Modelling the lethargic crab disease

The lethargic crab disease (LCD) is an emergent infirmity that has decimated native populations of the mangrove land crab (Ucides cordatus, Decapoda: Ocypodidae) along the Brazilian coast. Several potential etiological agents have been linked with LCD, but only in 2005 was it proved that it is caused by an ascomycete fungus. This is the first attempt to develop a mathematical model to describe the epidemiological dynamics of LCD. The model presents four possible scenarios, namely, the trivial equilibrium, the disease-free equilibrium, endemic equilibrium, and limit cycles arising from a Hopf bifurcation. The threshold values depend on the basic reproductive number of crabs and fungi, and on the infection rate. These scenarios depend on both the biological assumptions and the temporal evolution of the disease. Numerical simulations corroborate the analytical results and illustrate the different temporal dynamics of the crab and fungus populations.     

Specific primers for the detection of the black-yeast fungus associated with lethargic crab disease (LCD)

Lethargic crab disease (LCD) is an emerging infirmity that has been causing extensive mortalities in populations of the mangrove land crab Ucides cordatus (Ocypodidae) along the Atlantic coast of Brazil. Previous studies have indicated that LCD is associated with a dematiaceous fungus, Exophiala cancerae de Hoog et al. In the present study, we sequenced the internal transcribed spacer (ITS) of the rDNA region of this black yeast species and developed species-specific PCR primers. Sensitivity tests indicated that the developed protocol is capable of detecting very small amounts of target DNA. Also, the application of the protocol to a variety of other dematiaceous fungi did not generate any false positives. The specific primers provided in the present study represent an important tool for rapidly surveying a large number of crab individuals, as well as environmental samples. Such knowledge will be instrumental in understanding the epidemiological dynamics of LCD.     

Lethargic crab disease: multidisciplinary evidence supports a mycotic etiology

Although lethargic crab disease (LCD) is causing massive mortalities in populations of the mangrove crab Ucides cordatus of Northeastern Brazil, the identity of its etiological agent was hitherto unknown. In this study we provide robust evidence suggesting that LCD is caused by an anamorph Ascomycota (Fungi). We examined specimens of U. cordatus collected from stocks affected by LCD. Histological and TEM methods detected the presence of hyphae, conidia, and condiophores in several host tissues. Moreover, the abundance of fungal stages is negatively associated with crab health. Finally, DNA was isolated from the fungus and a region of its 18S ribosomal gene was sequenced Phylogenetic analyses not only confirm the diagnosis of the LCD fungus in crab tissues as an ascomycete, but also suggest a close relationship with members of the subphylum Pezizomycotina.     

Black mat syndrome,an invasive mycotic disease of the tanner crab, Chionoecetes bairdi

Black mat syndrome, caused by an encrusting fungus on the exterior of the carapace of tanner crabs, has been known for many years. Although it is a nuisance when processing crabs for the commercial market, it has been thought to be restricted to the external surfaces of the crab and, therefore, nonpathogenic. In the present study, 11 tanner crabs from the Kodiak area of Alaska with and 9 without grossly recognizable masses of the fungus on the carapace were necropsied and examined histologically utilizing special stains selective for fungi. In all individuals with the syndrome, hyphae of the fungus, previously identified as Phoma fimeti, penetrated the carapace and virtually replaced the underlying epidermis. In the more advanced cases, the eyestalk was invaded and the epidermis destroyed, and hyphae extended into the eyestalk musculature and nervous tissue. To date, infections of the connective tissue sheaths surrounding the esophagus, stomach, heart, hemopoetic tissue, thoracic ganglion, antennal gland, and ovary have also been observed. None of the crabs without the syndrome contained internal hyphae. Although data on the lethality of the disease are not yet available, the ease with which the hyphae penetrate the chitinous exoskeleton, their extensive proliferation in the epidermis, and their ability to invade deep tissues causing obvious pathological effects, are highly suggestive that it is a virulent, probably fatal, disease that may have a significant impact on tanner crab population dynamics.     

