绒螯蟹触角形态特征及其在分类中的应用

基于光镜和扫描电镜的观察,研究了绒螯蟹触角的形态结构,分析了绒螯蟹第1触角内、外肢节的组成,以及外肢节内侧感觉毛的特点;第2触角内肢节的形状以及极长感觉毛和表面鳞片的特征.探讨了在绒螯蟹系统分类中,触角形态特征作为新的分类性状的可能性.     

The effect of Sacculina carcini infections on the fouling, burying behaviour and condition of the shore crab, Carcinus maenas

Crabs infected by rhizocephalans usually do not moult. Because moulting is the ultimate defence against fouling epibionts, infected as opposed to uninfected crabs can be expected to harbour a diversity of hard-bottom organisms on their cuticula. Here we provide unequivocal evidence that this is the case in the Carcinus maenas-Sacculina carcini association. In a Danish sample of shore crabs, 75% of sacculinized individuals harboured macroscopic epibionts, whereas only 29% of the uninfected crabs were colonized. The mean numbers of fouling barnacles and serpulid tubeworms per individual were 7.7 and 47.3 for uninfected and infected crabs, respectively, corresponding to coverage of the cuticula by 0.7 and 5.4%. Infected crabs were 12% lower in tissue dry weight than uninfected individuals, which may be a factor causing the moult of sacculinized crabs to be postponed. Finally, a laboratory experiment suggested that infected crabs are less likely to bury than uninfected specimens. Because burying is an important fouling defence, such a parasite-induced behavioural change will favour the colonization by epibionts. It is argued that rhizocephalans infecting crabs from soft-bottom communities may indirectly affect the structure of the free-living benthic community by adding hard-bottom species that otherwise would be absent.     

Phylogenetic ecology of gall crabs (Cryptochiridae) as associates of mushroom corals (Fungiidae)

Coral‐associated fauna is a relatively understudied topic. Hence, the nature of the relationship between an associated organism and its host is usually unknown. In the present study, the obligate associations between gall crabs (Decapoda: Cryptochiridae) and mushroom corals (Scleractinia: Fungiidae) are reviewed from a phylogenetic perspective. Based on field surveys, examination of museum material and a literature review, a total of 35 fungiid species have been found that act as hosts for four gall crab species. Fungiid‐associated gall crabs appear to be more geographically widespread than previously known, with new records showing their occurrences from the Red Sea and western Indian Ocean all the way to the central Pacific Ocean. The obligate nature of the association between cryptochirids and their hosts makes them an ideal model taxon to test for possible cospeciation events. The congruence between their phylogenies was tested by using the program Jane 4.0, resulting in cospeciation and duplication events between the crabs and their host corals. The sharing of several closely related host coral species by the same gall crab species or genus may provide support to models indicating phylogenetic relationships within the Scleractinia.     

The relative importance of substratum characteristics and recruitment in determining the spatial distribution of the fiddler crab Uca uruguayensis Nobili

The roles of sediment characteristics and the pattern of recruitment in influencing the abundance of the fiddler crab Uca uruguayensis on Argentinean mudflats were evaluated. The density of adult crabs showed a patchy distribution related to the sediment thickness (depth at which a layer of fossil shells are buried), but the density of juvenile crabs was not coupled with the density of adult crabs. In a field experiment, fossil shells were removed and the density of crabs significantly increased, which demonstrates that the presence of the layer of shells is a structure that may hinder the establishment of burrows. The density of crabs was related to sediment thickness, sediment torque and organic matter content. The importance of each of these variables was different for adult and juvenile crabs, indicating that the distribution of adult crabs may be caused by mechanisms affecting adult crabs themselves and is not established by the recruitment pattern. Moreover, in a field experiment, the density of juveniles decreased when adult crabs were added, and increased when adult crabs were removed.The morphology of burrows was related to sediment characteristics. Burrows were deeper, longer and more voluminous when sediment thickness was high. The volume of burrows decreased with increasing sediment torque. These results suggest that the morphology of burrows is related to the space available and the ease with which sediment it can be excavated. However, an important amount of variability remained unexplained, suggesting the presence of additional environmental variables or behavioural plasticity not considered by this study. Together, these results demonstrate that the spatial heterogeneity in the environmental factors can be translated to a spatial heterogeneity in the distribution of fiddler crabs.     

