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     

Activity and feeding of Dotilla fenestrata (Brachyura: Ocypodidae) in a warm,temperate South African estuary

The activity of and consumption of organic material by the sand-bubbler crab Dotilla fenestrata was studied over neap and spring tides on a sheltered sand bank close to the mouth of the warm, temperate Kowie Estuary, South Africa. Crabs emerged from their burrows only after the tide receded, and it was light. Time to emergence therefore varied from about 30 minutes to three hours after exposure, depending on the time of low water in the early morning vs at midday. General activity of the crab population was longer on a spring tide (about five hours) than on a neap tide (about three hours). Maximum densities of active crabs were 140 m^–2 and 41 m^–2 on spring and neap tides, respectively. After emergence, crabs spent 60% to 80% of their time feeding. In the Kowie Estuary, D. fenestrata produced between 7 and 12 pseudofaecal pellets, average weight 0.0358 g per pellet, per minute. These pellets had a significantly lower organic and chlorophyll a content than the substratum, and it was estimated that crabs removed about 25% of the organic content from the sediment.     

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.     

Sex-specific mediation of foraging in the shore crab, Carcinus maenas

Experiments were conducted to investigate the sex-specific differences to feeding responses of the shore crab Carcinus maenas throughout the year. Results demonstrate that female shore crabs exhibit stronger feeding responses than males throughout the year with a significantly reduced feeding response in males during the summer months' reproductive season. We also studied the possible function(s) of the moulting hormone, 20-hydroxyecdysone (Crustecdysone) that has been described as a potential female-produced sex pheromone to initiate male reproductive behaviour in a number of crustaceans. We recently presented evidence that for shore crabs this is not the case and now show that the steroid is instead functioning as a sex-specific feeding deterrent protecting the moulting 'soft' female crabs. Whilst male shore crabs were deterred from prey (Mytilus edulis) and synthetic feeding stimulants glycine and taurine when these feeding stimulants were spiked with crustecdysone, intermoult female crabs were significantly less affected and rarely deterred from feeding. This sex specificity of the moulting hormone, in combination with the female sex pheromone, which has no anti-feeding properties, ensures that male crabs mate with soft-shelled, moulted females rather than engage in cannibalism, such as found frequently in cases when soft-shelled females are exposed to intermoult females.     

Effects of traditional clam harvesting on the foraging ecology of migrating curlews (Numenius arquata)

One of the main conservation priorities in many geographical regions for intertidal estuaries is to protect and maintain internationally important numbers of migratory shorebirds (Charadrii). Traditional harvesting practices at low tide cause alterations to the surface of the intertidal habitat by trampling and digging the substrate to obtain benthic resources, which may have negative effects on shorebirds. Here, we conducted a BACI (Before-After Control-Impact) experiment to monitor changes in foraging ecology of Eurasian curlews Numenius arquata, before and after an experimental disturbance of the intertidal mud in a key stopover area for migratory shorebirds. Mean density of curlews, foraging activity, feeding rate, percentage of crabs in the diet, size of crabs, and foraging speed differed neither between treatment and control plots, nor between before and after mud disturbance. The feeding technique was similar between treatment and control plots, but differed significantly between periods. These findings suggest that mud disturbance by harvesters working by hand did not affect curlew fitness by altering the energy deposition necessary for long-distance migration. Although conclusions must be extrapolated with caution to other sites and/or species, the Santoña Marshes Natural Park may provide a potentially good example of compatibility between shorebird conservation and traditional low tide harvesting practices.     

Antipredator behaviour mediated by chemical cues: the role of conspecific alarm signalling and predator labelling in the avoidance response of a marine gastropod

