Small- and large-scale effect of the SW Atlantic burrowing crab Chasmagnathus granulatus on habitat use by migratory shorebirds

Here we address the question of whether the presence of the burrowing crabs Chasmagnathus granulatus affects small- and large-scale habitat use by migrant shorebirds. This crab is the dominant species in soft bare sediments and vegetated intertidal areas along the SW Atlantic estuaries (southern Brazil 28°S to the northern Argentinean Patagonia 42°S). They generate very extensive burrow beds in soft bottom intertidal areas. Our information shows that this burrowing crab affects the small-scale habitat use by shorebirds, given that shorebirds never walk through the funnel-shaped entrances of burrows. Given that crab burrow entrances occupy up to 40% of the intertidal area, there is a large decrease of available shorebird habitat in crab beds, restricting their activity to the spaces between the burrows. The southern migratory shorebird Charadrius falklandicus maximize the use of these areas by foraging closer to the burrows than the other bird species. Neotropical migrants, such as Calidris fuscicollis, Pluvialis squatarola and Tringa melanoleuca, used foraging paths that tended to maximize the distance from burrows, especially the distance to larger burrows. A field experiment showed that this was not necessarily due to a decrease in the availability of polychaetes near the crab burrows. A combination of landscape measurements and satellite images showed that crab beds covered up to 40% of the intertidal area of the Mar Chiquita coastal lagoon (37°40′S, Argentina), and nearly 100% of the intertidal area of the Bahia Blanca estuary (38°48′-39°25′S, Argentina). These two estuaries are located along the migratory flyway of Neotropical migratory shorebirds, but the Bahia Blanca estuary (area∼110,000 ha) shows a much lower shorebird diversity than Mar Chiquita (area∼4500 ha). The most common species in Bahia Blanca is the two-banded plover C. falklandicus, the species least affected by crabs at Mar Chiquita and which prefers to use high-density crab areas as foraging sites. The oystercatcher Haematopus palliatus was also most abundant in high-density crab areas, but they used these areas for resting. The abundances of preys varied during the study period and between the crab density areas, indicating that the use of these areas by birds is independent of crab density. However, burrowing crabs affect the depth distribution of polychaete and thus their availability to shorebirds. We suggest that this shorebirds-burrowing organism interaction could be generalized for other intertidal estuarine habitats.     

The SW Atlantic burrowing crab Chasmagnathus granulatus Dana affects the distribution and survival of the fiddler crab Uca uruguayensis Nobili

In this paper, we address the question of whether the presence of the burrowing crab Chasmagnathus granulatus affects the habitat use of the fiddler crab Uca uruguayensis. Field samples showed that the species have a disjoint spatial distribution. Male fiddler crab density decreased in zones with C. granulatus, however, female density increased. Male fiddler crabs avoided feeding on sediment affected by C. granulatus and were more preyed. Predation was higher during the fiddler crab reproductive season and, probably due to predation risk, males showed lower reproductive display in shared zones. Field experiments shows that when C. granulatus were excluded, densities of U. uruguayensis increased mainly due to an increase in density of males. Habitat differentiation of these species may be because C. granulatus affects U. uruguayensis in several ways, including direct predation, disturbance and behavioural changes associated to predation risk. Males and females are affected differentially probably because of the extreme sexual dimorphism of this crab species. Coloration on enlarged claw and waving activities are all factors that increase predation risk for male and the presence of only one feeding claw may increase sediment-mediated effects.     

Colonization and substratum preference of an introduced burrowing crustacean in a temperate estuary

Habitat use in marine invertebrates is often influenced by multiple abiotic and biotic factors. Substratum composition is one factor known to have a dramatic effect on habitat selection. The Australasian burrowing isopod (Sphaeroma quoianum, H. Milne Edwards 1840) is a common introduced species in many estuaries on the Pacific coast of North America. S. quoianum burrows into a variety of firm substrata including marsh banks (composed of peat, clay, and/or mud), wood, friable rock, and Styrofoam floats. In some areas, isopods achieve high densities and may accelerate the rate of shoreline erosion and damage marine structures; thus, understanding the substratum preference of this species may be important for conservation and management efforts. Field experiments were conducted in Coos Bay, Oregon to examine substratum preference, burrowing rates, and the life stage of colonizers. In three experimental trials (Fall 2005, Spring 2006, Fall 2006), replicates of four intertidal substrata (marsh banks, decayed wood, sandstone, Styrofoam) were deployed near intertidal populations of S. quoianum. The numbers of burrows created in each substratum were enumerated weekly or daily (depending on trial). After the trials were completed, the total numbers of isopods inhabiting each substratum were counted. In weeks, S. quoianum extensively burrowed the substrata but exhibited a distinct preference for decayed wood. Significantly more isopods were present in wood than the other substrata at the end of the experiments and rates of burrowing were greatest in wood, although significance varied across time in one trial. Nearly 90% of colonizing isopods were under 5 mm in length suggesting that juvenile isopods primarily colonize intertidal substrata. Differences between burrow densities measured in the field and the results from these preference trials may indicate other factors, such as relative availability of substrata, recruitment and dispersal limitations, and possible gregarious behavior also influence local isopod densities.     