Paradoxical selective feeding on a low-nutrient diet: why do mangrove crabs eat leaves?

Sesarmid crabs dominate Indo West-Pacific mangroves, and consume large amounts of mangrove litter. This is surprising, since mangrove leaves have high tannin contents and C/N ratios that far exceed 17, normally taken as the maximum for sustainable animal nutrition. This paradox has led to the hitherto untested hypothesis that crabs let leaves age in burrows before consumption, thereby reducing tannin content and C/N ratio. We excavated burrows of Neosarmatium meinerti within high-shore Avicennia marina mangroves, and investigated whether burrow leaves had C, N or C/N values significantly different from those of senescent leaves. Leaves were found in <45% of burrows, mostly only as small fragments, and N concentrations and C/N ratios of burrow leaves never varied significantly from senescent leaves. The leaf-ageing hypothesis was therefore not supported. In the field N. meinerti and Sesarma guttatum fed on sediment in 76% and 66-69% of observations, respectively, and on leaves in <10% of observations. Sediments from two A. marina mangroves had a mean C/N ratio of 19.6. Our results, and the literature, show that mangrove leaves are unlikely to fulfil the N requirements of crabs, whether or not leaf ageing takes place. Sediment detritus could be a richer source of N, as shown by lower C/N ratios and regular ingestion by crabs. By fragmenting leaves crabs may be elevating the nutritional quality of the substrate detritus.     

Extraction of hermit crabs from their shells by white-faced capuchin monkeys (<Emphasis Type="Italic">Cebus capucinus</Emphasis>)

We observed two capuchin monkeys (Cebus capucinus) feeding on hermit crabs (Coenobita compressus) on the coast, and the tactics they used to extract this well-protected prey. The observations took place during the dry season at Playa Escondida beach, Puntarenas, Costa Rica. The capuchins descended from trees at the back edge of the beach to capture passing hermit crabs. Both capuchins extracted the hermit crabs from their protective shells by holding the shell with one hand and pulling the crab out with the other. Even though this was accomplished within seconds, the extraction of hermit crabs from their shells did not appear to be a straightforward task. Once the capuchins succeeded in pulling the crabs out of their shells, they consumed the soft abdomen and discarded the rest of the crab's body. To our knowledge, the consumption of hermit crabs has not been previously reported for any capuchin monkey (Cebus or Sapajus). Our observations provide a new example of extractive foraging by capuchins, and thus an additional natural context for which fine motor skills (which are highly developed in capuchins) are necessary.     

海南岛淡水蟹类分布格局与多样性保护

热带岛屿生物多样性是全球生物多样性保护研究的热点之一。海南岛是中国面积最大的热带岛屿, 丰富独特的淡水蟹类是维持岛内淡水生态系统功能完整性的关键类群。本文通过多年野外调查, 综合历史及最新文献资料, 对海南岛淡水蟹类物种多样性及其现状进行调查和评估, 并对淡水蟹类物种多样性保护现状进行了分析讨论。研究发现, 海南岛淡水蟹类物种多样性分布中心位于中南部山地, 主要集中于中部的霸王岭、鹦哥岭和猕猴岭, 南部的五指山和吊罗山, 以及西南部的尖峰岭一带。其物种多样性整体上呈现中南部山地高、平原台地低的特点。根据《IUCN物种红色名录濒危等级和标准》对海南岛淡水蟹类物种现状的评估结果显示, 全岛受威胁淡水蟹类物种的占比为16.7%。基于分布区预测, 以海南热带雨林国家公园为主体的保护地对淡水蟹类潜在适宜分布区的覆盖度明显优于此前碎片化的各级保护区。本文研究结果显示, 海南岛淡水蟹类的总体生存状况良好, 但一部分山地或平原种类处于受胁状态。国家公园体制的建立有望为岛内淡水蟹类物种多样性保护提供前所未有的机遇。基于物种多样性分布格局开展淡水蟹类等淡水生物多样性监测, 有助于促进海南岛淡水生态系统完整性的长效保护与可持续发展。     

