Homing in the swimming crab Thalamita crenata: a mechanism based on underwater landmark memory

We investigated whether Thalamita crenata, a swimming crab found on the East African intertidal flats, uses landmarks to locate its refuges. We modified the visual panorama of an intertidal flat, using conspicuous movable objects, and conducted homing trials with the local population of swimming crabs. In the first set of trials, after being moved away from their dens, the crabs were immediately able to find other known dens, using direct shortcut paths. In the second set of trials we moved all the artificial landmarks surrounding a crab's den 5 m away and then displaced the crab itself. The crabs made initial navigational errors in accordance with the new position of the landmarks; this shows that they oriented themselves by remembering the landmarks. We then repositioned the landmarks and released the crabs far from the familiar area, on a similar flat. This time the crabs could rely only on the artificial landmarks; they used this information and reached the point where home should have been according to the landmarks. Thus, T. crenata showed good spatial knowledge, based on the storage of landmark memories. This orienting mechanism is much more flexible and complex than those of other crabs and is comparable to the route-based memory of honeybees, Apis mellifera. Copyright 2000 The Association for the Study of Animal Behaviour.     

Genetic differentiation in the mud crab Scylla serrata (Decapoda: Portunidae) within the Indian Ocean

Scylla serrata (Decapoda: Portunidae) is a swimming crab that is widespread in the Indo-Pacific region and commonly found in estuarine and mangrove waters. An extended planktonic larval phase suggests high dispersal potential and the possibility of extensive gene flow between conspecific populations at least on a geographic mesoscale (tens to hundreds of kilometres).Intraspecific variation of the mitochondrial DNA cytochrome oxidase subunit I (mtDNA COI) gene was investigated in 77 individuals from four representative mangrove swamps of the African tropics (Kenya and Zanzibar) by means of DNA sequencing. We examined 535 base pairs (bp) and identified 24 different haplotypes. Each population sample is characterised by a single most frequent haplotype, shared among all four populations, and a small number of rare ones, typically present in only one or two individuals and representative of a specific population.Analysis of molecular variance (AMOVA), F_ST statistics and χ² contingency analysis of spatial distribution of mtDNA haplotype frequencies revealed in toto a significant genetic differentiation among populations. These results could indicate that gene flow might be reduced, even between geographically close sites, despite the high potential for dispersal; anyway, at the recorded level of divergence and owing to the abundance of rare haplotypes and singletons in our data set, repeated sampling over time is necessary to establish whether the recorded pattern of genetic differentiation is stable and biologically significant.Finally, integration of our data with those reported by Gopurenko et al. [Mar. Biol. 134 (1999) 227] on S. serrata from South Africa, Red Sea and Mauritius Islands allowed to infer S. serrata population structure within a larger area of the Indian Ocean region.     

Dana swimming crab growth Callinectes danae (Decapoda: Portunidae) from Margarita Island, Venezuela

Callinectes danae is a common species captured with crab traps in nearby areas of coastal lagoons in Margarita Island. Although its considerable economic importance as a fishery resource, few studies have been done on population dynamics and its fishery potential in local coastal environments to support decision making in fishery administration. We present growth pattern details of Callinectes danae to better estimate its population size and exploitation feasibility. For this, we analyzed a total of 3 623 specimens that were monthly captured in crab pots by artisanal fishermen in Las Marites lagoon, from October 2007 to September 2008. The length-weight ratio was determined, and growth parameters estimated from both length and weight curves of the von Bertalanffy model. The general sex ratio showed no significant difference between males and females (chi2 = 0.04, p > 0.05). However, values of slopes b between males and females were significantly different (t(s) = 2.75, p < 0.05), as well as intercepts a (t(s) = 2.44, p < 0.05). Thus, the length-weight ratio was determined separately: W = 7.48e(-5)*L(2.98) for males and W = 1.21e(-4)*L(2.87) for females, indicating a negative allometric growth in both sexes. Growth parameters were established as: L(infinity) =134.80mm, W(infinity) = 166.04g and k = 0.86/yr for males; L(infinity) = 122.35mm, W(infinity) = 118.45g and k = 0.63/yr for females. Lifespan was estimated at 3.05 years for males and 4.24 years for females. We concluded that Callinectes danae is a species with short lifespan and moderately rapid growth. The coefficient of variation values (CV), of the phi-prime growth performance index (?'), showed a different growth pattern compared to those obtained in other regions. We propose that a management strategy will be the periodical review of the minimum capture size for fishing area, after the great variability found in growth parameters.     

