Morphological and behavioural adaptations to the rocky substrate by the fiddler crab Uca panamensis (Stimpson, 1859): preference for feeding substratum and feeding mechanism

The fiddler crab Uca panamensis (Stimpson, 1959) inhabits rocky shores. We examined its preference for feeding substratum—sand or rock—and its manner of feeding. The crab made its burrow in the sand among rocks but preferred to feed on rocks. The feeding time decreased as the distance between the burrow and the rock increased. We consider this to be a result of exclusive interaction among the crabs because they defended their feeding area on the rocks against others.The crab wetted a small area of rock with water held in the branchial chambers before and during feeding. It pinched up the wetted surface in the minor chelipeds, which have bundles of setae on the posterior tips of the dactyl and pollex, and put the material into its buccal cavity. It never expelled sand pellets while feeding on rock, which indicates that it swallowed the food particles directly, without sorting. The bundles of setae retained water by capillary attraction, which suggests that they capture the suspended fine food particles scraped from the rock. The wetting action may prevent the fine materials from dispersing. We consider that morphological alteration of the minor chelipeds, the application of water from the branchial chambers, and direct swallowing permit the fiddler crab to feed on fine materials attached to rocks.     

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.     

Substratum preference of the tropical estuarine crabs,Uca tangeri Eydoux (Ocypodidae) and Ocypode cursor Linne (Ocypodidae)

The vertical and horizontal distribution of two burrowing mud crabs, (Ocypodidae) and Ocypode cursor Linne 1758 (Ocypodidae) are described for the Bonny Estuary (7° 00' E: 4° 20′ N), S. Nigeria. Substratum preference is the most prominent factor influencing distribution, but lack of tolerance to low salinities (< 6%_o) is also important, and prevents Ocypode cursor from occurring close to freshwater. The two species have slightly different sediment organic content and grade size requirements. Ocypode cursor was concentrated in well drained sandy sediment above mid-tide-level, whilst Uca tangeri was found in water-logged areas slightly above and below Mid-tide-level. This vertical demarcation is attributed to differential feeding and burrowing adaptations related to different substrata, rather than to differential tolerances to desiccation.     

Localization of substance P-like,leucine-enkephalin-like,methionine-enkephalin-like,and FMRFamide-like immunoreactivity in the eyestalk of the fiddler crab,Uca pugilator

Summary The occurrence and distribution of substance P (SP)-like, methionine-(Met)- and leucine-(Leu)-enkephalinlike, and FMRFamide-like immunoreactivities were determined in the neuroendocrine complex of the eyestalk of the fiddler crab, Uca pugilator, by immunocytochemistry. SP-like immunoreactivity was found in the optic peduncle, sinus gland, medulla externa, medulla interna, lamina ganglionaris, and retinular cells. Met-enkephalin-like and Leuenkephalin-like immunoreactivity was observed in most of the retinular cells, optic peduncle, sinus gland, medulla terminalis, and lamina ganglionaris. However, Met-enkephalin-like, but no Leu-enkephalin-like, immunoreactivity was seen in the medulla terminalis X-organ. FMRFamide-like immunoreactivity could be seen in all parts of the eyestalk except in the sinus gland, lamina ganglionaris, and retinular cells. FMRF-amide-like activity was especially strong in the three chiasmatic regions connecting the optic ganglia. The possibility that these four peptides may function as neuroregulators in the fiddler crab is discussed.This investigation was supported in part by Grant No. PCM-8300064 from the National Science Foundation to MF and Biomedical Research Support Grant No. 2 SO7RRO5373 SUB from the University of Kansas Medical Center to LLV     

Morphological diversity of setae on the second maxilliped of fiddler crabs (Decapoda: Ocypodidae) from the southwestern Atlantic coast

