Predation on exotic zebra mussels by native fishes: effects on predator and prey

SUMMARY 1. Exotic zebra mussels, Dreissena polymorpha, occur in southern U.S. waterways in high densities, but little is known about the interaction between native fish predators and zebra mussels. Previous studies have suggested that exotic zebra mussels are low profitability prey items and native vertebrate predators are unlikely to reduce zebra mussel densities. We tested these hypotheses by observing prey use of fishes, determining energy content of primary prey species of fishes, and conducting predator exclusion experiments in Lake Dardanelle, Arkansas. 2. Zebra mussels were the primary prey eaten by 52.9% of blue catfish, Ictalurus furcatus; 48.2% of freshwater drum, Aplodinotus grunniens; and 100% of adult redear sunfish, Lepomis microlophus. Blue catfish showed distinct seasonal prey shifts, feeding on zebra mussels in summer and shad, Dorosoma spp., during winter. Energy content (joules g⁻¹) of blue catfish prey (threadfin shad, Dorosoma petenense; gizzard shad, D. cepedianum; zebra mussels; and asiatic clams, Corbicula fluminea) showed a significant species by season interaction, but shad were always significantly greater in energy content than bivalves examined as either ash-free dry mass or whole organism dry mass. Fish predators significantly reduced densities of large zebra mussels (>5 mm length) colonising clay tiles in the summers of 1997 and 1998, but predation effects on small zebra mussels (≤5 mm length) were less clear. 3. Freshwater drum and redear sunfish process bivalve prey by crushing shells and obtain low amounts of higher-energy food (only the flesh), whereas blue catfish lack a shell-crushing apparatus and ingest large amounts of low-energy food per unit time (bivalves with their shells). Blue catfish appeared to select the abundant zebra mussel over the more energetically rich shad during summer, then shifted to shad during winter when shad experienced temperature-dependent stress and mortality. Native fish predators can suppress adult zebra mussel colonisation, but are ultimately unlikely to limit population density because of zebra mussel reproductive potential.     

Effects of temperature,salinity and agitation on byssus thread formation of zebra musselDreissena polymorpha

Byssus thread production ofD. polymorpha under different conditions of temperature, salinity and agitation were studied in the laboratory. The acclimation to salinity and temperature greatly affects the byssus production ofD. polymorpha. Byssus production of mussels was significantly reduced when temperature increased beyond 20°C and decreased below 10°C. Mussels with cut threads (for counting), produced a substantially increased number of threads. However, mussels with uncut byssus threads were comparatively more mobile. Byssus production of mussels did not vary significantly at salinities up to 3. Beyond this salinity byssus production was reduced significantly. Mussels increased their byssus production with increasing frequency of agitation.     

Wetlands as barriers: effects of vegetated waterways on downstream dispersal of zebra mussels

1. Stream flow is a major vector for zebra mussel spread among inland lakes. Veligers have been found tens to hundreds of km from upstream source lakes in unvegetated stream and river systems. It has been suggested, however, that the downstream transport of zebra mussels is restricted by wetland ecosystems. We hypothesized that vegetated waterways, (i.e. wetland streams) would hinder the downstream dispersal of zebra mussels in connected inland lake systems. 2. Veliger abundance, recruitment and adult mussels were surveyed in four lake‐wetland systems in southeastern Michigan, U.S.A. from May to August 2006. Sampling was conducted downstream of the lakes invaded by zebra mussels, beginning at the upstream edge of aquatic vegetation and continuing downstream through the wetland streams. 3. Veliger abundance decreased rapidly in vegetated waterways compared to previously reported rates of decrease in non‐vegetated streams. Veligers were rarely found more than 1 km downstream from where vegetation began. Newly recruited individuals and adults were extremely rare beyond open water in the wetland systems. 4. Densely vegetated aquatic ecosystems limit the dispersal of zebra mussels downstream from invaded sources. Natural, remediated and constructed wetlands may therefore serve as a protective barrier to help prevent the spread of zebra mussels and other aquatic invasive species to other lakes and ecosystems.     