Gastropods represent a challenge in the understanding of alarm signalling. We studied predator avoidance (climbing behaviour) of the marine snail Tegula funebralis in laboratory experiments. Snails were exposed to crude extract of conspecifics, and to water conditioned by actively feeding or non-feeding predatory crabs. Crabs had previously been maintained on different diets, and were accordingly labelled by chemical cues of various origins. Tegula -extract alone released climbing behaviour in May, but not in June. However, during both these months, snails responded to chemical cues from crabs that were actively feeding on Tegula . Crabs labelled by Tegula -diet, and actively feeding on Tegula , also caused more climbing responses compared to crabs labelled by other diets. Chemical cues derived from crabs actively feeding on another snail species, or from non-feeding crabs, did not induce snail climbing no matter the previous feeding history of the predators. When snails received Tegula -extract combined with water conditioned with a non-feeding, Tegula -labelled crab, no climbing occurred. However, when the non-feeding, Tegula -labelled crab was present in the solution of Tegula -extract, moderate climbing responses were obtained. The results imply that climbing responses of T. funebralis are in general caused by the action of a two-component system. This system seems to be a mixture of chemical cues leaking from the tissue of conspecifics when being eaten, and latent conspecific chemicals that are modified in crabs and presumably released with the urine of chemically labelled predators. The modified chemical labels appear to be fully released by crabs when feeding, and moderately released when detecting food. The responses obtained in May with crude extract alone may result from a seasonal change in alarm signalling properties, or a change in behavioural responsiveness of snails exposed to a variable predator regime.     

Entrainment of the activity rhythm of the mole crab Emerita talpoida

The mole crab Emerita talpoida migrates with the tide in the swash zone of sand beaches. A circatidal rhythm in vertical swimming underlies movement, in which mature male crabs show peak swimming activity 1-2 h after the time of high tides at the collection site. In addition, there is a secondary rhythm in activity amplitude, in which crabs are maximally active following low amplitude high tides and minimally active following high amplitude high tides. The present study determined the phase response relationship for entrainment of the circatidal rhythm with mechanical agitation and whether the cycle in activity related to tidal amplitude could be entrained by a cycle in the duration of mechanical agitation at the times of consecutive high tides. After entrainment with mechanical agitation on an orbital shaker, activity of individual crabs was monitored in constant conditions with a video system and quantified as the number of ascents from the sand each 0.5 h. Mechanical agitation at the times of high tide, mid-ebb and low tide reset the timing of the circatidal rhythm according to the timing relationship to high tide. However, mechanical agitation during flood tide had no entrainment effect. In addition, a cycle in duration of mechanical agitation entrained the rhythm in activity amplitude associated with tidal amplitude. Both rhythms and entrainment effectiveness over the tidal cycle may function to reduce the likelihood of stranding above the swash zone.     

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.     

Diet and some ecological features of the most widespread commercial crab species in the northwestern Sea of Japan in early spring

Results of the study of feeding habits of the commercial crab species Chionoecetes opilio, Ch. japonicus, Paralithodes platypus, P. camtschaticus, and Erimacrus isenbeckii in early spring 2009 are presented. The composition, distribution, and quantitative characteristics of benthos in the areas of sampling are analyzed. The generalized pattern of distribution of the considered crabs is shown with the maximum and mean densities. These crabs were found to consume at least three to four single portions of food during 10–12 hours of daylight. The lowest feeding intensity was observed in the opilio snow crab in March and April. Cannibalism proved to be typical for all the studied crab species in the spring. It was most developed in the deepwater red snow crab, in whose diet representatives of the same species constituted a one-third share. It was also noted that the diet of the studied crabs included a major proportion of crabs and shrimps, which was unusual for the feeding habits of the same species in other Far Eastern seas.     

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.     

Behavior of Four Species of Fiddler Crabs,Genus Uca,in Southeast Sulawesi,Indonesia

We studied the behaviors of four species of sympatric fiddler crabs on Kaledupa Island, Indonesia. Species differences in activity level, grooming, burrowing and feeding were related to their habitat and food. Uca chlorophthalmus, living in muddy mangrove areas, were inactive and spent most of the time feeding in place. Females fed 50% faster than males and spent more time feeding. U. vocans was the dominant species at the beach in silty sand and was very active. Its feeding rate was about twice that of the former species, females fed more rapidly than males, and many crabs of both sexes fed in droves at the water’s edge during ebb tides. During ebb tides, they spent most of their time feeding, while at flood tide they engaged in a greater variety of activities, including burrow maintenance. They frequently walked while feeding and interacted aggressively. U. tetragonon lived in a pebbly band along one edge of the beach, by a quay. Their feeding rate was comparable in both sexes and slower than that of U. vocans; they fed largely on filamentous algae growing on the quay, which provides better food, and fed faster during flood tide than ebb tide. They spent more time in waving and other sex-related activities, and were seldom aggressive, except during the week of the full moon. Burrowing activities included placing excavated mud balls some distance away from their burrows and re-arranging them. U. dussumieri inhabited the other end of the beach in muddier substrate. They did not have sex differences in feeding rates and their rate of scooping food into their mouths was slow, but feeding claws made multiple pinches of the substrate, thus accumulating more material in each clawful of food.     