Histopathology of the mangrove land crab Ucides cordatus (Ocypodidae) affected by lethargic crab disease

Lethargic crab disease (LCD) has caused extensive epizootic mortality of the mangrove land crab Ucides cordatus (Linnaeus 1763) (Brachyura: Ocypodidae) along the Brazilian coast. Direct culture of tissue samples from sick crabs and subsequent isolation and purification identified the causative agent as an Exophiala species of fungus. The histopathology of crabs with variable signs of LCD indicates that the most affected tissues are the epidermis, connective tissue, heart, hepatopancreas, nervous system, and gills. Gonads, somatic muscles, and digestive system are less affected by the fungus. The observed pathology is compatible with the clinical signs of LCD. Necrosis, tissue degeneration, and congestion of hemal sinuses and vessels are present in heavily infected organs. Nerve fibers may be compressed by accumulations of yeast-like cells. In heavy infections the tissue of gill lamellae is destroyed with subsequent dilation or compression. Cellular immune responses include hemocytic infiltration, agglutination and encapsulation, and phagocytosis. Phagocytosis of yeast-like cells is abundant in the connective tissue associated with the exoskeleton. These results indicate that LCD is the result of a systemic phaeohyphomycosis caused by a species of Exophiala. The present study also suggests that dispersal of the fungus within the crab occurs through the hemal system.     

基于MAXENT模型评估北部湾潮间带中国鲎和圆尾鲎稚鲎的潜在地理分布及种群保育对策

鲎具有极高的经济价值和科研意义。近年来由于过度捕捞和栖息地受损等原因,亚洲鲎种群数量正急剧下降。鲎漫长的生命周期使得鲎资源的保护和增殖迫在眉睫。生态位模型已经广泛应用于物种的潜在地理分布预测。基于实地调研数据和公开发表的北部湾中国海域中国鲎和圆尾鲎地理分布数据,运用MAXENT模型得到中国鲎和圆尾鲎在广西北部湾(中国部分)的栖息地适宜度指数(Habitat suitability index, HSI),确定了这两种稚鲎在北部湾中国海域潜在适生区。模型分析结果表明,潮间带坡度和地形指数是影响中国鲎分布的主要环境因子,而潮间带底质的有机物含量和植被指数是影响圆尾鲎分布的主要环境因子,根据研究结果建议在两种稚鲎适生区建立保护区,进行人工放流稚鲎,加强对海草和红树林的生态建设,进而促进鲎资源种群恢复和发展。     

Metamorphosis of mangrove crab megalopae, Ucides cordatus (Ocypodidae): Effects of interspecific versus intraspecific settlement cues

It has recently been shown that metamorphosis of Ucides cordatus megalopae is triggered by substrata from the mangrove forest habitat, and, in particular, adult conspecific odours. Here we demonstrate that the gender of the odour-emitting crabs is insignificant for the metamorphic response in this species. We further investigate whether other estuarine crabs (Goniopsis cruentata, Uca spp., and Callinectes danae) also induce settlement and metamorphosis of U. cordatus megalopae. This is of special interest for population recovery in areas hit by lethargic crab disease (LCD), a fungus that selectively kills U. cordatus but not co-occurring species. Ucides megalopae were reared in four treatments with interspecific-conditioned seawater and tested against the effects of conspecific-conditioned seawater (positive control) and pure seawater (negative control). All megalopae in the positive control metamorphosed successfully, while only one (2%) moulted in the negative control, with a delay of 10 days compared with the latest metamorphosis in the former treatment. In seawater conditioned with U. maracoani and C. danae, which occur on sediment banks and in tidal creeks respectively, all larvae died before reaching the juvenile stage. In the treatments with odours of species that share the same mangrove forest microhabitat as U. cordatus, i.e. G. cruentata and a group of five fiddler crab species (mixed-odour treatment), 20 and 10% respectively of the megalopae moulted with a delay of up to 11 days. No specimens metamorphosed after day 39, but megalopae lived up to 93 days. Since only the conspecific- and coexisting-species treatments stimulated development, we hypothesize that Ucides megalopae are able to precisely identify species-habitat-specific settlement cues. This will be investigated in more detail in future studies, which will also test the effects of the odours of the five forest fiddler crab species separately. The impact of the interspecific odour treatments was much smaller than that of the conspecific odours, nevertheless elevated moulting rates of up to 18% relative to seawater may still significantly accelerate the repopulation of U. cordatus in areas lacking conspecifics, e.g. after massive crab mortalities or at first colonization.     

