Holocene. Dating and soil morphological characters proved to be an effective support to discuss links between the strategies developed by macrobenthos and soil ecosystem functioning.

Population dynamics and secondary production of crabs in a Chinese salt marsh

In many salt marsh ecosystems, crabs are the most prominent members of the macrobenthic community, influencing material cycling and energy flow. Studies on population dynamics and secondary production are important for quantitative evaluation of the ecological function of crabs; however, few studies have been conducted in salt marsh ecosystems. In this study, we investigated the secondary production and population dynamics of two Thoracotremata species in an Asian salt marsh. A year-round sampling on a monthly basis in the Dongtan salt marshes of the Yangtze River estuary yielded 12 species of crabs belonging to 7 families and 10 genera. The dominant crabs were Chiromantes dehaani (67.57%) and Helice tientsinensis (17.28%). The overall size frequency distribution was bimodal for C. dehaani and unimodal for H. tientsinensis. The estimated annual production of C. dehaani and H. tientsinensis was 43.981 g and 4.247 g ash-free dry weight (AFDM) m−2 year−1, respectively. The annual P/B ratio was 1.22 for C. dehaani and 0.75 for H. tientsinensis. In total, the annual secondary production of all crabs at the Dongtan salt marshes was estimated to be approximately 49.405 g AFDM m−2 year−1. The consumption of vascular plants by C. dehaani and H. tientsinensis was 480.831 and 42.640 g AFDM m−2 year−1. About 20.4% of plant primary production was ingested by these two herbivorous crabs. The production of C. dehaani was high compared with those reported from other coastal wetland ecosystems. This suggests that the ecological functions of C. dehaani at the Yangtze River estuary salt marshes need more quantitative studies to be fully described.

Restoration of Aegiceras corniculatum mangroves in Jiulongjiang Estuary changed macro-benthic faunal community

Macro-benthic faunal communities were compared between non-vegetation mudflat and Aegiceras corniculatum mangroves with different ages in Jiulongjiang Estuary, China. Faunal species number was highest in the mature mangrove and was higher in mangroves than in the mudflat, as snails and some crustaceans species were only collected in mangroves. The 5-year-old mangrove had the highest infaunal abundance and crustacean biomass. Snails had more abundance in the young mangroves. Uca arcuata was the dominant crab species in the non-vegetation mudflat and 5-year-old mangrove. Mangrove vegetation and sediment characteristics analyses indicated different habitats due to A. corniculatum mangrove restoration. However, overall poor correlations between faunal assemblage and sediment properties indicated that sediment properties were not the major factors influencing faunal distribution.

Can land crabs be used as a rapid ecosystem evaluation tool? A test using distribution and abundance of several genera from the Seychelles

Habitat destruction and the introduction of exotic species are the primary causes of biodiversity loss in tropical island ecosystems. Conservation efforts on oceanic islands are often biased towards charismatic vertebrate faunas, neglecting invertebrate assemblages. We sampled the land crab community on five islands in the central Seychelles; in the intertidal zone, in the supralittoral zone and in nine different inland habitat types to explore the impacts of exotic vegetation and environmental variables on land crab abundance and community composition, and investigate whether land crabs can be used as a tool for the rapid assessment of habitat quality on tropical oceanic islands. We found that species richness and the abundance of the dominant ghost crab Ocypode cordimana was higher in native habitat types than habitats dominated by exotic vegetation. Available ground substrate suitable for burrowing may be a limiting factor for O. cordimana in exotic habitat types. Coenobita rugosus, the dominant crab in the supralittoral zone is largely absent where there is no supralittoral vegetation. These results suggest that land crabs could be reliable indicators of habitat quality on oceanic islands. The abundance of land crabs could be used in the rapid assessment of ecosystem perturbation and identification of sites requiring restoration or management.

Do mangrove root structures function to shelter benthic macrofauna from predators?

