Benthic resting periods of pelagic cyclopoids in an oligotrophic lake

The benthic resting stages of pelagic cyclopoids were studied in the oligotrophic lake Pääjärvi (maximum depth 87 m), southern Finland. Stage 5 copepodids of Thermocyclops oithonoides were found in the bottom from September to April, with highest abundances in the littoral at 1–2 m. Dormant Mesocyclops leuckarti (mainly stage 5 copepodids) were found from mid-August so April, with a strong concentration at the depth of 1.5 m, and resting stage 4 copepodids of Cyclops kolensis from mid-summer to late winter at the depths of 1–13 m. Single stage 4 and 5 copepodids of C. strenuus and C. lacustris were also found in the bottom during autumn and winter. Available information on the periodicity of these species conforms well with the hypothesis of a temperature-adjusted photoperiodic control of diapause.
In the sediment, the resting stages of pelagic cyclopoids were confined to the uppermost 2–3 cm, C. kolensis penetrating slightly deeper than the smaller Thermocyclops and Mesocyclops species.
Losses during the resting period were highest in T. oithonoides , in which the seasonal mortality rates were significantly correlated with temperature. Its overall mortality was slightly lower at 2 m than at 13 or 40 m. C. kolensis had the lowest mortality, with no clear relation to temperature.
The mean winter biomass of the benthic resting stages of pelagic cyclopoids exceeded that of the true benthic copepods, and the winter losses of the former were equal to about one fifth of the total annual production of the true benthic copepods.     

Predation by juvenile roach on the calanoid copepod Eudiaptomus gracilis and the cyclopoid copepod Cyclops vicinus: a laboratory investigation with mixed and single prey

We tested the outcome of predation by juvenile roach on the calanoid copepod Eudiaptomus gracilis and the similar sized cyclopoid copepod Cyclops vicinus in laboratory experiments with mixed and single-prey. When ovigerous females and adult non-ovigerous females of the calanoid copepod and the cyclopoid copepod were offered in equal numbers to the fish in the mixed-prey experiments, the cyclopoid copepods were significantly more heavily preyed upon than the calanoids. Between 14 and 16 females of the cyclopoid copepod and only between 2 and 4 calanoids had been consumed after the observation period of two hours. The single-prey experiments revealed that the conspicuousness of the calanoid vs the cyclopoid depended on sex and gravidity. Ovigerous females of the cyclopoid copepod were slightly earlier detected by the fish than ovigerous females of the calanoid, probably a result of the highly visible egg-sacs carried by the cyclopoid females. Females without eggs and males of the cyclopoid copepod were recognized later by the predator than females without eggs or males of the calanoid, probably a result of the different behaviours of the different copepod taxa. Cyclopoids frequently congregated near the aquarium bottom while calanoids were closer to the water surface where they were better visible. The calanoid copepod could better escape the fish's attacks than the cyclopoid copepod. Egg-bearing females of both the calanoid and the cyclopoid copepod could significantly better escape than non-ovigerous females or males. Probably ovigerous females react very early to water disturbances caused by the predator. Activity measurements showed that the cyclopoid copepod displayed 2 to 4 times more hops per time unit than the calanoid. Probably the high number of jerky movements displayed by the cyclopoid attracted attention of the predator and contributed to its greater vulnerability.     

A simple method for maintaining a laboratory supply of limnetic cyclopoid copepods

The following method is used for maintaining a laboratory supply of freshwater cyclopoid copepods with intervening diapause phase. Animals are collected with a core sampler from the upper sediment surface just after initiation of diapause, i.e. during early summer for summer resting species and mid autumn for winter resting species. The method makes it possible at all times of the year to keep a variety of cyclopoid copepod species ready for experimental work. The following widespread species have been used: Mesocyclops leuckarti, Thermocyclops oithonoides, Diacyclops bicuspidatus, Cyclops vicinus, C. strenuus and C. abyssorum. Probably all cyclopoid copepods with a diapause phase could be stored according to the present method.     

