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.     

Patterns of morphological and genetic variability in UK populations of the shore crab, Carcinus maenas Linnaeus, 1758 (Crustacea: Decapoda: Brachyura)

Previous research has identified extensive inter-population variability in the morphology of the shore crab (Carcinus maenas L.). To determine the source of this variation (genetic or environmental), morphological and genetic data were analysed from crabs collected from eight sites around the coast of the UK. Ten morphometric traits were measured from over 800 crabs and the degree of morphological similarity among sites was calculated using multivariate techniques. Allozyme electrophoresis was used to investigate patterns of genetic similarity. Extensive morphological variability was detected: eight out of the ten morphometric traits analysed were useful when discriminating between crabs from each site. Discriminant function analysis revealed that over 35% of individuals could be classified to their site of origin on the basis of their morphology. In contrast, the allozyme analysis revealed low levels of genetic variability, both within the meta-population and among the crab population at each site. Pairwise comparisons revealed a moderate correlation between the degree of morphological and genetic similarity of crabs at each site, which suggests that the observed phenotypic variability has a genetic component. However, only around 20% of the phenotypic variability detected was associated with the patterns of genetic similarity. This means that patterns of morphological variability in this species are largely determined by the local environmental conditions: local factors could have a within-generation selective influence on mean trait values or C. maenas may exhibit phenotypic plasticity.     

Characterization of microsatellite loci in the European green crab (Carcinus maenas)

Carcinus maenas (Decapoda: Portunidae) has proven a highly successful invasive marine species whose potential economic and ecological impacts are of great concern worldwide. Here, we characterize 14 polymorphic microsatellite loci in C. maenas and its sister species Carcinus aestuarii. These markers will prove useful for fine‐scale genetic analyses of native and introduced populations, for assessment of the sources and routes of invasion and for evaluation of post‐invasion population dynamics.     

A prediction come true: the green crab invades the Patagonian coast

We report the finding of the European green crab on the Atlantic coast of Patagonia. A total of 61 specimens were found in Camarones bay (Chubut Province, Argentina) in November 2003 and January 2004. Carapace widths ranged between 39.9 and 71.4 mm for females, and 51.4 and 81.4 mm for males. All of the females were sexually mature, and most had already spawned. Size frequency distributions and temperature conditions in the area suggest that the species had been introduced at least 3–4 years before our finding (1999–2000). The analysis of surface seawater temperature in the native and non-native ranges of distribution of the species suggests that the crab would be able to colonize the eastern coast of South America from southern Brazil (29°S) to the mouth of the Magellan Strait (52°S). Possible areas of introduction to the region and vectors are discussed.     

The invasive green crab and Japanese shore crab: behavioral interactions with a native crab species,the blue crab

Blue crabs, Callinectes sapidus (Rathbun), are an ecologically and commercially important species along the East coast of North America. Over the past century and a half, blue crabs have been exposed to an expanding set of exotic species, a few of which are potential competitors. To test for interactions with invasive crabs, juvenile C. sapidus males were placed in competition experiments for a food item with two common non-indigenous crabs, the green crab Carcinus maenas (L.) and the Japanese shore crab, Hemigrapsus sanguineus (De Haan). Agonistic interactions were evaluated when they occurred. In addition, each species’ potential to resist predators was examined by testing carapace strength. Results showed that C. maenas was a superior competitor to both C. sapidus and H. sanguineus for obtaining food, while the latter two species were evenly matched against each other. Regarding agonism, C. sapidus, was the “loser” a disproportionate number of times. C. sapidus carapaces also had a significantly lower breaking strength. These experiments suggest that both as a competitor, and as potential prey, juvenile blue crabs have some disadvantages compared with these common sympatric exotic crab species, and in areas where these exotics are common, juvenile native blue crabs may be forced to expend more energy in conflict that could be spent foraging, and may be forced away from prime food items toward less optimum prey.     

The geography of crushing: Variation in claw performance of the invasive crab Carcinus maenas

The major claws of predatory, durophagous decapods are specialized structures that are routinely used to crush the armor of their prey. This task requires the generation of extremely strong forces, among the strongest forces measured for any animal in any activity. Laboratory studies have shown that claw strength in crabs can respond plastically to, and thereby potentially match, the strength of their prey's defensive armor. These results suggest that claw strength may be variable among natural populations of crabs. However, very few studies have investigated spatial variation in claw strength and related morphometric traits in crabs. Using three geographically separate populations of the invasive green crab in the Gulf of Maine, we demonstrate, for the first time, geographic variation in directly measured claw crushing forces in a brachyuran. Despite variation in mean claw strength however, the scaling of claw crushing force with claw size was consistent among populations. We found that measurements of crushing force were obtained with low error and were highly repeatable for individual crabs. We also show that claw mass, independent of a linear measure of claw size, and carapace color, which is an indicator of time spent in the intermoult, were important predictors of claw crushing force.     

