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.     

Resource partitioning and competition for shells in a subtidal hermit crab species assemblage

Summary Shell and habitat utilization are quantified for 12 hermit crab species occurring subtidally in the San Juan Archipelago, Washington. The mechanism of competition for shells between these species is investigated using laboratory experiments to determine shell preferences, shell acquisition rates, and rates of exchange of shells via shell fighting. This information is used to estimate relative intensities of inter- and intra-specific competition for shells between the species in this assemblage. In contrast to earlier findings on intertidal hermit crab assemblages, a significant number (5) of the species in this assemblage appear to experience a greater reduction in their shell supply due to members of other species than due to other members of their own species. The relative amounts of inter- and intra-specific competition differ greatly for different species in the community. The high average figures for interspecific/intraspecific competition are largely a result of the presence of three abundant and very generalized species. In spite of the large number of species and relatively high ratios of interspecific to intraspecific competition, the species in this group are not close to a limiting similarity in resource use. There is suggestive evidence that greater selection pressures for divergence in habitat use may have resulted in the lower amounts of overlap observed in intertidal hermit crab assemblages in previous studies.     

Effect of fish predation on intertidal benthic fauna is modified by crab bioturbation

The burrowing crab Chasmagnathus granulatus is an important bioturbator in SW Atlantic estuaries where they generate dense and extended intertidal beds. Its bioturbation leads to profound changes in the structure, quality and dynamics of sediments with concomitant impacts on the entire benthic community. In this study, we evaluate whether the presence of this crab affects the predator-prey interaction between juvenile fishes and their benthic prey. Gut content and benthic prey selection by juvenile fishes inside and outside crab beds were evaluated, and predation effect was experimentally contrasted between areas using fish exclosures. The results show that in crab beds the percentage of fish with empty guts was lower and the number of polychaetes consumed by fish higher than outside crab beds. The silverside Odontesthes argentinensis and the catfish Pimelodella laticeps fed on larger polychaetes outside than inside crab bed areas, while the white mouth croaker Micropogonias furnieri preyed upon larger polychaetes inside crab beds. In addition, field experiments shows that fish predation decreases polychaete abundances only in crab beds. These results suggest that crab bioturbation facilitate fish predation on benthic prey.     

Habitat quality of estuarine nursery grounds: Integrating non-biological indicators and multilevel biological responses in Solea senegalensis

Estuaries are highly valuable ecosystems that provide various goods and services to society, such as food provision and supporting nursery habitats for various aquatic species. Estuarine habitat quality assessment is thus critical in managing both ecological and economic value. In this work, various biological and non-biological indicators of habitat quality in estuarine nursery areas were determined, encompassing local environmental conditions, chemical contamination, anthropogenic pressures, juvenile Solea senegalensis condition, biomarkers response to contamination and juvenile density. The various indicators provided an integrated view on habitat quality and their responses were broadly concordant. Nursery quality assessment based on anthropogenic pressure indicators and fish biomarker responses were very similar, signaling nursery areas with higher anthropogenic pressure in Tejo and Ria de Aveiro estuaries. Yet, favorable environmental conditions across all sites could have contributed to lessen the potential hazardous biological effects of exposure to anthropogenic stressors, resulting in soles’ fairly good condition and generally high juvenile density. Nevertheless, a mismatch between high juvenile density and high estuarine contribution to adult coastal populations was observed in areas with higher anthropogenic pressures. Although a causal relationship cannot be established, the results emphasize the need to fully understand how the estuarine period spent in estuaries and local processes determine the quantity and quality of juveniles exported to marine adult populations, which is critical to achieve the full potential of the fish production service of estuaries and coastal stock replenishment.     