Zooplankton in the Schelde estuary, Belgium and The Netherlands. Spatial and temporal patterns

The zooplankton fauna of the Zeeschelde estuary (Belgium) wasinvestigated over 10 months by means of monthly sampling. CanonicalCorrespondence Analysis (CCA) was used to relate the speciesdistribution to environmental factors. The variation in thespecies data was significantly (P < 0.05) related to a setof 10 environmental variables (chlorinity, NH₄⁺, temperature,PO₄-P^-, DW, Chl a and Chl b, NO₂-N, NO₃-N and pH). The mainspatial and seasonal gradients were associated with chlorinityand temperature respectively. The brackish water zone was dominatedby the calanoid Eurytemora affinis in spring, succeeded by Acartiatonsa and mysid species during summer. In the freshwater transect,cyclopoids dominated, together with several cladoceran species.Thermophilic cyclopoid species (Thermocyclops oithonoides, Th.crassus and Mesocyclops leuckarti) occurred during periods ofmaximal temperature. The cyclopoids Acanthocyclops robustus,Paracyclops poppei and Cyclops vicinus, the cladocerans Daphnialongispina, Chydorus sphaericus and Bosmina longirostris togetherwith the numerically dominant rotifers, oligochaetes, nematodesand juvenile copepods seemed little affected by environmentalgradients.     

Short term experiments on calanoid-cyclopoid-phytoplankton interactions

To investigate their potential effects on each other in nature, calanoid (Diaptomus clavipes and D. siciloides) and cyclopoid (Acanthocyclops vernalis and Mesocyclops edax) copepod populations were manipulated in 5 liter aquaria in laboratory experiments of 20–60 days duration. Diaptomus generally had a strongly negative effect on both cyclopoid species. The cyclopoids established populations more successfully when introduced to aquaria before calanoids than they did when calanoids were already present. On the other hand, whether introduced earlier or later than the cyclopoids, Diaptomus populations were unaffected by Acanthocyclops and were strongly depressed by Mesocyclops. Diaptomus effects on the phytoplankton were often strong but varied markedly among experiments. They included reduction of populations of edible algae, such as Chlamydomonas, which are essential for both calanoid and cyclopoid nauplii, and large increases in inedible algae, such as Kirchneriella. Feeding experiments revealed that under conditions of food scarcity Acanthocyclops nauplii survived less well than did Diaptomus nauplii. Competition for edible phytoplankton seemed to be a key factor in the calanoid-cyclopoid interactions, since the survival of herbivorous cyclopoid larvae determined the abundance of the predaceous adults. This indicates that the competitive effects of calanoids on cyclopoids often may exceed the predative effects of cyclopoids on calanoids.     

Long-term changes of Copepoda community (1957-1996) in a subtropical Chinese lake stocked densely with planktivorous filter-feeding silver and bighead carp

In contrast to the relatively well documented impact of particulate-feedingfish on zooplankton communities, little attention has been devotedto the impact of filter-feeding fish. Filter-feeding silverand bighead carp are the most intensively cultured fish speciesin Asia and comprise much of the production of Chinese aquaculture.However, little information is known about the impact of eitherfish on the zooplankton community. Long-term changes in theCopepoda community (1957–1996) were studied at two samplingstations of a subtropical Chinese lake (Lake Donghu) dominatedby silver and bighead carp. For both calanoids and cyclopoids,the littoral station (I) was much more resource profitable thanthe pelagic station (II). There has been a tremendous increasein the annual fish catch over the past 30 years due to the increasedstocking with fingerlings of the two carp species. There wasa notably higher fish density at Station I than at Station II.Cyclopoid abundance was notably higher at Station I than atStation II during the 1950s to the 1980s, while the reversebecame true in the 1990s. This is probably because when fishabundance increased to an extremely high level, the impact offish predation on the cyclopoids became more important thanthat of food resources at the littoral station. At both stations,cyclopoid abundance was relatively low in spite of the presenceof abundant prey. Similarly, calanoid density did not differsignificantly between the two stations in the 1950s and 1960s,but was significantly lower at Station I than at Station IIduring the 1980s and 1990s. Such changes are attributed to thegradient of fish predation between the stations and an increasingpredation pressure by the fish. The increased fish predationalso correlated with a shift in summer-dominant calanoids fromlarger species to smaller ones. In conclusion, the predaceouscyclopoids are affected by fish predation to a much lesser extentthan the herbivorous calanoids, and therefore increased predationby filter-feeding fish results in a definite increase in thecyclopoid/calanoid ratio. Predation by filter-feeding fish hasbeen a driving force in shaping the copepod community structureof Lake Donghu during the past decades.     

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.     

Patterns of prey size and taxonomic composition in larval fish: are there general size-dependent models?

The allometric relationship of maxilla length to larval fish weight approached a slope of 1/3 for 11 species from Conception Bay, Newfoundland. In 10 of these species, mean prey size increased with length of the maxilla but not constantly. In six species, niche breadth increased with length of maxilla. The rate of change in niche breadth was inversely related to the rate of change in mean prey size. Nauplii and copepodites of calanoids were selected positively, and cyclopoids negatively for almost all species of larval fish.     