Fiddler crabs are deposit feeders, and use the setae on their mouth appendages to manipulate sediment particles to extract food. The number of spoon‐tipped setae on the second maxilliped is frequently related to the distribution of fiddler crabs on estuarine sediments, but no study has compared the morphological diversity of these setae among multiple fiddler crab species. Here, we describe and classify the setae of the second maxillipeds of the nine Uca spp. known from the Brazilian coast. The second maxilliped of each species was examined by scanning electron microscopy. Six types of setae (five papposerrate, and one pappose) were described on the meropodite of the second maxilliped. Among the papposerrate setae, one type had a spoon‐like tip, and the morphology of this type, especially the degree of curvature, differed between species. Members of Uca leptodactylus, U. uruguayensis, and U. maracoani had highly concave spoon‐tipped setae. In U. rapax and U. cumulanta, the setal tip was moderately curved, while in U. thayeri, U. burgersi, and U. mordax, this curvature was slight. At the other extreme, the meropodite of the second maxilliped of U. vocator lacked setae altogether. This is the first study that describes differences in the degree of curvature of spoon‐tipped setae in fiddler crabs. This trait may be strongly related to the distribution of these fiddler crabs on different estuarine substrates.     

Morphology and function of the reproductive system of representatives of the genus Uca

The morphology of the reproductive organs of three species of fiddler crabs, Uca ecuadoriensis, Uca c.f. forcipata, and Uca tangeri were investigated to subsequently produce a model of their mode of operation. Vulva, vagina, and spermatheca in females, and the first and second gonopods in males were examined by applying histological techniques and electron microscopy. In all three species, vulva and vagina conform to the concave type, and the spermatheca complies with the ventral type. The tissue of the oviduct orifice is enlarged and bulges into the lumen of the spermatheca. Differences between the three species are apparent in the organization of the spermatheca, especially in the distribution and structure of glandular epithelium: In U. ecuadoriensis and U. c.f. forcipata the largest proportion of the spermathecal wall is lined with cuticle and only a small area consists of glandular epithelium, while in U. tangeri almost all of the lining is glandular. Furthermore, the glandular epithelia of the species differ in their histology and ultrastructure: In U. ecuadoriensis it is tubular and multilayered, while in U. c.f. forcipata it is mono‐layered. U. tangeri finally has both forms of this tissue. In the males, the terminal segments of the first gonopod exhibit a tight fit to female organs and narrow, tightly sealed sperm channels. These features suggest a tendency towards minimizing loss of fluids, which can be interpreted as an adaptation to mating on land. The tight fit of male gonopod and female opening seem to be protection from interbreeding, which points toward a strong sexual selection. In the terrestrial environment, these originally aquatic organisms experience serious competition for resources; therefore there is pressure on successful reproduction. According to the current results a model of the process of fertilization and egg‐laying involving the investigated organs was generated. J. Morphol., 2010. © 2010 Wiley‐Liss, Inc.     

Bioecology of the fiddler crab <Emphasis Type="Italic">Uca uruguayensis</Emphasis> and the burrowing crab <Emphasis Type="Italic">Chasmagnathus granulatus</Emphasis> (Decapoda,Brachyura) in the Refugio de Vida Silvestre Bahía Samborombón,Argentina

The aim of the present work conducted at the Refugio de Vida Silvestre Bahía Samborombón, Argentina, is to analyse the most relevant aspects of the life cycles of Chasmagnathus granulatus and Uca uruguayensis: their abundance, size distribution and sex ratio in order to be used for make management purposes. A total of 1200 individuals of U. uruguayensis (megalopae, juveniles and adults) and 957 individuals of C. granulatus (juveniles and adults) were collected from March 2001 to February 2003. U.␣uruguayensis had a maximun density of 42 ind./m² in February 2003, while the maximun density of C.␣granulatus was 52 ind./m² in June 2001. Minimun density was zero individual for both species in July 2002. Each environmental factor sampled (temperature of water, salinity and pH) was correlated with the abundance of each species by Pearson’s linear correlation analysis. Sex ratio did not significantly differ from the expected Mendelian ratio, except for U. uruguayensis in December 2002 (male-biassed) and C.␣granulatus in June and December 2001, April 2002 and February 2003 (female-biassed). Ovigerous females of U. uruguayensis and C. granulatus were found during the summer.     