A unique form of light reflector and the evolution of signalling in Ovalipes (Crustacea: Decapoda: Portunidae)

The first demonstration, to our knowledge, of an evolutionary shift in communication mode in animals is presented. Some species of Ovalipes display spectacular iridescence resulting from multilayer reflectors in the cuticle. This reflector is unique in animals because each layer is corrugated and slightly out of phase with adjacent layers. Solid layers are separated from fluid layers in the reflector by side branches acting as support struts. An effect of this reflector is that blue light is reflected over a ''broad'' angle around a plane parallel to the sea floor when the host crab is resting. Species of Ovalipes all possess stridulatory structures. The shallow-water species with the best developed stridulatory structures are non-iridescent and use sound as a signal. Deep-water species possess poorly developed stridulatory structures and display iridescence from most regions of the body. In deep water, where incident light is blue, light display is highly directional in contrast to sound produced via stridulation. Sound and light display probably perform the same function of sexual signalling in Ovalipes, although the directional signal is less likely to attract predators. Deep-water species of Ovalipes appear to have evolved towards using light in conspecific signalling. This change from using sound to using light reflects the change in habitat light properties, perhaps the hunting mechanisms of cohabitees, and its progression is an indicator of phylogeny. The changes in sexual signalling mechanisms, following spatial–geographical isolation, may have promoted speciation in Ovalipes.     

Optimal Mate Searching Strategy in the Symbiotic Spider Crab Inachus phalangium (Decapoda)

The reproductive strategy of the symbiotic spider crab, Inachus phalangium was studied from 1981 to 1983 in the natural environment. Main study focus was the efficiency of males in search of mates. I. phalangium females are site-constant, and live in the protection of one anemone or group. Males travel frequently between anemones harbouring females due to spawn; they copulate and guard the females until spawning, after which the male leaves again. A male operates in a patrol area containing 3–8 anemone groups and up to 8 females, visiting each female in turn repeatedly just before it is due to spawn. Patrol areas of different males may overlap, with resulting competition to fertilize a female's next brood. Large males have higher reproductive success than small ones. Females live up to 8 months after the moult of puberty and hatch up to six broods, and males live up to 7 months as adults. A male could fertilize a calculated 26,000 eggs, whilst a female's reproductive potential is ca. 4,200 eggs. Mortality risks are higher for males than for females, probably because of increased predation while leaving the protection of anemones in order to visit females. Males learn the positions of anemones harbouring females in their patrol areas, and when these are due to spawn. This allows a male to travel with a target and arrive punctually to fertilize the next brood due in his circuit. I. phalangium is the first marine invertebrate reported to use a “schedule” of localities and times for visiting prespawning females. In this way males minimize searching time and mortality risk, and maximize the number of broods fertilized.     

Influence of starvation on larval development of Carcinus maenas L. (Decapoda : Portunidae)

Lethal and sublethal effects of particular starvation events were investigated in larvae of Carcinusmaenas L. Mean survival times of continuously starved zoeae-1 were approximately twice the normal stage duration (12, 18, 25°C), and both increased with falling temperatures. At 6°C zoea-1 was unable to develop to stage-2. No larva retained the ability for successful further development if starved for half the stage duration time and was then refed. The zoea-1 larvae had to feed for at least 20 % of the normal stage duration for some larvae to moult to zoea-2. Some initial feeding was necessary to start zoea-1 development. Beyond a certain point of energy and accumulation of reserves development of the larvae seems to continue regardless of feeding rates. The demands for larval feeding correspond very well with the larval moulting cycle. Larvae of C. maenas proved to be well adapted to natural shortage of food.     

Molecular Markers Reveal Only Two Mud Crab Species of Genus Scylla (Brachyura: Portunidae) in Indian Coastal Waters

The taxonomic ambiguity of the Indian mud crab (genus Scylla de Hann 1833) is still a cause of concern as several papers have been published with misleading identification. This is the first attempt to resolve the taxonomic uncertainty of the mud crab commonly available in Indian coastal waters using molecular genetic markers (ITS-1 and sequencing of COI gene) combined with traditional morphometry. Additionally, we developed a PCR method by which Indian mud crab species can be identified rapidly and effectively. The results clearly indicate that the green morph of the Indian mud crab is Scylla serrata and the brown morph is S. olivacea. The S. serrata commonly mentioned in the literature from India is S. olivacea; the S. tranquebarica noted by many Indian researchers should belong to S. serrata. Caution should be taken when interpreting or implementing the biological, molecular, and aquaculture data in the literature.     

