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.     

Food availability in beach and marsh habitats and the size of the fiddler crab claw,a sexually selected weapon and signal

Males of the sand fiddler crab Uca pugilator possess a greatly enlarged claw that is used as a weapon in ritualized contests for control of breeding burrows and is waved to attract females to breeding burrows. Approximately 5400 crabs were collected along the Atlantic coast of North America at 14 localities, all of which had both beach and salt marsh habitats. Five measurements were made on each claw. Principal components analysis was used to generate a single measure of claw size from the seven correlated measures and scores of the claw. Carapace width was measured to index body size. Claw size was greater in beach than marsh habitats, controlling for body size. However, body size did not differ by habitat type. Claw size was also greater in laboratory‐reared males receiving more food, suggesting that differential access to food could influence habitat‐associated differences in claw size. Chlorophyll a concentration and total organic content, reflecting, respectively, the abundance of benthic algae and other food, were greater in beach than marsh habitats. Moreover, feeding opportunities were greater in the wetter beach habitat because crabs there, but not in marsh habitat, can feed at breeding burrows. Adult fiddler crabs continue to molt and grow in both body and claw size as they age. Energetic investment in the claw relative to the body is plastic. It appears that the availability of food can affect the amount of energy invested in the claw.     

Burrow ornamentation in the fiddler crab (Uca leptodactyla): female mate choice and male–male competition

Non-biological ornamentation is found in the nests and burrows of different kinds of animals. We evaluated here whether sand hoods constructed by male fiddler crabs (Uca leptodactyla) are one of the signals used by males to attract females during courtship. We observed females when they were walking among the males, and we quantified the proportion of females that visited male burrows with and without ornamentation and the choice to stay in a male’s burrow. Females visited more burrows with hoods than burrows without hoods, and they chose significantly more builder males. Male investment in ornamentation nevertheless decreased when the proportion of females increased in the area. Male investment was not correlated with the proportion of non-builder males nearby, but was positively correlated with overall density. The density sex ratio, however, was more male-biased in high-density than in low-density areas suggesting that even if building attracts females, the function could be related to male competition for mates.     

Burrow Architecture of the Ghost Crab Ocypode ceratophthalma on a Sandy Shore in Hong Kong

The ghost crab Ocypode ceratophthalma (Pallas) creates burrows of variety shapes at different ages. Juveniles (mean carapace length 11 mm) produced shallow J-shaped burrows, which incline vertically into the substratum (mean depth 160 mm). Larger crabs (17–25 mm carapace length) have Y-shaped and spiral burrows (mean depth 361 mm). These Y-shaped burrows have a primary arm, which extends to the surface forming the opening, and a secondary arm which terminates in a blind spherical ending. The two arms join in a single shaft and end with a chamber at the base. The secondary arms and chambers are believed to be used for mating or as a refuge from predation. The spiral burrows have spiral single channel ending in a chamber. Older crabs (mean carapace length 32.6 mm) had simple, straight single tube burrows, which inclined into the substratum at mean of 73° and had a mean depth of 320 mm. During summer daytime periods, the burrows shelter the crabs from heat and desiccation stress. The sand surface temperature at the burrow opening was ~48 °C but temperatures inside the burrows can drop to 32 °C at a depth of 250 mm. Variation in the burrow architecture with crab age appears to be related to the crab’s behaviour. Juvenile crabs have smaller gill areas and move out of the burrows regularly to renew their respiratory water and, as a result, they do not need a deep burrow. Larger crabs, in contrast, can tolerate prolonged periods without renewing their respiratory water and therefore create deeper and more complex burrows for mating and refuges.     