Effects of zebra mussels on food webs: interactions with juvenile bluegill and water residence time

We evaluated how water residence time mediated the impact of zebramussels Dreissena polymorpha and bluegill sunfish Lepomis macrochirus on experimental food webs established in1100-l outdoor mesocosms. Water residence time was manipulated asa surrogate for seston resupply – a critical variable affectinggrowth and survival of suspension-feeding invertebrates. We used a2×2×2 factorial experimental design witheight treatment combinations (3 replicates/treatment) including thepresence or absence of Dreissena (2000 per m²), juvenilebluegill (40 per mesocosm), and short (1100 l per d) or long (220 lper d) water residence time. Measures of seston concentration(chlorophyll a, turbidity and suspended solids) were greaterin the short- compared to long water-residence mesocosms, butintermediate in short water-residence mesocosms containing Dreissena. Abundance of rotifers (Keratella and Polyarthra) was reduced in Dreissena mesocosms and elevatedin short residence time mesocosms. Cladocera abundance, in general,was unaffected by the presence of Dreissena; densities werehigher in short-residence time mesocosms, and reduced in thepresence of Lepomis. The growth of juvenile Lepomiswere unaffected by Dreissena because of abundant benthicfood. The final total mass of Dreissena was significantlygreater in short- than long-residence mesocosms. Impacts of Dreissena on planktonic food webs may not only depend on thedensity of zebra mussels but also on the residence time of thesurrounding water and the resupply of seston.     

The effects of temperature, salinity and a simulated tidal cycle on the toxicity of fenitrothion to Callinectes sapidus

The 96 hr LC50 for Callinectes sapidus exposed to fenitrothion at 22 degrees C and a salinity of 34 ppt (parts per thousand) was estimated to be 8.6 micrograms/l with a 95% confidence interval of 7.4-9.9 micrograms/l. Acute toxicity was shown to increase with increasing temperature as well as increasing salinity. Exposure of Callinectes to a simulated tidal cycle of 17 ppt salinity change at 6 hr intervals increased the acute toxicity of fenitrothion to Callinectes. The autotomization response in Callinectes was shown to be affected at subacute exposure levels as low as 0.1 microgram/l.     

The effect of food availability on byssogenesis by the zebra mussel (Dreissena polymorpha Pallas)

The effect of varying algal availability on byssal thread productionby re–attaching zebra mussels (Dreissena polymorpha) wasquantified. The byssal apparatus was severed and mussels allowedto re–attach to a hard substratum for a 21 day periodduring which they were fed at algal concentrations of 0.0, 0.1,0.5, 1.0 or 2.0 mg C l^-1. Byssal thread production was quantifiedby counting the number of new attachment plaques present eachday. Results showed that starved mussels continued to partition organiccarbon towards thread production but the resultant byssal masswas compromised, containing fewer threads than those producedby fed mussels. The daily average byssal thread production bymussels fed at 2.0 mg C l^-1 was greater than that of starved musselsand the final mean dry soft tissue weight higher. At algal Cconcentrations below maintenance requirements byssal threadproduction was elevated compared to starved mussels, but therewas no concurrent increase in soft tissue. This suggests thatbelow maintenance levels assimilated carbon was solely partitionedtowards byssus production and stored reserves may have beenutilized. The ratio of organic carbon contained in the byssusto that in the soft tissues remained relatively constant acrossall feeding levels. This suggests that the carbon content ofthe byssus is a constant function of that of the soft tissuemass. These results may explain seasonal variation in attachmentstrength of numerous byssate species and seasonal vertical migrationsby D. polymorpha. Present Address-Queen Mary & Westfield College, Dept of Biology,University of London, Mile End Road, London E1 4NS, UK (Received 16 March 1998; accepted 30 September 1998)     

State-dependent effects of light-dark cycle on somatosensory and visual cortex EEG in rats