Effect of fish predation on intertidal benthic fauna is modified by crab bioturbation

The burrowing crab Chasmagnathus granulatus is an important bioturbator in SW Atlantic estuaries where they generate dense and extended intertidal beds. Its bioturbation leads to profound changes in the structure, quality and dynamics of sediments with concomitant impacts on the entire benthic community. In this study, we evaluate whether the presence of this crab affects the predator-prey interaction between juvenile fishes and their benthic prey. Gut content and benthic prey selection by juvenile fishes inside and outside crab beds were evaluated, and predation effect was experimentally contrasted between areas using fish exclosures. The results show that in crab beds the percentage of fish with empty guts was lower and the number of polychaetes consumed by fish higher than outside crab beds. The silverside Odontesthes argentinensis and the catfish Pimelodella laticeps fed on larger polychaetes outside than inside crab bed areas, while the white mouth croaker Micropogonias furnieri preyed upon larger polychaetes inside crab beds. In addition, field experiments shows that fish predation decreases polychaete abundances only in crab beds. These results suggest that crab bioturbation facilitate fish predation on benthic prey.     

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.     

Behavioral plasticity of the soft-shell clam, Mya arenaria (L.), in the presence of predators increases survival in the field

Soft-shell clams, Mya arenaria, are sessile, suspension-feeding bivalves that are preyed upon by the exotic green crab, Carcinus maenas. Clams evade crab consumers by burrowing deeper into the sediment after perceiving a threat from a nearby predator. The purpose of this study was to determine the types of signals that M. arenaria use to detect predators and the types of behaviors clams use to avoid being eaten. In a field study, clams increased their burial depth in the presence of green crab predators consuming conspecifics that were caged nearby, and also increased burial depth after artificial tactile stimulation in the laboratory assay. These results indicate that clams can use chemical and mechanical cues to detect potential predatory threats. We performed a field study to examine the difference in survivability of clams that had burrowed deeper into the sediment in response to predators vs. control clams that were burrowed less deeply. Significantly higher survival rates were observed in clams that had initially burrowed more deeply, suggesting that increasing burial depth is a valid predator avoidance strategy. Some bivalves also alter their pumping rates in the presence of predators, making them less apparent and providing more structural defense by covering soft tissue, and we measured pumping time of soft-shell clams in the presence and absence of predators, when burrowing was not an option for escape. Soft-shell clams did not alter their pumping time in the presence of green crab predators, possibly because they employ a burrowing method called “hydraulic” or “jet-propelled” burrowing, where it is necessary for the clam to pump in order to burrow. Chemical signals and tactile cues instigated behavioral changes in M. arenaria, and this change in behavior (increasing burial depth) increased clam survival in the field.     

Does the presence of the SW Atlantic burrowing crab Chasmagnathus granulatus Dana affect predator–prey interactions between shorebirds and polychaetes?

The burrowing crab Chasmagnathus granulatus is an important bioturbator that generates dense burrow assemblages (crab beds) characteristic of intertidal habitats of SW Atlantic estuaries. Crab bioturbation affects the topography and hydrodynamics of the sediment, increasing sediment water and organic matter content, decreasing sediment hardness and changing the grain size frequency distribution. In this study, we found that burrowing crabs can decrease the impact of predation by shorebirds on polychaetes. The polychaete Laeonereis acuta Treadwell has U-shaped burrows outside crab beds, which are associated with surface deposit-feeding while their burrows are mainly I-shaped inside which is associated with subsurface deposit feeding behavior. This pattern is likely the result of larger vertical sediment mixing inside crab beds due to crab burrowing. As a result of their feeding strategy, polychaetes appear on the surface more often outside crab beds, which increases their availability for shorebirds. In addition, shorebird species differentially use crab beds. The White-rumped Sandpiper, Calidris fuscicollis Vieillot, preferentially forage outside crab beds, meanwhile the Two-banded Plover Charadrius falklandicus Latham forage more frequently inside crab beds. However, experiments excluding shorebirds inside and outside crab beds showed negative effects of shorebirds only outside crab beds. Thus, our results show that the SW Atlantic burrowing crab C. granulatus affects the strength of the predator–prey interaction between shorebirds and polychaetes.     