Stereotypy and variation in the claw waving display of the fiddler crab <Emphasis Type="Italic">Uca tangeri</Emphasis>

We measured temporal and spatial components of the waving display in a Uca tangeri population to look for inter-individual differences in male waving structure that may convey information about individual identity. We found evidence that the spatial components of wave structure, especially “Maximum amplitude” are responsible for most of the between-male variation of the display. This variation could reflect differences in individuals’ condition and/or could be used by conspecifics to discriminate amongst familiar and unfamiliar individuals.     

A comparison of alternative methods for estimating population density of the fiddler crab Uca annulipes at Saco Mangrove, Inhaca Island (Mozambique)

Visual counts of surface-active crabs both by binocular and burrow counting methods have been used in many studies to estimate population density. However, their reliability has not yet been assessed comparatively. Three methods for estimating the abundance of fiddler crabs Uca annulipes in a mangrove forest (Inhaca Island, Mozambique) were compared from three different sub-areas: two sub-areas inundated only during spring tides and one sub-area inundated in both spring and neap tides. Burrow, binocular and direct (excavation) counting methods were performed by plotting ten 0.25 m² quadrats in each sub-area over the four moon phases. Overall densities (per 0.25 m²) differed according to method, sub-area and lunar phase. Burrow count overestimated crab density by up to 20%, while binocular count underestimated density by up to 41%. Correlation coefficient estimated for both counting methods showed that burrow count gives better density estimates than binocular count (0.91 and 0.56, respectively). Sex ratios were also investigated within the three sub-areas and at the moon phases. Males are dominant throughout the studied period except during new moon and first quarter, indicating that when the number of gravid females is low, sex ratio bias for binocular count is minimal.     

Impact scenario for the invasive red king crab <Emphasis Type="Italic">Paralithodes camtschaticus</Emphasis> (Tilesius, 1815) (Reptantia,Lithodidae) on Norwegian,native, epibenthic prey

Large invasive predators like the king crab, Paralithodes camtschaticus, deserve particular attention due to their potential for catastrophic ecological impact on recipient communities. Conspicuous, epibenthic prey species, such as the slow growing commercial scallop Chlamys islandica, are particularly exposed to the risk of local extinction. A research program integrating experiments and field monitoring is attempting to predict and track the impact of invasive king crab on scallop beds and associated fauna along the north Norwegian coast. The claw gape of the crab shows no limitations in handling the flat-bodied scallop. However, the potential impact of the crab on scallop may depend on the availability of other calcified prey associated with scallop beds, such as the sea star, sea urchin, and blue mussel, all species recorded in the diet of P. camtschaticus. To address this issue, a laboratory experiment on foraging behaviour of P. camtschaticus was conducted. The experimental results show that all size classes of red king crab prefer scallops, but small juveniles and medium sized crabs demonstrate active selection for starfish (Asterias rubens) that equals or surpasses the electivity of the large crab. The selection of sea urchin (Strongylocentrotus droebachiensis) and blue mussel (Mytilus edulis) is slightly positive or neutral for the three crab size classes. These results suggest that scallop beds with a rich associated fauna are less vulnerable to red king crabs predation and possibly more resilient than beds with few associated species. Also, crab size distribution is likely relevant for invasion impact, with increasing abundance of small and medium sized crabs being detrimental for alternative calcified prey associated with scallop beds. Successive stages of crab invasion will see an acceleration of scallop mortality rates associated with (i) decreasing availability of alternative prey, due to protracted predation pressure intensified by recruitment of juvenile crabs, and (ii) increased number of large crabs. Estimates of crab density and intake rates suggest that the accelerated loss rates will eventually endanger scallop beds persistence.     

Microgeographic genetic structure of the fidller crab,Uca arcuata De Haan (Ocypodidae) in Taiwan

The genetic structure of Uca arcuata in Tanshui mangrove swamp of northern Taiwan was examined. Using as genetic markers, isozymes identified through starch gel electrophoresis indicate that there was moderate genetic differentiation among subpopulations within the population (FST = .085). Gene flow appeared high when estimated indirectly (Nm = 2.69). The results suggest that the patterns of genetic structure of Uca arcuata were influenced by the interaction of local selection due to microhabitat differences and gene flow among fiddler crab colonies in the mangrove swamp.     

