Diapause in the calanoid freshwater copepod Eudiaptomus graciloides

The seasonal appearance and the intensity of diapausing-eggproduction in Eudiaptomus graciloides in five lakes of differentsize and trophic level were studied. In all lakes, diapausingeggs were produced in autumn. In the large mesotrophic lakeSelenter See, the population’s shift to the productionof diapausing eggs was more complete than in other lakes. Weexamined day length, temperature and food as proximate factorsfor the production of diapausing eggs with laboratory experiments.Eudiaptomus graciloides produced diapausing eggs in all treatments,but a significantly higher percentage of diapausing eggs wasfound under short day conditions except when algal food wasabundant and temperature was high. To investigate the adaptivesignificance of diapause in E. graciloides, we compared thesurvival of adult and juvenile at different temperatures forE. graciloides with Eudiaptomus gracilis, a sympatric speciesthat does not exhibit diapause. At 8°C, adult E. graciloidessurvived better than adult E. gracilis and exhibited reducedgut contents and accumulation of storage lipids, traits characteristicof adult diapause. Eudiaptomus graciloides nauplii did not reachthe copepodid stage at 6°C, but E. gracilis nauplii exhibitedhigh mortality and developed very slowly. We hypothesize thatadult diapause and production of diapausing eggs facilitatethe survival of E. graciloides during cold periods and enhancecoexistence with its congener, E. gracilis, in temperate zones.     

Identification and characterization of 10 microsatellite primers for the calanoid freshwater copepods Eudiaptomus gracilis and E. graciloides using enriched genomic libraries

The calanoid copepods Eudiaptomus gracilis and Eudiaptomus graciloides are widespread among European lakes. We constructed enriched genomic libraries for both species in order to isolate and characterise microsatellite markers. The obtained 7 polymorphic microsatellite‐loci for E. gracilis and 3 for E. graciloides are the first for any freshwater copepod. They display an expected heterozygosity ranging from 0.60 to 0.96 and 0.63 to 0.94 and observed heterozygosity ranging from 0.53 to 0.87 and 0.57 to 0.87, respectively. We were not able to cross‐amplify the isolated loci across species, indicating long divergence among both congeneric species despite morphological similarity.     

Resting stages in the life cycle of Eudiaptomus graciloides (Lill.) (Copepoda: Calanoida) in Lake Glubokoe

Seasonal changes in the abundance, age composition, vertical distribution, lipid and protein body content, development rates in the older copepodids (CIV and CV) and reproduction characteristics were studied in Eudiaptomus graciloides in the mesotrophic Lake Glubokoe (Moscow) region). Two periods of reproduction were observed: spring and late summer/autumn. The older copepodids, having accumulated lipid reserves during phytoplankton bloom, descended into the meta- and hypolimnion at the beginning of the summer. These copepods did not feed, but instead consumed their lipid reserves. A considerable delay in development (up to 40-60 days) of CIVs and CVs was found. The descended CIVs and CVs incubated under ambient temperature developed 4-5 times slower than their surface counterparts, while temperature dependence could only account for a 2-fold slow down. The descended copepods are believed to form a summer resting stock. In October, females began to produce resting eggs. The period of switching from subitaneous to resting egg production was very brief, not longer than 2-3 weeks. The percentage of subitaneous eggs produced in mid-October tended to increase when females were exposed to an increased photoperiod. At the beginning of February, latent eggs kept in a cooling container in the darkness started hatching. This process could be speeded up under natural photoperiod and increased temperature. The over-wintering population consisted largely of adults. Thus, E.graciloides includes three types of diapause in the life cycle: resting eggs, summer resting stage at CIV-CV, and overwintering adults.      

Gut evacuation rates and ingestion rates of Eudiaptomus graciloides measured by means of the gut fluorescence method

Diel variation in feeding rates of Eudiaptomus graciloides wasmeasured in situ with the gut fluorescence method during Mayand June in the eutrophic Frederiksborg Slotss Pronounced dielperiodicity occurred in both periods with highest feeding ratesduring the night. The food concentration as well as the periodicitywas almost constant with depth, indicating that the feedingactivity represents a behavioral periodicity. The gut evacuationmeasured in the laboratory on field-collected copepods was exponentialand temperature-dependent. The exponential decline in gut contentwas found for both copepods feeding continuously and when feedinghad ceased. Considering spatial and temporal variation in feedingactivity, daily ratios were 61–68 body carbon per day,and higher than if calculated from existing laboratory dataof short-term feeding dates.     