Important crab resources inhabiting Hokkaido waters

In this paper the fishing grounds, fluctuations in catches, history of exploitation, classification, distribution, migration, life history and resource trends of the edible crabs of the waters around Hokkaido are discussed. The Hokkaido crab fisheries developed along with the canning industry and there is now an increasing demand for boiled crab. At first, the most popular species was the king crab, Paralithodes camtschatica, but as its numbers declined other species such as the horsehair crab, Erimacrus isenbeckii, the banasaki crab, Paralithodes brevipes, the snow crab, Chionoecetes opilio, and the red snow crab, Chionoecetes japonicus, became popular. This report is based on the results of the latest research into the ecology and resources of the Erimacrus isenbeckii and Paralithodes brevipes species, which are currently the focus of resource conservation in Hokkaido.     

Evidence for a free-living life stage of the blue crab parasitic dinoflagelate, Hematodinium sp.

Hematodinium sp. is a parasitic dinoflagellate reported to cause disease and death in a variety of crustacean species including the blue crab (Callinectes sapidus). However, because of difficulties in the culture of Hematodinium sp. associated with blue crabs, little is known about its life cycle or mode of transmission. Here, we report the first detection of this organism outside of a metazoan host and provide evidence that this life stage can act as an infective agent. Observations of dinospores in crab hemolymph samples suggest that dinospores may be responsible for waterborne disease transmission. Additionally, we developed and validated a quantitative Real Time PCR assay for the detection of Hematodinium sp. inside and outside of a host organism that will be useful for future investigations of Hematodinium biology and Hematodinium sp.-infection etiology. Based on the observations of a free-living form of Hematodinium sp. and the association of this parasite with a widespread epizootic in blue crab populations, we propose that Hematodinium sp. be considered a Harmful Algal Bloom species.     

Conceptual Model Development for Invasive Species and a Regional Risk Assessment Case Study: The European Green Crab,Carcinus maenas,at Cherry Point,Washington, USA

The goal of this article was to generate a method of regional scale ecological risk assessment using an adaptation Relative Risk Model (RRM). As a case study we performed a quantitative, regional risk assessment of an invasive species, the European green crab (Carcinus maenas) at Cherry Point, Washington, USA. The conceptual model was modified from the RRM and incorporates the structure of the hierarchical patch dynamic paradigm. The ranks and filters were integrated to determine the relative contribution of each source of C. maenas to risk as well as the risk to selected biological endpoints, habitats and sub-regions for two source scenarios: (1) current conditions (2004) and (2) future conditions during an El Nino year. The results suggest that the habitat and endpoint with the greatest risk are the eelgrass habitat and the juvenile Dungeness crab, respectively. The Cherry Point subregion was identified as the area having the most risk in the first source scenario, while the Lummi Bay sub-region is most at risk during an El Nino event. The risk of impacts is substantially higher for all endpoints, habitats and sub-regions when El Nino–driven current dispersal is considered. The methodology applied in this case study can be modified and applied to determine the risk of introduction and impacts of other invasive species to the Strait of Georgia, Puget Sound, and other coastal areas.     