The blue streak: a dynamic trait in the mud fiddler crab, Uca pugnax

Mud fiddler crabs, Uca pugnax, have a streak of blue coloration located on the front of the carapace above the mouth and centered between the eyes. We documented that this blue streak is absent in juveniles and develops as crabs become sexually mature. By photographing male crabs under controlled conditions in the laboratory, we demonstrated that the brightness of the blue streak (in comparison with the rest of the carapace) is dynamic, and can dim from bright blue to nearly black in fewer than two minutes. We examined blue streak variability in male crabs in response to physical factors (light and temperature) and social context to begin to understand what causes its dynamic response. The blue streak darkens in response to decreased ambient light, but does not respond to changes in temperature. In the field, it is brighter when crabs are roaming on the mudflat or fighting, but darker when crabs are basking or performing waving displays. The highly visual nature of fiddler crabs and the dynamic character of the blue streak suggest that it may communicate information about the state of a crab or its environment.     

Hair-trigger autotomy in porcelain crabs is a highly effective escape strategy

The benefits of autotomy, the voluntary shedding of limbs, havebeen adequately demonstrated in vertebrates but are poorlystudied in invertebrates. We provide strong experimental evidencefor an antipredatory benefit to autotomy in two porcelain crabs(Petrolisthes cinctipes and P. manimaculis). Since the outcomeof autotomy depends critically on the species of predator andprey involved, we first surveyed field populations of porcelaincrabs to identify ecologically relevant predators to use insubsequent experiments. We then examined the escape tacticsof the porcelain crabs in response to the four potential predatorswe identified, all larger predatory crabs. Cheliped autotomywas induced by three of the predator species (Cancer antennarius,Hemigrapsus nudus, Pachygrapsus crassipes); the fourth didnot attack porcelain crabs. Autotomy occurred in response to34% of all attacks, and in 67% of attacks in which the preywas held at some point by only the cheliped. Autotomy was ahighly effective escape mechanism against these predators;58 of 59 crabs that autotomized escaped, usually while thepredator stopped pursuit to eat the shed cheliped. Relianceon autotomy as a primary mechanism for escaping predators maybe particularly common in small crabs that cannot adequatelydefend themselves by other means and in suspension-feedingcrabs that do not need their chelipeds for feeding.     

Vacancy Chains Provide Aggregate Benefits To Coenobita clypeatus Hermit Crabs

Vacancy chain theory describes a unique mechanism for the sequential distribution of animal resources across multiple individuals. This theory applies to any resources, such as shelters or nest sites, that are discrete, reusable, and limited in use to single individuals or groups at one time. Hermit crabs rely on gastropod shells for shelter, and a single vacant shell can initiate a chain of sequential shell switches that distributes new resources across many individuals. Using the terrestrial hermit crab Coenobita clypeatus , we examined the previously untested theoretical prediction that this process will yield trickle-down resource benefits to vacancy chain participants (aggregate benefits). In laboratory experiments, we measured improvements in shell quality when a single vacant shell was provided to groups of eight crabs. We found that crabs participating in vacancy chains (averaging 3.2 individuals) gained significant reductions in their shell crowding. In addition, vacancy chains terminated early when experimental groups included a single crab occupying a damaged shell, because damaged vacancies always remained unoccupied. Hermit crabs in damaged shells were more likely to win resource contests for high quality shells against size-matched hermit crabs in crowded shells. Finally, field additions of many new shells to an island population of C. clypeatus hermit crabs reduced average shell crowding for crabs of all sizes, possibly from propagation of benefits through vacancy chains. These results provide empirical support for the theoretical prediction that vacancy chains should provide benefits distributed across many vacancy chain participants. Since shelter-based vacancy chains likely occur in other animals, additional studies of vacancy chain processes should provide new insights into resource acquisition behaviors in diverse animal groups.     