Coexistence of two similar copepod species, <Emphasis Type="Italic">Eudiaptomus gracilis</Emphasis> and <Emphasis Type="Italic">E. graciloides</Emphasis>: the role of differential predator avoidance

Habitat choice in relation to environmental factors of two coexisting calanoid copepod species, Eudiaptomus gracilis and E. graciloides, was studied in a mesotrophic lake and in large indoor mesocosms. Both species and sexes showed pronounced diel vertical migration (DVM) in the field. In 12 m deep mesocosms with free ranging fish DVM was observed and species increased day depth over time. No changes were observed in copepod day depth over time in experiments with fish kairomone. It is hypothesized that fish kairomone acts as an early warning system to copepods which respond by moving deeper, but only as far as the thermocline. For full DVM, a nearby mechanical stimulus is necessary. Thus, as fish go deeper to feed, copepods retreat. The response of copepods to fish predation, in the presence of low and high numbers of Daphnia,shows that copepods effectively use Daphniaas living shields to avoid predation. The two species adopt different vertical migration strategies depending on whether there are high or low numbers of Daphniapresent. A dominant feature of mesocosm experiments was the night time aggregating (lekking) of E. gracilis males at the surface. When the spring and autumn percentages of risk takers in the epilimnion were compared, E. gracilis, particularly males, suffered the greatest cost.     

Investigations into the Food Selectivity of the Planktic Crustaceans Daphnia hyalina,Eudiaptomus gracilis and Cyclops vicinus

The food selectivity of Daphnia hyalina, Eudiaptomus gracilis and Cyclops vicinus was investigated in laboratory experiments using algae of different sizes and at different concentrations as food sources. Vanderploeg and Scavia's (1979a, b) coefficient, which is most suitable for characterising selectivity, was selected from the literature and used to analyze the results. None of the three species was found to feed selectivity, however. The results show, on the other hand, that the leaky-sieve model can be used to describe the feeding patterns of these species. There were clear differences between the particle size ranges consumed by our experimental species, and these showed that, during the experiments, Daphnia hyalina acted as a microfilter-feeder, whereas Eudiaptomus gracilis and Cyclops vicinus acted as macrofilter-feeders.     

Top-down effects of crustacean zooplankton on pelagic microorganisms in a mesotrophic lake

A series of single-factor in situ experiments was conductedin a mesotrophic lake in Brandenburg, North Germany, to studythe predatory impact of Eudiaptomus graciloides (adults, copepodites,nauplii), cyclopoid copepods (adult Diacyclops bicuspidatus,Thermocyclops oithonoides) and daphnids (adult Daphnia hyalina,Daphnia cucullata) on the microbial community (bacteria, autotrophicpicoplankton, flagellates, ciliates). All zooplankton speciestested reduced the ciliate community significantly and ingestionrates were always higher for ciliates in the 20–55 µmsize category as compared to smaller ciliates (10–20 µm).Adult E.graciloides, which exhibited the highest predatory impacton ciliates, differed from cyclopoids and daphnids by theirability to decimate ciliates to very low abundances. Ingestionrates of ciliates by the crustacean zooplankton followed thesequence E.graciloides > daphnids = cyclopoids = copepodites.While top-down control was evident for ciliates, top-down effectsdown to the autotrophic picoplankton and flagellates were mostlyrestricted to Daphnia-dominated treatments. Top-down effectswere never strong enough to produce negative bacterial growthrates. For all zooplankton tested, clearance rates for ciliatesexceeded those for phytoplankton. Besides the potential of thecrustacean zooplankton to influence the structure of ciliatecommunities, ciliates may contribute to the energy demands ofcopepods and daphnids, especially when phytoplankton resourcesare limited.     

