Interrelations of rotifers with predatory and herbivorous Cladocera: a review of Russian works

Publications in Russian on the influence of epibiontic rotifers on cladocerans as well as on predator-prey interrelations between cladocerans and rotifers are reviewed. Proales daphnicola and Brachionus rubens are common epibionts of Cladocera. The biology of these species is described including the choice of host, feeding, reproduction and impact on the host. Representatives of the families Daphniidae and Moinidae are most readily colonized. At high densities of epibiontic rotifers, a high percentage of young Cladocera die. Predators consuming cladocerans (mainly Bosminidae) belong to the family Asplanchnidae. Rotifers are consumed by the predatory cladocerans Leptodora and Polyphemus. The results of functional response experiments by the author with P. pediculus feeding on Synchaeta pectinata, Asplanchna priodonta, colonial and solitary Conochilus unicornis, and Platyias patulus are given. In the range of prey densities of 100–1600 1^–1 a functional response was found in all the rotifers except P. patulus. Colonies of C. unicornis were not consumed. The highest level of feeding rate saturation was observed in Synchaeta pectinata. Various adaptations in prey morphology prevented effective predation: coloniality, large size of the prey and hard lorica with spines.     

Zooplankton composition, dominance and abundance as indicators of environmental compartmentalization in Jurumirim Reservoir (Paranapanema River), São Paulo, Brazil

Spatial and temporal patterns in the distribution of the zooplankton in a large tropical reservoir were investigated for a year. The zooplankton was sampled at 10 limnetic stations. Rotifera were richest in number of species and individuals, especially in transitional river–lake zones. They were dominant during the summer in nine sampling stations, and decreased in spring. The main species were Polyarthra vulgaris, Keratella americana, K. cochlearis and Conochilus unicornis. Polyarthra vulgaris was widely distributed. Keratella was more abundant at upstream stations, and a dense population of C. unicornis was observed in a lateral, sheltered compartment. Among copepods, Calanoida were more abundant in spring and Cyclopoida in autumn. Longitudinal gradients in the Calanoida/Cyclopoida relation were observed, with the predominance of Cyclopoida at upstream sampling stations and Calanoida in more lacustrine zones towards the dam. Notodiaptomus iheringi, Thermocyclops minutus and T. decipiens were the main species. Diaphanosoma birgei, the most abundant cladoceran, mainly occurred in lacustrine zones, while Moina minuta was more abundant at riverine sampling stations, generally in association with Bosminopsis deitersi. Peaks of tintinnid protozoans were observed in upstream zones during summer and spring.     

Indirect food web effects of <Emphasis Type="Italic">Bythotrephes</Emphasis> invasion: responses by the rotifer <Emphasis Type="Italic">Conochilus</Emphasis> in Harp Lake,Canada

As a recent invader of North American lakes, Bythotrephes longimanus has induced large changes in crustacean zooplankton communities through direct predation effects. Here we demonstrate that Bythotrephes can also have indirect food web effects, specifically on rotifer fauna. In historical time series data, the densities of the colonial rotifer Conochilus unicornis significantly increased after Bythotrephes invasion in Harp Lake, Ontario. No such changes were observed in a non-invaded reference lake, the nearby Red Chalk Lake. Evidence for two mechanisms explaining the Conochilus increase was examined based on changes to the crustacean zooplankton community over time. Rapid and severe declines in several herbivorous species of cladoceran zooplankton after Bythotrephes detection indicated a decrease in exploitative competition pressure on Conochilus. Secondly, a later and significant decline to virtual extinction of native invertebrate predators (Mesocyclops and Leptodora) could account for the observed Conochilus increase which also began 1–2 years after invasion by Bythotrephes. Ultimately, it appears that both reduced competition followed by a loss of native invertebrate predators were necessary to lead to the large Conochilus densities observed following invader establishment. From this analysis of long-term community data, it appears that Bythotrephes has important indirect, as well as direct, food web effects in newly invaded North American lakes with implications for trophic relationships.     

