Predation impact of Leptodora kindtii on population dynamics and morphology of Bosmina fatalis and B. longirostris in mesocosms

1. We assessed the impact of predation by the invertebrate predator Leptodora kindtii on Bosmina longirostris and B. fatalis, which show seasonal and reciprocal succession patterns in Lake Suwa, in a mesocosm experiment using 20‐L tanks with or without the predator under different food conditions. We also analysed morphological responses of the two Bosmina species to the predator in the tanks. 2. Bosmina fatalis dominated B. longirostris regardless of predator presence under high food density. However, the presence of Leptodora induced the dominance of B. fatalis more rapidly than its absence. On the contrary, no dominance of B. fatalis was observed in tanks with low food density, irrespective of the presence of the predator. Only B. fatalis showed morphological changes in response to the presence of Leptodora. 3. Mucrone length and antennule shape (angle between body and antennule and angle between antennules) showed marked responses at both high and low food densities, but antennule length responded only at high food density. Mucrone length seems to be a more effective defence against Leptodora. 4. The results suggest that B. fatalis is a superior competitor against B. longirostris and is more resistant to Leptodora predation, especially in good food conditions. The repeatedly observed seasonal succession of the two Bosmina species in the eutrophic Lake Suwa – the replacement of B. longirostris by B. fatalis following the occurrence of abundant Leptodora– seems to be caused by the selective predation of Leptodora on B. longirostris as well as the competitive ability of B. fatalis.     

Contaminated sediments in the Elbe estuary: Ecological and economic problems for the Port of Hamburg

The effects of food density on competition between Bosmina longirostris and Bosmina fatalis from Lake Kasumigaura were examined in the laboratory. When the animals were reared with a high concentration of Chlorella, B. fatalis, which is slightly larger than B. longirostris in body length, had larger brood sizes and a higher rate of population growth than B. longirostris. When the species were reared together at high food density, B. fatalis overcame B. longirostris. But B. longirostris won the competition at low food density.The effects of Microcystis on the competition were also examined. When the two species of Bosmina were reared in a mixture of Chlorella and Microcystis, the populations of both species hardly increased, although individual B fatalis showed slightly better individual growth and reproduction than B. longirostris. Decomposed Microcystis, however, was utilized well by both species.In Lake Kasumigaura, the decomposition of abundant Microcystis seemed to raise the food level, favouring B. fatalis over B. longirostris.     

Impact of selective predation by Mesocyclops pehpeiensis on a zooplankton community: experimental analysis using mesocosms

Predation by cyclopoid copepods is an important factor affecting zooplankton communities in freshwater habitats. Experiments provide strong evidence of the role of selective predation by cyclopoid copepods in structuring zooplankton communities. To assess the predation impact of a cyclopoid copepod, Mesocyclops pehpeiensis, we conducted a mesocosm experiment using 20-l polyethylene tanks in which the density of the predator and the food available to herbivorous zooplankton varied. M. pehpeiensis had a notable but selective effect on the zooplankton community. The population of a small cladoceran, Bosmina fatalis was affected negatively, but M. pehpeiensis did not have any apparent impact on the population dynamics of another Bosmina species, B. longirostris. On the other hand, the population of small rotifers responded positively to the presence of M. pehpeiensis, and their densities increased in mesocosms with a high density of M. pehpeiensis. It seems that suppression of B. fatalis by M. pehpeiensis predation indirectly affected rotifers by releasing them from competition with B. fatalis. The results suggest that copepod predation is a powerful factor regulating zooplankton communities directly and indirectly.     

Seasonal and reciprocal succession and cyclomorphosis of two Bosmina species (Cladocera, Crustacea) co-existing in a lake: their relationship with invertebrate predators

The seasonal succession and cyclomorphosis of Bosmina longirostrisand Bosmina fatalis (also known as Sinobosmina fatalis) wereinvestigated in the shallow eutrophic Lake Suwa from 1996 to1999. Two different invertebrate predators, cyclopoid copepodsand Leptodora kindtii, co-existed with Bosmina in the lake.A pattern of seasonal succession of the two species was apparent,with B. fatalis dominant in summer and B. longirostris in springand fall. The two species also showed different seasonal changesin morphology. During summer, when the density of the invertebratepredator Leptodora was high, B. fatalis changed its morphologyand maintained long features, with elongation of the antennulesand mucrones. This species also maintained the protruding shapeof the antennules by increasing the angles between the antennulesand the body, and between two antennules, during the same period.The seasonal differences and magnitude of changes in morphologywere more marked for larger individuals. However, such markeddevelopment of protruding structures was not observed in B.longirostris, which showed only slight changes in the antennuleand mucrone lengths related to the density of copepods. Theresults suggest that the morphological traits of B. fatalisare a morphological defence against Leptodora, while those ofB. longirostris are a response to copepods. In Lake Suwa, thepredators show different patterns of seasonal changes in density.Different morphological traits and the resulting interactionswith invertebrate predators seem to be important in determiningthe reciprocal succession of the two Bosmina species.     