CYP330A1 and CYP4C39 enzymes in the shore crab Carcinus maenas: sequence and expression regulation by ecdysteroids and xenobiotics

Cytochrome P450 enzymes (CYP enzymes) catalyse important metabolic reactions of exogenous and endogenous substrates, including steroid hormones. Here, we report the first two CYP sequences from the shore crab, Carcinus maenas. Two complete cDNAs isolated from crab hepatopancreas encode CYP enzymes named CYP330A1, the first member of a new family, and CYP4C39. CYP330A1 is closest related to members of the CYP2 family (37.3% identical to mouse CYP2J6) and CYP4C39 is most identical to crayfish CYP4C15 (59.5%). CYP330A1 gene expression was induced in hepatopancreas of male green intermoult crabs by ecdysone and ponasterone A, but also by benzo(a)pyrene and phenobarbital. CYP330A1 induction was not observed in red crabs. The present results indicate that the CYP330A1 enzyme may be involved in ecdysteroid metabolism, presumably catabolism, and in the detoxification of environmental pollutants. Ecdysteroids or xenobiotics did not affect CYP4C39 gene expression. The fact that both ecdysteroids and xenobiotics affect CYP330A1 gene expression indicates that mutual interactions between chemical exposures and endocrine functions may exist in the shore crab.     

A histopathological survey of shore crab (Carcinus maenas) and brown shrimp (Crangon crangon) from six estuaries in the United Kingdom

Invertebrates show considerable potential as sentinel organisms for the monitoring of the health status of aquatic systems. They are generally small, abundant, relatively sessile, and may readily bioaccumulate toxins. Cascade-like stress responses can occur following acute or chronic exposures to contaminated environments and as such, the overall health status of individuals within those environments, both in terms of histopathological lesions and the presence of infecting organisms, may ultimately reflect the general health status of these sites. The current study provides baseline multi-organ histopathological data for two common crustacean species, the shore crab (Carcinus maenas) and the brown shrimp (Crangon crangon) collected from six UK estuarine sites. Changes in the metabolic condition of crustaceans from these sites (measured in terms of connective tissue storage cell status) were interpreted in relation to other health measures (including parasite load and the presence of microbial pathogens). The relative ease at which a holistic assessment of health can be made using histopathology and the suitability of these species as environmental sentinels provide support for the inclusion of crustaceans as indicators of aquatic environmental health. Studies linking disease status to burdens of industrial contamination in these environments are now required.     

The hair-peg organs of the shore crab,Carcinus maenas (Crustacea,Decapoda): Ultrastructure and functional properties of sensilla sensitive to changes in seawater concentration

Summary The hair-peg organs of the shore crab, Carcinus maenas, are modified hair-sensilla. A small hair shaft (peg) is surrounded by a tuft of solid cuticular bristles (hairs). Each hair-peg organ is innervated by 6 sensory neurons, 2 of which have scolopidial (type-I) dendrites. The outer segments of all dendrites pass through a cuticular canal extending to the articulated hair base in which the 2 type-I dendrites terminate. The other 4 (type-II) dendrites reach the clavate tip of the hair shaft and have access to a terminal pore and a large sickle-shaped aperture. Three inner and 8–12 outer enveloping cells belong to a hair-peg organ. The innermost enveloping cell contains a scolopale, which has desmosomal connections to the ciliary rootlets of the type-I dendrites. An inner and an outer sensillum lymph space are present. The ultrastructural features of the dendrites and the cuticular apparatus indicate that the hair-peg organs are bimodal sensilla, comprising 2 mechano- and 4 chemosensitive sensory neurons. Extracellular recordings from the leg nerve indicate that the chemosensitive neurons of the hair-peg organs respond to changes in seawater concentration in the physiological range of Carcinus maenas.Supported by the Deutsche Forschungsgemeinschaft (SFB 45/A1; W. Gnatzy)     

Frequency of moulting by shore crabs Carcinus maenas (L.) changes their colour and their success in mating and physiological performance