Moulting synchrony in green crabs (Carcinus maenas) from Prince Edward Island,Canada

The growth and spread of non-indigenous green crabs (Carcinus maenas) in Atlantic Canada is of concern to the sustainability of shellfish resources, particularly in areas recently invaded. Commercial green crab fishing has been initiated on Prince Edward Island to help control this species and provide a new resource for inshore fishermen. Developing a soft-shell crab product modelled after the Venetian ‘Moleche’ would provide an economic incentive beyond the existing hard-shell crab bait market. However, answers to questions such as the timing and characteristics of green crab moulting are required. A pilot study conducted in 2014–2015 collected seven groups of crabs and held them in individual compartments for 2–4 weeks to record moulting rates and physical characteristics. We found that a synchronized ‘moulting window’ occurs during July for male crabs. Field experiments in 2015 had an average moulting rate of 34%, with group-specific rates as high as 60%. The same cohort of crabs held in the laboratory had an average moulting rate of 48%, with group-specific rates as high as 75%. We observed a gradual increase in moulting rates from early to mid-July, after which all crabs caught had recently moulted, with evidence of new carapaces on all crabs. In 2015, the moulting window followed a 5°C increase in water temperature. Regarding morphology, the presence of a ‘halo’ on the episternites of the carapace was an indicator that a crab would soon moult. These promising results represent the first step in assessing the feasibility of a soft-shell, green crab industry.     

Stimulation of feeding behavior in three species of fiddler crabs by hexose sugars

Feeding responses to five hexoses were examined in three closelyrelated species of fiddler crabs, Uca minax, U.pugnax and U.pugilator.Hexoses tested were glucose, galactose, sorbose, fructose andmannose. Intact crabs and eyestalk-ablated crabs were tested.Responses to sugars were species specific. Eyestalks are directlyinvolved in vision and overall neural integration as well aswith chemosensory and metabolic pathways associated with feeding.Overall, eyestalk-ablated crabs were more sensitive uian intactcrabs. Studies of responses of individuals within a populationto hexoses showed tiiere are individual U.pugnax that respondto galactose and others that do not. Similarly, there were U.pugilatorindividuals that were mannose responsive and others that didnot respond to mannose. An additional study of differences inpopulation responses to hexoses would provide valuable toolsin studying geographic relationships between fiddler crab populations.     

Evolution, insular restriction, and extinction of oceanic land crabs, exemplified by the loss of an endemic Geograpsus in the Hawaiian Islands

Most oceanic islands harbor unusual and vulnerable biotas as a result of isolation. As many groups, including dominant competitors and predators, have not naturally reached remote islands, others were less constrained to evolve novel adaptations and invade adaptive zones occupied by other taxa on continents. Land crabs are an excellent example of such ecological release, and some crab lineages made the macro-evolutionary transition from sea to land on islands. Numerous land crabs are restricted to, although widespread among, oceanic islands, where they can be keystone species in coastal forests, occupying guilds filled by vertebrates on continents. In the remote Hawaiian Islands, land crabs are strikingly absent. Here we show that absence of land crabs in the Hawaiian Islands is the result of extinction, rather than dispersal limitation. Analysis of fossil remains from all major islands show that an endemic Geograpsus was abundant before human colonization, grew larger than any congener, and extended further inland and to higher elevation than other land crabs in Oceania. Land crabs are major predators of nesting sea birds, invertebrates and plants, affect seed dispersal, control litter decomposition, and are important in nutrient cycling; their removal can lead to large-scale shifts in ecological communities. Although the importance of land crabs is obvious on remote and relatively undisturbed islands, it is less apparent on others, likely because they are decimated by humans and introduced biota. The loss of Geograpsus and potentially other land crabs likely had profound consequences for Hawaiian ecosystems.     

Shell fights in the hermit crabPagurus geminus: Effect of cheliped use and shell rapping

In shell fights of the hermit crab,Pagurus geminus, frequently it is observed that large crabs (attackers) grasp the thoracic appendage of small crabs (defenders) with the major cheliped and pull the smaller crabs out of their shells. If this is a standard occurrence and result, then the interaction should not be called a “negotiation” (Hazlett 1978). The role of cheliped use by the attckers in the eviction of defenders was therefore studied using crabs with tubes on their chelipeds, and the effect of shell rapping, which is thought to be necessary for eviction, was studied using crabs without shells. The experimental crabs evicted the defenders but fighting was significantly prolonged. Therefore, the negotiation model cannot be rejected. Specific aspects of shell fights in hermit crabs are discussed.     

