Valve closure responses of the Asiatic clam Corbicula fluminea exposed to cadmium and zinc

The valve movement patterns of immobilized Asiatic clams (Corbicula fluminea) were monitored during exposure to constant concentrations of cadmium (0.0, 0.1, 0.2, 0.3, and 0.4 mg 1^–1) or zinc (0.0, 0.1, 0.3, 0.5, and 0.9 mg 1^–1) for 24 h following a 24-h acclimation period. Data indicate that the duration of response was concentration dependent and toxicity related. Durations of periods with valves parted declined as the concentration of heavy metal increased. Behavior was consistent for both mean time to first closure following the initial exposure and mean time per valve parting episode over a 24-h exposure period. Mean time per valve parting episode during the 24-h exposure period ranged from 600 minutes for control trials to 36 and 69 minutes for the highest concentrations of cadmium and zinc tested, respectively. There was no association between durations of periods with valves sealed and solutions of cadmium; however, lengthening durations of periods with valves sealed coincided with exposure of clams to progressively more concentrated zinc solutions. In addition, Asiatic clams demonstrated a greater rate of response (decline in the duration of periods with valves parted) to progressively more concentrated solutions of cadmium than to comparable increases in the concentrations of zinc solutions.     

Ecological resistance to the invasion of a freshwater clam,Corbicula fluminea: fish predation effects

Summary Fish predation is shown to have a twenty nine fold effect on the abundance of the invasive freshwater clam, Corbicula fluminea, in a Texas reservoir. This predation has prevented the clam from establishing the high densities commonly reported for it elsewhere. The high magnitude of the fish effect is attributed to Corbicula being an invader to this reservoir and not being able to cope well with the mix of resident fish species. In the absence of fish, colonization of the reservoir by Corbicula is spatially patchy. When fish interact with these clams, they remove sufficient numbers of individuals from dense patches to create the appearance of a spatially uniform distribution.     

Metallothionein responses in the Asiatic clam (Corbicula fluminea) after exposure to trivalent arsenic

The main objective of this work was to evaluate arsenic effects on metallothionein (MT) induction by exposing a freshwater Asiatic clam (Corbicula fluminea) to different concentrations of this metalloid. The presence of MT-like proteins was detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis and compared with a standard rabbit MT. In addition, the polarographic response showed good correspondence between standard MT and MT-like curves from C. fluminea, allowing MT quantification. The results show that clams exposed to different concentrations of arsenic are able to induce significant levels of MTs. Although variability was found in MT induction, significant differences in MT levels were found after 28 days of exposure in all treatments in comparison with the controls, suggesting that exposure to arsenic induced MT-like proteins in C. fluminea.     

Metallothionein responses in the Asiatic clam (Corbicula fluminea) after exposure to trivalent arsenic

The main objective of this work was to evaluate arsenic effects on metallothionein (MT) induction by exposing a freshwater Asiatic clam (Corbicula fluminea) to different concentrations of this metalloid. The presence of MT-like proteins was detected by sodium dodecyl sulfate polyacrylamide gel electrophoresis and compared with a standard rabbit MT. In addition, the polarographic response showed good correspondence between standard MT and MT-like curves from C. fluminea, allowing MT quantification. The results show that clams exposed to different concentrations of arsenic are able to induce significant levels of MTs. Although variability was found in MT induction, significant differences in MT levels were found after 28 days of exposure in all treatments in comparison with the controls, suggesting that exposure to arsenic induced MT-like proteins in C. fluminea.     

Secondary production of introduced Asiatic clam,Corbicula fluminea,in a central Arizona canal

Asiatic clam, Corbicula fluminea, is often a serious pest where introduced and established outside its native range. This includes many canals of the southwestern U.S.A. Because of its potential role in organic matter processing, nutrient dynamics, and sedimentation, the clam is an important component of the benthic community and of the aquatic ecosystems which it inhabits. To better understand the ecology of Corbicula in canals, secondary production and life history of an introduced population of clams in a small, earthen canal in the Phoenix metropolitan area, central Arizona, were studied over a 12-month period in 1981–1982. Clams had a monthly mean density of 2 255 m^–2, single annual spawning, 2 year life span, and overlapping cohorts. Annual secondary production (size-frequency method) was 25.62 g DM m^–2 and cohort production (composite Allen curve) was 29.32 g DM m^–2, with turnover ratios of 2.70 and 5.81, respectively. These are among the highest known single-species estimates for molluscs, and illustrate the importance of clams in these unique southwestern aquatic systems.     