Gut evacuation rates and ingestion rates of Eudiaptomus graciloides measured by means of the gut fluorescence method

Journal of Plankton Research, 8, 973–983, 1986 FIg. 2. Time-dependent changes in the gut content (percentageof initial ng pigment) of E. gro.ciloides at different temperaturesunder simultaneous feeding. Fig. 4. The relationship between instantaneous evacuation rateand temperature of E. graciloides. The regresston equation forfeeding animals: y = 0.0044 e(0.141 ) (r² = 0.90). For comparisonthe results of non-feeding animals are indicated with open circles.     

Results Regarding the Food of the Planktic Crustaceans Daphnia hyalina and Eudiaptomus gracilis

The graph of maximum filtering and grazing rates versus particle size shows that Daphnia hyalina is a microfiltrator and Eudiaptomus gracilis is a macrofiltrator. In contrast to Eudiaptomus gracilis, Daphnia hyalina is also able to utilize larger bacteria (c. 1 μm³) as a food source.     

Seasonal variability of the embryonic development time of three planktonic crustaceans: dependence on temperature,adult size,and egg weight

The seasonal variability of the temperature dependence of embryonic development in Daphnia galeata, D. hyalina and Eudiaptomus gracilis of Lake Constance was investigated. We found a significant correlation between the adult size and the variability of the temperature function in all three species, and only in case of D. hyalina a significant influence of egg weight on the temperature function. We could demonstrate no general trend in the influence of the thermal adaptation of parent animals on the temperature function, but some specifically different responses among the investigated species.     

Feeding of freshwater filter-feeders at very low food concentrations: Poor evidence for "threshold feeding" and "optimal foraging" in Daphnia longispina and Eudiaptomus gracilis

The feeding behaviour of two freshwater zooplankton species,the calanoid copepod Eudiaptomus gracilis and the cladoceranDaphnia longispina, at extremely low food concentrations hasbeen studied in order to test whether the animals react as predictedby the models of optimal foraging. Three different sized algae were offered in concentrations downto 1µg C/litre at 7°C during winter and 19°C duringsummer. Ingestion rates were measured by use of a flow-through¹⁴C-technique In contrast to the findings of several authors working withmarine copepods a threshold concentration where the animalsceased filtering could be detected only in one experiment byregression analysis (Daphnia, 19°C, food: Stichococcus).A depression of the filtering rate at low concentrations wasobserved only in Daphnia at 19°C with Stichococcus and Scenedesmusas food. It is supposed that in this case the reduction of thefiltering rate is not a direct response of the filtrator, butis caused by exhaustion due to the experimental conditions. As freshwater zooplankton is very rarely exposed to food abundancesas low as in the marine environment, there seems to be no selectionpressure favouring the evolution of an "optimal forager" behaviour. ⁽¹⁾Supported by Deutsche Forschungsgemeinschaft     

Grazing and nutrient influences of Daphnia and Eudiaptomus on phytoplankton in laboratory microcosms

In two experiments, top-down and bottom-up influences of theherbivorous crustaceans Daphnia pulicaria and Eudiaptomus grac$$$loideson phytoplankton were compared in laboratory microcosms. Ina long-term experiment (63 days), both grazers were able toestablish populations. The Daphnia population exerted strongergrazing pressure, whereas Eudiaptomus fed more selectively.Daphnia retained relatively more phosphorus (P) and thus causedalgal P limitation; with Eudiaptomus as a grazer, both nitrogen(N) and P remained limiting. In a short-term experiment (1 day),N and P release rates and algal-specific grazing rates by bothconsumers were measured. In this experiment, the increase inconcentrations of soluble mineral nutrients (     

Feeding of nauplius stages of Eudiaptomus gracilis on mixed plastic beads

Eudiaptomus gracilis makes up 30–40 and 80–90% ofthe zooplankton in Lake Balaton during the summer and the winterrespectively. More than half of the species population consistsof nauplii We studied feeding and size selectivity of naupliiin suspensions which contained polystyrol latex beads in a concentrationdose to the natural seston. Guts of NI nauplii were free ofboth beads and remnants of natural food Of NII–NVI nauplii,67–87% took in beads. Older animals consumed more andlarger particles. The maximum diameter of ingested beads reached29 µm On an average, NII nauplii took in 128 µm³of beads in 10 min, whereas older animals consumed 615–900µm³. The clearance rate remained below 0 01 µl h^–1NII nauplii strongly preferred 1 2 µm particles Oldernauplii did not show any preference or selected only slightlyfor the smallest particles. Nauplii rejected 4–11 µmbeads. In some cases a weak positive selection could be observedtoward 12 µm or larger beads.     