Antennule shape and body size of <Emphasis Type="Italic">Bosmina</Emphasis>: key factors determining its vulnerability to predacious Copepoda

Cyclopoid copepods are common in lakes and ponds, and they have a significant predation impact on the communities of the small zooplankton species. To reduce the predation risk, some cladoceran zooplankters develop protuberant (defensive) morphologies in the presence of the copepods. In the case of the small cladoceran Bosmina, they elongate their appendages (antennule and mucrone) and change the antennule morphotype. However, information about the effectiveness of these defensive devices against copepod predation is still insufficient. In our study, to find the compositive effects of these appendages on the vulnerability of Bosmina, we exposed two bosminid species (B. longirostris and B. fatalis) of different body sizes and with appendages of different lengths and shapes to copepod (Mesocyclops) predation. The experiment revealed that the shape of the antennule is a main factor determining the bosminid’s vulnerability to copepod predation and indicated that the protection of the opened ventral carapace must be a key strategy by which Bosmina avoids copepod predation.     

The development and reproductive output of three species of Cladocera (Crustacea,Branchiopoda) with different size spectra as the result of vertebrate and invertebrate predation impact

Seasonal changes in development and reproduction of three species of Cladocera, Bosmina longirostris, Bosmina coregoni and Diaphanosoma brachyurum, were studied in regard to impact of vertebrate (planktivorous fish) and invertebrate (cladoceran Leptodora kindtii and cyclopoid copepods Mesocyclops leuckarti and Thermocyclops oithonoides) predators. Each of the cladocerans represented a different prey type. The population of the largest body-sized D. brachyurum was not affected by predators suggesting unidentified mechanisms of predation-avoidance. Diaphanosoma displayed low reproductive output coinciding with high density. Medium-sized B. coregoni, influenced by planktivorous fish, invested in reproduction and carried large egg-clutches resulting in gradual increase in population density. The population of small body-sized B. longirostris, that was influenced by invertebrate predators, displayed high density and reproductive effort in spring, followed by extreme collapse in abundance and reproduction during summer months. The results suggest that invertebrate predators may suppress populations of some Cladocera more strongly than planktivorous fish.     

Experimental analysis of the predation impact of the larvae of pond smelt (<Emphasis Type="Italic">Hypomesus transpacificus nipponensis</Emphasis>) on zooplankton populations established in mesocosms

The predation impact of the larvae of pond smelt Hypomesus transpacificus nipponensis on a zooplankton community was studied using mesocosms. The fish significantly depressed the abundances of copepod nauplii and rotifers, especially Hexarthra mira. The vulnerabilities of these prey might be determined by their swimming behavior and population density, suggesting that larval fish selectively prey on zooplankton that have a high encounter rate with the predator. The larvae did not have a negative effect on the densities of cladocerans, but fish predation altered the cladoceran community structure from the dominance of B. longirostris to that of B. fatalis. This result suggests that larval fish predation is an important factor that shifts the species composition of Bosmina in some lakes, the shift occurring in the season when fish larvae are abundant. Our results have shown that predation by the larval fish would control not only the abundance, but also the community structure of the small-sized zooplankton prey.     

Aging in Brachionus plicatilis: The evolution of swimming as a function of age at two different calcium concentrations

Species composition and density of cladoceran populations changed in Lakes Zürich, St. Mortiz and Baldegg as human populations increased in these watersheds. Lake Zürich sediments became annually laminated in the 1890's as a result of increased organic input as the size of the cities surrounding the lake grew. At the same time, the Bosmina species changed from a oligotrophic form (longispina) to a eutrophic form (longirostris). An increase in Daphnia spp. populations also occurred at this time in the lake's history. Bosmina longispina reappeared in the lake in 1965 as the lake's trophic status changed from eutrophic to mesotrophic due to effective sewage treatment facilities. Annual laminations appear in the Lake St. Moritz sediments about 1910. Shortly thereafter, a shift from B. longispina to B. longirostris occurred. This change in trophic status is associated with increased tourism in the area. Lake Baldegg sediments also show annual laminations beginning in 1885 and a similar shift in the Bosmina species. Other cladoceran remains were too scarce to be useful in interpreting the histories of these lakes.     