Male shore crabs, Carcinus maenas [Linnaeus, 1758], compete aggressively for access to receptive females to mate. Size is the single most important factor for the outcome of these conflicts, large males with carapace width (CW) over 60 mm being much more likely to gain access to receptive females than smaller males. To compete aggressively, large male shore crabs decrease moulting frequencies and may potentially terminate moulting to enter a state of anecdysis, in which further growth is suspended to increase reproductive output. This change from a “growth” strategy to a “reproduction” strategy results in the creation of two morphs, which can roughly be separated by their colouration. As the new exoskeleton created during moult is always green in appearance, crabs tend to be green in appearance during periods where they grow rapidly and moult frequently. Green crabs are found in all size classes. However, as the exoskeleton becomes older, the colour gradually changes to a darker red colour, and large crabs that have spent an extended period in intermoult are therefore often red in appearance. Also, the exoskeleton of red crabs exhibits a higher incidence of epibionts and wear and tear. Red crabs can also be found in all size classes, but their relative proportion in the population increases dramatically in size classes above 60-mm CW. Size for size, the red morph has a thicker carapace and larger master chelae than the green morph. Also, the reproductive indices (RI) for red crabs are higher than for green crabs, and they experience higher mating success. However, this mating success appears to be achieved at the expense of a lower physiological tolerance, green crabs being better adapted to deal with changes in the surrounding environment. This increased tolerance is not only observed with regard to natural variations in the habitats where shore crabs live, but green crabs also appear to be more tolerant to variations caused by anthropogenic pollution. Consequently, the shift from growth to reproduction exerts a profound effect on the behaviour, physiology and ecotoxicology of male shore crabs. The present paper reviews the studies conducted so far, proposes a mechanism by which some of these differences between the two morphs are created and discusses their ecological and ecotoxicological significance.     

Effect of an invasive crab upon a marine fishery: green crab, Carcinus maenas, predation upon a venerid clam, Katelysia scalarina, in Tasmania (Australia)

Nonindigenous species are increasingly recognized as altering marine and estuarine communities, causing significant changes in abundance and distribution of native species. Such effects are of particular concern to coastal fisheries. We experimentally determined the effect of the nonindigenous European green crab, Carcinus maenas, upon the stepped venerid clam, Katelysia scalarina, the basis for a fledgling clam fishery in Tasmania, Australia. First, we observed a trend of decreased juvenile (<13-mm shell length or SL) abundance of K. scalarina at sites with C. maenas relative to those without this invasive predator. Additionally, relative predation intensity on these juveniles was significantly higher in invaded areas. To better understand the dynamics of predation by this invader, we conducted a number of manipulative experiments. In cage experiments testing per capita predation rates, we found that: (1) of the various sizes of C. maenas, large C. maenas were the most significant predators; (2) the smallest size class of K. scalarina tested (6-12-mm SL) was preferred by C. maenas; (3) C. maenas had much higher predation rates than any native predator tested; and (4) while the native shore crab, Paragrapsus gaimardii, was found to have a constant predation rate over an eightfold range of densities of juvenile K. scalarina (16-128 individuals·m⁻²), C. maenas significantly increased its per capita predation with increasing prey density. Notably, in open field plots at a site where C. maenas was abundant, predation was constant over the range of tested prey densities. We predict, therefore, that the invasion of C. maenas will have significant negative consequences for the Tasmanian K. scalarina fishery.     

Temporal genetic homogeneity among shore crab (Carcinus maenas) larval events supplied to an estuarine system on the Portuguese northwest coast

Despite the importance of larval biology in the life histories of many marine animals, relatively little information exists on the dynamics and genetic composition of larval cohorts. The supply of megalopae larvae of the shore crab, Carcinus maenas, was measured on a daily basis during 8 months spread along two larval periods (2006 and 2007) at the Ria de Aveiro estuary, on the Portuguese northwest coast. A total of 10 microsatellite DNA loci were employed to explore the genetic structure, variability and relatedness of temporally distinct megalopal events, selected from the major pulses of supply. Larval variation was also compared genetically with that of a previously studied adult crabs sample, at the same loci, collected in 2006 and 2007 along the Iberian Peninsula. Results revealed a lack of genetic differentiation and identical diversity levels among larval events over time. No evidence of reduced genetic diversity between megalopae relative to the diversity assessed from the pooled sample of adults was found. Moreover, there was no evidence of any family relatedness among larvae from temporal events. The results obtained for C. maenas contradict predictions made by the sweepstakes reproduction hypothesis, in which large variance in reproductive success is expected, which is presumably detectable as sharp genetic discontinuities among separate larval events. Data here indicate conversely a high degree of temporal genetic stability among larval supply to a given estuary under variable oceanographic conditions, consistent with the hypothesis that sampled larvae were drawn from a large number of adults that do not differ in reproductive success.     