Selective crab predation on native and introduced bivalves in British Columbia

Experiments were conducted to determine whether locally abundant crab species prefer co-occurring littleneck clams, Protothaca staminea (Conrad, 1837) and Tapes philippinarum (A. Adams and Reeve, 1850), relative to a recently introduced species, the varnish clam, Nuttallia obscurata, (Reeve, 1857). Prey preference, handling time, pick-up success, profitability and consumption rates were investigated for two crab species, Dungeness crab, Cancer magister (Dana, 1852) and red rock crab, Cancer productus (Randall, 1839) crabs. Both crab species preferred varnish clams over the native species. This may be attributable to the lower handling time, higher pick-up success and increased profitability of consuming varnish clams. Handling time appeared to be a factor not only in species preference, but also in the degree of preference, with shorter handling times corresponding to stronger preference values. Both native and introduced bivalves burrow into the substratum, with the varnish clam burrowing deepest. When feeding on clams in limited substratum both crab species preferred the varnish clam. In the unlimited substratum trials Dungeness crabs preferred varnish clams (although to a lesser degree) while red rock crabs preferred littleneck clams. This was likely due to the significantly deeper burial of the varnish clam, making it less accessible. Although the morphology (i.e. thin shell, compressed shape) of the invader increases its vulnerability to predation, burial depth provides a predation refuge. These results demonstrate how interactions between native predators and the physical characteristics and behaviour of the invader can be instrumental in influencing the success of an invasive species.     

An analysis of competitive interactions between 3 hermit crab species

Summary Competition for empty gastropod shells in a group of three sympatric hermit crabs (Pagurus hirsutiusculus, Pagurus granosimanus, and Pagurus beringanus) was studied in the San Juan Archipelago, Washington State. Estimates of the competitive effects of each species on the others' shell supplies were derived using field data on shell utilization and the results of laboratory experiments to determine rates of acquisition and exchange of shells and preferences for different shell species. Each species experienced approximately an order of magnitude more intraspecific competition than interspecific competition for empty shells. This resulted from differences in preference for shell shapes, shell size use, and habitat use between P. hirsutiusculus and P. granosimanus, and largely from differences in habitat use between P. beringanus and the other two species. Experiments involving the release and recensusing of marked empty shells were used to estimate competitive effects more directly for the interaction between P. hirsutiusculus and P. granosimanus. Results were consistent with the estimates derived from data on resource partitioning. Possible causes of the low levels of interspecific competition are discussed, and results are compared with studies of other organisms that estimated both inter- and intra-specific competition.     

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.     

Interactive effects of episodic hypoxia and cannibalism on juvenile blue crab mortality

We hypothesized that as the spatial extent of hypoxic bottom water increased, (1) adult blue crab predator densities would increase in shallow habitats as they avoided hypoxia, and that (2) juvenile blue crabs, which use shallow unvegetated habitat as a predation refuge from adult conspecifics, would experience increased mortality rates during crowding by cannibalistic adult blue crabs. These hypotheses were tested along a depth gradient of sandy-mud shoreline in the Neuse River Estuary (NRE), North Carolina, USA using a combination of (1) hydrographic measurements to characterize the spatial extent of hypoxia, (2) beach seines to quantify the density of adult blue crab predators in relatively shallow water as a function of 1, and (3) tethering experiments to quantify relative rates of predation on juvenile blue crabs as a function of 1 and 2. During our seven tethering experiments, the NRE study site experienced a range of DO scenarios including normoxia, chronic hypoxia, and hypoxic upwelling. No known predators of juvenile blue crabs, other than adult conspecifics, were collected in any of our shallow-water seines. During the transition from normoxia to chronic hypoxia, blue crab predator densities in shallow refuge habitats increased 4-fold, and relative mortality rates of juvenile blue crabs in shallow habitats increased exponentially with the density of adult conspecifics. Conversely, during hypoxic upwelling events, the density of adult blue crabs in shallow water declined, which may explain why the relative mortality of juvenile crabs did not increase significantly with the increasing spatial extent of hypoxia. Thus, juvenile blue crabs may be relatively safe from adult conspecifics during hypoxic upwelling events, but not during chronic hypoxia. These experimental results highlight the need to consider the effects of dynamic water quality on mobile consumers emigrating from degraded habitats when considering indirect trophic impacts beyond the immediate area of impact.     