Abrupt Changes in Bosmina (Cladocera,Crustacea) Assemblages During the History of the Ostrowite Lake (Northern Poland)

During the 10000-year history of the Ostrowite Lake, there have been several episodes of change in dominance amongst species of the genus Bosmina. The dominants were alternately B.longirostris and Eubosmina spp. Amongst the subgenus Eubosmina, two species prevailed in different periods:Bosmina coregoni and Bosmina reflexa. The first species, Bosmina coregoni, is characterized by long antennae and short carapace mucrones, while the second one, Bosmina reflexa, by short antennae and very long mucrones. Bosmina reflexa was dominant at the beginning of the lake's history (Preboreal, 10000--9000 BP) and during the early Subboreal Period (5000–3500 BP). Only a small number of remains of Bosmina longispina(Eubosmina spp.) were present along the entire profile. Such strong changes in the domination of Bosmina species were observed for the first time in the sediments of Polish lakes. A similar transition from one species to another has been described in a few German lakes and was linked to climate changes. However, the changes in the Ostrowite Lake do not correlate with climate changes, but most probably, with changes in the lake's trophic level. This is suggested by the simultaneous increase of Bosmina reflexa and a good indicator of eutrophy, Bosmina longirostris.     

The Filtering Rates of Four Herbivorous Crustaceans on Nannoplankton and Net Plankton Fractions in an Acid Lake in West Virginia

Four experiments were conducted in Cheat Lake, West Virginia, to determine the filtering rates of zooplankton on the size fractions of phytoplankton in an acid lake. The results showed that Bosmina longirostris filtered significantly more net plankton than nannoplankton. Results for other species were not consistent in the size fraction selected. The dominance of Peridinium inconspicuum may have made the nannoplankton fraction unavailable as food for Bosmina.     

Inhibition of development of anti-predator morphology in the small cladoceran Bosmina by an insecticide: impact of an anthropogenic chemical on prey–predator interactions

1. The combined effect of the insecticide carbaryl and the predator (Leptodora kindtii) kairomone was assessed on the development of protuberant morphology in the small cladoceran Bosmina fatalis, a feature which evolved originally as a response to the kairomone. The experiment showed that Bosmina changed its morphology in response to the kairomone, but development was inhibited by carbaryl at a sub‐lethal concentration even in the presence of the kairomone. At the same time, reduction of fecundity was observed in animals exposed to carbaryl. 2. A short‐term feeding experiment (B. fatalis versus Leptodora) indicated that such low concentrations of the insecticide had no impact on predation by Leptodora. Thus, it is suggested that the inhibition of development of anti‐predatory defences in Bosmina can increase its vulnerability to predation. 3. Such disturbance of chemical communication by the insecticide reduces the individual survival rate of prey Bosmina in the environment with high predator density. On the other hand, reduction of fecundity may result in decreased population growth rate of animals. 4. The impact of the insecticide on the anti‐predator morphology in Bosmina (inhibition) was opposite to that in Daphnia (enhancement). This suggests that biochemical induction processes in the development of anti‐predatory morphology are evolutionally different between Daphnia and Bosmina.     

Can extreme morphology in Bosmina reduce predation risk from Leptodora? An experimental test

Bosmina (Cladocera) populations, especially within the subgenus Eubosmina, show a variety of phenotypes that exhibit large differences in body size and shape and antennule length. In some populations, the morphological traits also vary during the season, with the most extreme forms occurring in periods with high densities of certain invertebrate predators. However, while temporal phenotypic variation in other cladocerans, as in the family Daphnidae, has been shown to be an adaptation to reduce the risk of predation by invertebrate predators, the reason for such changes in Bosmina is much less clear. We examined whether certain morphological traits in Bosmina species could act as a defence against invertebrate predators. We tested three Bosmina forms (subgenus Eubosmina), differing in morphology from each other, which are found in lakes together with the predator Leptodora kindtii (Cladocera). Bosmina (E.) longispina has a relatively low and elongated carapace with a caudal mucro, and short antennule, B. (E.) coregoni gibbera has a higher and more protruding carapace without caudal mucro, and a much longer antennule. Finally, B. (E.) coregoni retro extensa has a carapace like that of B. longispina but with no caudal mucro and a much longer antennule. In one experiment, B. longispina and B. gibbera were exposed for 12 h to Leptodora in Petri dishes. In a second experiment, we observed directly the escape efficiency of B. longispina, B. gibbera and B. retro extensa, and the handling time of Leptodora. The two Bosmina forms with more extreme morphological features had a lower death rate and higher escape efficiency than B. longispina. Prey that escaped did so, in most cases, within 5 min. Predator handling time was correlated to predator body length and antennule length of the prey. The results suggests that Bosmina species with extreme morphological traits may be less vulnerable to invertebrate predators. Received: 8 June 1998 / Accepted: 5 October 1998     