Subtidal-intertidal trophic links: American lobsters [Homarus americanus (Milne-Edwards)] forage in the intertidal zone on nocturnal high tides

Homarus americanus (Milne-Edwards), the American lobster, is a predator in New England subtidal communities, feeding on ecologically important grazers (sea urchins), mesopredators (crabs), and basal species (mussels). In this study, we provide the first report of adult American lobsters foraging in rocky intertidal habitats during nocturnal high tides. Censuses by SCUBA divers in the low intertidal (Chondrus crispus Stackhouse) zone showed mean densities of 2.2 lobsters/20 m² on nocturnal high tides, with contrasting low densities of 0.18/20 m² during diurnal high tides. Nocturnal high-tide intertidal densities were 62% of those reported in a previous study of lobsters in nearby subtidal rocky areas (Novak, 2004). The average carapace length of lobsters in the intertidal at night was > 50 mm. These lobsters were actively foraging in the intertidal with collected individuals having a mean stomach fullness of 67%. Prey found in the stomach contents primarily consisted of crabs, mussels and snails. Field experiments showed that lobsters rarely fed on medium to large size individuals of the common intertidal snail, Littorina littorea (L.). In contrast, experiments with local crab species demonstrated that lobsters actively and readily prey on Cancer irroratus (Say) and Carcinus maenas (L.), but were significantly less likely to consume Cancer borealis (Stimpson). The abundance of Carcinus maenas and blue mussels (Mytilus edulis L.) in the intertidal zone may explain the upshore movement of lobsters. Since nocturnal migration of Homarus americanus into the intertidal zone has not been documented before, our understanding of the dynamics of New England intertidal communities needs to be expanded to include this predator.     

Effect of the snail Ilyanassa obsoleta (Say) on dynamics of the amphipod Corophium volutator (Pallas) on an intertidal mudflat

We investigated interactions between two dominant invertebrate species of intertidal soft-sediment environments of the northwest Atlantic, the mud snail Ilyanassa obsoleta and the burrowing amphipod Corophium volutator, on a mudflat of the upper Bay of Fundy, Canada. Distribution of I. obsoleta on the mudflat was highly patchy and negatively correlated with density of C. volutator. Manipulation of snail density in cages showed that I. obsoleta influences C. volutator; specifically, increasing density of snails reduced density, increased patchiness in distribution, decreased recapture rates and decreased immigration of C. volutator. Ilyanassa obsoleta seems to be affecting C. volutator through an influence on survival rate and emigration rate, although temporal variation in these effects was observed. Given that both I. obsoleta and C. volutator show a preference for tide pools, an important microhabitat on mudflats, snails might have a profound impact on C. volutator population dynamics.     

Effect of the substratum in the recruitment and survival of the introduced barnacle Balanus glandula (Darwin 1854) in Patagonia, Argentina

The barnacle Balanus glandula was introduced in Argentina in the 1970s, and today it dominates the high intertidal level in most Argentinean rocky shores. The aim of this work was to evaluate the effect of the type of substrata and intertidal height on a population of Balanus glandula by conducting field surveys and one-year field experiments in which we combined different substrata (hardness: hard and soft, and texture: smooth and rough) at two intertidal heights (mid and high). In natural populations, the highest density of adults and recruits occurred on soft-rough substratum and in the high intertidal. The different textures were important only on the soft substrata and high intertidal, and the density of barnacles of the soft-rough substrata was higher than soft and smooth ones. The most suitable experimental substratum was the soft-rough of the high intertidal, which had the highest recruitment, survival and final density of barnacles at the end of the experiment. In contrast, the hard and smooth of the high and middle intertidal were the least suitable in all cases. Although the recruitment of B. glandula occurred throughout the year, it was higher in the high intertidal, and it showed a recruitment peak in the winter and a second in the summer. While most studies on this barnacle investigated the effects of granite or other volcanic hard substrata, our study also focused on soft substrata. The effects of soft substrata are particularly important because soft sedimentary rocks characterise the southern Atlantic coast of South America and the presence of soft rocks appears to optimize the success of Balanus glandula.     