Leptodora kindti: efficient predator and preferred prey item in Neusiedler See,Austria

In the Neusiedler See, Leptodora kindti appeared in the plankton in April/May; the growing season lasted until November. Its optimum conditions prevailed during the summer months, when densities of >500 ind. m^–3 were recorded. First, males appeared by the mid/end of August, while maximum resting egg production occurred in September. At temperatures below 10 °C the population declined.At high population levels, L. kindti caused suppression of its preferred prey item, the juveniles of Diaphanosoma mongolianum. Twenty-five to 45% of juvenile Diaphanosoma can be eliminated by Leptodora. This predation impact is a pronounced but short-term event. During the periods of high Leptodora densities, a significant relationship between Leptodora predation and the death rate of Diaphanosoma is found.The older developmental instars of L. kindti are themselves endangered by the impact of planktivorous fish (i.e. Pelecus cultratus, Alburnus alburnus), which show a preference for Leptodora larger than 6 mm. Frequently, elimination by fish corresponds with high death rates of Leptodora. During the periods when adult Leptodora occurred in numbers >50 ind. m^–3, 55% of the variance connected with its death rate can be attributed to fish predation. The success of this population of L. kindti is discussed within the framework of predator-prey interactions.     

Life cycle patterns of rotifers in Lake Peipsi

Life cycle strategies of rotifers in Lake Peipsi (Estonia) were examined. Bisexual (mictic) reproduction was detected in 26 species. Life cycle patterns were determined for 17 species. All three basic life cycle patterns were represented, with some evidence of intraspecific variability. The midcycle pattern, with mixis occurring near the population maximum, prevailed. Extended mixis was observed in littoral populations of Polyarthra luminosa and P. remata. Most species probably are monocyclic. Polyarthra dolichoptera and Synchaeta oblonga may be dicyclic. Mictic periods were not detected in Conochilus hippocrepis and C. unicornis, which probably are acyclic in L. Peipsi. Mictic ratios and egg ratios were calculated for some of the dominant species. The highest amictic egg ratios were observed slightly before or at the population maxima. High mictic ratios (1.0) indicated very intense bisexual reproduction in populations of A. fissa, P. dolichoptera and S. verrucosa. Definite periods of bisexual reproduction could not be distinguished in the rotifer community. During May–October, the spread of mictic reproduction merely reflects the general seasonal distribution of rotifers. Their life cycle strategies represent specific adaptations to unpredictability of their habitat. The results of the study confirm that mixis is an anticipatory event, and not a response to environment deterioration, or an ending of a rotifer population cycle.     

Production of planktonic Rotatoria in Ormajärvi,an eutrophicated lake in southern Finland

The production of planktonic rotifers was studied in eutrophic Lake Ormajärvi. Of the total annual production of rotifers (2.9 g org. C m^–2 or 231 mg dry weight m^–3) 49% was achieved during one month (July) and 88% during 3 months of summer. The most important producers were Keratella cochlearis (1.2 g C m^–2), Asplanchna priodonta (0.8 g C m^–2) and Conochilus unicornis (0.6 g C m^–2). The P/B ratio for the total rotifer community during the growing season (7 months) was 25.0; monthly P/ B values varied between 0.3 and 5.2. The daily P/ B values were highest among species of Collotheca. The relationships of rotifers to some biotic and abiotic factors (invertebrate predators — Mesocyclops, Cladocera, planktonic Protozoa and temperature) are briefly discussed.     

Variability and possible adaptive significance of day-time vertical distribution ofLeptodora kindtii (Focke) (Cladocera) in a shallow eutrophic lake

The day-time vertical distribution of different size classes ofLeptodora kindtii in Tjeukemeer was studied during two contrasting years: 1978 and 1979. During the first year (1978) the predation pressure exerted by the planktivorous fish was rather high, whilst this was ca. 15 times lower during 1979. The diel vertical migration of different size classes ofLeptodora, was studied twice during 1979. Both studies support the predator avoidance hypothesis and also emphasize the idea that diel vertical migration is a dynamic rather than a fixed behaviour trait within a population.     

The effect of rainfall on tick challenge at Kyle Recreational Park,Zimbabwe

The effect of rainfall pattern on tick challenge was investigated at Kyle Recreational Park, Zimbabwe, from 1991 to 1992 using drag and removal plot methods to sample environmental tick density. The abundance of adults and nymphs of the brown ear-tick Rhipicephalus appendiculatus and larvae of the bont tick Amblyomma hebraeum was positively correlated with monthly rainfall, whereas no relationship with rainfall was revealed for larval R. appendiculatus, adults of the red-legged tick R. evertsi, or larvae of the blue tick Boophilus decoloratus. A comparison between 1991 (490 mm rainfall) and the drought year of 1992 (161 mm) revealed significant differences in the abundance of R. appendiculatus, A. hebraeum, and B. decoloratus. During the wet season, R. appendiculatus adults were 2–3 time more numerous in the environment during the higher rainfall year of 1991. A. hebraeum larval abundance exhibited a similar pattern to that of R. appendiculatus adults, but B. decoloratus larvae were more abundant in the drought year of 1992 during both the wet and dry seasons. Comparable tick abundance data collected at Kyle during the above-average rainfall years of 1975–1977 (mean = 1029 mm) were compared with tick challenge during the below-average rainfall years of 1991–1992 (mean = 326 mm). In grassland sand habitat and all habitats combined R. appendiculatus adults, nymphs, and larvae were much more abundant during the high rainfall years. In contrast, larvae of B. decoloratus were more numerous during the drier years. A. hebraeum larvae were also more abundant during the drier years. The strong positive correlation of adult R. appendiculatus abundance with rainfall and the coincidence of increased adult tick challenge with increased rainfall indicates that adult R. appendiculatus tick burden on hosts would be heaviest during the wet season and high rainfall years.     