Copepod and cladoceran success in an oligotrophic lake

Cladocerans have proved to be much less successful members ofthe Lake Tahoe limnetic community than the calanoid copepods.An analysis of temporal and spatial heterogeneity of the cladoceranBosmina longirostris and the herbivorous calanoid copepod Diaptomustyrrelli revealed important differences between the two typesof zooplankton. Although the productivity of both species islimited to varying degree by the abundance of certain algalspecies and the availability of particulate nitrogen, they differstrongly in the mechanisms responsible for mortality. Correlationanalysis suggests that B. longirosris is severely limited temporallyand spatially by predation from the omnivorous calanoid copepodEpischura nevadensis. In contrast, D. tyrrelli is not noticeablylimited by predators. Instead, abiotic seasonal events and starvationcontrol its population dynamics. Mortality and natality differencesbetween Bosmina and Diaptomus in Lake Tahoe help to explainthe general lack of success of cladocerans in oligotrophic lakes.     

Demographic and life history response of the cladoceranBosmina longirostris to variation in predator abundance

Population dynamics, demography and body size of the cladoceranBosmina longirostris were examined in an experimental study in which the abundance of its predator (the cyprind fishPhoxinus eos) was varied in an unproductive lake. Four densities of fish were used, encompassing the biomass of fish in the lake.Bosmina was most abundant at low and medium fish densities (1.06 and 2.12 g fish biomass · m^-3) and less abundant when fish were either absent or present at high density (3.71 g fish biomass · m^-3). The unimodal response to predator abundance resulted from effects on both birth and death rates.Bosmina birth rates increased as fish biomass increased, in response to increasing food (phytoplankton) biomass. Death rates were highest at high fish biomass (because of fish predation) and in the absence of fish (because of predation by the dipteranChaoborus, which was most abundant in the absence of fish). Size-frequency distributions revealed that fish eliminated the larger size classes ofBosmina, and mean carapace length ofBosmina populations was inversely proportional to fish biomass.Bosmina initiated reproduction at smaller size in the presence of fish than in their absence, and size at maturity was inversely proportional to fish biomass. Size at birth also tended to decrease with increasing fish biomass, but this trend was not as strong as that of size at maturity. Decreased size at maturity apparently allowedBosmina individuals to reproduce before becoming vulnerable to fish predation. Flexibility in size at maturity, together with low abundance of invertebrate predators and large herbivores (which were preyed upon by fish), allowedBosmina to become abundant in low and medium fish treatments. In the high fish treatment, mortality due to fish predation was too severe to be offset by decreased size at maturity, andBosmina population density was low. The net response ofBosmina populations to fish predation results from interactive effects of predation on mortality, natality, and life history traits.     

Zooplankton of a flooded opencast sulphur mine

The zooplankton community of a flooded opencast sulphur mine was investigated during two years. The complicated physical and chemical relationships in this water-filled pit caused an atypical plankton assemblage. At the time of the investigation 20 species of rotifers, 6 of Cladocera, and 8 species of Copepoda were found. A single Ostracod and Chaoborus flavicans were also noted in the samples. Usually the epilimnion was monopolised by rotifers whereas the hypolimnion had a more diversified assemblage of rotifers, cladocerans and copepods. The result of clustering analysis indicated that each predator was associated with its own group of prey. Discrimination analyses showed weak seasonal differences in the zooplankton and three zones in the vertical profile – epilimnetic, upper hypolimnetic and anaerobic. Principal component analysis reduced the chemical parameters to the main contributors of conductivity (the main anions and cations) plus toxic sulphide and hydrogen sulphide. The animals were classified by the first two components into two groups: cold stenotherms and medium-sized filtrators, such as Bosmina longirostris. A specific analysis of vertical profiles generated five variants of diel vertical migrations. When time–space distributions of the plankters were analysed the highest numbers of different species were usually in different times and depths. The epilimnion was inhabited by small fishes and was dominated by rotifers but was free from C. flavicans which inhabited the cold hypolimnion, together with the remaining invertebrate predators (copepods and Asplanchna).Chaoborus and the other invertebrate predators participated more evenly in the contributions of species to the total density. In the presence of invertebrate predators the structure of the rotifer population shifted from soft-bodied species to spined or fast-moving species. When the predation pressure was too strong, as in the hypolimnion, the soft-bodied species were, in practice, absent. When the pit was in an oligotrophic state the hypolimnion population was composed of up to 60–90% rotifers. When the trophic state shifted to mesotrophy, the proportion of cladocerans increased.     