Burrow structure and use in the sand fiddler crab, Uca pugilator (Bosc)

Uca pugilator, the sand fiddler crab, constructs two kinds of burrows in protected, sandy upper-intertidal and supratidal substrates on the west coast of Florida. Temporary burrows are built and used as a refuge by non-breeding crabs during high tide periods and at night when crabs cease feeding in the intertidal zone. Breeding burrows are constructed and defended by courting males and are the site of mating, oviposition and the incubation of eggs by females. Up to three ovigerous females may be accommodated in a single breeding burrow, each female sequestered in a separate terminal chamber. The construction and defence of burrows specialized for breeding may be an adaptive response by males to the preferences females exhibit when selecting a breeding site.     

Social and maintenance behaviour in two Australian ocypodid crabs (Crustacea: Brachyura)

The feeding, burrowing and fighting behaviour and courtship display of the two ocypodids Heloecius cordiformis (H. Milne Edwards) and Hemiplax latifrons (Haswell) are described from daytime observations in the field over a two-year period and from four months study under laboratory conditions. The behaviour of the two species is compared with that of other ocypodid crabs. Heloecius cordiformis and Hemiplax latifrons live intertidally in estuaries in semi-permanent burrows and feed on detritus sifted from the mud and sand by the mouth-parts. The two species each possess considerably stereotyped patterns of behaviour, particularly in relation to fighting and courtship display in which the two differ most. The courtship display of Heloecius cordiformis closely resembles that of vertically waving, narrow-fronted species of fiddler crabs (genus Uca). The fighting behaviour of Hemiplax latifrons is similar to that of the New Zealand Hemiplax hirtipes (Jacquinot) which apparently lacks courtship display.     

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.     

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.     

Number of ghost crab burrows does not correspond to population size

Ghost crabs are distributed worldwide on sandy beaches, and several studies have associated the number of ghost crab burrows with the levels of anthropogenic impacts on the beaches under study. However, our results show that the use of ghost crab Ocypode quadrata burrows to assess levels of anthropogenic impacts on sand beaches may not be accurate, as previously thought, because the number of burrows does not represent an estimate of the population size. In addition, we propose three hypotheses to explain the extremely low number of individuals/number of burrows ratio: the “secret chamber”, the “multiple openings”, and the “one crab, several burrows” hypotheses. We also observed an unusual sex ratio.     

Parental care and reproductive behavior of the clown goby, <Emphasis Type="Italic">Microgobius gulosus</Emphasis>, with observations on predator interactions

Describe reproductive behavior and mating system of the clown goby from field observations. Clown gobies exhibit a loosely haremic mating system. Pairs construct burrows at the base of cattails, the roots of which provide structural support and a spawning substrate. Larger males monopolize multiple burrows, each with an individual female. After spawning, males camouflage burrow entrances with sand and females brood developing young for 4 days. Males continue to guard the covered nests in 50% of observed brooding periods. Burrows are also used as shelter from predators. Both sexes confront intruders but only males exhibit a distinct color response to juvenile blue crabs, Callinectes sapidus, the most significant predator. The male color response appeared to mimic the color of adult blue crabs, a known predator of juvenile crabs, perhaps acting as a deterrent. The presence of the predatory blue crab may require one parent to perform deterrent displays, promoting female care in this mating system.     

Heart rate and gill ventilation in ovigerous and non‐ovigerous edible crabs,Cancer pagurus: The effects of disturbance,substrate and starvation

This study compared the cardiac and ventilatory behaviour of disturbed and settled (48 h recovery) ovigerous Cancer pagurus with the behaviour of non‐ovigerous crabs. It also examined the effects of starvation and a sand substrate on ovigerous females. Ovigerous crabs had significantly lower heart rates than non‐ovigerous crabs. This implies that they have a reduced metabolic rate, confirmed by an earlier study which described lower rates of oxygen uptake in ovigerous crabs. Scaphognathite beat frequency was unaffected by either the presence of eggs, starvation or a sand substrate, but the amplitude of ventilatory pressure changes was higher in settled ovigerous females, implying greater force. The duration of periods of reversed ventilation was also substantially extended in ovigerous crabs, and disturbance caused a ten‐fold increase in the frequency of these reversals, resulting in ventilation being carried out in a predominantly reversed direction. It is suggested that these respiratory adaptations serve to ventilate the egg mass. Disturbance also led to a greater elevation of heart rate in ovigerous crabs, indicating that they are more prone to handling stress. Fed ovigerous crabs spent a higher percentage of time ventilating unilaterally; this response was lost following starvation. Starved ovigerous crabs had the lowest mean heart rate, highest mean ventilatory pressures and, when disturbed above a sand substrate, they showed the highest frequency of reversals of ventilation, possibly in association with their efforts to bury their abdomen bearing the egg mass.     