Somatosensory (SSctx) and visual cortex (Vctx) EEG were evaluated in rats under a 12:12-h light-dark (LD) cycle and under constant light (LL) or constant dark (DD) in each sleep or wake state. Under LD conditions during light period, relative Vctx EEG slow-wave activity (SWA) was higher than that of the SSctx, whereas during dark period, relative Vctx EEG SWA was lower than in the SSctx. These effects were state specific, occurring only during non-rapid eye movement sleep (NREMS). Under LL conditions, the duration of REMS and NREMS during the period that would have been dark if the LD cycle had continued (subjective dark period) was greater than under LD conditions. DD conditions had little effect on the duration of NREMS and REMS. SSctx and Vctx EEG SWA were suppressed by LL during the subjective dark period; however, the degree of Vctx SWA suppression was smaller than that of the SSctx. DD conditions during the subjective light period enhanced SSctx SWA, whereas Vctx SWA was suppressed. Under LL conditions during the subjective dark period, Vctx EEG power was higher than that of the SSctx across a broad frequency range during NREMS, REMS, and wakefulness. During DD, SSctx EEG power during NREMS was higher than that of the Vctx in the delta wave band, whereas SSctx power during REMS and wakefulness was higher than that of the Vctx in frequencies higher than 8 Hz. We concluded that the SSctx and Vctx EEGs are differentially affected by light during subsequent sleep. Results provide support for the notion that regional sleep intensity is dependent on prior regional afferent input.     

A comparative study of the gradual degradation of exines,resulting from the effects of temperature

The gradual degradation of three types of pollen exines from different plant groups (gymnosperms and angiosperms) with rising temperature has been observed and comparisons made. Pollen grains are heated to different temperatures (100°C–350°C) in a sealed copper tube, placed in a nichrome wire resistance furnace. In each case the pollen grains are heated for 100 hours.The colour change and the size reduction with rising temperature are common to all pollen types. The sexine or ornamented part of the pollen exine is affected first by rising temperature. In angiosperm pollen, the sexibe pattern is not recognisable at 300°C, but pine pollen retains its pattern up to 350°C. The nexine seems to be more stable at high temperatures than the sexinous elements and either remains unaltered with remnants of the sexinous pattern, or becomes altered and amorphous.The lamellar part of the nexine appears to be important and the evolutionary significance of the exine is discussed. The present work shows that the gymnosperm pollen has more stable exines, and may be better adapted for survival than angiosperm exines.     

Seasonal effects of zebra mussels on littoral nitrogen transformation rates in Gull Lake, Michigan, U.S.A.

• 1 Zebra mussels (Dreissena polymorpha) are successful colonisers of lake littoral habitats and they interact strongly with littoral benthos. Previous research suggests that localised areas colonised by zebra mussels may be hotspots of nitrogen (N) cycling.
• 2 The effects of zebra mussels on nitrification and denitrification rates were examined approximately every other month for 1 year in Gull Lake, Michigan, U.S.A. Littoral sediment was collected from an area free of zebra mussels and distributed into shallow trays; rocks colonised with zebra mussels were placed in half of the trays, while uncolonised rocks were placed in the remaining trays. After an incubation period of 6–8 weeks in the lake, sediment and zebra mussels were collected from the trays, replaced with new sediment and zebra mussels, and placed in the lake for the next interval. In the laboratory, sediment nitrification and denitrification rates were measured for each tray.
• 3 Sediment nitrification rates did not increase in the presence of zebra mussels; instead nitrification rates were sensitive to changes in water temperature and increased with increasing exchangeable sediment ammonium. In contrast, denitrification rates increased in sediment trays with zebra mussels in the winter when nitrate (NO₃⁻) availability was high and when Chara did not grow in the trays.
• 4 Sediment denitrification was NO₃⁻‐limited in all seasons, regardless of zebra mussel treatment. However, sediment in the presence of zebra mussels responded less to NO₃⁻ addition, suggesting that NO₃⁻ limitation of denitrification can be reduced by zebra mussel activity. Zebra mussels have a seasonally variable impact on sediment denitrification rates, and this may translate into altered seasonal patterns of N cycling in localised areas of lakes where they are particularly abundant.