The relevance of life-history traits in the establishment of the invader <Emphasis Type="Italic">Eudiaptomus gracilis</Emphasis> and the extinction of <Emphasis Type="Italic">Eudiaptomus padanus</Emphasis> in Lake Candia (Northern Italy): evidence for competitive exclusion?

The niches invaded by exotic species are generally not entirely vacant, and one possible factor affecting the success of the invader to establish a permanent large population may be the ability of the former to outcompete native species. Eudiaptomus gracilis, which was not present in Italy before the 1980s, is becoming established in an increasing number of Northern Italian lakes, ultimately replacing the endemic E. padanus. Coexistence of the two species in Lake Candia lasted only 7 months, suggesting that species replacement was determined either by environmental changes or by strong competition. To assess whether the potential for interspecific competition existed and to identify species' traits which could explain the competitive superiority of E. gracilis, we examined field seasonal patterns, reproductive parameters and body size of the two species. E. padanus abundance was probably reduced by predation and parasitism, which favoured the displacement of the species shortly after invasion by E. gracilis. The temporarily underexploited niche provided an opportunity for the successful establishment of the invader. The reproductive patterns of the two species were found to be similar in most of their features, with the exception of a markedly larger clutch size and a smaller egg volume in the resident species. By contrast, the invader showed a higher adult:egg ratio and a lower death rate. Thus, despite the greater fecundity of E. padanus, the competitive success of the invader might be attributable to interspecific differences in developmental rates and/or juvenile mortality.     

Dissolved organic carbon released by zooplankton grazing activity-a high-quality substrate pool for bacteria

Experiments were designed to investigate whether processes relatedto zooplankton feeding have a positive effect on bacterial growth.Bacterial abundance and [³H]thymidine incorporation rates werefollowed in grazer-free batch cultures originally containingeither Scenedesmus quadricauda or Rhodomonas lacustris as foodsources, and Daphnia cucullata or Eudiaptomus graciloides asgrazers. Compared with controls lacking either animals or algae,a significantly higher bacterial abundance and productivityoccurred in cultures which contained both phyto- and zoo-plankton.The same experimental methodology was tested during the declineof a diatom spring bloom in a eutrophic, temperate lake. A significantincrease in bacterial biomass was observed due to the grazingactivity of in situ mesozooplankters during the clear-waterphase. Our results demonstrated that the dissolved carbon pathwayfrom mesozooplankton to bacteria averaged 57% (26–78%)of the algal carbon filtered from suspension.     

The year cycle of Eudiaptomus vulgaris (Schmeil, 1896) (Copepoda,Calanoida) in a small,acid water body during 1973. Development in the natural habitat and relationships between temperature and duration of development stages

The development of a population of Eudiaptomus vulgaris (Schmeil, 1896) in the Meeuwenven, a shallow acid guanotrofic moorland pool, has been described during one year. The population hibernates as copepodite 5 stages, adults and, to a small extent as naupliar stages N1, N2 and N3 (which could not develop further at low temperatures in autumn). In spring the population development starts at temperatures above 10°C and shows 3 or 4 pulses a year. An attempt has been made to explain seasonal changes in the size of adult males and females and in the sex ratio.In order to establish the duration of the various development stages, the animals have been cultured at different temperatures under illumination with 2000 Lux at a daylength of 14 hours. An adequate quantity of food from the natural habitat was available.Total egg development and total naupliar and copepodite development have been compared with the results of other workers, especially with those from Eckstein (1964), who studied Eudiaptomus vulgaris in the deep Schluchsee. The duration curves do not differ markedly with those of Eckstein and are strongly temperature dependent.The relation between the development times of the various stages with temperature can be generally expressed as parabolic regressions of the type D = a + b₁T + b₂T², the C₅ and adult stages being the only exception at higher temperatures. Comparison of the relative duration of the stages at different temperatures did show that younger stages can take a larger share of the total development time at lower temperatures, stage N6 being the most temperature-sensitive.     