Ecological consequences of ontogenetic shifts in predator diet: Seasonal constraint of a behaviorally mediated indirect interaction

Predators play an important role in structuring assemblages through direct and cascading indirect effects. While there has been recent interest in how the strength and direction of trophic cascades vary spatially, seasonal variability in trophic links is seldom considered. In North Carolina, recruitment-failure of bay scallops typically occurs following the spring but not the fall spawning despite the presence in each of these seasons of predatory blue crabs. One explanation for this pattern is that in the fall, seasonally abundant predators of blue crabs reduce the foraging efficiency of crabs on scallops and thus the overall magnitude of top-down effects. Quantification of bay scallop consumption by blue crabs in closed mesocosms with or without pinfish supported the hypothesis that seasonally abundant adult pinfish indirectly increase survivorship of bay scallop recruits in fall by reducing predation by blue crabs. Despite voracious consumption of bay scallops during both the day and night in mesocosms to which only small blue crabs were added, blue crabs in mesocosms with visually-foraging adult pinfish consumed bay scallops only by night. Juvenile pinfish that dominate estuarine populations in spring did not impede consumption of bay scallops by blue crabs. In mesocosms from which animals could not emigrate, the addition of neither adult nor juvenile pinfish increased the mortality of blue crabs, indicating a behaviorally mediated interaction. Blue crabs restricted by adult pinfish to nocturnal feeding did not compensate for lost feeding time by increasing their night-time consumption of bay scallops. These results strongly suggest that greater survivorship of bay scallops in fall than spring is due to adult pinfish, potential predators of small blue crabs, restricting blue crab foraging to hours of dark. In spring, when pinfish are small and incapable of consuming blue crabs, blue crabs consume bay scallops by day and by night. Such seasonal variation in the number of trophic links in a system may have important evolutionary implications. By timing reproduction to occur in fall when the pinfish-crab-scallop cascade is in operation, bay scallops maximize recruitment.     

Host specificity of <Emphasis Type="Italic">Sacculina carcini</Emphasis>, a potential biological control agent of the introduced European green crab Carcinus maenas in California

The European green crab, Carcinus maenas, is an introduced marine predator established on the west coast of North America. We conducted laboratory experiments on the host specificity of a natural enemy of the green crab, the parasitic barnacle Sacculina carcini, to provide information on the safety of its use as a possible biological control agent. Four species of non-target, native California crabs (Hemigrapsus oregonensis, H. nudus, Pachygrapsus crassipes and Cancer magister) were exposed to infective larvae of S. carcini. Settlement by S. carcini on the four native species ranged from 33 to 53%, compared to 79% for green crabs. Overall, cyprid larvae tended to settle in higher numbers on individual green crabs than on either C. magister or H. oregonensis. However, for C. magister this difference was significant for soft-shelled, but not hard-shelled individuals. Up to 29% of the native crabs arrested early infections by melanizing the rootlets of the parasite. Most native and green crabs settled on by S. carcini became infected, especially when settled on by >3 cyprids. Infected green crabs died at more than twice the rate of uninfected green crabs. In contrast to green crabs, all infected native crabs died without producing an externa (reproductive sac). At high settlement intensities, infected native crabs frequently exhibited neurological symptoms (twitching, loss of movement) before death. These results indicate that use of S. carcini as a biological control agent could result in the death of native crabs. The magnitude of this effect would be proportional to the density of infected green crabs in the environment and the probability that cyprids would contact native crabs in the wild. Potential benefits of biological control should be assessed in relation to these potential non-target effects.     

Phylogenetic analysis of intestinal bacteria in the Chinese mitten crab (Eriocheir sinensis)

AIMS: To identify the dominant intestinal bacteria in the Chinese mitten crab, and to investigate the differences in the intestinal bacteria between pond-raised and wild crabs. METHODS AND RESULTS: The diversity of intestinal bacteria in the Chinese mitten crabs was investigated by denaturing gradient gel electrophoresis (DGGE) fingerprinting, 16S rRNA gene clone library analysis and real-time quantitative PCR. The principal component analysis of DGGE profiles indicated that substantial intersubject variations existed in intestinal bacteria in pond-raised crab. The sequencing of 16S rRNA genes revealed that 90-95% of the phylotypes in the clone libraries were affiliated with Proteobacteria and Bacteroidetes. Some genera were identified as unique in wild crabs and in pond-raised crabs, whereas Bacteroidetes was found to be common in all sampled crab groups. Real-time quantitative PCR indicated that the abundance of Bacteroides and the total bacterial load were approximately four-to-10 times higher in pond-raised crabs than in wild crabs. A significant portion of the phylotypes shared low similarity with previously sequenced organisms, indicating that the bacteria in the gut of Chinese mitten crabs are yet to be described. CONCLUSIONS: The intestinal bacteria of pond-raised crabs showed higher intersubject variation, total diversity and abundance than that observed in wild crabs. The high proportion of the clones of Proteobacteria and Bacteroidetes in the clone library is an indication that these bacteria may be the dominant population in the gut of the Chinese mitten crab. SIGNIFICANCE AND IMPACT OF THE STUDY: This study demonstrated obvious differences in the intestinal bacterial composition of pond-raised crabs and wild crabs. This knowledge will increase our understanding of the effects of aquaculture operations on bacterial community composition in the crab gut and provide necessary data for the development of probiotic products for crab cultivation.     