Dusk-copulation of the rock-dwelling ocypodid,Ilyoplax integra (Brachyura)

Pre-copulatory behaviour and surface copulation of the rock-dwelling ocypodid crabIlyoplax integra was studied from May 1989 to June 1990, at Horikawa, Okinawa Island, southern Japan. A male crab would grasp a female near the entrance to her burrow and copulatein situ. Copulation lasted 12.7 minutes on average. Surface copulations were frequently observed during neap tide days, especially in the first quarters during May and June. On these days copulations were quite frequent in the dusk period, during the 1.5 h around sunset. Surface copulation has not been described in otherIlyoplax species, therefore the sexual behaviour ofI. integra is unique within the genus. On rocks, where the number of burrows is in shortage, surface copulation in which a male can mate without evacuating his burrow to the female may be adaptive. Two possibilities were suggested to explain the high frequency of surface copulation during dusk hours; 1) to reduce the risk of predation by plovers and 2) to avoid exposing crabs to higher temperature and stronger radiation in daytime hours during surface copulation.     

Branch-fidelity in the tree crab Sesarma leptosoma (Decapoda, Grapsidae)

Sesarma lepzosoma is the only species of crab inhabiting the mangrove swamps of East Africa which can climb to the top of tall mature trees, where it feeds on fresh leaves. Twice daily, in the morning and evening, this crab performs mass migrations towards the canopy and spends about three hours on the tree-tops. At dusk it descends to spend the night among the aerial roots of the tree. Two observation stations were built on a Rhizophora mucronata tree, up to 13 metres above ground level, to observe and record the feeding and spatial aspects of the behaviour of this species during its time in the canopy. Observations on the daily migrations of marked specimens showed that the crabs are faithful to a well-defined feeding site; moreover, the migration paths of the marked specimens were exceptionally constant over the whole of the two-month study period. S. leptosoma was observed to feed on mature fresh leaves by scraping the lower surface of the leaf with its chelae. It was also found to spend most of its time on leaf-buds in a particular stage of development. Water which accumulates among the scales covering these buds represents an important water supply for these crabs, which they prestmably use to reduce physiological stress due to water loss. Feeding-site fidelity in S. leptosoma seems to be related more to the chance of finding suitable leaf-buds, which are relatively rare and patchily distributed, than finding leaves, which are abundant and uniformly distributed all over the canopy.     

Short-range Homing in Fiddler Crabs (Ocypodidae, Genus Uca): A Homing Mechanism not Based on Local Visual Landmarks

This field study investigated the spatial strategies and homing ability of two East African fiddler crabs, Uca lactea annulipes and Uca vocans hesperiae, using various experimental procedures. A clear-cut spatial segregation between the two species was observed, with U. l. annulipes occupying areas even a few centimetres above U. v. hesperiae . Females of both species were extremely faithful to their holes while the males exhibited behavioural differences. U. l. annulipes males were quite faithful to their holes; in contrast, U. v. hesperiae males, during a single low tide, visited several holes which they did not defend. Two displacement experiments were conducted in order to identify the stimuli that fiddler crabs use in their homing. In the 'dislocation experiment', the crabs were moved (by hand) 1 or 2 m away from their refuge and released. None of these crabs returned to its burrow and, moreover, the initial directions they followed were not homeward orientated. In the 'translation experiment', the crabs were passively translated while actively feeding. When the crabs attempted to go 'home' after the translation, they darted along a straight path that led them to points that would have corresponded to their homes if the translation had not taken place. After this first response crabs then commenced searching strategies that led half of them home. Short-range homing seems to depend strongly on information actively gathered during the outward journey, other than local visual landmarks.     