Habitat partitioning between prey soldier crab Mictyris brevidactylus and predator fiddler crab Uca perplexa

Interaction and habitat partition between the soldier crab Mictyris brevidactylus (prey) and the fiddler crab Uca perplexa (predator) were examined at a sandy tidal flat on Okinawa Island, Japan, where they co-occur. Both live in dense colonies. When the soldier crabs were released in the densely populated habitat of the fiddler crab, male fiddler crabs, which maintain permanent burrows in hard sediment, preyed on small soldier crabs and repelled large ones. Thus, the fiddler crabs prevented the soldier crabs from trespassing. It was also observed whether soldier crabs burrowed successfully when they were released 1) where soldier crab burrows just under the sand were abundant, 2) in a transition area between the two species, 3) an area without either species, and 4) where artificial tunnels simulated soldier crabs' feeding tunnels were made by piling up sand in the area lacking either species. In contrast to the non-habitat area, many soldier crabs burrowed in the sediment near the release point in the tunnel, transition and artificial tunnel areas. This indicates that the feeding tunnels on the surface attracted other crabs after emergence. When the large male fiddler crabs were transplanted into the artificial burrows made in soft sediment of the soldier crab habitat, all left their artificial burrows by 2 days. In the fiddler crab habitat, however, about one-third of the transplanted male fiddler crabs remained in the artificial burrows after 3 days. The soldier crabs regularly disturb the sediment by the up and down movement of their burrow (small air chamber) between tides. This disturbance probably prevents the fiddler crab from making and occupying permanent burrows. Thus, it appears that these crabs divide the sandy intertidal zone by sediment hardness and exclude each other by different means.     

Selective crab predation on native and introduced bivalves in British Columbia

Experiments were conducted to determine whether locally abundant crab species prefer co-occurring littleneck clams, Protothaca staminea (Conrad, 1837) and Tapes philippinarum (A. Adams and Reeve, 1850), relative to a recently introduced species, the varnish clam, Nuttallia obscurata, (Reeve, 1857). Prey preference, handling time, pick-up success, profitability and consumption rates were investigated for two crab species, Dungeness crab, Cancer magister (Dana, 1852) and red rock crab, Cancer productus (Randall, 1839) crabs. Both crab species preferred varnish clams over the native species. This may be attributable to the lower handling time, higher pick-up success and increased profitability of consuming varnish clams. Handling time appeared to be a factor not only in species preference, but also in the degree of preference, with shorter handling times corresponding to stronger preference values. Both native and introduced bivalves burrow into the substratum, with the varnish clam burrowing deepest. When feeding on clams in limited substratum both crab species preferred the varnish clam. In the unlimited substratum trials Dungeness crabs preferred varnish clams (although to a lesser degree) while red rock crabs preferred littleneck clams. This was likely due to the significantly deeper burial of the varnish clam, making it less accessible. Although the morphology (i.e. thin shell, compressed shape) of the invader increases its vulnerability to predation, burial depth provides a predation refuge. These results demonstrate how interactions between native predators and the physical characteristics and behaviour of the invader can be instrumental in influencing the success of an invasive species.     

Population structure of the South American Estuarine crab, Chasmagnathus granulatus (Brachyura: Varunidae) near the southern limit of its geographical distribution: comparison with northern populations

Crabs are among the most conspicuous and ecologically important invertebrates of the large intertidal zones that characterize estuarine and protected coastal areas in temperate regions. The habitat, population structure and breeding cycle of Chasmagnathus granulatus (Brachyura: Varunidae), a semiterrestrial burrowing crab endemic to the warm temperate coasts of the Southwestern Atlantic, were studied in San Antonio Bay (Argentina), near the southern limit of its range. San Antonio Bay has no freshwater input, winter is relatively colder, and summer warmer, than northern habitats of this species. Crabs lived both in vegetated and unvegetated zones, but density and sex ratio varied among dates and zones. The maximum observed density was 136 crabs/m² , the maximum carapace width (CW) was 32 mm (males) and 29.8 mm (females), ovigerous females were found only in November and January, and the smallest ovigerous female measured 17 mm CW. The population structure, spatial distribution, and recruitment pattern of C. granulatusdid not differ between San Antonio Bay and northern habitats. The higher density, smaller maximum size and shorter reproductive cycle observed in San Antonio cannot be atributed to changes associated with a latitudinal cline and other factors, such as thermal amplitude and food availability, need to be studied.     