Development and characterization of new microsatellite loci from Chinese mitten crab (Eriocheir sinensis)

A set of new microsatellite loci were isolated from Chinese mitten crab and characterized in two Chinese mitten crab populations. The number of the alleles ranged from 3 to 8 and observed and expected heterozygosity varied from 0.0870 to 0.9565 and 0.3092 to 0.8367 in each population, respectively. Loci HLJEsin1, HLJEsin4, HLJEsin30 in CLN population and HLJEsin6 in CJS population presented significant deviation from Hardy–Weinberg equilibrium (PHWE < 0.001). These markers will be useful for population studies.     

Environmentally-controlled,density-dependent secondary dispersal in a local estuarine crab population

The mechanisms driving the pelagic secondary dispersal of aquatic organisms following initial settlement to benthic habitats are poorly characterized. We examined the physical environmental (wind, diel cycle, tidal phase) and biological (ontogenetic, density-dependent) factors that contribute to the secondary dispersal of a benthic marine invertebrate, the blue crab (Callinectes sapidus) in Pamlico Sound, NC, USA. Field studies conducted in relatively large (0.05 km²) seagrass beds determined that secondary dispersal is primarily undertaken by the earliest juvenile blue crab instar stages (J1 crabs). These crabs emigrated pelagically from seagrass settlement habitats using nighttime flood tides during average wind conditions (speed ~5 m s^–1). Moreover, the secondary dispersal of J1 crabs was density-dependent and regulated by intra-cohort (J1) crab density in seagrass. Our results suggest that dispersal occurs rapidly following settlement, and promotes blue crab metapopulation persistence by redistributing juveniles from high-density settlement habitats to areas characterized by low postlarval supply. Collectively, these data indicate that blue crab secondary dispersal is an active process under behavioral control and can alter initial distribution patterns established during settlement. This study highlights the necessity of considering secondary dispersal in ecological studies to improve our understanding of population dynamics of benthic organisms.     

Inferring the dynamics of a spatial epidemic from time-series data

Spatial interactions are key determinants in the dynamics of many epidemiological and ecological systems; therefore it is important to use spatio-temporal models to estimate essential parameters. However, spatially-explicit data sets are rarely available; moreover, fitting spatially-explicit models to such data can be technically demanding and computationally intensive. Thus non-spatial models are often used to estimate parameters from temporal data. We introduce a method for fitting models to temporal data in order to estimate parameters which characterise spatial epidemics. The method uses semi-spatial models and pair approximation to take explicit account of spatial clustering of disease without requiring spatial data. The approach is demonstrated for data from experiments with plant populations invaded by a common soilborne fungus, Rhizoctonia solani. Model inferences concerning the number of sources of disease and primary and secondary infections are tested against independent measures from spatio-temporal data. The applicability of the method to a wide range of host-pathogen systems is discussed.     

Chinese mitten crab <Emphasis Type="Italic">Eriocheir sinensis</Emphasis> in the Baltic Sea—a supply-side invader?

Although the Chinese mitten crab Eriocheir sinensis (H. Milne-Edwards, 1853) (Crustacea, Decapoda, Varunidae) invaded the Baltic Sea about 80 years ago, published information on its present distribution and abundance in this region is lacking. We provide here information on its Baltic-wide distribution and long-term population dynamics. The species has been found all over the coastal Baltic Sea and also in some adjacent rivers and lakes. The Chinese mitten crab appears to have increased in abundance in recent years in the northeastern part of the Baltic Sea (Gulf of Finland, Gulf of Riga, northern Baltic Proper). Higher catch rates were observed in spring (April–June) and autumn (September–November). The size variation of crabs in different samples was low (mean carapace width 6.1–6.3 cm). Despite findings of gravid females, the reproduction of the mitten crab in the central, northern and eastern Baltic region is considered unlikely due to low salinity and the individuals caught are assumed to actively migrate into the region from the species’ main European distribution area (southeastern North Sea), certainly over 1500 km migration distance. Thus, the dynamics of the North Sea population is probably regulating, at least in part, the occurrence of the Chinese mitten crab in the Baltic Sea area.     