Behavioural and ecological investigations of the crabOcypode ceratophthalmus (Crustacea: Ocypodidae)

Observations conducted at different places along the Moçambique coast indicate that the behaviour of Ocypode ceratophthalmus is extremely plastic, permitting a high degree of adaptation to the often vastly different shores on which they are found. When the crabs are nocturnal and when, as is usual, the burrows are intertidally placed, the greater part of the crab's life is spent beneath the sand and the periods of activity are much influenced by the tidal cycles. Apart from feeding on items of prey conveyed into the burrows and stridulating when disturbed there is no evidence that they are anything but inactive when underground. On emergence from the sand the sole major activities observed were searching for food and the digging and clearing of burrows. They are very aggressive and although cannibalism is common, conflicts between equal sized crabs are generally ritualized. Social behaviour appears limited and, except for protest stridulation and ritualized posturing during intraspecific encounters, none was observed.     

Ecological consequences of ontogenetic shifts in predator diet: Seasonal constraint of a behaviorally mediated indirect interaction

Predators play an important role in structuring assemblages through direct and cascading indirect effects. While there has been recent interest in how the strength and direction of trophic cascades vary spatially, seasonal variability in trophic links is seldom considered. In North Carolina, recruitment-failure of bay scallops typically occurs following the spring but not the fall spawning despite the presence in each of these seasons of predatory blue crabs. One explanation for this pattern is that in the fall, seasonally abundant predators of blue crabs reduce the foraging efficiency of crabs on scallops and thus the overall magnitude of top-down effects. Quantification of bay scallop consumption by blue crabs in closed mesocosms with or without pinfish supported the hypothesis that seasonally abundant adult pinfish indirectly increase survivorship of bay scallop recruits in fall by reducing predation by blue crabs. Despite voracious consumption of bay scallops during both the day and night in mesocosms to which only small blue crabs were added, blue crabs in mesocosms with visually-foraging adult pinfish consumed bay scallops only by night. Juvenile pinfish that dominate estuarine populations in spring did not impede consumption of bay scallops by blue crabs. In mesocosms from which animals could not emigrate, the addition of neither adult nor juvenile pinfish increased the mortality of blue crabs, indicating a behaviorally mediated interaction. Blue crabs restricted by adult pinfish to nocturnal feeding did not compensate for lost feeding time by increasing their night-time consumption of bay scallops. These results strongly suggest that greater survivorship of bay scallops in fall than spring is due to adult pinfish, potential predators of small blue crabs, restricting blue crab foraging to hours of dark. In spring, when pinfish are small and incapable of consuming blue crabs, blue crabs consume bay scallops by day and by night. Such seasonal variation in the number of trophic links in a system may have important evolutionary implications. By timing reproduction to occur in fall when the pinfish-crab-scallop cascade is in operation, bay scallops maximize recruitment.     

Responses of benthic scavengers to fishing disturbance by towed gears in different habitats

The aggregation and feeding behaviour of invertebrate scavengers in areas disturbed by trawling was investigated at three different localities. At each site a fishing disturbance was created using a commercial 4 m beam trawl and scavenger density was quantified using a light beam trawl. At one site two diver surveys were also carried out; along a line fished with a scallop dredge or a beam trawl on two separate occasions. For all experiments the fished and adjacent unfished control areas were sampled before, and at intervals after, the initial fishing disturbance. Sampling with the light beam trawl revealed that hermit crabs Pagurus bernhardus moved into areas which had been fished with a 4 m beam trawl at an experimental site near Anglesey. The density of these hermit crabs increased significantly in the fished area after fishing had taken place, but no change in density occurred in the adjacent control (unfished) area. At two other sites (Red Wharf Bay, Anglesey and a site offshore from Walney Island) there were no detectable increases in scavenger numbers in the fished areas. Furthermore, at the site near Walney Island, numbers of hermit crabs P. bernhardus, swimming crabs Liocarcinus depurator and starfish Asterias rubens actually decreased after fishing. Thus the responses of scavengers to towed fishing gears varied considerably between different communities. At Red Wharf Bay, divers observed similar responses of scavengers to both beam trawl and scallop dredge disturbance. Four predatory species were observed feeding in the fished area; starfish A. rubens, hermit crabs P. bernhardus, brittlestars Ophiura ophiura and whelks Buccinum undatum. These predators fed on damaged bivalves, echinoderms, crustaceans, whelks and polychaetes. The proportion of starfish feeding in the fished area was significantly higher after fishing had taken place. Demersal fishing activities provide food for scavengers in the form of damaged animals which are left in the tracks of the trawl or dredge. The responses of scavengers to fishing disturbance are not always manifested as a large increase in their abundance. It is clear that the magnitude of response varies between species and between habitat types.     

