Seasonal variations in size and morphology of Acanthocyclops robustus (Copepoda Cyclopoida)

The variations in the body length, swimming legs and some setaeof the freshwater cyclopid Acanthocyclops robustus were analysedduring an annual cycle in the field, and in laboratory experimentsat different culture temperatures. In Lake Créteil. ashallow temperate sand-pit lake, there is a seasonal morphologicalvariation of the setae: the plumose forms were restricted tothe warmer season and the spinose ones to colder periods. Contraryto other years, the specific ‘spine formula’ ofthe swimming legs (3-4-4-4) did not vary during the referenceyear. In laboratory experiments, both sex and post-embryonicdevelopmental temperatures had an effect on the length of adults.A significant parental effect on body length was detected. Pairswith anomalous spine formulae produced offspring either withanomalous spine formulae or typical 3-4-4-4 formulae. No significanteffects of parental culture temperatures, offspring culturetemperatures and sex on the total number of spines were found.Modification of plumose into spinose setae did not occur, whateverthe offspring developmental temperatures. The results presentedhere suggest that the alterations of the appendages of the cyclopidA.robustus could be compared to the cyclomorphosis in otherzooplanktonic groups, and contribute to our understanding ofthe role of phenotypic induction in aquatic biology.     

Characteristics of the reproductive cycles and development times of Copidodiaptomus numidicus (Copepoda: Calanoida) and Acanthocyclops robustus (Copepoda: Cyclopoida)

The female reproductive cycles of the calanoid copepod Copidodiaptomus numidicus and the cyclopoid Acanthocyclops robustus were characterized by periodic changes in the oviducts. Females oscillated between a gravid and a non-gravid condition, and the whole cycle was strongly dependent on temperature. In both species, the maturation of new oocytes took place when the old egg sacs were still being carried, thus ensuring a rapid clutch succession. The embryonic and post-embryonic development duration of C.numidicus and A.robustus was investigated at five and four different temperatures, respectively. Embryonic development time was related to oviductal cycle duration at each temperature to estimate the minimum amount of non-ovigerous females expected under non-limiting mate and food conditions. Copidodiaptomus numidicus females spent 35-19% of the cycle without carrying eggs at 7-20C. Acanthocyclops robustus females spent 31-33% of the cycle without carrying eggs at the same temperature interval. Both naupliar and copepodid durations were inverse functions of temperature with nauplii developing at a faster rate than copepodids. In general, A.robustus showed reproductive advantage over C.numidicus due to a faster oviductal cycle and shorter embryonic and post-embryonic development times. However, the influence of food quality may be crucial. Acanthocyclops robustus copepodid development may lengthen well beyond the corresponding development stages of C.numidicus when growing only on algae. This retardation of development affects especially the later development stages of the cyclopoid.      

Prey preference of predatory mite Amblyseius swirskii (Acari: Phytoseiidae) on Tetranychus urticae (Acari: Tetranychidae) and Bemisia tabaci (Hemiptera: Aleyrodidae)

Tetranychus urticae Koch (Acari: Tetranychidae) and Bemisia tabaci Gennadius (Hemiptera: Aleyrodidae) are major pests in greenhouse crops. Recently, Amblyseius swirskii Athias-Henriot (Acari: Phytoseiidae) was shown to be an effective biological control agent of both pests. Therefore, the prey preference of A. swirskii was determined using immature stages of T. urticae and B. tabaci in three various treatments based on Manly's β preference index (β). These treatments consisted of immature stages of two prey species (egg, first and second instar nymphs) with densities 12:12, 6:6 and 3:3, respectively, and with 13 replicates. After 24?h starvation, same-aged females of A. swirskii were added to the leaf discs. All experiments were done on bean leaf discs in Petri dishes (8?cm in diameter) in laboratory conditions with 25?±?2°C, 70?±?5% relative humidity and the photoperiod of 16L:8D hours. Comparing the preference indices using t-tests indicates a significant preference of the predator on eggs (t?=?10.80, df?=?24, P?t?=?8.17, df?=?24, P?T. urticae than B. tabaci. Our findings suggest that developmental stages of prey have effect on the prey selection by A. swirskii.     