Diel vertical migrations of invertebrate predators (Leptodora kindtii ,Thermocyclops taihokuensis, andMesocyclops sp .) in a shallow, eutrophic lake

We studied vertical distribution patterns of three invertebrate predators – Leptodora kindtii, Mesocyclopssp., and Thermocyclops taihokuensis– in a shallow eutrophic lake, Lake Suwa , Japan. From June to October in 2000 and 2001, we collected samples in the lake center in order to examine the vertical distribution patterns and the densities of the predators in the water column during the day (0900) and at night (2330). We also examined phototactic behavior of Leptodora in the laboratory. The three invertebrate predators showed clear migration patterns. Leptodora and Thermocyclops displayed a typical migration, avoiding the surface and maintaining a high abundance in deeper water during the day, and being distributed uniformly during the night. Mesocyclops, on the other hand, showed no clear vertical distribution pattern in the water column. However, Mesocyclops showed higher densities in the water column during the night than during the day. It suggests that they stayed just above the bottom during the day and migrated upward during the night. Leptodora also showed such a density difference between day and night. In the laboratory, Leptodora showed strong negative phototactic behavior. The observed density changes between day and night in Leptodora and Mesocyclops suggests the possible underestimation of their population density by usual sampling methods, and thus the impact of predation on populations of prey zooplankton species may also be underestimated in shallow water bodies.     

Zooplankton and their relationships with water quality and fisheries

Zooplankton population structure in Rutland Water changed over the first five years of the reservoir's existence. Daphnia pulex, initially dominant was replaced by D. hyalina in late 1975 and since 1979 this latter cladoceran has coexisted for part of each year with Bosmina longirostris. Population fluctuations of all the main crustacean zooplankton — D. hyalina, Cyclops spp. and Diaptomus gracilis — were erratic in the first three years (1975–1977) but have since become more regular. The relationships of the zooplankton with their potential algal food supply and with their potential predators, the fish population, are discussed.     

Investigations on fecundity of Bythotrephes longimanus in Lake Lucerne (Switzerland) and on Niche Segregation of Leptodora kindti and Bythotrephes longimanus in Swiss lakes

In Lake Lucerne, Switzerland, the predaceous cladocerans Leptodora kindti and Bythotrephes longimanus segregate along spatial and temporal dimensions. In spring (April–May/June), Bythotrephes longimanus occurs below 0–20 m, while Leptodora is absent. In summer and early autumn (July–September/October), when Leptodora dominates during daytime in the 0–20 m depth, Bythotrephes longimanus also lives in deeper zones. Food competition and fish predation pressure may be the cause of differences in ecology of Leptodora and Bythotrephes acquired during evolution. Due to its transparency and tolerance of higher temperature, Leptodora could avoid fish predation and, therefore, competes with Bythotrephes longimanus successfully. In addition, the differences between the two species may account for the spatial and temporal niche segregation in oligotrophic Swiss Lakes. But spatial niche segregation is less important in mesotrophic lakes with high prey density than in oligotrophic lakes with low prey density. In small, eutrophic lakes importance of temporal niche segregation also decreases, and Bythotrephes is seldom or not present. The preference of Bythotrephes to live in deeper water to avoid fish predation during summer may be the cause of its difficulties to establish itself in small and eutrophic lakes with high prey densities, where the hypolimnion is missing or anoxic.In the spring, Bythotrephes exhibits r-strategy (smaller body size and a higher fecundity), the female is already fertile after the first molt. In the summer, a K-strategy prevails (larger body length and lower fecundity than in the spring), and female Bythotrephes are fertile only after the second molt. Shortage of prey (biomass of Bosmina and Daphniadecreased after June especially in the surface layers) and the maximum fish predation pressure in summer may change the life strategy of Bythotrephes: while fecundity decreases from generation to generation, body length increases. Enhanced prey densities (e.g. during mesotrophic conditions in L. Lucerne) lead to larger individuals in summer and autumn.     