Spatial-temporal patterns in intra-annual gray whale foraging: Characterizing interactions between predators and prey in Clayquot Sound, British Columbia, Canada

In Clayquot Sound, British Columbia, gray whales (Eschrichtius robustus) forage primarily on mysids (Family Mysideae) and also on crab larvae (Family Porcellanidae) that are constrained to specific habitat, which relate to bathymetric depths. In this paper we characterize the interactions of gray whales and their prey by analyzing fine scale spatial‐temporal patterns in foraging gray whale distribution within a season. Kernel density estimators are applied to two seasons (1998 and 2002) of high‐resolution data on foraging by gray whales. By partitioning data from each foraging season into several time periods (12 in 1998 and 11 in 2002), using a temporal autocorrelation function, and generating kernel density estimated surfaces for each time period, it is possible to identify discrete areas of increasing and declining foraging effort. Our results indicate that gray whales forage on mysids throughout a season and opportunistically forage on crab larvae. The episodic crab larvae feeding may reduce, but not eliminate, pressure to mysid populations enabling mysids to reassemble swarms and continue to support gray whale foraging in the latter part of the season. Results suggest that when managing marine environments, gray whale populations require multiple and connected habitats for summer foraging.     

Quantifying two-dimensional dichromatic patterns using a photographic technique: case study on the shore crab (<Emphasis Type="Italic">Carcinus maenas</Emphasis> L.)

Contrasting patterns of pigmentation, such as those associated with crypsis and aposematism, are common in many taxa. In order to determine why patterning varies among individuals or populations, it is important to quantify how these patches of pigment are arranged. Here we present a simple technique for measuring areas of pigmentation as well as their spatial distribution, and demonstrate its application to the study of substrate-associated patterning in shore crabs (Carcinus maenas L.). The results, based on a virtual grid laid over digital images of crab carapaces, allow for correlations to be made among sample populations. The technique, or variations of it, can be applied to any situation where two-dimensional dichromatic patterns need to be quantified.     

Genetic Variation and Population Structure of Hair Crab (<Emphasis Type="Italic">Erimacrus isenbeckii</Emphasis> ) in Japan Inferred from Mitochondrial DNA Sequence Analysis

Genetic variation and population structure of hair crab (Erimacrus isenbeckii) were examined using nucleotide sequence analysis of 580 base pairs (bp) in the 3′ portion of the mitochondrial cytochrome c oxidase subunit I gene (COI) of 20 samples collected from 16 locales in Japan (the Hokkaido and Honshu Islands) and one in Korea. A total of 27 haplotypes was defined by 23 variable nucleotide sites in the examined COI region. Pairwise population F _ST estimates and neighbor-joining tree inferred distinct genetic differentiation between the representative samples from the Pacific Ocean off the Eastern Hokkaido Island and the Sea of Japan, while others were intermediate between these two groups. AMOVA also showed a weak but significant differentiation among these three groups. The present results suggest a moderate population structure of hair crab, probably influenced by high gene flow between regional populations due to sea current dependent larval dispersal of this species.     

A modelling approach to estimate the effect of exotic pollinators on exotic weed population dynamics: bumblebees and broom in Australia