Prey preference and prey switching in Anthocoris minki Dohrn (Hemiptera: Anthocoridae)

The understanding of host specificity of predacious insects is one of the most important problems in the investigation of biological control. Prey preference and the potential for prey switching in Anthocoris minki Dohrn (Hem.: Anthocoridae) was assessed in two- choice experiments under environmentally controlled conditions. Manly's preference index was used to compare A. minki predation on Agonoscena pistaciae Burckhardt and Lauterer (Hem.: Psyllidae) versus Psyllopsis repens Loginova (Hem.: Psyllidae) but showed no statistically significant evidence for preference. Anthocoris minki did prefer psyllids (Agonoscena pistaciae and Psyllopsis repens) over aphids (Forda hirsuta Mordvilko (Hem.: Pemphigidae) and Aphis craccivora Koch (Hem.: Aphididae)). The predator also consumed significantly more 2nd instar than 4th instar nymphs of P. repens. Switching behavior experiments showed that A. minki did not preferentially attack the most abundant prey when the prey combination was either biased toward A. pistaciae or toward F. hirsuta. Linear regression between preference indices and prey ratios indicated an overall preference for psyllids over aphids. The results suggest that a psyllid-specialized feeder such as A. minki is more likely to be effective biocontrol agent against psyllid species and less effective at reducing the numbers of other pest species that might co-occur with psyllids.     

Effect of Experience on Prey Species Selection by the Bivalve Feeder Halla Okudai (Polychaeta: Lysaretidae)

We report on the effects of previous foraging experience on prey-selection by the bivalve feeding polychaete Halla okudai, including whether there was evidence of frequency-dependent predation. Three separate batches of H. okudai were maintained for 30 days on clams, mussels, or oysters, before being offered a choice among these three prey. Initially individuals from all three treatments consumed more clams than mussels, and no oysters. As the number of clams was depleted the polychaetes shifted their diet to include a greater proportion of mussels, but even after 20 days oysters were only eaten by polychaetes that had been previously acclimated to them. Nevertheless, polychaetes from each treatment inspected significantly more of the prey species to which they had initially been acclimated, suggesting that previous experience may increase the likelihood of certain prey being detected. When individuals of H. okudai were repeatedly offered the same prey species, handling time did not decrease (and therefore prey profitability did not increase) with experience, which may be because H. okudai paralyses its prey with toxic mucus. Since repeated experience of the same prey species gives no advantage in terms of reduced handling time, we suggest this may be why this species does not show frequency-dependent prey-selection.     

Effect of Experience on Prey Species Selection by the Bivalve Feeder Halla Okudai (Polychaeta: Lysaretidae)

We report on the effects of previous foraging experience on prey-selection by the bivalve feeding polychaete Halla okudai, including whether there was evidence of frequency-dependent predation. Three separate batches of H. okudai were maintained for 30 days on clams, mussels, or oysters, before being offered a choice among these three prey. Initially individuals from all three treatments consumed more clams than mussels, and no oysters. As the number of clams was depleted the polychaetes shifted their diet to include a greater proportion of mussels, but even after 20 days oysters were only eaten by polychaetes that had been previously acclimated to them. Nevertheless, polychaetes from each treatment inspected significantly more of the prey species to which they had initially been acclimated, suggesting that previous experience may increase the likelihood of certain prey being detected. When individuals of H. okudai were repeatedly offered the same prey species, handling time did not decrease (and therefore prey profitability did not increase) with experience, which may be because H. okudai paralyses its prey with toxic mucus. Since repeated experience of the same prey species gives no advantage in terms of reduced handling time, we suggest this may be why this species does not show frequency-dependent prey-selection.     

Effects of UV-B radiation on ingestion, fecundity, population dynamics and antioxidant enzyme activities of Schmackeria inopinus (Copepoda Calanoida)

Effects of enhanced ultraviolet B (UV-B, 280-320 nm) on copepods have gained particular attention in recent years. In this study, we investigated the effects of UV-B radiation on ingestion, fecundity, population dynamics and antioxidant enzyme activities of copepod Schmackeria inopinus exposed to varying doses of UV-B irradiance. Artificial UV-B radiation resulted in an increased mortality of nauplii, copepodites and adults with increasing UV-B doses. Nauplii and copepodites were more sensitive to UV-B radiation than adults, and adult males had a higher UV-B radiation susceptivity in comparison with adult females. Both ingestion rates and proportion of gravid females decreased with the increase of UV-B doses; and at the same time, we also observed that adult females had higher ingestion rates as compared with adult males. In comparison with the control, the abundance of the treatment significantly decreased. Antioxidant enzyme (GPx and GR) activities attained a significant increase at lower UV-B radiation doses when compared to the control, but declined at higher UV-B doses. These results suggested that enhanced UV-B radiation might change the species composition of copepods. Our study also showed that antioxidant enzymes might protect S. inopinus against UV-induced oxidative damage.     