Temporal and spatial distribution of the crab Callinectes sapidus (Decapoda: Portunidae) in Chetumal Bay, Quintana Roo, Mexico

In order to determine temporal and spatial distribution patterns of Callinectes sapidus, samplings were carried out during the cold-front (January-February), dry (May-June) and rainy (August-September, 2002) climatic seasons, in 30 sampling stations of Chetumal Bay, grouped in sectors A (14 stations), B (eight stations) and C (eight stations). In each sampling station crabs were collected from two transects parallel to the coast, each with three traps, separated by 30 m. Sediments were calcareous coarse and medium sand, white or lightly gray. A total of 1 031 specimens were collected. CPEU (Capture Per Effort Unit) differed spatially and temporally. Highest CPEU was found in sector C with 1.3 ind.trap(-1), and in the rainy season with 1.1 ind.trap(-1). Population was predominantly composed of male individuals. The male:female ratio was 15:1. Males and adults (group II) CPEU was significant different between sectors and climatic seasons. Both males and adults (group II) had a greater CPEU in sector C (1.2 ind.trap-) and in the rainy season (1.1 ind.trap(-1)). Abundance of female and juvenile individuals (group I) was low during the sampling period whereas group 0 juvenile individuals were not found. A greater relative frequency between sectors and climatic seasons were observed in 130-139 mm and 140-149 mm size interval (CW). C. sapidus occurred on sandy sediments in Chetumal Bay. Pearson product moment correlations exhibited significant relationships between CPEU and temperature, salinity and dissolved oxygen. In Chetumal Bay, the spatial and temporal distribution of C. sapidus can be related to salinity, temperature, habitat quality, food availability, recruitment and reproduction events of individuals.     

PERMANENT GENETIC RESOURCES: Isolation and characterization of microsatellite loci in the Mediterranean shore crab Carcinus aestuarii (Decapoda, Portunidae)

We characterized nine polymorphic microsatellites in the Mediterranean shore crab Carcinus aestuarii (Decapoda: Portunidae). Microsatellites were isolated from a partial genomic library enriched for multiple motifs. All loci were polymorphic, with number of alleles ranging from two to 16 and a mean observed heterozygosity of 0.75. Seven loci were in Hardy-Weinberg equilibrium, and two showed weak heterozygote deficiency. No linkage disequilibrium was found between loci. In addition, we tested Hardy-Weinberg and linkage equilibrium of three Carcinus maenas loci, already reported to cross-amplify in C. aestuarii. These molecular markers will be potentially useful to investigate genetic structure of this species.     

Genetic Heterogeneity of the Blue Swimming Crab (Portunus pelagicus) in Thailand Determined by AFLP Analysis

Genetic diversity and population differentiation of the blue swimming crab (Portunus pelagicus) in Thailand, originating from Ranong and Krabi located in the Andaman Sea (west) and Chanthaburi, Prachuap Khiri Khan, and Suratthani located in the Gulf of Thailand (east), were examined by AFLP analysis. High genetic diversity of P. pelagicus in Thai waters was found (N=72). The four primer combinations generated 227 AFLP fragments, and the percentage of polymorphic bands in each geographic sample was 66.19-94.38%. The mean genetic distance between pairs of samples was 0.1151-0.2440. Geographic heterogeneity analyses using the exact test and FST-based statistics between all pairwise comparisons were statistically significant (P<0.01), indicating a fine-scale level of intraspecific population differentiation of Thai P. pelagicus. The estimated number of migrants per generation (Nem) was 0.26-0.76, suggesting restricted gene flow levels of P. pelagicus in Thai waters.     