Nocturnal feeding by Dark-bellied Brent Geese Branta bernicla bernicla

The occurrence and extent of nighttime feeding by Dark-bellied Brent Geese Branta bernicla bernicla on salt marshes adjacent to their tidal roost site were assessed on the north Norfolk coast using position-sensitive radio transmitters. Nine birds were monitored on 143 bird-nights, and feeding was recorded on 87. The frequency of feeding by night varied widely between individuals. The mean proportion of the night spent feeding for one bird was 19.7 ± 8%. Of this, 59% occurred within ± 1.5 h of high tide. Neither the proportion of the whole night spent feeding nor the intensity of feeding around high tide varied with the length of the night or with time after sunset at which high tide occurred. The extent of nocturnal feeding was also independent of the brightness and duration of moonlight. The birds fed for significantly longer on colder nights, and there was a significant positive relationship between the proportion of the night spent feeding and maximum temperature the preceding day. Analysis of daytime activity budgets on inland pastures showed that the proportion of time engaged in nonfeeding, energy-expensive activities was also positively related to maximum daytime temperature. Increases in the proportion of the night spent feeding after warmer days were achieved by increasing the period of time spent feeding on either side of the high tide, not by increasing feeding intensity at high tide. It is concluded that the geese compensate for increased energy expenditure during the day by increasing the extent to which they feed at night. Implications of increasing energy expenditure as a result of disturbance during the day are discussed.     

Winter feeding strategies of individually marked herons

The winter feeding behaviour of individually marked herons was studied in an estuarine habitat with adjacent streams in north-east Scotland. Feeding rates in the estuary were highest around low tide, and no feeding took place 3 h before and after low tide. Feeding rates at streams were significantly lower than in the estuary, but were constant throughout the day.Individual herons fed consistently with one of the following strategies: (1) feeding in a stream only: (2) feeding in the estuary only; or (3) switching between stream and estuary. The feeding rate was measured as a basis for a comparison of the benefits of alternative feeding strategies. A comparison between the three feeding strategies showed that herons that switched between estuary and streams achieved a significantly higher food intake than birds that used a stream feeding site only. The timing of arrival on the estuary and departure from the estuary to the streams was consistent with an energy maximization hypothesis. In midwinter, switchers were significantly heavier than non-switchers.     

Crab: snail size-structured interactions and salt marsh predation gradients

We studied size-structured predator-prey interactions between blue crabs (Callinectes sapidus) and marsh periwinkles (Littoraria irrorata) with a combination of field studies, laboratory experiments and individual-based modeling. Size distributions of Littoraria differed among years at the same sites in a salt marsh and could largely be explained by dominance of strong cohorts in the population. At a given site, abundance increased with elevation above tidal datum. Size-selective predation by blue crabs does not appear to be an important regulator of snail size distributions but may have a major effect on local abundance. Laboratory studies indicated that predator-prey interactions between Callinectes and Littoraria are strongly size-dependent. Crabs were generally effective at feeding on periwinkles at size ratios greater than approximately 6 (crab width: snail length). At lower size ratios crabs were far less effective at manipulating the snails, which often survived but with damaged shells. An individual-based model which incorporated information about incidence of snail shell scarring (resulting from non-lethal interactions) and snail density, predicted reduced predation rates and smaller average crab size with distance from the low tide refugium for crabs.