Development of Orius vicinus (Ribaut) (Heteroptera: Anthocoridae) on Different Prey

The survival, development and adult size of Orius vicinus were studied on five prey species, the mites Aculus schlechtendali, Panonychus ulmi and Tetranychus urticae, and larvae of the insects Dasineura mali and Thrips obscuratus. Survival was high (74-100%) and O. vicinus was able to complete development on all prey species in an average of 22-26 days at 20 C. The most rapid development occurred on T. obscuratus, T. urticae and P. ulmi. Adult size was estimated from the length of the fifth instar cast skin, which averaged 2.18 to 2.48 mm on different prey. The largest bugs were obtained from feeding on T. obscuratus or D. mali. O. vicinus, which were fed A. schlechtendali, were the slowest to develop and produced the smallest adults. The results are discussed in relation to prey availability and observed predation in the field, and are compared with previous feeding studies.     

Enhanced excretion rates induced by small-scale turbulence in Acartia (Copepoda: Calanoida)

Several experiments have been performed on three congenericspecies of the calanoid copepod Acartia to determine the effectsof small-scale turbulence on metabolic rates. Both inorganicnitrogen and phosphorus excretion rates significantly increased     

Feeding of Cyclops vicinus (Uljanin) (Copepoda: Cyclopoida) in Lake Balaton on the basis of gut content analyses

Consumption of planktonic algae by Cyclops vicinus living in Lake Balaton were studied in eighteen feeding experiments during 1980–1981. Gut contents of 284 adult C. vicinus were analysed in comparison with the natural phytoplankton assemblage. Rates of algal cells elimination were also studied and compared to controles which did not contain animals. C. vicinus in Lake Balaton was found to be an omnivore.     

First observations of predation by New Zealand Greenshell mussels (Perna canaliculus) on zooplankton

Observations made overseas of predation by blue mussels and zebra mussels on mesozooplankton (>200 μm) have raised concern within New Zealand that the Greenshell mussel, Perna canaliculus, which is cultured in large tonnages throughout hundreds of marine farms within the New Zealand coastal zone, could exert ecologically detrimental effects by preying on zooplankton. We conducted experiments at Clova Bay, Pelorus Sound in May 2002 to determine the rates that P. canaliculus ingests prey, up to and including the mesozooplankton size range. Single mussels from farms were incubated with seawater enriched with zooplankton (>60 μm) in gently circulated 15-l pails. Depletion of chlorophyll-a (chl-a), ciliate microzooplankton, and nauplii, copepodites, and adults of copepods was determined over 5 h, relative to controls with no mussels. Two experiments were made over consecutive days. Gut contents of these experimental mussels, and of mussels examined soon after collection from a farm, were described.Gut contents of experimental and of freshly collected mussels (standard shell length ∼90 mm) had numerous copepod parts, whole copepods and larval bivalves present. Experimental mussels cleared chl-a and ciliates from 59- to 137-l individual⁻¹ day⁻¹, respectively, averaged across the two experiments. Faster ciliate than chl-a clearance was probably caused by the high proportion (56%) of phytoplankton below the retention size for P. canaliculus (ca. 5 μm) and by faster ciliate grazing in controls than treatments. The average clearance rates of adult, copepodite, and naupliar copepod stages by mussels were 20, 31, and 49 l individual⁻¹ day⁻¹, respectively. The clearance rates of each copepod stage were not significantly different between the two experiments. Clearance of nauplii was significantly greater than of adults and copepodites, while adult and copepodite clearance rates were nearly significantly different. The mean lengths of the adult, copepodite, and naupliar copepods were 430, 265, and 165 μm, respectively. The decreasing clearance rates with increasing size and development of prey (from ciliates, through naupliar, copepodite to adult copepods), suggested that prey escape ability, related to body size and/or morphology, affected capture rates. Mussel faecal samples indicated complete digestion of the gut contents. Pseudofaecal samples showed very low rejection rates of mesozooplankton by mussels. The results are considered in context of current biophysical modelling studies of impacts of large mussel farms in New Zealand. Designs of future experiments to improve accuracy of estimates of mesozooplankton clearance rates by P. canaliculus are considered.     