Temperature and growth in Carcinus maenas L. (Decapoda: Portunidae) larvae reared in the laboratory from hatching through metamorphosis

Larvae of Carcinus maenas L. were reared in the laboratory from hatching through metamorphosis at 9, 12, and 18°C. Dry weight (DW) and elemental contents of carbon (C), nitrogen (N), and hydrogen (H) were analysed at short intervals through successive larval moulting cycles (four zoea-stages, megalopa), and newly metamorphosed crabs. C. maenas larvae grew significantly during all instars, at all temperatures tested. Biomass (DW, C, N, H) and energy (Joule) slightly declined shortly before ecdysis in zoea stages. This terminal decrease was more distinct in the megalopa stage, where ≈39 and 83% of the maximum energy attained, was lost at 12 and 18°C, respectively. Changes of biomass and energy in successive moult cycles showed best fits to quadratic equations, whereas their maximum in successive larval instars formed exponential sequences with time. Due to parabolic growth curves, biomass and energy accumulation within single instars were discussed as maximum (MG) and effective growth (EG), considering gain both at times of maximum biomass, and shortly before ecdysis. Metamorphosing larvae achieved EG with 1137% (DW), 1195% (C), 1108% (N), 1395% (H), 1339% (Joule) at 12°C, and 1140% (DW), 1099% (C), 1133% (N), 1225% (H), 1107% (Joule) at 18°C, relative to newly hatched zoea-1. Ash content and inorganic C in newly hatched zoea-1, were 29.4% and 5.5% ash, respectively. The stoichiometric C H N method of Gnaiger & Bitterlich was used to assess protein, lipid, and carbohydrate compositions. Obviously proteins formed the major part of larval biomass (>50% DW). C: N ratios indicate that more lipid than protein was built up shortly after moulting, but relatively more protein was subsequently accumulated. Temperature effects on larval growth (MG, EG), growth rates (GR), and gross growth efficiencies (K₁) were discussed. C. maenas zoea stages accumulated energy and biomass with higher efficiencies at 18 than at 12°C. Megalopa growth seemed to be limited at 18°C, showing lower K₁ values than at 12°C. N was accumulated with higher efficiencies than C in all larval stages. Characteristic variations in larval K₁ values between premoult and ecdysis were discussed. Cumulative gross growth efficiencies (MG-related) were calculated as ≈11 and 10%, at 12 and 18°C, respectively.     

Black Yeast Biota in the Mangrove, in Search of the Origin of the Lethargic Crab Disease (LCD)

Knowledge of natural ecology is essential for a better understanding of pathogenicity and opportunism in black yeast-like fungi. Although etiological agents of diseases caused by these fungi are supposed to originate from the environment, their isolation from nature is difficult. This is probably due to their oligotrophic nature, low competitive ability, and, overall, insufficient data on their natural habitat. We obtained environmental samples from mangrove areas where mortalities by lethargic crab disease (LCD) are reported and areas without disease recorded. Isolation of chaetothyrialean black yeasts and relatives was performed using a highly selective protocol. Species-specific primers were used to determine if these isolates represented Exophiala cancerae or Fonsecaea brasiliensis, two proven agents of LCD, in order to test hypotheses about the origin of the disease. Isolates, identified by morphology as Fonsecaea- or Exophiala-like, were tested specific diagnostic markers for the fungi associated with LCD. Although several black fungi were isolated, the main causative agent of the LCD, E. cancerae, was not found. Molecular markers for F. brasiliensis revealed 10 positive bands for isolates from biofilms on mangrove leaves, branches, and aerial roots, of which four were confirmed by ITS sequencing. The absence of E. cancerae in environmental samples suggests that the species is dependent on the crab, as a genuine pathogen, different from F. brasiliensis, which is probably not dependent on the host species, U. cordatus. However, we did not attempt isolation from the marine water, which may represent the pathway of dispersion of the black yeast species between neighbor mangroves.     

Environmental factors modulating the abundance and distribution of Callinectes danae (Decapoda: Portunidae) from two areas of the southeastern coast of Brazil

The distribution and abundance of marine organisms are directly or indirectly a function of biotic and abiotic features such as sediment and water properties. In this study we analyzed the spatial and temporal distribution of Callinectes danae in two distinct littoral areas of the northern coast of the State of São Paulo, Brazil and associated them with local geomorphologic features and environmental factors. We collected specimens on a monthly basis, from July 2001 to June 2003, using a fishing boat equipped with double-rig net, in two locations: Ubatuba (UBA) and Caraguatatuba (CAR). Collecting was carried out in seven transects with different depths, which were further subdivided into two regions: bay region (5, 10, 15 and 20 m deep); and inner shelf region (25, 30 and 35 m deep). Samples from different periods and transects differed significantly in abundance. A positive correlation was found between the abundance of C. danae and the following parameters: temperature at the bottom, organic matter content and Phi (grain diameter). By contrast, a negative correlation was established between specimen abundance and salinity at the bottom. The total abundance was highest in 5 m deep transects in both regions. Summer was the season when individuals were least abundant in both areas. We concluded that the spatial and temporal distribution of C. danae are determined by a set of environmental factors and not just by one single factor.     