Clarification of the role of mangrove root structures as shelter from predators for benthic animals was investigated by considering (1) the impact of predation on benthic faunal communities in a mangrove forest as indicated by a predator exclusion experiment, and (2) the uses and effects of mangrove root structures by benthic animals under laboratory conditions. In the exclusion experiment, three treatments (predator exclusion cages, partial cages and un-caged controls) were established on surface substrates in each of 3 microhabitats (tidal creek, mangrove cover present and mangrove cover absent) in a Rhizophora mangrove forest in Southern Thailand. After 1 month, benthic invertebrates were sampled and comparisons made of their species' richness, abundance and biomass among the treatments. Laboratory experiments were undertaken in tanks containing artificial mangrove roots, using 2 dominant crab species ( Perisesarma indiarum and Uca bengali) as prey and the barred mudskipper Periophthalmus argentilineatus as predator. Crab preference for root structure and predation rates were recorded. Predator exclusion from the mangrove area resulted in enhanced species' richness, abundance and biomass of benthic invertebrates in all microhabitats, including those with mangrove roots. However, the partial cage treatment did not change such characteristics, indicating that the artificial cage itself did not affect the animals. The laboratory experiments demonstrated that crab preferences for root structures were not influenced by predator presence, and that root structures did not reduce crab predation rates. Such process of dominant species might be cause for significant predation in field benthic community. The overall results indicated that predation was significant for species' richness, abundance and biomass of benthic mangrove forest communities, and vertical root structures did not necessarily provide effective shelter for benthic animals.

Fate of mangrove organic matter along a subtropical estuary: small-scale exportation and contribution to the food of crab communities

The export of mangrove primary production to adjacent habitats has long been regarded as playing a significant role in estuarine food webs. In the Logan River Estuary, a sub- tropical estuary situated in the southern part of Moreton Bay in Queensland, Australia, we used the fatty acid markers approach to investigate this export and its contribution to the estuarine food web, compared to other sources. During summer and autumn of 2004, 4 stations were sampled along a salinity gradient. Surface sediment samples were collected, using pre-cut syringes, in the mangrove forest of each station and in the adjacent tidal flats. Individuals of dominant crab species were also collected from the mangrove forests. Seasonal differences in fatty acid compositions of the surface sediments within the mangrove forests and along the adjacent banks were recorded, indicating that organic matter inputs varied between summer and autumn. Results also showed that mangrove production was accumulated at the surface sediments in some parts of the estuary during autumn and consequently was not exported to the tidal flat. The fatty acid composition of crab species present in the mangrove showed obvious differences between the grapsid and the ocypodid crabs collected from the estuary. These differences most likely reflect the distinctive feeding strategies of the crab groups, although there was some indication of species-specific biosynthesis/accumulation of certain polyunsaturated fatty acids. The grapsid of this study, Paras- eserma erythodactyla, in addition to being more dependent on mangrove leaves than the ocypodid crabs, is shown to potentially feed on fungal biomass.

PATTERNS OF LARVAL RELEASE BY THE ASIAN SHORE CRAB HEMIGRAPSUS SANGUINEUS (DE HAAN): PERIODICITY AT DIEL AND TIDAL FREQUENCIES

The Asian shore crab, Hemigrapsus sanguineus, is an invasive species along the Atlantic coast of North America. It has spread rapidly since its discovery in the USA in 1988 and has become the dominant crab species in rocky intertidal habitats at many locations in the Middle Atlantic Bight and in the southern Gulf of Maine. The reproductive biology of H. sanguineus has received only cursory study in native east-Asian waters. Available information consists mainly of descriptions of the larval stages and general characterizations of the natural history of the species. This study examined the periodicity of larval release in an invasive population of H. sanguineus near its original site of introduction at the mouth of Delaware Bay. We investigated larval release at diel and tidal frequencies. Analysis of laboratory data showed that larval release always occurs near the time of nocturnal high tide. Field sampling confirmed the presence of stage I zoeae in waters near adult habitat at the expected time of larval release. This pattern of release minimizes the impact of visual predators on newly hatched zoeae and takes advantage of circulation at tidal frequency to transport zoeae away from intertidal habitats where conditions may not be favorable for larval development. However, H. sanguineus is not unique among intertidal crabs in releasing larvae near the time of nocturnal high tide. This pattern has been reported for a large number of species with wide representation among families. Thus, the pattern of release observed in H. sanguineus does not provide any special competitive advantages over co-occurring native species, and the remarkable success of the species in North American habitats may be attributable to competitive interactions occurring after settlement and metamorphosis.