Grazing on toxic and non-toxic Microcystis aeruginosa PCC7820 by Unio douglasiae and Corbicula fluminea

To explore the potential grazing effects of mussels on Microcystis aeruginosa, a common bloom-forming phytoplankton, Unio douglasiae and Corbicula fluminea were fed with Scenedesmus obliquus, toxic and non-toxic strains of Microcystis aeruginosa as single food and as mixtures in the laboratory. When fed with single foods, U. douglasiae has similar clearance rates on the three algae populations, while C. fluminea has significantly lower clearance rate on toxic M. aeruginosa than those on the other two algae populations. When fed with mixture foods, both the mussels show significantly higher clearance rates than on single foods. The clearance rates of U. douglasiae on the different food mixtures are not significantly different, and C. fluminea has a significantly lower clearance rate on the toxic food mixtures than that on non-toxic food mixtures. Although the relative lower clearance rates of C. fluminea on toxic food, we may still deduce that both the mussels can exert grazing pressure on phytoplankton. The deduction is supported by the composition of the excretion products. The excretion products (faeces and pseudofaeces) of both mussels contained mainly S. obliquus. In both mixed-food treatments, the ratios of S. obliquus to M. aeruginosa in the excrete products are significantly higher than those in the foods. Therefore, it can be concluded that both mussels prefer M. aeruginosa to S. obliquus, and can cause grazing pressure on M. aeruginosa.     

Linking environmental warming to the fitness of the invasive clam Corbicula fluminea

Climate warming is discussed as a factor that can favour the success of invasive species. In the present study, we analysed potential fitness gains of moderate warming (3 °C above field temperature) on the invasive clam Corbicula fluminea during summer and winter. The experiments were conducted under seminatural conditions in a bypass-system of a large river (Rhine, Germany). We showed that warming in late summer results in a significant decrease in the clams' growth rates (body mass and shell length increase) and an increase in mortality rate. The addition of planktonic food dampens the negative effect of warming on the growth rates. This suggests that the reason for the negative growth effect of temperature increase in late summer is a negative energetic balance caused by an enhanced metabolic rate at limited food levels. Warming during early summer revealed contrasting effects with respect of body mass (no warming effect) and shell length (increased shell growth with warming). This differential control of both parameters further enhances the loss of the relative (size-specific) body mass with warming. In contrast, warming in winter had a consistently positive effect on the clams' growth rate as demonstrated in two independent experiments. Furthermore, the reproduction success (as measured by the average number of larvae per clam) during the main breeding period (April) was strongly enhanced by experimental warming during winter, i.e. by eight times during the relatively cold winter 2005/2006 and by 2.6 times during the relatively warm winter 2007/2008. This strong, positive effect of moderate winter warming on the clams' fitness is probably one reason for the recent invasion success of C. fluminea in the northern hemisphere. However, warm summer events might counteract the positive winter warming effect, which could balance out the fitness gains.     

Low dissolved oxygen levels reduce anti-predation behaviours of the freshwater clam Corbicula fluminea

1. When dissolved oxygen levels decline in aquatic systems, prey may be unable to maintain behaviours protecting them from predators. We examined how oxygen availability affected anti‐predator responses in the freshwater clam, Corbicula fluminea. 2. When attacked, bivalves protect their soft tissues by closing their protective valves. This reduces vulnerability to small predators, but ventilation and oxygen uptake are suspended. We found that after a simulated attack, clams under low oxygen conditions reopened their valves sooner than clams under high oxygen conditions, suggesting that hypoxia increases vulnerability to predation. 3. Bivalves may also evade predators through burial into the substratum. Deeper burial confers greater refuge from predators, but increases the costs of ventilation. In a second experiment, we studied how burial depth of C. fluminea is affected by oxygen availability. Additionally, we examined whether clams changed burial depth following a simulated attack by a small predator, and whether this response was affected by oxygen availability. Our results offered partial support for the hypothesis that burial depth is reduced under hypoxic conditions, but the simulated attack did not affect burial depth in any oxygen treatments.     