Food and feeding behaviour of a planktivorous cyprinid, Pelecus cultratus (L.), in a shallow eutrophic lake, Neusiedler See (Austria)

Pelecus cultratus (razor fish), a cyprinid fish, has become a dominant species in Neusiedler See. Gut content analyses of 400 specimens collected in 1989 and 1990 showed that Pelecus fed mainly on large zooplankton (Diaphanosoma, Leptodora and Arctodiaptomus), although their diet also included Insecta (larvae, pupae and adults) and Arachnida (spiders), occasionally small fishes. Comparison of the relative abundance of the zooplankton species in the stomach to the lake indicated that Pelecus strongly selected cladocerans over copepods, and fed mostly on large-sized individuals of Diaphanosoma, Leptodora and Arctodiaptomus. The fish showed a significant positive selectivity only for individuals of Diaphanosoma > 1.0 mm and Leptodora > 4.0 mm. In contrast, selectivity increased continuously in relation to the diameter of the compound eye of both prey species. This suggested that prey visibility was a key factor in determining the prey selectivity by Pelecus. It also seems likely that the persistence of the Leptodora population in Neusiedler See can be attributed to negligible predation pressure on the smaller sized individuals of this species.     

Observations on the susceptibility of some protists and rotifers to predation by asplanchna girodi

Direct observations of behavioral interactions show that the predator A. girodi: 1) easily ingested Synchaeta pectinata, two forms of Keratella cochlearis cochlearis, and individuals of Conochilus unicornis and C. dossuarius enzymatically dissociated from their matrix; 2) rarely, if ever, captured Kellicottia bostoniensis and intact Conochilus; and 3) generally rejected the peritrich ciliate Rhabdostyla sp. and the dinoflagellate Peridinium sp. Coloniality and secretion of a gelatinous matrix in Conochilus can be viewed as adaptations to limit mortality from invertebrate predation. Intraspecific variability in the feeding responses of A. girodi is considered.     

Effects of <Emphasis Type="Italic">Bythotrephes longimanus</Emphasis> (Crustacea,Cladocera) on the abundance,morphology, and prey community of <Emphasis Type="Italic">Leptodora kindtii</Emphasis> (Crustacea,Cladocera)

We hypothesized that native Leptodora kindtii would be shorter and have smaller feeding baskets in central Ontario lakes with greater abundances of small-bodied zooplankton prey, and that differences in zooplankton size among lakes could be attributed to the invasive cladoceran Bythotrephes longimanus. We evaluated these conjectures by comparing size metrics of Leptodora and the size of their preferred cladoceran prey in lakes invaded or not by Bythotrephes. Leptodora was less abundant in invaded lakes, but were smaller bodied with smaller feeding baskets only in lakes with long invasion histories. Small cladoceran abundance was greater in non-invaded lakes and was directly related to Leptodora abundance although not to Leptodora size. Mean Leptodora body size declined with increasing abundance of Bythotrephes. We evaluated three possible explanations for these patterns in Leptodora—(a) competition with Bythotrephes for zooplankton prey, (b) direct predation by Bythotrephes, and (c) size-selective predation by fish. While we were unable to unequivocally distinguish among these hypotheses, our observations are most consistent with predation by Bythotrephes changing zooplankton community composition and size structure in a manner that is detrimental to Leptodora. Our results indicate that Bythotrephes invasion may trigger more complex and subtle changes in food webs than previously thought.     

The response of a harpacticoid copepod community to sediment disturbance in a semi-enclosed lagoon

The harpacticoid fauna of a sandy sediment in the Sluice Dock of Ostend, Belgium, was dominated by a single species, Canuella perplexa, that attained an average annual density of over 400 ind. 10 cm^–2 in 1976 and 1977, whereas other species had a total mean density of less than 10 ind. 10 cm^–2. In the summer of 1979 a dystrophic crisis with abundant growth of Ulva occurred and in March 1980 the lagoon was drained. After refilling of the lagoon, the sediment was immediately colonized by good swimming harpacticoids, that attained densities of 200 ind. 10 cm^–2. Three months after refilling Canuella perplexa reappeared and the system returned to its original state within one month.     