The relative importance of habitat-specific settlement, predation and juvenile dispersal for distribution and abundance of young juvenile shore crabs Carcinus maenas L.

Young juveniles of many motile benthic species are concentrated in structurally complex habitats, but the proximate causes of this distribution are usually not clear. In the present study, I assessed three potentially important processes affecting distribution and abundance of early benthic stages in the shore crab (Carcinus maenas): (1) selection of habitat by megalopae (postlarvae); (2) habitat-specific predation; and (3) post-settlement movements by juveniles. These processes were assessed concurrently over 3-9 days at two spatial scales: at the scale of square meters using cage techniques within nursery areas, and at the scale of hectares using isolated populations of juvenile shore crabs in small nursery areas as mesocosms. The results were compared to habitat-specific distribution in the field.Shore crab megalopae and first instar juveniles (settlers) were distributed non-randomly among micro-habitats in the assessed nursery areas, with great densities in both mussel beds, eelgrass and filamentous algal patches (on average 114-232 settlers m⁻²), and significantly smaller densities on open sand habitats at all times (on average 4 settlers m⁻²). The same habitat-specific settlement pattern was found in cages where predators were excluded, suggesting that active habitat selection at settlement was responsible for the initial distribution. Older juveniles (second to ninth instar crabs) were also sparse on sand, but in contrast to settlers, were concentrated in mussel beds, which showed significantly greater densities than eelgrass and algal habitats. The cage experiment demonstrated a dynamic distribution of juvenile crabs. Young juveniles constantly migrated over open sand habitats (20 m or further) and colonized the experimental plots in a habitat-specific pattern that reflected the distribution in the field. This pattern was also found for very small crabs colonizing predator-exclusion cages, suggesting that selection of habitat by migrating juveniles caused the ontogenetic change in habitat use. Although post-settlement movements were great within nursery areas, juvenile dispersal at a regional scale appeared to be small, and the recruitment of juvenile shore crabs to the shallow bays occurred mainly through pelagic megalopae.Conservative estimates at the scale of whole nursery areas, based on migration trap data and field samples, indicated great mortality of settlers and early benthic stages of shore crabs. Results from the cage experiment suggest that predation by crabs and shrimp were responsible for the high settlement mortality. Both enclosed cannibalistic juvenile crabs and local predators on uncaged habitat plots caused significant losses of settlers in all habitats (on average 22% and 64% 3 day⁻¹, respectively). The effect of predators was highly variable between trials, but differed little between habitat types, and predation had no detectable proximate effect on juvenile distribution, despite the great losses. Small settlement densities on sand habitats in combination with a refuge at low prey numbers, and an aggregation of cannibalistic juvenile crabs in nursery habitats appear to decrease the effect of habitat-specific predation rates on the distribution of juvenile shore crabs. This study demonstrates that active habitat selection at settlement followed by a dynamic redistribution of young juveniles can be the proximate processes responsible for habitat-specific distribution of epibenthic juveniles, and indicate that predation represents a major evolutionary process reinforcing this behavior.     

Big fish in shallow water; use of an intertidal surf-zone habitat by large-bodied teleosts and elasmobranchs in tropical northern Australia