Predatory impact of the green crab (Carcinus maenas Linnaeus) on post-settlement winter flounder (Pseudopleuronectes americanus Walbaum) as revealed by immunological dietary analysis

Predation on flatfish during the early juvenile stage is an important factor regulating year-class strength and recruitment. In this study, immunological dietary analysis was performed on green crabs (Carcinus maenas) collected from the Niantic River, Connecticut, in an effort to evaluate the predatory impact of this species on post-settlement winter flounder (Pseudopleuronectes americanus). Through the use of species-specific antiserum, winter flounder proteins were identified in 4.8% of the green crab stomachs analyzed (n = 313, size range = 14-74 mm carapace width, CW), revealing that crabs ≥ 29 mm CW are predators of post-settlement winter flounder in natural populations. The most significant factor underlying the predator-prey interaction was the relative size relationship between species, such that the incidence of winter flounder remains in the stomach contents of green crabs was positively correlated with predator-to-prey size ratio. Results from dietary analysis were incorporated into a deterministic model to estimate the average daily instantaneous mortality and cumulative mortality of winter flounder owing to green crab predation. Accordingly, green crabs may account for 0.4% to 7.7% (mean = 2.2%) of the daily mortality of winter flounder and consume 1.1% to 32.3% (mean = 10.2%) of the flounder year-class. Model simulations further indicate that variations in green crab abundance and size-structure account for the greatest variability in winter flounder mortality. Relative to other macro-crustacean predators, however, predation by green crabs has a minimal effect on winter flounder survival, due in large part to the low densities of these crabs in temperate estuaries.     

Corrective note about R* cells of the digestive gland of the shore crab Carcinus maenas

In a previous paper, we described and discussed the possible functions of calcospherite-rich cells (R^* cells) in the digestive gland of the shore crab, Carcinus maenas. We recently realised that electron micrographs in this publication presented neither typical R^* cells nor their calcium phosphate granules. Indeed, our pictures showed spermatophores (filled with typical spermatozoa) that had contamined hepatopancreatic cell suspensions. As the present study indicates, this contamination is difficult to detect by optical microscopy because unstained R^* cells closely resemble spermatophores. However, morphological differences between these cell types appear clearly when observed by electron microscopy. The present paper describes a comparative study of cell populations isolated from female digestive glands; it validates our previous results obtained with male hepatopancreas and suggests a low containation of those male cell fractions by spermatophores.     

Agonistic interactions between the invasive green crab, Carcinus maenas (Linnaeus) and juvenile American lobster, Homarus americanus (Milne Edwards)

Invasive organisms have the potential for competition with native organisms. In the Southern Gulf of St. Lawrence, juvenile American lobsters have a potential spatial overlap with adult green crabs. Crustaceans use agonistic behaviour to settle disputes, with the larger organism often winning contests for limited resources such as food and shelter. Two experiments were carried out using adult green crabs (53-76 mm carapace width) and juvenile American lobsters (28-57 mm carapace length). The first experiment used a limited food resource. We found that green crabs were the first to the food in all trials, fed in significantly more trials than lobsters and spent a significantly greater proportion of time with the food. The lobsters were only able to displace the green crabs from the food in 2 of 65 attempts. The second experiment was designed to examine shelter competition; unexpectedly some predation by green crabs on lobsters occurred, which allowed us to test hypotheses about how relative size and shelter use affect predation. Green crabs captured and consumed juvenile lobsters in 6 of 11 trials. The lobsters that survived spent significantly more time in shelter. There was no clear relationship between shelter use and size of lobster. The lobsters that were larger in relation to the green crabs suffered a higher rate of predation, which we believe was due to more conspicuous activity and less use of shelter. It appears that green crabs have the potential to negatively impact native juvenile lobster.     

Occurrence of immunoreactive enkephalins in a neurohemal organ and other nervous structures in the eyestalk of the shore crab,Carcinus maenas L. (Crustacea,Decapoda)

Summary Light-microscopical observations with immunofluorescence and peroxidase staining procedures revealed leu-enkephalin-like immunoreactivity in axon profiles of the sinus gland (SG) and in single small neurons in the optic ganglia of the eyestalk of Carcinus maenas. Electron microscopy of the SG showed reactivity to be associated with neurosecretory granules 82±23 nm in diameter. High performance liquid chromatography of SG-extracts revealed radioimmunoreactive substances with the retention times of synthetic met- and leu-enkephalin and met-enkephalin-Arg⁶-Phe⁷, respectively.