Corbicula fluminea (Bivalvia: Corbiculidae): a possible second molluscan intermediate host of Echinostoma cinetorchis (Trematoda: Echinostomatidae) in Korea.

More than 1,500 clams of Corbicula fluminea, the most favorable food source of freshwater bivalves in Korea, were collected from 5 localities to examine cercarial and metacercarial infection with Echinostoma cinetorchis. Although 3 clams infected with suspicious E. cinetorchis metacercariae out of 200 specimens collected at Kangjin, Chollanam-do were detected, no cercarial and metacercarial infections with E. cinetorchis were observed in field-collected Corbicula specimens. In the susceptibility experiments with laboratory-reared clams, those infected with miracidia of E. cinetorchis did not release their cercariae up to 60 days after infection. To confirm the identity of second intermediate host of E. cinetorchis experimentally, a total of 30 clams were exposed to the cercariae from Segmentina hemisphaerula that had been infected with miracidia of E. cinetorchis. The clams were susceptible to cercariae of E. cinetorchis with an infection rate of 93.3%. Metacercariae from clams taken more than 7 days after cercarial exposure were fed to rats (S/D strain), and adult worms of E. cinetorchis, characterized by 37-38 collar spines on the head crown, were recovered from the ileocecal regions. This is the first report of C. fluminea as a possible second intermediate host of E. cinetorchis.     

Heat shock protein expression in fish

Heat shock proteins (HSP) are a family of proteins expressed in response to a wide range of biotic and abiotic stressors. They are thus also referred to as stress proteins. Their extraordinarily high degree of identity at the amino acid sequence level and the fact that this cellular stress response has been described in nearly all organisms studied, make this group of proteins unique. We provide a brief historical overview of HSP research, as a background to summarizing what is known about HSP expression in fish. The expression of HSPs in fish has been described in cell lines, primary cultures of various cells, and in the tissues of whole organisms. Collectively, the data show that the expression of HSPs are affected in a wide variety of fish cells and tissues, in response both to biological stressors such as infectious pathogens, as well as to abiotic stressors such as heat and cold shock, and environmental contaminants. HSP research in fish is in its early stages and many studies are describing the expression of proteins in response to various stressors. Several studies have contributed to our understanding of the molecular nature and the molecular biology of HSPs in fish. Recent studies have shown a relationship between HSP expression and the generalized stress response in fish, but further research is needed to clarify the complex relationships between stress hormones and the cellular HSP response. In general, the HSP response seems to be related to the sensing of the stressor and the subsequent cellular effects which may adapt the cells to cope with the stressors. Consequently, such data may be of central importance in understanding the significance of HSP expression to the whole organism. We conclude with sections on laboratory methods used in HSP research and on potential applications of this knowledge in biomonitoring.     

The effect of dissolved oxygen,sediment, and sewage treatment plant discharges upon growth,survival and density of Asiatic clams

The biology of Corbicula fluminea, the Asiatic clam, in the Vermilion River, Louisiana, as affected by sediment, dissolved oxygen (DO) levels, and sewage treatment plant (STP) effluents was investigated. A point source of high DO water to the Vermilion River established a gradient of DO that decreased as the river moved towards the Gulf of Mexico. Lowering DO levels were exacerbated by municipal sewage treatment plant discharges in the 20 km reach studied. Low dissolved oxygen was associated with reduced Corbicula density in the river and 30-day in-stream growth studies (weight and length) demonstrated that low DO inhibited growth. Generally, if DO was < 1.0 mg l^–1 in sediment pore water and/or < 3.0 mg l^–1 at the sediment-water interface, growth was significantly impaired (p < 0.05). Corbicula experienced substantial mortality near the STP discharges (up to 70% in 30 days) and laboratory toxicity tests with Ceriodaphnia dubia, a sensitive cladoceran, also strongly suggested discharges were chronically toxic at 6.25–25.0% effluent. Respiration experiments along with environmental measurements of DO, temperature, and STP discharge chemistry support a hypothesis that clam populations are adversely affected by the suite of environmental conditions present in the Vermilion River. Further, growth studies were consistent with observed population densities in situ.     