Shallow intertidal habitats are recognised as critical for larval and juvenile fish, and are often assumed to function as refuge areas where predation risk is reduced. Yet there is growing evidence that suggest these areas may also be regularly inhabited by large bodied fish and be the site of high levels of juvenile fish predation. In the present study we examined the use of an intermittently available surf zone habitat in tropical northern Australia by a diverse community of large-bodied teleosts (mean total length 444 mm), sharks (mean total stretched length 658 mm) and rays (mean total stretched length 1,108 mm). Drawing on the methods and ecological knowledge of a local commercial fishery, gillnets were used to capture fish as they entered the surf zone on the flooding tide. Monthly surveys over a one-year period revealed a dynamic assemblage consisting of 30 species of teleosts (mostly caught as adults) and 14 species of sharks and rays (mostly caught as juveniles or young of the year). Although it is unclear why these fish use this habitat, we conclude that it may support a broad range of biological benefits including spawning for teleosts, parturition for elasmobranchs, as well as foraging and refuge. The unique findings of this study highlight existing knowledge gaps and the need to better understand what fish use the intertidal zone and why. This should be a high priority given the increasing anthropogenic pressures on coastal margins.     

Modelling the dynamics of invasion and control of competing green crab genotypes

Establishment of invasive species is a worldwide problem. In many jurisdictions, management strategies are being developed in an attempt to reduce the environmental and economic harm these species may cause in the receiving ecosystem. Scientific studies to improve understanding of the mechanisms behind invasive species population growth and spread are key components in the development of control methods. The work presented herein is motivated by the case of the European green crab (Carcinus maenas L.), a remarkably adaptable organism that has invaded marine coastal waters around the globe. Two genotypes of European green crab have independently invaded the Atlantic coast of Canada. One genotype invaded the mid-Atlantic coast of the USA by 1817, subsequently spreading northward through New England and reaching Atlantic Canada by 1951. A second genotype, originating from the northern limit of the green crabs European range, invaded the Atlantic coast of Nova Scotia in the 1980s and is spreading southward from the Canadian Maritime provinces. We developed an integrodifference equation model for green crab population growth, competition and spread, and demonstrate that it yields appropriate spread rates for the two genotypes, based on historical data. Analysis of our model indicates that while harvesting efforts have the benefit of reducing green crab density and slowing the spread rate of the two genotypes, elimination of the green crab is virtually impossible with harvesting alone. Accordingly, a green crab fishery would be sustainable. We also demonstrate that with harvesting and restocking, the competitive imbalance between the Northern and Southern green crab genotypes can be reversed. That is, a competitively inferior species can be used to control a competitively superior one.     

Simple ecological trade-offs give rise to emergent cross-ecosystem distributions of a coral reef fish

Ecosystems are intricately linked by the flow of organisms across their boundaries, and such connectivity can be essential to the structure and function of the linked ecosystems. For example, many coral reef fish populations are maintained by the movement of individuals from spatially segregated juvenile habitats (i.e., nurseries, such as mangroves and seagrass beds) to areas preferred by adults. It is presumed that nursery habitats provide for faster growth (higher food availability) and/or low predation risk for juveniles, but empirical data supporting this hypothesis is surprisingly lacking for coral reef fishes. Here, we investigate potential mechanisms (growth, predation risk, and reproductive investment) that give rise to the distribution patterns of a common Caribbean reef fish species, Haemulon flavolineatum (French grunt). Adults were primarily found on coral reefs, whereas juvenile fish only occurred in non-reef habitats. Contrary to our initial expectations, analysis of length-at-age revealed that growth rates were highest on coral reefs and not within nursery habitats. Survival rates in tethering trials were 0% for small juvenile fish transplanted to coral reefs and 24-47% in the nurseries. As fish grew, survival rates on coral reefs approached those in non-reef habitats (56 vs. 77-100%, respectively). As such, predation seems to be the primary factor driving across-ecosystem distributions of this fish, and thus the primary reason why mangrove and seagrass habitats function as nursery habitat. Identifying the mechanisms that lead to such distributions is critical to develop appropriate conservation initiatives, identify essential fish habitat, and predict impacts associated with environmental change.     

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.     