     

The effects of natural biofilms on the reattachment of young adult zebra mussels to artificial substrata

This laboratory study examined the effects of natural biofilms on the reattachment of young adult zebra mussels, Dreissena polymorpha, in Petri dishes. Natural biofilms were developed in glass and polystyrene Petri dishes using water samples collected at various times of the year. Biofilms were developed over 1, 3, 8, and 14 d. Controls were clean glass and polystyrene Petri dishes. Zebra mussels collected from the field (< or = 10 mm, ventral length) were placed in the dishes and their reattachment by byssal threads was recorded after 1 d. Zebra mussels reattached to the dish surface or the shells of other mussels in the dish, or remained unattached. The data indicate that reattachment to clean glass was greater than to clean polystyrene (p < or = 0.05, ANOVA), but there were no consistent differences between reattachment to filmed polystyrene and filmed glass dish surfaces. Zebra mussels in control and filmed glass dishes reattached in higher percentages to the dish surface compared to the shells of other mussels (p < or = 0.05, ANOVA). There was no difference in mussel of reattachment between the dish surface and the shells of other mussels in most control polystyrene dishes (p > 0.05, ANOVA), whereas in filmed polystyrene the percentage of reattachment to the dish surface was greater than to the shells of other mussels (p < or = 0.05, ANOVA). These results indicate that substratum wettability and the presence of biofilms on some types of substrata can be factors in the reattachment of young adult zebra mussels.     

Effects of culture temperature shift and light-dark time cycle on the cellular sugar accumulation of Chlorella pyrenoidosa

We investigated the effect of culture temperature on the maximum specific growth rate and the cellular sugar accumulation, and the effect of a temperature shift on the sugar accumulation of Chlorella pyrenoidosa cells in a batch culture system. Increase in temperature below 30?°C appeared to correlate with increase in the maximum specific growth rate, on the contrary the cellular sugar content showed a reverse tendency against temperature. We attempted to utilize this tendency for improving sugar productivity in Chlorella. First, we cultured Chlorella at 28?°C during the logarithmic growth phase to obtain a high specific growth rate. The culture temperature was then shifted from 28?°C to 22?°C at the late logarithmic growth phase in order to reduce the specific growth rate and enhance the cellular sugar accumulation. As a result, we obtained a 15% increase in sugar production over that obtained by cultivation at 28?°C throughout the culture. We also investigated the effect of light-dark time cycle on the sugar productivity and found that this operating variable did not affect the cellular sugar content but influenced the final cell concentration. Among the examined light-dark time cycles, maximum sugar productivity was obtained in the case of 12?h light and 12?h dark period.     

Effects of zebra mussels on phytoplankton and ciliates: a field mesocosm experiment

Many observational studies in North American lakes have documenteddecreases in phytoplankton abundance after the invasion of thezebra mussel (Dreissena polymorpha). However, few field experimentshave examined in detail the effect of zebra mussels on phytoplanktonabundance and species composition over an extended period. Replicatedin situ mesocosms were used to evaluate the impact of naturaldensities of zebra mussels on phytoplankton and ciliate biovolume,and algal species composition over a 5-week period in a habitatthat lacked extant mussel populations. Mussel biomass used inthe experiment was determined using a regression model basedon a data analysis that predicts zebra mussel biomass from totalphosphorus concentration. Within 1 week, zebra mussels decreasedphytoplankton biovolume by 53% and ciliate biovolume by 71%.The effect of zebra mussels on ciliate biovolume was sustainedthroughout the study. However, the effect of zebra mussels onphytoplankton abundance gradually waned over the remaining 4weeks of the experiment, such that the declining effect of zebramussels could not be explained by a shift towards less edibleand/or faster growing algal species. The mussels’ decliningcondition could help to explain the effect observed over thecourse of the experiment.     