Crustacean zooplankton assemblages in freshwaters of tropical Australia

Throughout tropical Australia eulimnetic zooplankton is composed of 7 calanoid copepods, 5 cyclopoid copepods and 12 cladocerans. A further 10 cyclopoids and 9 cladocerans occur as littoral strays, making 43 species in all. Dominants include Diaptomus lumholtzi, Mesocyclops notius, Thermocyclops decipiens, Diaphanosoma excisum, Ceriodaphnia cornuta, and Moina micrura. Momentary species composition averages 1.0 calanoids, 1.3 cyclopoids and 2.0 cladocerans. These assemblages are not only less complex than in temperate Australia, but also are not as rich as in most other tropical regions. The main contributors to these differences are calanoids and cladocerans respectively. Nevertheless, as elsewhere, most zooplankters in tropical Australia are eurytropic, widely distributed and small.     

Observations on the taxonomic composition and vertical distribution of cyclopoid copepods in the central Red Sea

The taxonomic composition and vertical distribution of cyclopoid copepods, including very small species, in the central Red Sea were analyzed. Samples were taken in a multiple opening and closing net with 0.1 mm mesh size to a depth of 450 m. Most species belong to the genus Oncaea (13 identified and 6 unidentified forms). Nearly one half of these are smaller than 0.4 mm in length. Nine cyclopoid species and three groups of very similar species and forms of Oncaea predominate. In the upper 450 m of the water column, each of these species or groups account for more than 1 % of all cyclopoid specimens. In the epipelagic zone (0–100 m) Oithona simplex predominates, accounting for 20 % of the total. In the upper mesopelagic zone Paroithona sp. and one of the unidentified Oncaea forms are most numerous from 100 to 250 m, and two groups of Oncaea are most abundant from 250 to 450 m. The dominance of single species among cyclopoids is less pronounced than that reported for calanoids in the mesopelagic zone of the central Red Sea.     

Predation on the Invasive Copepod,Pseudodiaptomus forbesi,and Native Zooplankton in the Lower Columbia River: An Experimental Approach to Quantify Differences in Prey-Specific Feeding Rates

Invasive planktonic crustaceans have become a prominent feature of aquatic communities worldwide, yet their effects on food webs are not well known. The Asian calanoid copepod, Pseudodiaptomus forbesi, introduced to the Columbia River Estuary approximately 15 years ago, now dominates the late-summer zooplankton community, but its use by native aquatic predators is unknown. We investigated whether three species of planktivorous fishes (chinook salmon, three-spined stickleback, and northern pikeminnow) and one species of mysid exhibited higher feeding rates on native copepods and cladocerans relative to P. forbesi by conducting `single-prey’ feeding experiments and, additionally, examined selectivity for prey types with `two-prey’ feeding experiments. In single-prey experiments individual predator species showed no difference in feeding rates on native cyclopoid copepods (Cyclopidae spp.) relative to invasive P. forbesi, though wild-collected predators exhibited higher feeding rates on cyclopoids when considered in aggregate. In two-prey experiments, chinook salmon and northern pikeminnow both strongly selected native cladocerans (Daphnia retrocurva) over P. forbesi, and moreover, northern pikeminnow selected native Cyclopidae spp. over P. forbesi. On the other hand, in two-prey experiments, chinook salmon, three-spined stickleback and mysids were non- selective with respect to feeding on native cyclopoid copepods versus P. forbesi. Our results indicate that all four native predators in the Columbia River Estuary can consume the invasive copepod, P. forbesi, but that some predators select for native zooplankton over P. forbesi, most likely due to one (or both) of two possible underlying casual mechanisms: 1) differential taxon-specific prey motility and escape responses (calanoids > cyclopoids > daphnids) or 2) the invasive status of the zooplankton prey resulting in naivety, and thus lower feeding rates, of native predators feeding on invasive prey.     