Plant Exclusion of a Herbivore; Crayfish Population Decline caused by an Invading Waterweed

The population of the crayfish Astacus astacus was recorded in Lake Steinsfjorden (southeast Norway) over the period 1979–1996 by studies of total catch effort (baited traps) and catch per unit effort over the season. Over the years 1979–1987, total population of exploitable crayfish (>9.5 cm) was 194,000 (±62,000 SD), and the catches were evenly distributed over the shallow areas. After 1987 there was a sudden decrease in population size, reaching a level of 92,000 (±22,000 SD) for the 10 years that followed. From 1977 onwards the Canadian pondweed Elodea canadensis invaded the lake and established dense covers over large parts of the shallow areas. The crayfish were gradually excluded from areas covered by Elodea, while the yield from remaining areas remained relatively constant. Direct observation by test-fishing and diving confirmed the virtual absence of crayfish within the Elodea stands. Dense stands may directly interfere with movement of adult crayfish, and also strong fluctuations in O₂ and pH were recorded within the stands, probably acting as a stress factor. Although anoxia was not observed in this survey, it may occur during years of massive dieback of Elodea. Thus, while crayfish are able to feed on Elodea, they are unable to control the rapid growth over large areas of this invader, and somewhat paradoxically the herbivore is spatially excluded by its potential resource.     

The relationship between Trichocerca pusilla (Jennings), Aulacoseira spp. and water temperature in Loch Leven,Scotland, U.K.

Loch Leven is a shallow, eutrophic lake in the Scottish lowlands that is famous for its brown trout (Salmo trutta L.) fishery. Studies of planktonic rotifer populations began here in January 1977. Since then, samples have been collected and analysed at more or less weekly intervals. Additional information on the composition and abundance of phytoplankton and crustacean zooplankton species, and on a variety of physical and chemical determinants, has been recorded on each sampling occasion.Long-term datasets, such as that described above, are invaluable for identifying interactions between components of the plankton that only appear for short periods each year, as these interactions would probably be overlooked in data spanning a shorter period of time. This study uses the long-term data from Loch Leven to examine the food and temperature requirements of the summer rotifer species Trichocerca pusilla (Lauterborn). The results suggest that T. pusilla prefers water temperatures above 12 °C and that it feeds, primarily, on the filamentous diatom Aulacoseira spp. During the summer months, its abundance was closely related to the availability of this diatom. When filaments of Aulacoseira spp. were abundant, rotifer densities reached 1000–3000 ind. l^–1 and when they were scarce (e.g. 1980, 1997 and 1998) T. pusilla densities also remained low (i.e. less than 100 ind. l^–1). The reason for the success or failure of Aulacoseira during the summer months each year is unclear but, in general, its abundance was related to the availability of dissolved silica in the water.     

Rotifer population dynamics in response to increased bacterial biomass and nutrients: a mesocosm experiment

The impact of organic nutrients and massive addition of bacteria was followed in lake water mesocosms in a eutrophic lake. Increased DOM initiated a sequence of trophic responses indicated by rapid increases in bacterioplankton, protozoa, and algal biomass. The populations of Keratella cochlearis and Keratella quadrata showed a distinct response by rapid increase in birth rate followed by maxima of production and abundance. This succession clearly reflected the trophic position of these rotifer populations in the food chain. A reverse response was observed in Conochilus unicornis.     

Fish impact on rotifer community structure

Larger species, e.g. Asplanchna priodonta and Conochilus unicornis, and grasping species, e.g. Gastropus stylifer and Ascomorpha spp., became more abundant, while smaller filter feeders, e.g. Keratella cochlearis, decreased after an experimental reduction of the fish population. This development is probably caused by changes in basic regulating factors. The change to invertebrate predator dominance may affect smaller species (e.g. Keratella cochlearis) more than others, the increase of net phytoplankton (e.g. peridineans) will favour grasping species, and the change in seasonal occurrence of certain crucial food species may affect the competitive balance between certain rotifers (e.g. Polyarthra spp.).     

The development of Hexarthra spp. in a shallow alkaline lake

In Neusiedler See, a shallow alkaline lake with fluctuating water level and salinity, four species of Hexarthra occur: H. mira, H. fennica, H. jenkinae (occasional) and H. polyodonta. The analysis of longterm data reveals a general phenological pattern which does not change from year to year. They first occur in May, develop a maximum in June/July, sometimes a second one in August/September and disappear in October. But the species succession is different in the various years, occasionally only one species (H. mira or H. polyodonta) being present. There is a fairly consistent relation between the chemical conditions and the prevalent species; an increase in salinity favours the development of H. polyodonta. Low temperature and wind generated suspended particles have a negative influence on the development of the Hexarthra populations. Smaller populations of Hexarthra are in a relation to the occurrence of Leptodora indicating predation pressure of the latter species. In Neusiedler See the Hexarthra populations seem to be controlled to a great extent by abiotic factors, but predation by Leptodora and most probably by young fish seems to play an important role too.