Sexual selection and the physiological consequences of habitat choice by a fiddler crab

In mid-Atlantic salt marshes, reproductively active male sand fiddler crabs, Uca pugilator, use a single greatly enlarged major claw as both a weapon to defend specialized breeding burrows from other males and an ornament to attract females for mating. During the summer breeding season, females strongly prefer to mate with males controlling burrows in open areas high on the shore. Food availability decreases while temperature and desiccation stress increase with increasing shore height, suggesting that the timing and location of fiddler crab mating activity may result in a potential trade-off between reproductive success and physiological condition for male crabs. We compared thermal preferences in laboratory choice experiments to body temperatures of models and living crabs in the field and found that from the perspective of a fiddler crab, the thermal environment of the mating area is quite harsh relative to other marsh microhabitats. High temperatures significantly constrained fiddler crab activity on the marsh surface, a disadvantage heightened by strongly reduced food availability in the breeding area. Nevertheless, when the chance of successfully acquiring a mate was high, males accepted a higher body temperature (and concomitantly higher metabolic and water loss rates) than when the chances of mating were low. Likewise, experimentally lowering costs by adding food and reducing thermal stress in situ increased fiddler crab waving display levels significantly. Our data suggest that fiddler crabs can mitigate potential life history trade-offs by tuning their behavior in response to the magnitude of both energetic and non-energetic costs and benefits.     

Pattern of Dispersion of Young Wild Rabbits,Oryctolagus cuniculus L., in Relation to Burrows

The development of dispersion in relation to burrows of young rabbits, Oryctolagus cuniculus L., was studied in a sand dune habitat between May and September 1984–1985. Generally, young rabbits did not show a close association with their original burrow. From the first week of life on the surface they used different burrows as well as the original one. No significant age-related changes in the mean distance from different kinds of burrows were observed. The mean distance from the nearest burrow remained always under 3 m, but this distance may have been due largely to the high density of burrows. The apparent freedom of movements of young rabbits around different burrows may be related to the social system of the adults in a sand dune habitat.     

Density affects mating mode and large male mating advantage in a fiddler crab

Fiddler crabs show two different mating modes: either females search and crabs mate underground in male burrows, or males search and crabs mate on the surface near female burrows. We explored the relationship between crab density, body size, the searching behavior of both sexes, and the occurrence of both mating modes in the fiddler crab Uca uruguayensis. We found that crabs change their mating mode depending on their size and crab density. Crabs mated mostly on the surface at low densities, and underground at high densities. The proportion of wandering receptive females but not courting males accounted for the variation in mating modes. This suggests that whether crabs mate underground (or on the surface) is determined by the presence (or absence) of searching females. We found that the change in the mating mode affected the level of assortative mating; males mating underground were bigger than those mating on the surface, suggesting active female choice. Given that fiddler crabs experience multiple reproductive cycles, they are prone to showing behavioral plasticity in their mating strategy whenever the payoffs of using different mating modes differ between reproductive events. Our results suggest that the incorporation of different levels of environmental variability may be important in theoretical models aimed at improving our understanding of the evolution of alternative mating tactics and strategies.     