The seasonal cycle of Thermocyclops crassus (Fischer, 1853) (Copepoda: Cyclopoida) in a shallow,eutrophic lake

The seasonal cycle of Thermocyclops crassus was studied from 1985 to 1987, in the Gronne, a shallow, productive lake. T. crassus was present from late April to early October, while water temperature was above 10 °C. It produced three generations per year. Population peak was usually reached by the second generation, in August. Abundance was positively correlated with water temperature. Females carried 18.3 to 32.3 eggs on average. Reproduction rates were highest in July and August. Sex ratio was low, as females generally outnumbered males. Between October and April copepodites 4 went in diapause, predominantly in the deepest part of the lake and 0–4 cm deep in the bottom mud. Variations in body size were low across the year.T. crassus coexisted with the cyclopids, Cyclops vicinus and Mesocyclops leuckarti. In 1985 and 1986, abundances of M. leuckarti were low, while those of T. crassus and C. vicinus were high. In 1987 lower water temperatures, caused by cold weather, resulted in a marked decrease of the population density of T. crassus, while the abundance of M. leuckarti increased, and density of C. vicinus remained high. Comparison of the egg duration times of T. crassus, C. vicinus and M. leuckarti showed that at 15 °C and below T. crassus may be outcompeted by its comparatively longer egg development times. However, experiments in limnocorrals showed that T. crassus has an advantage over C. vicinus when fish predation is high.Department of Biology III, University of Ulm     

Prey use of the fishing spider Dolomedes triton (Pisauridae,Araneae): an important predator of the neuston community

Summary Prey of feeding juvenile and adult Dolomedes triton (Walckenaer 1837) were sampled over two seasons on three small ponds in central Alberta, Canada. Prey were mainly insects active at the water surface with truly aquatic species making up about 14% of the diet. Throughout the season aquatic and semi-aquatic Heteroptera represented about 30% of the prey. Diptera and adult Odonata were also important prey items but their abundance in the diet was more variable seasonally. Of the 625 prey items recorded nearly 50% were represented by taxa taken no more than once by spiders in one of the five size classes (adult females, adult males, large, intermediate and small juveniles). Large spiders did not take the smallest prey available, although small and intermediate-sized spiders fed on nearly the full size range taken by larger spiders. Cannibalism was common, accounting for 5% of the observations, with females and large juveniles as the most frequently observed cannibals. We hypothesize that intraguild predation (including cannibalism) could be an important coevolutionary force structuring phenology, population dynamics and microhabitat use of the predatory guild of the neuston community.     

Foraging behaviour by an intraguild predator blowfly, <Emphasis Type="Italic">Chrysomya albiceps</Emphasis> (Diptera: Calliphoridae)

Optimal foraging theory assumes that predators use different prey types to maximize their rate of energetic gain. Studies focusing on prey preference are important sources of information to understand the foraging dynamics of Chrysomya albiceps. The purpose of this investigation is to determine the influence of larval starvation in C. albiceps on the predation rate of different prey blowfly species and instars under laboratory conditions. Our results suggest that C. albiceps prefers Cochliomyia macellaria larvae to Chrysomya megacephala under non-starvation and starvation conditions. Nevertheless, predators gained more weight consuming C. macellaria. This result suggests that C. albiceps profit more in consuming C. macellaria rather than C. megacephala. The foraging behaviour displayed by C. abiceps on their prey and the consequences for the blowfly community are also discussed.     

Seasonal dynamics of zooplankton community in four Mediterranean reservoirs in humid area (Beni Mtir: north of Tunisia) and semi arid area (Lakhmes,Nabhana and Sidi Saâd: center of Tunisia)

The zooplankton community was studied in four Mediterranean reservoirs to assess the relative importance of environmental factors as determinants of zooplankton dynamic in the four seasons. The water temperature and hydrology variations affect the distribution of zooplankton. A positive correlation was established between the total zooplankton and the water temperature (r=0.9, n=9, p<0.05). Fourteen zooplankton species were identified. Seasonal changes in the density (ANOVA, F=3.7, d.f=36, p<0.01) and the biomass of total zooplankton (ANOVA, F=4.4, d.f=36, p<0.001) were observed. Our results suggest that planktivorous fish may not modify the zooplankton dynamics in Beni Mtir reservoir (oligotrophic). On the contrary, in Sidi Saâd reservoir (mesotrophic), fish predation has major effects on seasonal zooplankton dynamics.     

Intraguild predation of Serangium parcesetosum (Coleoptera: Coccinellidae), on whitefly Bemisia tabaci (Homoptera: Aleyrodidae) parasitized by Eretmocerus mundus (Hymenoptera: Aphelinidae)

We studied intraguild predation of Serangium parcesetosum on Eretmocerus mundus, both natural enemies of Bemisia tabaci, on eggplant under laboratory conditions. Fourth instar and adult predators attacked parasitized and unparasitized hosts equally if immature parasitoids were larvae, but avoided predating on parasitized hosts containing pupae.     