Distribution and drift of the crab Carcinus maenas (L.) (Decapoda, Portunidae) larvae over the continental shelf off northern Portugal in April 1991

The coastal distribution and abundance of Carcinus maenas larvalstages were studied from plankton samples collected in a gridof 79 sampling station organized into six cross-shore transectsextending from the coast to –170 km offshore, betweenthe Mondego and Ave rivers, on the northwest coast of Portugal.The samples were collected in April 1991 with a modified Bémultinet plankton sampler, which was towed from a maximum depthof 200 m and provided a vertical resolution of up to five standardstrata at each station. Current and wind data were availablefor a period that extended beyond the period covered by theobservations All the species' larval stages were found in thesamples, but their distribution was confined to the inner andmiddle shelf stations Vertically, 88% of the larvae occurredin the top 30 m and another 11% in the 30–60 m stratum.The zonal stages I and II were concentrated (–90%) inthe surface layer, but a gradual ontogenic displacement to deeperwaters was observed from then on, the megalopa being equallydistributed between the 0–30 and the 30–60 m strata.Horizontally, there was a dear association of the first zoeawith the estuarine inlet while the older meal stages were dispersedprogressively offshore. Evidence was found that the megalopaexperienced an onshore transport that did not affect the previousstages.This transport is wnsistent with the observation of anonshore flow component at 40 m. It was not possible, however,to examine the hypothesis that this flow conveys the last larvalstage to the coast, but not the previous ones. The dependenceof the along-shore flow component on wind stress lends supportto the hypothesis that the larvae are advected from the northas the upwelling season progresses.     

Respiration,energy balance and development during growth and starvation of Carcinus maenas L. larvae (Decapoda:Portunidae)

In all larval stages of Carcinus maenas L. oxygen consumption was measured at three temperatures (12,18,25 °C). Values increased during development and were in the range of 0.037 ± 0.01 (zoea-1, 12°C, $\text{x}\text{?}\text{± 95\% CL}$) to 0.734 ± 0.047 μl O₂ · h^?1 · ind^?1 (megalopa, 25 °C). Growing larvae showed temperature dependent trends in weight specific respiration rates (referred to dry wt; DW), with values between ≈2.4 and 9.4 μl O₂· h^?1·mg DW^?1. Increase in oxygen consumption of megalops did not differ much at temperatures between 18 and 25 °C. This points to an exceptional physiological position of this stage. Fed zoea-1 of C. maenas (18 °C) revealed growth rates in terms of 40% DW, 20% carbon (C), 30% nitrogen (N) and 65% hydrogen (H). At the same time larvae gained individual energy by 13% (J · ind^?1), while weight specific energy dropped by ≈ 19% (J · mg DW^?1) during the first day and remained constant until the moult. Starved zoea-1 of C. maenas (18 ° C) gained ≈ 20 % in DW through the first day, probably caused by inorganic salts which enter the organism after the moult of the prezoea. DW dropped to ≈ 25 % of initial value, when starvation continued. Single components decreased by ≈50% (C), 54% (N), 57% (J · ind^?1). Weight specific energy (J · mg DW^?1) decreased by 40% during the first 4 days of starvation, remaining constant thereafter. Individual respiration rate (R) dropped by 61 %, weight specific respiration rate (QO₂) by 55 %. Individual energy loss in starved zoea-1 was 0.077 J over a period of 11 days. In this period ≈ 9.3 μl O₂·ind^?1 were consumed. Thus effective oxygen capacity was lower than in growing larvae. It dropped to 5.3 J·mlO₂^?1 after 4 days and remained constant if starvation continued, i.e. 65 % of possible energy loss occurred during the first 4 days. Decrease in requirement for oxygen and its effective capacity were both recognized as independent components of survival during starvation. Partitioning of energy through individual larval development of C. maenas was investigated for all five larval stages. The cumulative budget could be calculated: consumption (C) = 28.23 J, growth (G) = 0.92 J, exoskeleton (Ex) = 0.20 J, metabolism (M) = 5.30 J, egestion and excretion (E) = 21.82 J. Mean gross and net growth efficiency were, K₁ = 3.3% and K₂ = 14.8%, respectively.