Feeding and digestive activity of cyclopoid copepods in active diapause

Three different methods were used to compare the feeding patterns of active cyclopoids and cyclopoids in active diapause from different habitats. First, we offered ¹⁴C- labeled algal detritus to stage III, stage IV (two different physiological groups), and adult females of Acanthocyclops vernalis from temporary water-bodies of the Volga Delta. Second, we compared the functional responses of active and actively diapausing cyclopoids preying on copepod nauplii. Stage III, stage IV (various physiological groups), stage V, adult males and females of Acanthocyclops viridis, Cyclops kolensis, C. abyssorum sevani from the Volga Delta, Lake Sevan (Armenia), and Mozhaisk Reservoir (Moscow District) were tested. Finally, we examined the activity of digestive enzymes (proteases) of CIV of C. kolensis from Mozhaisk Reservoir at different temperatures and temperature acclimation regimes. All three approaches used have led to similar conclusions. Actively diapausing cyclopoids were capable of feeding on both plant and animal prey; however, their feeding rates and digestive enzyme activity were 2–8 times lower than those of active cyclopoids of similar size. At the initial stage of active diapause the feeding rates were comparable with those of active cyclopoids then declined to a low level during approximately two weeks. Transfer of cyclopoids undergoing winter diapause from ambient temperature (4 °C) to warmer water (20 °C) resulted in dramatic increase of proteases activity only if the acclimation period was relatively long (two weeks).     

Intraspecific Autochthonous and Allochthonous Resource Use by Zooplankton in a Humic Lake during the Transitions between Winter,Summer and Fall

Seasonal patterns in assimilation of externally produced, allochthonous, organic matter into aquatic food webs are poorly understood, especially in brown-water lakes. We studied the allochthony (share biomass of terrestrial origin) in cladoceran, calanoid and cyclopoid micro-crustacean zooplankton from late winter to fall during two years in a small humic lake (Sweden). The use of allochthonous resources was important for sustaining a small population of calanoids in the water column during late winter. However, in summer the calanoids shifted to 100% herbivory, increasing their biomass several-fold by making efficient use of the pelagic primary production. In contrast, the cyclopoids and cladocerans remained at high levels of allochthony throughout the seasons, both groups showing the mean allochthony of 0.56 (range in mean 0.17-0.79 and 0.34-0.75, for the respective group, depending on model parameters). Our study shows that terrestrial organic matter can be an important resource for cyclopoids and cladocerans on an annual basis, forming a significant link between terrestrial organic matter and the higher trophic levels of the food web, but it can also be important for sustaining otherwise herbivorous calanoids during periods of low primary production in late winter.     

Possible impact of mild winters on zooplankton succession in eutrophic lakes of the Atlantic European area

1. We report on a long-term study (1975–94) of water temperatures and plankton in a eutrophic lake (Heiligensee, Berlin, Germany). Using a phenomenological approach, we use historical data to infer how an increase in air temperature has influenced a natural zooplankton community.
2. Air temperatures in Berlin showed a significantly rising trend between 1975 and 1994. Mean winter air temperatures in the last 8 years always exceeded the long-term mean.
3. A rising trend was also found for April water temperature, which increased significantly beginning in 1988–89. An increase of 2.58°C in the last 21 years was recorded using a linear model. A significantly decreasing trend was found in June but no trend was noted for the other summer months.
4. Phytoplankton composition shifted from a dominance of diatoms and cryptophytes during winter and spring in the 1980s towards a dominance of cyanobacteria in 1990–94.
5. The dominant zooplankton species in spring shifted in recent years from the large-bodied Daphnia galeata to the smaller D. cucullata . Cyclops kolensis , previously the only invertebrate predator during winter, decreased in abundance while C. vicinus , usually present during spring and autumn, increased in abundance and was numerous during winter, a season passed in diapause in the earlier years.
6. Because direct and indirect temperature effects are species specific, we put forward the hypothesis that zooplankton species, rather than functional groups, are the nexus between environmental stress, such as rising air temperatures, and ecosystem changes.     

Carbon content of some common tropical Andaman Sea copepods

Individual carbon contents of eight common copepod speciesfrom the Andaman Sea were determined. Length–carbon regressionsare presented for four calanoids (Acrocalanus gibber, Centropagesfurcatus, Temora discaudata and Euchaeta spp.), two cyclopoids(Oncaea spp. and Corycaeus spp.) and two harpacticoids (Macrosetellagracilis and Microsetella spp.). The copepod specimens wereobtained from different localities and times of the year during1996–1997. The regression coefficients are good in calanoidand cyclopoid copepods, but poor in the harpacticoids. The slopevalues range from 2.3 to 3.8 in the calanoids to 1.2 and 1.6in the harpacticoids, while the cyclopoids have slope valuesof 2.0 and 2.9. The equations derived from this study are expectedto be general characteristics, which are applicable for thecalculation of copepod biomass and production in tropical areas.     