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.     

Some factors influencing the density of subarctic populations of Bosmina longirostris,Holopedium gibberum,Codonella cratera and Ceratium hirundinella

Changes in the density of 2 populations of Bosmina longirostris, Holopedium gibberum and Codonella cratera were correlated through multiple regression analyses with shifts in temperature, light, nannoplankton densities and seston weight. Sampling took place every 2 weeks between June 1975 and November 1976 in the Yellowknife River and Yellowknife Bay, both in the Canadian subarctic. Temperature was usually the main factor influencing all 3 species, accounting for up to 60% of the seasonal variability in B. longirostris, 37% in H. gibberum and 62% in Codonella cratera. Seston (the major food source) ranked second overall and in one instance accounted for 60% of density changes in B. longirostris. Nannoplankton were consumed in only small numbers and had little influence on densities. Light was of minor importance to all species. Temperature was also the main factor effecting reproduction and feeding in B. longirostris and H. gibberum (the only species studied). The simultaneous collections of Ceratium hirundinella made in the river and bay indicated that temperature accounted for 30–55% of the seasonal variability of this species, followed by light (4% and nutrients (3–4%).     

Predation of freshwater jellyfish on Bosmina : the consequences for population dynamics,body size,and morphology

Invertebrate predators may cause strong changes in behaviour, life-history, and morphology of prey species. However, little is known about the influence of jellyfish on such characteristics of their prey. This study analyses the impacts of the freshwater jellyfish Craspedacusta sowerbii on life history and morphological defenses in a population of the cladoceran Bosmina longirostris . Length of mucro and antennule, size-dependent number of eggs, size at maturity, and size of juveniles, adults, and egg-carrying females were investigated during a 23 days experiment using medusae-enriched and control enclosures filled with natural plankton populations. Significant differences in parameters investigated were found not only between treatments, but also within treatments over time. Changes in Bosmina life-history parameters and morphology in controls were probably due to predation by cyclopoid copepods. The significant increase in the size of adults and egg-carrying females as well as the increase in mucro and antennule length in medusae-enriched enclosures are discussed as defense strategies against the freshwater jellyfish.     

Proximate factors controlling the morphologic plasticity of <Emphasis Type="Italic">Bosmina</Emphasis>: linking artificial laboratory treatments and natural conditions

Recent studies have shown that some biotic/abiotic signals such as predator-mediated chemical/physical cues and disparate water temperature are important factors for the induction of anti-predator morphs in cladocerans. As much of the evidence has come from laboratory studies, it is important to investigate the extent to which these studies reproduce the real cyclomorphic conditions occurring in nature. The present study was planned to compare the differences in the potential intensity of cyclomorphogenic factors in the laboratory and in nature. For this, we conducted two experiments in the laboratory and the field at the same time using the small cladoceran Bosmina longirostris and a predacious copepod. The results of the present experiments suggested that an abiotic factor, lower temperature acted as a key factor enhancing the changes in both the length and shape of antennules in Bosmina, which had been governed by different signals from predators. The lower temperature can be effective in helping prey bosminids to detect a severe predation threat by cyclopoid copepods, since that condition is a feature of spring and fall, a time when copepods are abundant. Handling editor: S. I. Dodson     

Food Size Spectra and Species Replacement within Herbivorous Zooplankton

The relationship between two commonly co-occurring filter-feeders; Bosmina longirostris and Eudiaptomus gracilis at different nutrient levels, was investigated in bag experiments in Lake Gjersjøen. At the start of the experiment, the numerical relationship between Bosmina and Eudiaptomus was less than 1 : 10, but during the experimental period, a strong increase in Bosmina and a corresponding decrease in Eudiaptomus and medium-sized algae (5–25 μm) was observed. The shift from Eudiaptomus to Bosmina was probably due to strong competition on mediumsized algae while Bosmina in addition may utilize smaller algae and bacteria, not available to Eudiaptomus. Their coexistence in the lake at much lower densities is probably due to a strong selective fish predation, suppressing Bosmina. Daphnia longispina, not naturally occurring in the lake, showed strong competitive abilities in a fish-free bag.