Effects of invasive zebra mussels on phytoplankton,turbidity, and dissolved nutrients in reservoirs

Few experiments have quantified the effects of invasive zebra mussels (Dreissena polymorpha) on man-made reservoirs relative to other aquatic habitats. Reservoirs, however, are the dominate water body type in many of the states that are at the current front of the zebra mussel invasion into the western United States. The objective of this research, therefore, was to determine how zebra mussels affected phytoplankton, turbidity, and dissolved nutrients in water that was collected from three Kansas reservoirs that varied in trophic state (mesotrophic to hypereutrophic), but all experienced frequent cyanobacterial blooms. Laboratory mesocosm experiments were conducted to document the effects of zebra mussels on cyanobacteria and general water quality characteristics in the reservoir water. Zebra mussels significantly reduced algal biomass, and the total biovolume of cyanobacteria (communities were dominated by Anabaena) in each reservoir experiment. The effects of zebra mussels on other major algal groups (diatoms, flagellates, and green algae) and algal diversity were less consistent and varied between the three reservoir experiments. Similarly, the effects of zebra mussels on nutrient concentrations varied between experiments. Zebra mussels increased dissolved phosphorus concentrations in two of the reservoir experiments, but there was no effect of zebra mussels on dissolved phosphorus in the mesotrophic reservoir experiment. Combined, our results strongly suggest that zebra mussels have the potential to significantly impact reservoirs as they continue to expand throughout the western United States. Moreover, the magnitude of these effects may be context dependent and vary depending on the trophic state and/or resident phytoplankton communities of individual reservoirs as has similarly been reported for natural lakes.     

The effects of different soil water potentials, temperature and salinity on germination of seeds of the desert shrub Zygophyllum dumosum

The effects of different soil water potentials, temperature and NaCl concentration on seed germination of Zygophyllum dumosum Boiss., a common shrub in Israeli deserts, was investigated. Seeds had to be exposed to constant field capacity conditions (–0.0316 MPa) for a minimal period of two days before germination could start. Maximal germination under such conditions occurred after four days or more. Under simulated conditions of gradual dehydration of the soil, seeds were inhibited either at low soil water potentials (–0.10 to –10.00 MPa) or at high ones (–0.002 to –0.0398 MPa). Germination of Z. dumosum was independent of temperature in the range of 10–25°C, but strongly inhibited at 30 and 35°C. At 20°C germination was inhibited by salinity of the medium but still occurred (0.5%) even at a concentration of 0.5 M NaCl.     

The effects of the light-dark cycle and its inversion on the susceptibility of mice to lead acetate

The effects of the light-dark (LD) cycle and its inversion on the susceptibility of mice to lead were investigated. Groups of mice were injected once with a high (200 mg Pb/kg body weight) or low (10 mg Pb/kg body weight) dose of lead acetate at one of four-hourly intervals, i.e., 09.00 h, 13.00 h, 17.00 h, 21.00 h, 01.00h, and 05.00 h, on the days before and after a phase shift in the LD cycle. The main indices used to measure the susceptibility of the mice were dying rate and percent mortality of ICR mice following injection together with the Pb concentration (Pb-B) and -aminolevulinic acid dehydratase (ALAD) activity in the blood. In the present study, we ascertained the following facts: (1) Dying rate (time to death following injection) and Pb-B exhibited a circadian rhythm with a peak initiated by injection in the dark period. (2) Injection with a low dose of lead within a few days after an LD inversion significantly decreased the level of ALAD activity and increased the Pb in the blood in comparison with control animals kept under a non-inverted LD cycle. Both the dying rate and the percent mortality following a high dose of lead, however, were hardly affected.     

温度、盐度和pH对尼罗罗非鱼性别分化的影响

采用Box-Behnken设计及响应曲面法研究了温度(20～36℃)、盐度(0 ～16)和pH(5.5 ～8.5)3个主要环境因子对吉富品系尼罗罗非鱼性别分化的影响.结果表明:温度的一次和二次效应分别对吉富罗非鱼性别分化的影响显著.盐度和pH的一次和二次效应分别对吉富罗非鱼性别分化的影响不显著,3个环境因子中任意2个因子之间的互作效应都不显著.采用响应曲面法分析发现,随着温度的升高,吉富罗非鱼的雄性率呈逐渐上升的趋势;温度为36℃、盐度为8、pH为8.5时,吉富罗非鱼最大雄性率可达80％.建立雄性率与温度、盐度和pH三者之间关系的模型方程,并剔除相关不显著的因子后,得到雄性率与温度之间的模型方程,可用于预测吉富罗非鱼雄性率的变化.