Burrowing behavior and ecology of the crab-eating Indian snake eel Pisoodonophis boro

Synopsis The snake eel Pisoodonophis boro burrows, causing leaks in the embankments and damaging the paddy fields and salt pans near estuaries. Field observations and laboratory experiments were made to study this behavior. P. boro was burrowing to eat the fiddler crab Uca annulipes in the mud flats. The eel showed a patchy distribution within the Uca zone. Salinity and the physical nature of the deposits controlled the distribution of the eel. Eel population density was low when the estuary was completely filled with neritic waters during the summer and fresh water during the monsoon period. The region of greatest abundance contained a good mixture of sand, silt and clay. Eels were not found where medium and fine sand formed the bulk of the substratum. The laboratory experiments showed that P. boro preferred loam soil although it could invariably burrow into hard substratum like sand for protection. The eel adapted itself to the experimental substrates ranging from sand to fine clay. However, their natural distribution was determined by Uca distribution. As U. annulipes is not found either in salt pans or in paddy fields P. boro rarely occurs in these habitats.     

A phylogeny of Pacific fiddler crabs of the subgenus Minuca (Crustacea,Brachyura, Ocypodidae: Uca) with the description of a new species from a tropical gulf in Pacific Costa Rica

The phylogeny of all Pacific fiddler crab representatives of the subgenus Minuca Bott, 1954 (sensu Beinlich and von Hagen, 2006) is reconstructed. For the molecular analysis, Cox1 mitochondrial and 28S ribosomal nuclear DNA sequences were used. According to these data, same transisthmian sister species relationships are confirmed and a new species of the genus Uca Leach, 1814, Uca osa sp. n., is described from Golfo Dulce, a tropical gulf in Pacific Costa Rica. Morphological as well as molecular data confirm distinctness of this species compared with all other members of the subgenus Minuca, to which it belongs. Distinctive morphological traits are presented to distinguish Uca osa sp. n. from its congeners in the Eastern Pacific.     

Predation by red-jointed fiddler crabs on congeners: interaction between body size and positive allometry of the sexually selected claw

The enlarged (major) claw of male fiddler crabs is used in contestsover breeding burrows and is waved to attract females. We recentlydiscovered that males of the red-jointed fiddler crab, Uca minax,also use the claw to kill smaller-sized fiddler crabs, U. pugnaxand U. pugilator, with which they co-occur in Atlantic coastsalt marshes. Large U. minax males use walking legs or the enlargedclaw to capture prey feeding on moist sand flats. On sand flats,small U. minax males and females are much less common than largemales, suggesting that large males move onto sand flats to seekprey. Males of prey species use the major claw against attackingpredators and, consequently, are more likely than females toescape. In laboratory experiments, large U. minax males weremore likely to attack and kill small-clawed males and femalesthan large-clawed males, consistent with a preference for morevulnerable, less threatening prey. The size of the major clawis a positive allometric function of body size. The allometricfunction varies little among species. Also, the mechanical advantageand indices of closing speed and closing force of the majorclaw, when corrected for body size, are not consistently greaterin U. minax relative to prey species. Thus, predation by U.minax males may reflect the opportunity afforded by larger bodysize and positive allometric growth, which result in a majorclaw that is more massive than the prey it is directed against.     

Population and reproductive biology of the fiddler crab Uca thayeri Rathbun, 1900 (Crustacea: Ocypodidae) in a tropical mangrove from Northeast Brazil

Population and reproductive biology of Uca thayeri Rathbun, 1900 were studied for the first time in a tropical mangrove. Absolute density, sex ratio, population structure, handedness, breeding season and fecundity were investigated. Seven transects were delimited in a mangrove area of the Pacoti River, Northeast of Brazil (3° 43′ 02″ S/38° 32′ 35″ W). On each transect, ten 0.25 m² squares were sampled on a monthly basis during low tide periods from September 2003 to August 2004. A total of 483 crabs were obtained, of which 250 were males, 219 non-ovigerous females, and 14 ovigerous females. The U. thayeri population presented bi-modal size frequency distribution, with males and non-ovigerous females not differing significantly size-wise. Ovigerous females were larger than males and non-ovigerous females. The overall sex ratio (1:1.07) did not differ significantly from the expected 1:1 proportion. The major cheliped was the right one in 50% of the males. The observed density was of 8.5 individuals/m², with the specimens being found mostly in shaded areas. Ovigerous females were found in 5 months of the year, coinciding with the rainy season, suggesting that the population of U. thayeri presents seasonal reproductive events. Juvenile crabs were more abundant during the dry period, while larger crabs were found mainly during the rainy period. The fecundity of the studied population was much smaller than that of subtropical populations of this species. The regression analysis shows that the number of eggs increases linearly with the increase of carapace width.     