七种捕食性鱼类对中华绒螯蟹幼蟹捕食风险的评估

分别以鲤、鳜、斑点叉尾、黄颡鱼、瓦氏黄颡鱼、大口鲇和乌鳢作为捕食者,以中华绒螯蟹幼蟹作为猎物,在室内水泥池(2.4 m3)进行捕食试验。以日捕获率和日摄食率为指标,评估这些鱼类对幼蟹的捕食作用和危害程度,为提高湖泊幼蟹放流效果、建立蟹—鱼复合的优质高效养殖模式提供科学依据。在幼蟹完全暴露的条件下,经过多次(至少9次)重复的试验(短期1d和长期7d),鳜对不同大小的硬壳和软壳(刚蜕壳的)幼蟹没有任何捕食作用;黄颡鱼对硬壳和软壳幼蟹也没有捕食作用,但还需做进一步观察;虽然鲤、瓦氏黄颡鱼对硬壳蟹的捕获率低,但对软壳的幼蟹有较大的危害性,对幼蟹的日摄食率分别为0.070%、0.012%;大口鲇、斑点叉尾、乌鳢对幼蟹具有较强的捕食能力,对幼蟹的日摄食率分别为0.122%、0.188%和0.284%。根据这些研究结果,可以建议:(1)在池塘和湖泊河蟹养殖中,完全可以将鳜作为套养或混养对象,以期提高养殖效益;(2)在河蟹放养的湖泊,需要抑制乌鳢和大口鲇种群,适当减少鲤和瓦氏黄颡鱼丰度,以期减少这些鱼类的捕食作用,提高幼蟹存活率;(3)在河蟹养殖池塘,不能放养乌鳢、大口鲇、斑点叉尾、瓦氏黄颡鱼和鲤。     

Effects of marine reserves versus nursery habitat availability on structure of reef fish communities

No-take marine fishery reserves sustain commercial stocks by acting as buffers against overexploitation and enhancing fishery catches in adjacent areas through spillover. Likewise, nursery habitats such as mangroves enhance populations of some species in adjacent habitats. However, there is lack of understanding of the magnitude of stock enhancement and the effects on community structure when both protection from fishing and access to nurseries concurrently act as drivers of fish population dynamics. In this study we test the separate as well as interactive effects of marine reserves and nursery habitat proximity on structure and abundance of coral reef fish communities. Reserves had no effect on fish community composition, while proximity to nursery habitat only had a significant effect on community structure of species that use mangroves or seagrass beds as nurseries. In terms of reef fish biomass, proximity to nursery habitat by far outweighed (biomass 249% higher than that in areas with no nursery access) the effects of protection from fishing in reserves (biomass 21% lower than non-reserve areas) for small nursery fish (≤ 25 cm total length). For large-bodied individuals of nursery species (>25 cm total length), an additive effect was present for these two factors, although fish benefited more from fishing protection (203% higher biomass) than from proximity to nurseries (139% higher). The magnitude of elevated biomass for small fish on coral reefs due to proximity to nurseries was such that nursery habitats seem able to overrule the usually positive effects on fish biomass by reef reserves. As a result, conservation of nursery habitats gains importance and more consideration should be given to the ecological processes that occur along nursery-reef boundaries that connect neighboring ecosystems.     

Spatial and temporal patterns in the contribution of fish from their nursery habitats

Because anthropogenic influences threaten the degradation of many ecosystems, determining where organisms live during early life-history stages and the extent to which different areas contribute individuals to adult populations is critical for the management and conservation of a species. Working in Puget Sound, Washington State in the United States, and using a common flatfish (English sole, Parophrys vetulus), we sought to establish (using otolith chemistry) which areas contribute age-0 fish to age-1 population(s), the extent to which this pattern was consistent between two years, and whether this spatial pattern of contribution coincides with surveys of age-0 fish and/or the available area of nearshore habitat. Our study indicated completely different spatial patterns of fish nursery use between the two years of sampling. We highlight that the contribution of individuals from nursery areas is not related to density of recently settled English sole or the available area of nearshore habitat (depth <10 m) in Puget Sound, nor can we draw conclusions based on environmental data (precipitation, water salinity, light transmission, pH, dissolved oxygen, and water temperature). The results of this study highlight (1) the need for assessing the temporal patterns of nursery habitat use, and (2) that, in order to conservatively manage a species and its population(s), it may be necessary to protect several areas that are used intermittently by that species.     