Chemically induced predator avoidance behaviour in the burrowing bivalve Macoma balthica

The responses of the burrowing bivalves Macoma balthica and Cerastoderma edule to chemical cues emitted by feeding shore crabs Carcinus maenas were investigated. M. balthica held in the laboratory and exposed to chemical signals in effluent water discharging from tanks containing C. maenas fed 20 M. balthica day^− 1 reacted by increasing their burial depths from approximately 30 mm to depths of > 60 mm, over a period of several days. When the signal was removed the bivalves gradually returned to their original depth over 5 days. C. edule similarly exposed to effluent from crabs feeding on conspecifics showed no response. In an attempt to identify the signal inducing this burrowing response, M. balthica were exposed to a variety of chemical signals. Crabs fed M. balthica elicited the strongest response, followed by crabs fed C. edule. There were also small responses to effluent from crabs fed on fish, crabs previously fed on M. balthica and to crab faeces, but no responses to starved crabs, crushed M. balthica, or controls. We conclude that increased burrowing depth of M. balthica is induced by some as yet unidentified chemical cue produced by feeding crabs and is strongest when the crabs were fed on M. balthica. Unexpectedly, neither the presence of crabs themselves, nor of damaged conspecifics, was effective in eliciting a burrowing response. The mortality rates of M. balthica and C. edule selected by crabs when burrowed at normal depths and after exposure to effluent from feeding crabs were different. Crabs selected 1.5 times more C. edule than M. balthica when both species were burrowed at their normal depths, but 15 times more after the tanks had been exposed to effluent from feeding crabs for 5 days. The burrowing response of M. balthica thus appears to reduce mortality significantly by displacing predation pressure on to the more accessible C. edule.     

Changes in rainfall pattern affect crab herbivory rates in a SW Atlantic salt marsh

Climatic fluctuations usually change the intensity of existing interactions. Thus, in the context of the global climate change, it is important to consider new potential interactions or changes that may appear. Heavy rainy periods (one of the consequences of global climate change in eastern-central Argentina) can promote flooding in some estuaries (mainly on coastal lagoons), and thus, affect interactions between species. In this work we investigate if climatic fluctuations can affect Spartina densiflora Brong. (dominant marsh plant) survival through a chain of biotic and abiotic interactions in a SW Atlantic costal lagoon (37° 40′S, 57° 23′W; Mar Chiquita, Argentina). To achieve this, the long-term rainfall behavior of this region, and the effect of rainy periods on submergence of estuarine marsh areas (using satellite images) were analyzed. Then, the effect of flooding on the activity of the dominant herbivore of this system, the burrowing crab Neohelice granulata (= Chasmagnathus granulatus), was studied using pitfall traps. Finally, the effect of flooding on crab herbivory rates and plant survival were analyzed using transplants, stem-marking and flooding experiments. Long-term rainfall behavior showed that mean annual rainfall has increased during the last century, with the occurrence of more rainy years, and increases in cumulative monthly rainfall increased the submerged area of the S. densiflora marsh. Also, crab activity in the marsh largely increased during periods of flooding, associated with more than 100% increments in herbivory rates and stem mortality. These results reveal that increments in rainfall regime can trigger a cascade of abiotic and biotic interactions leading to increased marsh mortality, and stresses the importance of considering both, biotic and abiotic factors, together to predict changes in community organization.     

Response to environmental salinity of Na-KATPase activity in individual gills of the euryhaline crab Cyrtograpsus angulatus