Heat shock response ofChlorella protothecoides during greening

Heterotrophically grown cells ofChlorella protothecoides were transferred to autotrophic medium and allowed to green at 25°C. The protein synthetic activity of the greening cells measured in terms of incorporation of [³5S]-methionine showed a maximum around 20 h of greening and thereafter started declining. Similarly, an analysis of densitometric tracings of the fluorographic profile of the polypeptides associated with both total cellular fraction and membrane fractions during different hours of greening revealed that maximum number of polypeptides were getting labelled around 20 h of greening. At 20 h of greening, the cells were shifted to 40°C and the effect of heat shock on protein synthesis was studied. The heat shock treatment caused a definite decrease in the incorporation of [³⁵S]-methionine into proteins. Due to heat shock, the synthesis of total soluble proteins was affected much more than that of the thylakoid membrane bound proteins. When the cells were transferred back to 25°C after a brief period of heat shock at 40°C, there was a considerable recovery in the protein synthesis and this recovery was found to be significant in the case of soluble proteins, while there was no such definite recovery in the synthesis of thylakoid membrane bound proteins.     

Basic features of the Staphylococcal heat shock response

The major heat shock proteins of Staphylococcus aureus had apparent M_rs of 84,000, 76,000, and 60,000, and other prominent proteins of M_rs 66,000, 51,000, 43,000 and 24,000 were also induced. Staphylococcus epidermidis showed a similar response. These proteins were also induced by CdCl₂, ethanol and apparently osmotic stress (1.71 M NaCl or 2.25 M sucrose). Most of the proteins sedimented with the membrane fraction, but the M_r 60,000 protein remained in the cytoplasm.     

Growth and overwinter survival of the Asiatic clam,Corbicula fluminea,in the St. Clair River,Michigan

We report the discovery in April 1986 of the first population of the Asiatic clam, Corbicula fluminea, known to occupy a lotic environment in the Laurentian Great Lakes system. This population occupied a 3.8 km long sandy shoal in the discharge plume of a steam-electric power plant on the St. Clair River (Michigan), the outflow of Lake Huron. Samples collected April 1986 to April 1987 revealed the growth of one-year-old Corbicula (1985 cohort) began after mid-May and ended by mid-November, while water temperatures were higher than 9 °C. Maximum growth (0.78 mm wk^-1) occurred between mid-August and mid-September, while water temperatures were about 16–23 °C. We recorded a substantial overwinter mortality of the 1986 cohort, but not the 1985 cohort; this was particularly evident at sampling locations more remote from the heated discharge of the power plant, suggesting low water temperature was the major mortality agent. The available information suggests low water temperature in the St. Clair River may limit the success of Corbicula in the river, including portions of populations inhabiting thermal plumes, by reducing growth, delaying the onset of sexual maturity and reproduction, and by causing heavy overwinter mortality in the first year of life.This paper is contribution 730 of the National Fisheries Research Center-Great Lakes, U.S. Fish and Wildlife Service, 1451 Green Road, Ann Arbor, Michigan 48105.     

Heavy metal indicator potential of the Asiatic clam (Corbicula fluminea) in artificial stream systems

The potential of the Asiatic clam, Corbicula fluminea, as a bioindicator of cadmium, copper, and zinc was studied during 28-day exposures in field artificial streams receiving river water on a once-through basis. Copper, at aquatic concentrations of 0.016 and 0.057 mg l^-1, showed the greatest degree of tissue uptake and had bioconcentration factors (BCF) of 22 571 and 17 720, respectively. A significant correlation (coefficient = 0.639) was observed between water concentration and tissue accumulation. Cadmium was intermediate relative to BCF (3 770 and 1 752 at aquatic exposures of 0.023 and 0.055 mg l^-1, respectively), and had a correlation coefficient of 0.758. Zinc had the lowest potential for concentration (631, 358, and 511 BCF at 0.218, 0.433, and 0.835 mg l^-1, respectively) with a correlation coefficient of 0.478. The rate of accumulation in Corbicula reached a maximum after 11 days for cadmium while a steady state condition for copper was not observed in 28 days. Zinc accumulation, like copper, showed a relative increase throughout the 28-day exposure period. Data from this study show that the Asiatic clam may be a reliable indicator of uptake for exposure to selected heavy metals.     