Variation in fish density,assemblage composition and relative rates of predation among mangrove,seagrass and coral reef habitats

We tested the hypothesis for several Caribbean reef fish species that there is no difference in nursery function among mangrove, seagrass and shallow reef habitat as measured by: (a) patterns of juvenile and adult density, (b) assemblage composition, and (c) relative predation rates. Results indicated that although some mangrove and seagrass sites showed characteristics of nursery habitats, this pattern was weak. While almost half of our mangrove and seagrass sites appeared to hold higher proportions of juvenile fish (all species pooled) than did reef sites, this pattern was significant in only two cases. In addition, only four of the six most abundant and commercially important species (Haemulon flavolineatum, Haemulon sciurus, Lutjanus apodus, Lutjanus mahogoni, Scarus iserti, and Sparisoma aurofrenatum) showed patterns of higher proportions of juvenile fish in mangrove and/or seagrass habitat(s) relative to coral reefs, and were limited to four of nine sites. Faunal similarity between reef and either mangrove or seagrass habitats was low, suggesting little, if any exchange between them. Finally, although relative risk of predation was lower in mangrove/seagrass than in reef habitats, variance in rates was substantial suggesting that not all mangrove/seagrass habitats function equivalently. Specifically, relative risk varied between morning and afternoon, and between sites of similar habitat, yet varied little, in some cases, between habitats (mangrove/seagrass vs. coral reefs). Consequently, our results caution against generalizations that all mangrove and seagrass habitats have nursery function.     

Why do juvenile fish utilise mangrove habitats?

Three hypotheses to discern the strong positive association between juvenile fish and mangrove habitat were tested with field and laboratory experiments. Artificial mangrove structure in the field attracted slightly more juvenile fish than areas without structure. Artificial structure left to accumulate fouling algae attracted four-times the total number of juvenile fish than areas without structure or areas with clean structure. Community composition of fish attracted to structure with fouling algae was different when compared with areas with no structure or clean structure; five species were attracted by structure with fouling algae whilst two species were associated with structure regardless of fouling algae. Algae were linked to increased food availability and it is suggested that this is an important selection criteria for some species. Other species were apparently attracted to structure for different reasons, and provision of shelter appears to be important. Predation pressure influenced habitat choice in small juvenile fish in laboratory experiments. In the absence of predators, small juveniles of four out of five species avoided shelter but when predators were introduced all species actively sought shelter. Large fish were apparently less vulnerable to predators and did not seek shelter when predators were added to their tank. Feeding rate was increased in the mangrove habitat for small and medium-sized fish compared with seagrass beds and mudflats indicating increased food availability or foraging efficiency within this habitat. Larger fish fed more effectively on the mudflats with an increased feeding rate in this habitat compared with adjacent habitats. The most important aspect of the mangrove habitat for small juvenile fish is the complex structure that provides maximum food availability and minimises the incidence of predation. As fish grow a shift in habitat from mangroves to mudflat is a response to changes in diet, foraging efficiency and vulnerability to predators.     

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。     

Learning in an invasive and a native predatory crab

The invasion of the green crab Carcinus maenas in the northeastern U.S. and its competition with the native blue crab Callinectes sapidus and other native crustaceans has been well-documented and researched. Various reasons for the invader’s success against native crabs have been examined (juvenile predation, food source flexibility, etc.), but another possibility is a difference in the learning ability of invasive versus native crab species. In this study, the learning ability of C. maenas and C. sapidus was tested by their increased speed in locating hidden food over successive days. The data suggest that C. maenas possesses a learning ability significantly greater than that of C. sapidus, which may partially contribute to its success.