Percussive Foraging: Stimuli for Prey Location by Aye-Ayes (Daubentonia madagascariensis)

Aye-ayes (Daubentonia madagascariensis) use the thin middle finger to tap on wood in search of subsurface cavities containing insect larvae. When a cavity is located, they gnaw away wood until the prey can be extracted. Previous researchers suggested that acoustical cues reveal cavity location. We designed five studies to identify the cavity features that provide acoustical cues. When cavities were backfilled with gelatin or acoustical foam, excavation was still successful, suggesting that the reverberation of sound in air-filled cavities is not necessary for detection. Moreover, when the density of cavity content was varied, there was no difference in excavation frequency. On the other hand, a one-dimensional break in the subsurface wood was an effective stimulus for excavation. These studies suggest that a simple interface beneath the surface is sufficient to elicit excavation and that neither prey nor cavity nor even small air pockets are necessary to elicit the behavior. These results raise provocative questions as to how the aye-aye manages to forage efficiently.     

Direct and indirect impacts of predation by fish on the zooplankton community: an experimental analysis using tanks

The impact of Pseudorasbora parva, a common zooplanktivorous fish species in Japan, on a zooplankton community was analyzed in experimental tanks, half of which were stocked with the fish. Different zooplankton species showed different responses to the introduction of the fish. In the presence of the fish, the populations of the large cladoceran Ceriodaphnia and the predatory copepod Mesocyclops were reduced, but the population of the herbivorous copepod Eodiaptomus and the small cladocerans Bosmina fatalis and Bosminopsis deitersi increased relative to the controls. The increase of Mesocyclops seen in the control tanks might have suppressed the populations of the small cladocerans, which are vulnerable to invertebrate predation. The results suggest that the population densities of the large prey items preferred by the fish, Ceriodaphnia and Mesocyclops, were controlled directly by fish predation, but the population densities of the smaller and less preferred zooplankton were controlled indirectly through the food-web cascade.     

A comparison of the effects of prey and non-prey neighbours on foraging rates of Epischura nevadensis (Copepoda: Calanoida)

SUMMARY. 1. Adults of the calanoid copepod, Epischura nevadensis , aggregate in situ near the thermocline in Lake Tahoe, California-Nevada. together with adults of another species of calanoid copepod, Diaptomus tyrelli and juveniles of both species. With a series of laboratory predation and algal clearance trials, we show that foraging rates of adult E. nevadensis are determined not only by the density of co-occurring potential prey (small copepods), but also by the presence of co-occurring non-prey neighbours (large, adult copepods). These effects occur at densities and in zooplankton assemblages found naturally, emphasizing the ecological importance of neighbours other than prey on zooplankton feeding.
2. Neighbours are distinguished primarily by size. Although predation rates increase linearly with the densities of small copepods. both algal clearance and predation rates decrease in the presence of large copepods. We also show, with a field predation experiment using small enclosures, that adults are size selective within species and that Diaptomus are selected over conspecifics of the same size.
3. We hypothesize that by reducing foraging rates in the presence of large zooplankton. E. nevadensis avoids predators and reduces predation risk at the cost of reduced energy consumption.     

The effect of the rate of successful dispersal of a phytoseiid mite,Phytoseiulus persimilis Athias-Henriot (Acarina: Phytoseiidae) on the persistence in the interactive system between the predator and its prey

Dispersal behaviour was studied on a predacious phytoseiid mite, Phytoseiulus persimilisAthias-Henriot in response to the density of its prey, Tetranychus kanzawaiKishida . And the effect of the change in the rate of successful dispersal of the predators among patches was tested on the persistence of the predator-prey system. The results of the study are summarized as follows:

1. With a severe decline in prey density available per individual predator, the predators exhibited a marked behavioural change and dispersed to other areas.
2. When two neighboring host plants did not touch with each other and the dispersal of the predators was possible only through a 20cm plywood “bridge” which connected the two plants, the rate of successful dispersal of the predators was only 20 to 25%, whereas it was 95% when the plants touched with each other.
3. In the system where 32 host plants touched with each other, the predators succeeded in immigrating into all the plants by the 16th day, and they completely eliminated all the prey by the 33rd day.
4. In another system, the 32 plants were equally divided into eight “patches” which were connected with each other with two of the bridges and the predators could move between them only through the bridges as mentioned above. In such a system the predators and their prey coexisted about three times longer than in the other one.
5. It is suggested that for a longer continued coexistence of the predators and prey, it would be necessary to introduce more physical barrier against the dispersal of the predators and to provide more chances for the prey to disperse.