Heat shock stress in Bacteroides fragilis

The response to heat shock was investigated in the obligate anaerobe Bacteroides fragilis. The cells responded quickly to stress and synthesised seven heat shock proteins immediately upon exposure to heat. The apparent molecular weights of the seven proteins differed from the apparent molecular weights of the proteins induced by UV irradiation, O₂ and H₂O₂. Heat shock did not induce phage reactivation whereas UV irradiation, O₂ and H₂O₂ did induce phage reactivation systems. Ethanol did not elicit the heat shock response. Two heat resistant B. fragilis mutants were isolated. Both mutants lost the ability to synthesise the same two heat shock proteins. It is concluded that the heat shock response and the responses to UV irradiation, O₂ and H₂O₂ represent two independent groups of stress responses in B. fragilis.     

水体腐殖酸影响下河蚬对低浓度镉的蓄积和释放

研究了水体中低浓度镉(1μg?L-1)单独存在以及低浓度镉(1μg?L-1)和腐殖酸(1mg?L-1、10mg?L-1)共存情况下河蚬对低浓度镉的蓄积和释放,以了解实验室条件下水体腐殖酸对河蚬蓄积和释放的影响。在实验室中曝露15d内分别于1d、2d、3d、10d、15d取样,对比实验结果发现两种处理方式均存在镉在河蚬体内存在蓄积的现象,该蓄积效应与其蓄积时间呈显著线性正相关,但在腐殖酸影响下,河蚬对低浓度镉的蓄积减少。释放10d和20d的实验结果发现低浓度镉进入河蚬体内后难以释放。腐殖酸对镉从河蚬体内的释放效应影响极小。     

非生物因子对河蚬重金属富集量的影响

对影响长江口滨岸带河蚬体内重金属富集量的4个主要非生物因子依次进行分析。结果表明,较高温度有助于河蚬对Cu、Zn、Cr、Ni的吸收;盐度对河蚬的重金属累积量影响比较复杂,仅在秋季表现出一定相关性,春季盐度影响不大;河蚬体内Cu含量与沉积物粒径存在显著正相关,Zn与沉积物中有机质含量存在一定正相关,Pb与沉积物中有机质含量存在着负相关;河蚬体内重金属元素两两间的相关性明显,多种元素间的相互作用机理值得进一步探讨。     

Reproduction and development of the freshwater clam Corbicula australis in southeast Australia

The freshwater clam Corbicula australis is an important component of the macrobiota of the river systems of southeast Australia. Reproduction of two populations of this clam in the Nepean River at Douglas Park and Menangle was investigated to document the gametogenic cycle, larval morphology and to determine when they incubate embryos. C. australis is a simultaneous hermaphrodite and broods its young in the inner demibranchs. The gonads are ovotestes with oogenic and spermatogenic regions in each ascinus. The sperm are biflagellate, a condition unique in the Bivalvia to clonal corbiculids. Gametogenesis was continuous and did not exhibit a seasonal pattern. In contrast, spawning and incubation of embryos was limited to the warmer months of the year. Embryos were present in the gills from October to May. C. australis develops through a modified veliger larva with a vestigially ciliated velum which is not used for swimming or particle capture. The velum is covered by microvilli and it is suggested that the velar epithelium may be specialised for nutrient uptake in the marsupial environment. C. australis produces several clutches each year and the young are released as advanced juveniles with a well-developed foot. Reproductive output differed between the two populations. This was in part due to the larger size of the clams from Menangle and may also reflect the enhanced productivity at this site. The suite of life history traits exhibited by C. australis: hermaphroditism, potential for self-fertilization/androgenesis, brooding progeny to the crawl-away juvenile stage and a high reproductive output, provide for rapid colonization and population growth in this clam which typically inhabits disturbance prone sandy lotic habitats.