The behaviour of Corophium volutator (Crustacea: Amphipoda)

The behaviour of Corophium volutator (Pallas) is outlined. Swimming, crawling, burrowing and feeding activities are described in detail. Animals usually swim on their backs. Every few seconds, swimming alternates with passive sinking. Animals can crawl over surfaces in and out of water. Out of water they do so by a looping motion using their second antennae and telson. When out of water animals crawl away from light and down slopes. In water they swim towards light. Burrowing is initiated by rapid beating of the pleopods; the animal then sinks below the surface by a concerted action of pereiopods, pleopods, telson, uropods and second antennae; within a few minutes, a shallow burrow is formed. The formation of permanent burrows is dependant on particle size, on adhesive properties of detritus and primary films on sand particles, and on a secretion produced by the animal itself. Individuals can turn about in permanent burrows. The species is essentially a detritus feeder. Animals normally feed only when in their burrows, by using their second antennae to scrape material from the substrate surface into the entrance of the burrow. This material is then transported to the mouth by the feeding appendages and respiratory current. The behaviour of small and large animals differs; small animals burrow rapidly and permanently, and do not emerge spontaneously; furthermore, they only swim occasionally. Large animals swim more frequently, spend more time on the substrate surface, and periodically move burrows. It is suggested that new habitats are colonized by large animals which have already bred once.     

Adaptive Visually-Directed Orientation in Uca pugilator

Sand fiddler crabs are ill-equipped to inhabit either the terrestrialor permanently submerged littoral zones. They are obligate inhabitantsof the intertidal region in which, at any point, extremes ofconditions continually fluctuate. The crabs must constantlymove about to specific areas in order to carry out life-supportingprocesses while avoiding detrimental physical conditons. Forthis reason, directional orientation is basic to their existence. The typical activities of adult sand fiddlers during diurnallow tides have charateristic directional components. Some directedmovements have immediate survival value under conditions ofstress, such as the approach of a predator. For example, crabson the lower beach (well away from their burrow area) oftenrun landward and enter burrows or vegetation, thereby obtainingrefuge. Those which are chased offshore or inland are able toreorient to the beach. Experiments conducted both in the field and under controlledconditions indicate that these adaptive oriented movements areguided primarily by visual mechanisms. Adult crabs exhibit atime-compensated, menotactic orientation to the sun and polarizedsky-light that enables them to maintain a heading coincidentwith the landward compass bearing of their particular shoreline.New directional preferences can be induced by holding crabsin simulated habitats with specific shore-water spatial configurations. The crabs also exhibit a telotaxis toward gross landmarks, suchas mangroves or clumps of beach grass, that stand in opticalcontrast to the background. This orientation occurs in mostindividuals if celestial cues are obscured or if the animalsbecome desiccated. Upon reaching the object, crabs enter suitableinterstices affording refuge. Since they can orient by eithercelestial cues or landmarks, or both, there are probably fewtimes in nature when they are disoriented from lack of guidance-stimuli.     

Size-dependent Mating Behaviours of Male Sand-bubbler Crab,Scopimera globosa: Alternative Tactics in the Life History

Factors leading to the separation of mating behaviours were investigated in the sand-bubbler crab, Scopimera globosa. The crabs mated on the surface (surface copulation, SC) and underground (UC). UC males were large (old) whilst SC males were small (young). Burrowless females bred in the UC males' burrows. These females accepted UC in exchange for access to a burrow. UC occurred much more frequently than SC in the burrow area in which females oviposited. Most SC occurred in the water-saturated area affording a rich diet. SC was accepted by most large and small females in both areas and most UC by small females in the burrow area. SC was an alternative to UC for males in that there was a size dependence between types of copulation. These two mating behaviours involved different degrees of interaction with neighbouring males. Males attempting to carry a female to their burrows for UC were more often disturbed by other males than were males attempting SC. In the interaction for both UC and SC, larger males were likely to resist the disturbances. UC males needed their own burrows, but these burrows were not enlarged before mating. UC males have a higher paternity of eggs than SC males, because SC males' sperm is often displaced by other males. Thus, UC was a behaviour with relatively higher costs and benefits for male crabs than SC behaviour. Alternative mating behaviours in male S. globosa are conditional, and explained by intrasexual interactions and a male life history strategy with a trade-off between growth and reproduction. It is not likely that the size dependence of male mating behaviour is caused by mate preference of females for UC males in the burrow area.     