POSITIVE PLANT–ANIMAL INTERACTIONS IN THE HIGH MARSH OF AN ARGENTINEAN COASTAL LAGOON

Although plant-plant facilitations have been shown to be important struc- turing forces in salt marshes, less attention has been given to the potential role played by plant-animal facilitations in structuring these communities. We used a combination of sampling and field experiments to evaluate the effect of microenvironmental changes pro- duced by plant cover on the distribution of the burrowing crab Chasmagnathus granulata, a dominant macroinvertebrate of high marshes of the southwestern Atlantic coast. Four questions were explored. Is there a relationship between the spatial distribution of C. granulate and the spatial distribution of rooted macrophytes or distance from the marsh edge? How important is plant cover for the establishment and survival of crabs in the high marsh? Does plant cover affect critical physical variables for crab establishment? How important are environmental conditions for the survival of crabs in the high marsh? Surveys of the marsh surface showed that: (1) there was a strong relationship between the presence of plant cover and the spatial distribution of Chasmagnathus granulate in the high marsh, and (2) both mean crab size and burrow density decreased from the marsh edge to high-marsh levels. By shading the substrate, live plants and experimental plant mimics were found to be equally efficient at buffering high temperature, dehydration, and soil hardness in the high marsh. Experimental amelioration of these harsh physical con- ditions led to higher crab densities. Crab burrows also buffered harsh environmental con- ditions, diminishing surface air temperature from -460C to 230C. Finally, tethering ex- periments showed that stressful heating in the high marsh is lethal for crabs, and that plant cover is crucial for the establishment and long-term success of crabs in the high marsh. No predation was observed in tethering experiments. Our results suggest that plant cover is largely responsible for determining the spatial distribution of this dominant crab in the high marsh through facilitation. Thus, our work shows that plant-animal facilitations as well as plant-plant facilitations are important struc- turing forces in salt marsh communities.

Seasonal abundance and recruitment in an estuarine population of mud crabs, Scylla paramamosain, in the Mekong Delta, Vietnam

Artisanal fisheries for mud crabs are an important source of income in coastal communities of the Mekong Delta. However, populations are subject to pressure from increasing fishing effort to provide seed crabs for pond culture, as well as diminishing mangrove habitats. In the present study, Scylla paramamosain was found to be the dominant mud crab species within an estuarine mangal system, representing over 96.8% of mud crab fishery landings between February 1998 and March 1999. S. olivacea was also present in relatively low abundance (3.2% of landings). Analysis of catch-per-unit-effort (CPUE) data collected throughout 1997 and 1998, for an intertidal hand-fishery, shows that mud crabs were present in a fairly narrow range of mean abundance throughout the year, despite protracted periods of freshwater conditions during the rainy season. Recruitment of juvenile S. paramamosain (3–4 cm carapace width) was continuous through 1998, with a significant peak at the beginning of the dry season. No juvenile S. olivacea (< 5 cm carapace width) were found during the study. The mangal appears to provide a nursery habitat for S. paramamosain, with predominantly juveniles and sub-adults present, while a higher proportion of adults were recorded in the sub-tidal component of the population. Mature females were present throughout the year, with a peak in September–October 1997. The CPUE analysis provides a baseline of seasonal variation in crab abundance that can be used to monitor the effectiveness of resource management and potential impacts of future changes in habitat.