The role of mutualisms in contributing to species invasions is rarely considered, inhibiting effective risk analysis and management options. Potential ecological consequences of invasion of non‐native pollinators include increased pollination and seed set of invasive plants, with subsequent impacts on population growth rates and rates of spread. We outline a quantitative approach for evaluating the impact of a proposed introduction of an invasive pollinator on existing weed population dynamics and demonstrate the use of this approach on a relatively data‐rich case study: the impacts on Cytisus scoparius (Scotch broom) from proposed introduction of Bombus terrestris. Three models have been used to assess population growth (matrix model), spread speed (integrodifference equation), and equilibrium occupancy (lattice model) for C. scoparius. We use available demographic data for an Australian population to parameterize two of these models. Increased seed set due to more efficient pollination resulted in a higher population growth rate in the density‐independent matrix model, whereas simulations of enhanced pollination scenarios had a negligible effect on equilibrium weed occupancy in the lattice model. This is attributed to strong microsite limitation of recruitment in invasive C. scoparius populations observed in Australia and incorporated in the lattice model. A lack of information regarding secondary ant dispersal of C. scoparius prevents us from parameterizing the integrodifference equation model for Australia, but studies of invasive populations in California suggest that spread speed will also increase with higher seed set. For microsite‐limited C. scoparius populations, increased seed set has minimal effects on equilibrium site occupancy. However, for density‐independent rapidly invading populations, increased seed set is likely to lead to higher growth rates and spread speeds. The impacts of introduced pollinators on native flora and fauna and the potential for promoting range expansion in pollinator‐limited ‘sleeper weeds’ also remain substantial risks.     

Timing is crucial for consequences of migratory connectivity

Migratory connectivity can have important consequences for individuals, populations and communities. We argue that most consequences not only depend on which sites are used but importantly also on when these are used and suggest that the timing of migration is characterised by synchrony, phenology, and consistency. We illustrate the importance of these aspects of timing for shaping the consequences of migratory connectivity on individual fitness, population dynamics, gene flow and community dynamics using examples from throughout the animal kingdom. Exemplarily for one specific process that is shaped by migratory connectivity and the timing of migration – the transmission of parasites and the dynamics of diseases – we underpin our arguments with a dynamic epidemiological network model of a migratory population. Here, we quantitatively demonstrate that variations in migration phenology and synchrony yield disease dynamics that significantly differ from a time‐neglecting case. Extending the original definition of migratory connectivity into a spatio‐temporal concept can importantly contribute to understanding the links migratory animals make across the globe and the consequences these may have both for the dynamics of their populations and the communities they visit throughout their journeys. Synthesis Migratory connectivity quantifies the links migrant animals make across the globe and these can have manifold consequences – from individual fitness, population dynamics, gene flow to transmission of pathogens and parasites. We show through the use of empirical examples and a conceptual model that these consequences not only depend on which sites are used but importantly also on when these are used. Therefore, we specify three dimensions of migration timing – phenology, synchrony and consistency, which describe the timing of migration 1) relative to development of key resources; 2) relative to the migration of other individuals; and 3) relative to previous migration events. Each of these dimensions can alter the consequences, but typically through different mechanisms.     

Temporal genetic analysis of the endangered tidewater goby: extinction–colonization dynamics or drift in isolation?

Extinction and colonization dynamics are critical to understanding the evolution and conservation of metapopulations. However, traditional field studies of extinction–colonization are potentially fraught with detection bias and have rarely been validated. Here, we provide a comparison of molecular and field‐based approaches for assessment of the extinction–colonization dynamics of tidewater goby (Eucyclogobius newberryi) in northern California. Our analysis of temporal genetic variation across 14 northern California tidewater goby populations failed to recover genetic change expected with extinction–colonization cycles. Similarly, analysis of site occupancy data from field studies (94 sites) indicated that extinction and colonization are very infrequent for our study populations. Comparison of the approaches indicated field data were subject to imperfect detection, and falsely implied extinction–colonization cycles in several instances. For northern California populations of tidewater goby, we interpret the strong genetic differentiation between populations and high degree of within‐site temporal stability as consistent with a model of drift in the absence of migration, at least over the past 20–30 years. Our findings show that tidewater goby exhibit different population structures across their geographic range (extinction–colonization dynamics in the south vs. drift in isolation in the north). For northern populations, natural dispersal is too infrequent to be considered a viable approach for recolonizing extirpated populations, suggesting that species recovery will likely depend on artificial translocation in this region. More broadly, this work illustrates that temporal genetic analysis can be used in combination with field data to strengthen inference of extinction–colonization dynamics or as a stand‐alone tool when field data are lacking.