Effect of lipid reserves on individual carbon content of two planktonic freshwater copepod species

The effect of the variation of lipid droplet volume on individual carbon content was studied in two species of planktonic freshwater copepods, Eudiaptomus gracilis G.O. Sars and Thermocyclops oithonoides G.O. Sars. It was found, that when a multiple regression of carbon content on prosoma and lipid droplet volume instead of a simple regression of carbon content on prosoma volume was applied, the unexplainied variance of individual carbon content of Eudiaptomus gracilis could be reduced. However, the use of multiple regression on Thermocyclops oithonoides data did not lead to significantly higher coefficient of determination than was obtained by using a simple regression.     

The influence of year-to-year variations in winter weather on the dynamics of Daphnia and Eudiaptomus in Esthwaite Water, Cumbria

1. Micro-crustacea of the genus Daphnia and Eudiaptomus have evolved different physiological mechanisms for coping with life in a rapidly changing environment. In this paper, we analyse some of the physical and biological factors influencing the winter abundance of the two species in a small lake (Esthwaite Water in Cumbria).
2. The results demonstrate that much of the year-to-year variation in their relative abundance can be related to long-term changes in the weather. The highest numbers of Daphnia were typically found in cold, calm winters when small flagellates were relatively abundant. In contrast, the highest numbers of Eudiaptomus were found in mild, windy winters when the phytoplankton community was dominated by colonial diatoms.
3. Year-to-year variations in the winter abundance of Eudiaptomus had no effect on their subsequent development but the numbers of overwintering Daphnia had a significant effect on the size of their first spring 'cohort'. The most important factor influencing the overwintering performance of the two species was the water temperature.
4. Winter air temperatures over much of Europe are influenced by the atmospheric pressure variation known as the North Atlantic Oscillation (NAO). Winter water temperatures in Esthwaite Water were strongly correlated with this empirical index and there was a significant positive correlation between the NAO and the number of overwintering Eudiaptomus.     

Effects of invading vendace (Coregonus albula L.) on species composition and body size in two zooplankton communities of the Pasvik River System, northern Norway

Species composition and body-size distribution were studiedin the crustacean zooplankton communities of two limnologicallysimilar lake localities situated 50 km apart in the Pasvik RiverSystem, northern Norway. A recent invasion and successive downstreamexpansion of vendace (Coregonus albula), a specialized zooplanktivorousfish, allowed comparisons between sites with different predationpressures. Vendace had established a high population densityand was the dominant fish species in the pelagic of the upperlocality, but had just invaded the lower locality with a smallnumber of individuals. Whitefish (Coregonus lavaretus), a closelyrelated but less specialized zooplanktivore species, dominatedthe native fish community of both lakes. The zooplankton communityof the upper locality was dominated in June and August by Bosminalongirostris, the smallest zooplankton species represented inthe water course, and in September by Daphnia cristata. Thelower locality was dominated by the larger Holopedium gibberumand Eudiaptomus graciloides in June, by D.cristata in August,and by D.cristata and B.longirostris in September. The meanbody size of the three most abundant cladoceran species wassignificantly smaller in the upper locality, compared to thelower locality. It was concluded that the invasion and establishmentof a dense vendace population in the upper locality had increasedthe predation pressure in the pelagic, resulting in a reductionof body size and a shift towards smaller species in the zooplanktoncommunity.     

A simple method for fitting Bělehrádek's equation to embryonic development data of zooplankton

A method is described whereby a Bělehrádek function is fitted to experimentally derived data for embryonic development duration at a range of temperatures. The method is applied to data for the freshwater copepods Cyclops abyssorum and Eudiaptomus gracilis from Lough Neagh, Northern Ireland. Differences in duration of embryonic development of Lough Neagh populations and that of previously published data for these species are discussed.