Feeding and assimilation of mangrove leaves by the crab Sesarma meinerti de Man in relation to leaf-litter production in Mgazana, a warm-temperate southern African mangrove swamp

Consumption of Avicennia marina (Forks.) Vierh. leaves by the large detritivorous mangrove grapsid Sesarma meinerti de Man was measured in the laboratory. This was compared with abscission rates for this mangrove species in the field (Mgazana estuary, Transkei, southern Africa) to estimate how much leaf material was directly utilised by this ecologically dominant crab. Although a significant correlation was found between crab size and dry litter massingested per day, no correlation was found between crab size and assimilation efficiency, which was high (82.44%). A seasonal trend in leaf litter fall was discernable with a mean annual rate of 1.79 g m -2d -1(653.4 g m -2y -1). With amean crab density of 4 m -2 and mean crab size of 40 g, leaf consumption was calculated at 0.78 g m -2d -1DM (284.7 g m -2y -1 or 6462 kJ m 2y -1), accounting for 43.58% of the leaf fall. For the entire mangrove area (150 ha) a total of 75t of frasse 955 kJ m -2y -1) enters the estuary per annum. This is considerable contribution to the detrital pool of the estuary and increases leaf turnover an estimated four-fold.

Biogenic Sedimentary Structures Produced by Crabs in Lagoon Margin and Salt Marsh Environments Near Beaufort, North Carolina

ABSTRACT Intertidal burrowing crabs produce distinctive biogenic sedimentary structures in protected lagoon margin and salt marsh environments in the vicinity of Beaufort, North Carolina. Distribution of crabs is determined primarily by substrate characteristics, salinity, and vegetation cover in the intertidal zone. Protected sand beaches and flats are heavily burrowed by the sand fiddler, Uca pugilator, which produces gently curved J-shaped and sharply curved L-shaped burrows 1 to 2 cm in diameter. Larger Y- and U-shaped burrows are formed by the ghost crab, Ocypode quadrata, in the backshore and foredune ridge zones above the high tide line. Salt marshes influenced by normal to near normal marine salinities are burrowed by U. pugilator, U. pugnax, and Sesarma reticulatum. U. pugnax and S. reticulatum burrow in muddy substrates, particularly along tidal creek banks, whereas U. pugilator is restricted to sandy substrate areas. U. pugnax burrows are 1 to 2 cm in diameter and have complex twisting and turning forms. The larger communal burrows of S. reticulatum consist of a complex of interconnected shafts with several oval-shaped surface openings. Panopeus herbstii is often found inhabiting these burrows. In salt marshes where the influence of fresh water is great, U. minax and U. pugnax are the dominant crabs. U. minax forms burrows 2 to 5 cm in diameter with ho ded entrances and long vertical shafts. Intertidal crabs are important agents of bioturbation in the environments that they inhabit. Their burrows have excellent potential for preservation in the sedimentary record. Recognition of the record of biogenic sedimentary structures produced by intertidal crabs would be of significant value in identifying paleoenvironments, fixing ancient lagoon margin and estuarine shorelines, and determining paleosalinities.

The Role of Shell Adequacy in Behavioral Interactions Involving Hermit Crabs

Field studies have suggested that the intertidal hermit crabs of the San Juan Islands of Washington normally occupy snail shells smaller than preferred. In this study the effects of shell size on protection from predation and on hermit crab shell fighting were studied in the laboratory. A predator (Cancer) presented with two hermit crabs (Pagurus granosimanus), identical except in size of occupied shell, preyed upon the hermit in the smaller shell first in 15 out of 16 trials. This results suggests that large shell size confers a selective advantage on the occupying crab. Shell fights involving two hermit crabs (P. hirsutiusculus) of unequal size were observed in which replicates differed only in the shell size of the larger crab. The probability of the larger crab effecting a shell exchange through fighting was shown to increase as the size of its shell decreased. However, shell size was shown to have no effect on the level of aggressiveness as measured by four criteria. The mechanism underlying the former result thus appears to involve a continual high level of general aggressiveness together with an increased tendency associated with occupancy of an inadequate shell by the dominant crab for that crab to evoke a shell exchange during an aggressive interaction.