Dormancy, dispersal and the survival of cyclopoid copepods (Cyclopoida, Copepoda) in a lowland floodplain

1. The survival of cyclopoid copepods was investigated in a floodplain for 2 years where flooding occurred during the cold season. The cyclopoid community was studied in three waterbodies with distinct hydroperiods: a permanent pond connected to the flooded area during inundation, a temporary pool that is part of the flooded area and an isolated temporary pool.
2. Field studies, including data obtained from samples of water, sediment and soil, showed the overall predominance of species with a summer diapause over those with a winter diapause or without diapause. Emergence of cyclopoid copepods at the onset of flooding, examined using emergence traps and data from recently filled or still isolated temporary pools, showed that some species can survive several months of drying.
3. The ability of the diapausing fourth copepodid stages of Cyclops strenuus and C. insignis , the two cyclopoids most abundant during winter and spring flooding, to survive terrestrial conditions was tested in laboratory experiments. Both species survived for several months, but rates differed among the species. A higher percentage of C. strenuus survived for a longer period, possibly explaining why this species was relatively more abundant in more temporary habitats.
4. Both dormancy and dispersal facilitated survival of cyclopoid copepods in transient habitats connected to each other during flood periods. Dormancy was the most important survival strategy, whereas dispersal could be more important following prolonged periods without flooding.     

Possible impact of mild winters on zooplankton succession in eutrophic lakes of the Atlantic European area

1. We report on a long-term study (1975–94) of water temperatures and plankton in a eutrophic lake (Heiligensee, Berlin, Germany). Using a phenomenological approach, we use historical data to infer how an increase in air temperature has influenced a natural zooplankton community.
2. Air temperatures in Berlin showed a significantly rising trend between 1975 and 1994. Mean winter air temperatures in the last 8 years always exceeded the long-term mean.
3. A rising trend was also found for April water temperature, which increased significantly beginning in 1988–89. An increase of 2.58°C in the last 21 years was recorded using a linear model. A significantly decreasing trend was found in June but no trend was noted for the other summer months.
4. Phytoplankton composition shifted from a dominance of diatoms and cryptophytes during winter and spring in the 1980s towards a dominance of cyanobacteria in 1990–94.
5. The dominant zooplankton species in spring shifted in recent years from the large-bodied Daphnia galeata to the smaller D. cucullata . Cyclops kolensis , previously the only invertebrate predator during winter, decreased in abundance while C. vicinus , usually present during spring and autumn, increased in abundance and was numerous during winter, a season passed in diapause in the earlier years.
6. Because direct and indirect temperature effects are species specific, we put forward the hypothesis that zooplankton species, rather than functional groups, are the nexus between environmental stress, such as rising air temperatures, and ecosystem changes.     

The midsummer crustacean plankton communities of seven small impoundments

The crustacean zooplankton communities of seven, small, eutrophic West Virginian impoundments were examined during midsummer. The more eutrophic lakes had as dominants, 2 cyclopoid, 1 calanoid, and 2 cladoceran species. Cluster analysis showed that the 4 more eutrophic lakes clustered together, further cluster analysis showed there was a very similar group of 5 species which characterized the most eutrophic lakes. Principal components analysis showed that the more eutrophic lakes were characterized by having more cyclopoids and cladocerans than calanoids. Niche breadth showed that the most often dominant species had much higher values than the species ohich could be considered secondary dominants.     

Ecological and evolutionary significance of cyclopoid predation on fish larvae

Of 412 cyclopoids found preying on fish larvae in Lake Michigan,411 were adult females. It is suggested that adult, female cyclopoidpredation on fish larvae may represent an important mechanismboth to reduce intraspecific competition for food (i.e. betweensexes) and maximize net energy intake for reproduction. Ourobservations of cyclopoid predation on fish larvae in naturemay also provide complementary evidence on how freshwater parasiticcyclopoids evolved from free-living ancestors. Freshwater, parasiticcyclopoids may have evolved from an ancestral free-living cyclopoidwhich was predaceous on small organisms, to a free-living cyclopoid(e.g. Acanthocyclops vernalis and Diacyclops thomasi) whichwas predaceous on larger prey such as fish larvae. Evolutionthen could have proceeded to a free-living cyclopoid with intermediatemorphological modification of its second antennae (e.g. Ergasiluschautauquaensis), and finally to a parasitic cyclopoid whichmorphologically exhibits a high degree of modification in itssecond antennae (e.g. many species of Ergasilus).