The ectosymbiont crab Dissodactylus mellitae-sand dollar Mellita isometra relationship

The presence of the ectosymbiont Dissodactylus mellitae on Mellita isometra was studied at Tybee Island in Georgia. The number of crabs found on sand dollars, stage of maturity, sex, numbers of eggs, and size of eggs produced by crabs were noted. The test diameter of sand dollars, and the number and diameter of eggs produced were also noted. Infestation rates of crabs on sand dollars varied over time. The maximum number of crabs found on a sand dollar was 10. The three types of population dispersion, random, uniform, and clumped, were observed for crabs on sand dollars throughout the sampling period. Clumped or gregarious settling was observed when juvenile crabs were abundant, uniform and random distribution when mature crabs were abundant. Female crabs were significantly larger than male crabs, with carapace width of the largest mature female crab being 4.6 mm and the largest male 3.1 mm. Crabs produced between 80 and 300 eggs from 0.188 to 0.291 mm in diameter. Bigger crabs produced significantly more eggs than smaller crabs. Sand dollar sizes were 50-110 mm, with a mode between 60 and 70 mm. The effect of crab burden on egg production in the sand dollar was time-dependent. The presence of crabs on sand dollars correlated with total egg production of sand dollars in May, the peak of the spawning season, with sand dollars carrying one or two crabs having a lower egg production than those without crabs. Overall, variation in egg size was attributed to variation among females followed by variation between seasons and crab burden.     

The response of meiofauna and microphytobenthos to engineering effects of fiddler crabs on a subtropical intertidal sandflat

Fiddler crabs are key bioturbators on tidal flats. During their intense bioturbation process, they manipulate large amounts of sediment, altering the physical state of existing materials. We investigated whether different types of sediment bioturbation produced by fiddler crabs modulate meiofaunal assemblages and microphytobenthic content. We hypothesized that sedimentary structures produced by burrowing (the burrow itself and the excavation pellets) and feeding (feeding pellets) generate different microenvironments compared with areas without apparent signs of fiddler crab disturbance, affecting both meiofauna and microphytobenthos, independent of the sampling period. Our results indicate that the engineering effects of burrow construction and maintenance and the engineering effects of fiddler crab foraging modulate meiofaunal assemblages in different ways. Overall, meiofauna from burrows and excavation pellets was more abundant and diverse than at control sites, whereas feeding pellets contained poor meiofaunal assemblages. By contrast, only foraging effects were detected on microphytobenthos; independent of the sampling period, Chl a and phaeopigment content were higher in the feeding pellets, but similar among burrows, excavation pellets and control sites. The present study demonstrates that the different engineering effects of fiddler crabs are an important source of habitat heterogeneity and a structuring agent of meiofaunal assemblages on subtropical tidal flats.     

Orientation during short-range feeding in the crab Dotilla wichmanni

The ocypodid crab Dotilla wichmanni is a common inhabitant of tropical sandy shores, where it feeds at low tide by sorting the material deposited by the ebbing tide. Feeding occurs in the immediate vicinity of the burrow by means of systematically sampling the sand surface. While feeding, the crabs move along trenches radiating from the burrow and produce pseudofaecal pellets which are amassed over the already excavated area. When disturbed, the crabs rapidly vanish into the burrow, where they remain hidden for a while. Upon re-emerging, they recommence feeding moving along the same trench as before retreat. The crabs, however, were found to assume the same feeding direction held before retreating even if any sign that could be derived from their previous activity was removed experimentally. To uncover the orientation cues used in this behaviour, the area near the burrow was manipulated in particular ways. The crabs were found to rely mainly on the skylight polarization pattern, while visual landmarks near the feeding area may play a role when astronomic cues provide no useful information, such as under overcast skies. Both cues were used by the crabs as references to assume the same feeding direction as that used before retreat. Accepted: 9 June 1997