The occurrence, localization and response to environmental salinity changes of Na⁺-K⁺ATPase activity were studied in each of the individual gills 4-8 of the euryhaline crab Cyrtograpsus angulatus from Mar Chiquita coastal lagoon (Buenos Aires Province, Argentina). Na⁺-K⁺ATPase activity appeared to be differentially sensitive to environmental salinity among gills. Upon an abrupt change to low salinity, a differential response of Na⁺-K⁺ATPase activity occurred in each individual gill which could suggest a differential role of this enzyme in ion transport process in the different gills of C. angulatus. With the exception of gill 8, a short-term increase of Na⁺-K⁺ATPase specific activity was observed in posterior gills, which is similar to adaptative variations of this activity described in other euryhaline crabs. However, and conversely to that described in other hyperregulating crabs, the highest increase of activity occurred in anterior gills 4 by 1 day after the change to dilute media which could suggest also a role for these gills in ion transport processes in C. angulatus. The fact that variations of Na⁺-K⁺ATPase activity in anterior and posterior gills were concomitant with the transition to hyperregulation indicate that this enzyme could be a component of the branchial ionoregulatory mechanisms at the biochemical level in this crab. The results suggest a differential participation of branchial Na⁺-K⁺ATPase activity in ionoregulatory mechanisms of C. angulatus. The possible existence of functional differences as well as distinct regulation mechanisms operating in individual gills is discussed.     

The effects of burrowing of Helice tridens (De Haan) on the soil of a salt-marsh habitat

The physical and chemical effects of the burrowing activity of the mud crab Helice tridens (De Haan) on the soil of a salt-marsh habitat were investigated. Soil-turnover rate caused by burrowing activity was found to be ≈ 3% of the soil from the surface to a depth of 40 cm every day during the summer. The vertical distributions of leaf and stem fragments of the salt-marsh plant Phragmites australis (Trin.) and the vertical distribution of ammonium N concentration in the soil were also investigated. At locations in the marsh where there were many large burrows, numerous leaf and stem fragments were recognized in the soil, while in areas in the marsh containing only a few small burrows these fragments were scanty. The soil depths at which leaf and stem fragments were abundant, corresponded to the depths of the burrows. These results show that mud crabs bury fallen plant fragments in the soil by their burrowing activity. Ammonium N in the soil was also abundant at locations in the marsh where there were many burrows, indicating that organic matter, such as fallen leaves and stems, may be decomposed to inorganic nutrients which are useful to the salt-marsh plants.     

Habitat preferences of two estuarine burrowing crabs Helice crassa Dana (Grapsidae) and Macrophthalmus hirtipes (Jacquinot) (Ocypodidae)

The horizontal and vertical distributions of two species of endemic, burrowing mud crabs Helice crassa Dana 1851 (Grapsidae) and Macrophthalmus hirtipes (Jacquinot 1853) (Ocypodidae) are described for the Avon-Heathcote Estuary (43°33′S: 172°44′E), Christchurch, New Zealand. Substratum preference is shown to be the most important factor influencing mud crab distribution, but lack of tolerance to salinities below 4‰ is also a significant factor preventing M. hirtipes from occurring at points close to freshwater input. Both species had similar sediment organic content and particle size requirements. Helice crassa was concentrated in well-drained, compacted sediments above mid-tide level, whilst Macrophthalmus hirtipeswas found in waterlogged areas below mid-tide level. This vertical separation is shown not to be caused by differential desiccation tolerances, but by feeding and burrowing adaptations related to these different substrata.     

The relative importance of substratum characteristics and recruitment in determining the spatial distribution of the fiddler crab Uca uruguayensis Nobili

The roles of sediment characteristics and the pattern of recruitment in influencing the abundance of the fiddler crab Uca uruguayensis on Argentinean mudflats were evaluated. The density of adult crabs showed a patchy distribution related to the sediment thickness (depth at which a layer of fossil shells are buried), but the density of juvenile crabs was not coupled with the density of adult crabs. In a field experiment, fossil shells were removed and the density of crabs significantly increased, which demonstrates that the presence of the layer of shells is a structure that may hinder the establishment of burrows. The density of crabs was related to sediment thickness, sediment torque and organic matter content. The importance of each of these variables was different for adult and juvenile crabs, indicating that the distribution of adult crabs may be caused by mechanisms affecting adult crabs themselves and is not established by the recruitment pattern. Moreover, in a field experiment, the density of juveniles decreased when adult crabs were added, and increased when adult crabs were removed.The morphology of burrows was related to sediment characteristics. Burrows were deeper, longer and more voluminous when sediment thickness was high. The volume of burrows decreased with increasing sediment torque. These results suggest that the morphology of burrows is related to the space available and the ease with which sediment it can be excavated. However, an important amount of variability remained unexplained, suggesting the presence of additional environmental variables or behavioural plasticity not considered by this study. Together, these results demonstrate that the spatial heterogeneity in the environmental factors can be translated to a spatial heterogeneity in the distribution of fiddler crabs.