Homing in fiddler crabs (Uca lactea annulipes and Uca vomeris : Ocypodidae)

Fiddler crabs emerge from burrows on intertidal sand- and mudflats to feed during low tide. In the species studied here (Uca lactea annulipes, Uca vomeris) a crab normally wanders no more than about 1 m away from its burrow and, when frightened, dashes back along a straight line to take cover. Feeding crabs tend to move sideways, without changing orientation, along paths radiating from the burrow. When they move along circumferential paths they adjust their orientation so that one side continues to point towards the burrow. The crabs do not need to see the burrow in order to stay aligned with the home vector, and they are not misled by a dummy hole close to their own burrow unless they have come to within about 10 cm of it. The home runs of crabs end within a few centimeters of a burrow that is covered with a sheet of sandpaper and then give way to search runs, centred upon a position slightly short of the burrow location. Feeding crabs can be displaced on sandpapers and their subsequent home runs end at a position where the burrow would be, had there been no displacement. Landmarks close to the burrow do not influence the home runs of displaced crabs. Crabs that are rotated on a sheet of sandpaper, counter-turn to keep their original orientation constant. Fiddler crabs thus employ path integration with external compass information and close range visual guidance for homing. Accepted: 11 May 1998     

Prevalence of bacteria in the spermathecae of female snow crab, Chionoecetes opilio (Brachyura: Majidae)

The spermathecal contents of primiparous, multiparous and barren female snow crabs were observed by a scanning electron microscope. Bacteria colonies were observed with a significantly higher frequency in the spermathecae of primiparous and old barren females. Bacteria infect the spermathecae and destroy the spermatophores and spermatozoa inside. These observations suggest that bacteria in the spermathecae do not exclude opportunistic microbes by modifying pH of the medium as suggested in the literature. The prevalence of bacteria in primiparous and old barren females suggests that they infect individuals with a weak anti-microbial protection. The absence of bacteria in the highly acidic seminal fluid derived from males upon copulation suggests that it may provide anti-microbial protection. Bacteria do not seem to be able to survive in an anaerobic environment for a long period.     

The effect of salinity on experimental infections of a Hematodinium sp. in blue crabs, Callinectes sapidus

The parasitic dinoflagellate Hematodinium sp. parasitizes blue crabs along the Atlantic seaboard of the United States. Infections in blue crabs have only been reported from waters where salinity is >11 practical salinity units (psu). Blue crabs maintain a hyperosmotic internal concentration at low salinities (0-5 psu), roughly comparable to 24 psu, and should be capable of maintaining an infection in low-salinity waters even if Hematodinium spp. cells are intolerant of low salinities. We tested this notion by observing the effect of low salinity on the progression of disease in crabs experimentally infected with the parasite. Blue crabs were acclimated to 5 psu or 30 psu salinity treatments. They were inoculated with Hematodinium sp. and necropsied 3, 7, 10, and 15 days post-inoculation. The low-salinity treatment did not have an effect on the proliferation of Hematodinium sp. infections in blue crabs; moreover, a greater proportion of infections in crabs in the low-salinity treatment developed dinospore stages than did those in the high-salinity treatment, indicating that salinity may affect the development of the parasite. However, dinospores from in vitro cultures rapidly became inactive when held in salinities <15 psu. Our experiments indicate that Hematodinium spp. can develop in blue crabs at low salinities, but that the parasite is incapable of transmission in this environment, which explains the lack of natural infections in crabs at low salinities.