Feeding Habits of Percnon gibbesi (H. Milne Edwards, 1853) in the Sicily Strait

The subtropical grapsid crab Percnon gibbesi (H. Milne Edwards, 1853) is one of the most recent alien decapods found in the Mediterranean, where it was discovered at Linosa (Pelagie Islands, Sicily Strait) in summer 1999. At present, the invasion of this species has been recorded in several other insular localities. We studied the feeding habits of the crab in an attempt to understand its success in the Mediterranean. The morphological characteristics of the chela, the feeding adaptations of the gastric mill and the analysis of stomach contents indicate that P. gibbesi is a strictly herbivorous species, a characteristic not shared with any other large-sized infralittoral Mediterranean crab.     

Semilunar courtship rhythm of the fiddler crab<Emphasis Type="Italic"> Uca lactea</Emphasis> in a habitat with great tidal variation

Semilunar courtship rhythm is a widely distributed phenomenon among fiddler crabs in the genus Uca (Decapoda, Ocypodidae). Typically, synchronous courtship has been reported to peak near spring tides. To determine whether a region of large tidal variation shifts reproductive activity, we measured the frequency of specific courtship behaviors including claw-waving and semidome building for U. lactea males on Kanghwa Island, Korea. We found that synchronized courtship for U. lactea peaked near neap tides, whereas near the spring tides, seawater flooded the habitat and males predominantly fed on the mudflat. Although active females, which hold their burrows and usually feed on the mudflat, are abundant near to spring tides, males rarely claw-waved to attract females. This pattern is atypical for the species because other populations of U. lactea on Japan and Taiwan are synchronous around spring tides. We suggest that males invest most of their time in feeding during spring tides because foraging is limited during neap tides. During neap tides, males feed infrequently and thus expend stored energy on courtship signals. We conclude that patterns of reproductive synchrony may be dependent on food availability in periodically changing environments.     

Neurosecretion in the sinus gland of the fiddler crab,Uca pugnax

The ultrastructure of the sinus gland of the fiddler crab, Uca pugnax, was investigated and found to be similar to that in other crustaceans. Five types of neurosecretory axon terminals were tentatively identified on the basis of the size, shape, and electron density of granules within the axons. Release of neuro-secretory material appears to be by exocytosis.     

Male-like mudballing behavior of some female fiddler crabs (Uca tangeri)

At each low tide, male and female Uca tangeri remove mudballs from inside their burrows and place them on the surface. Previous studies have shown clear intersexual differences in mudball arrangements. However, we noticed that some females placed their mudballs in an arrangement similar to that of males. In this study, we investigated several factors that may have been responsible for this change in female mudballing behavior. We found no significant effect of the lunar cycle, female size and reproductive state, or burrow features. We briefly discuss the avoidance of sexual coercion or parasite modification of host behavior as possible factors. Our study shows that intersexual differences in mudballing behavior are more complex than previously thought. Received: October 18, 2000 / Accepted: May 7, 2001     

Acanthodiaptomus denticornis another omnivorous calanoid copepod: description of its mouth appendages and feeding experiments on animal prey

The mouthpart morphology of the freshwater calanoid copepod Acanthodiaptomus denticornis was examined with optical microscopy. The mouthparts have sharp teeth and stout appendages with clawlike setae, typical of omnivorous calanoid mouthpart morphology. Observation of the buccal aperture with Scanning Electron Microscopy shows a large opening permitting feeding on prey as large as Keratella cochlearis. These observations agree with our feeding experiments which show that A. denticornis feed on K. cochlearis.