Cryptic diversity within the rotifer Polyarthra dolichoptera along an altitudinal gradient

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Suppression of the rotifer Polyarthra remata by the omnivorous copepod Tropocyclops extensus: predation or competition

Indirect evidence from observations in the field suggested thata common and often abundant cyclopoid copepod, Tropocyclopsextensus, despite its small size (0.5 mm) and largely algaldiet, is an important predator of the rotifer Polyarthra remata.Laboratory experiments showed that copepodids and adults, butnot nauplii, markedly suppressed the population growth of thisrotifer. The suppression could be attributed entirely to predation,rather than to exploitative competition for shared food resourcesor to interference. Mortality rates predicted from a separatefeeding-rate experiment weresufficient to account for the declinein population size observed in the Tropocyclops treatment ofculture experiments. Also, monitoring the availability of cryptomonadfood in treatments with and without Tropocyclops throughoutthese experiments showed that cryptomonad densities always remainedhigh and were just as high or significantly higher in the copepodtreatments. Furthermore, direct videographic observations ofP. remata from cultures with and without Tropocyclops demonstratedno significant differences in swimming velocity, tendency todeviate from a straight-line path, or frequency and length ofspontaneous escape responses. Per capita ingestion rates ofT. extensus on P. remata varied from 2 to 8 day^-1, were significantlyhigher for adult females than adult males or copepodids V, andincreased significantly with rotifer density. A tendency ofcopepods to have higher ingestion rates on young than adultrotifers was not significant. Copepods cultured since birthwith cryptomonads and P. remata ate significantly fewer rotifersthan those cultured only with cryptomonads; this may be explainedby a more sated condition of the former and provided no evidencefor the idea that previous experience with this rotifer mightincrease predation efficiency. The results show that T. extensusin natural communities has the potential to deplete naturalpopulations of susceptible rotifer prey. Accordingly, it mayshift the species structure of rotifer assemblages in favourof resistant species, and provide selection pressure for avoidanceresponses.     

Apogamic development of plasmodia in the myxomycetePhysarum polycephalum: a cinematographic analysis

Summary Strain CL ofPhysarum polycephalum forms multinucleate plasmodia within clones of uninucleate amoebae. The plasmodia have the same nuclear DNA content as the amoebae. Analysis of plasmodial development, using time-lapse cinematography, showed that binucleate cells were formed as a result of nuclear division in uninucleate cells. Binucleate cells developed into plasmodia by further nuclear divisions and cell fusions. No fusions involving uninucleate cells were observed. A temporary increase in cell and nuclear size occurred at the time of binucleate cell formation.     

The Polyarthra escape from response: Defense against interference from Daphnia

Direct observations show that almost all (97%) Polyarthra remata caught in the inhalant current of Daphnia pulex avoid inhalation by initiating escape responses. Microvideographic analysis shows that these responses are initiated, on average, 0.24 mm (about 2.5 body lengths) from the inhalant opening of the branchial chamber, when the Polyarthra are moving at a velocity of about 1.4 mm s^–1 (about 6 times their normal swimming speed — 0.24 mm s^–1). The stimulus for the escape response could be rapid acceleration or, more likely, shear. The average distance traveled during an escape response is at least 1.25 mm (about 12 body lengths).The Polyarthra escape response can provide a very effective defense against interference from Daphnia and may greatly increase the ability of Polyarthra spp. to coexist with Daphnia.     

Pharyngeal movements during feeding sequences inNavanax inermis: a cinematographic analysis

Summary Pharyngeal movements during feeding inNavanax inermis were filmed and correlated with known neural activity controlling the pharynx. Seven distinct components of feeding were identified. Occurrence of a component was in some cases fixed, in that once initiated the act went to completion, and in other cases reflex, in that tonic stimulus control was needed for the act to be maintained. As few as 2 or as many as 7 different motor acts could occur in a feeding sequence. The specific acts which make up a sequence were dependent upon the nature of the prey stimuli that elicited feeding: qualitatively as well as quantitatively different feeding sequences were elicited by prey of differing sizes or by prey which was withdrawn fromNavanax at different stages in a movement. The data indicate that the sequence of pharyngeal movements is not preprogrammed, but rather the sequence is appropriate to a specific type of prey. Flexibility in fitting a feeding sequence to the prey that elicits the sequence is achieved by combining in different ways a limited number of specific, fairly stereotyped motor acts.     

Morphological response of a bdelloid rotifer to desiccation

We desiccated bdelloid rotifers (Macrotrachela quadricornifera), submitting the animals to four desiccation procedures (protocols A, B, C, D) that differed in the rate of water evaporation, in the time of desiccation, and in the substrates provided. We observed external morphological changes of the rotifer bodies during drying with scanning electron microscopy and, in parallel, assessed rates of recovery after a 7-day period of dormancy. Two protocols produced disorganized morphologies of the anhydrobiotic animals, with no (A) or very poor (B) recovery. Protocols C and D gave rather high rates of recovery and dry rotifers appeared unaltered and well organized. The different protocols affected rotifer morphology during the 7-day anhydrobiosis and rates of recovery after the 7-day anhydrobiosis; high recovery rates corresponded to well-organized morphologies of anhydrobiotic bdelloids, suggesting that a proper contraction of the body into a tun shape and probably a rigorous packing of internal structures are necessary for survival after anhydrobiosis. These features are affected by the time between water shortage and full desiccation, but also by the surrounding relative humidity and by the nature of the substrate. Possible adaptations of anhydrobiotic rotifers are discussed.     

Modeling the dynamics of natural rotifer populations: Phase-parametric analysis

A model of the dynamics of natural rotifer populations is described as a discrete non-linear map depending on three parameters, which reflect characteristics of the population and environment. Model dynamics and their change by variation of these parameters were investigated by methods of bifurcation theory. A phase-parametric portrait of the model was constructed and domains of population persistence (stable equilibrium, periodic and a-periodic oscillations of population size) as well as population extinction were identified and investigated. The criteria for population persistence and approaches to determining critical parameter values are described. The results identify parameter values that lead to population extinction under various environmental conditions. They further illustrate that the likelihood of extinction can be substantially increased by small changes in environmental quality, which shifts populations into new dynamical regimes.     

Isozyme analysis of rotifer proteins

An analysis of isozyme variation of five enzymes among 17 strains of the brackish-water rotifer, Brachionus plicatilis, using thin-layer polyacrylamide electrophoresis, showed that it was possible to resolve genetic differences. It is suggested that isozyme characters will be useful in distinguishing sub-species of the group.     

Genetic differentiation and phylogeography of rotifer Polyarthra dolichoptera and P. vulgaris populations between Southeastern China and eastern North America: High intercontinental differences

Genetic differentiations and phylogeographical patterns of small organisms may be shaped by spatial isolation, environmental gradients, and gene flow. However, knowledge about genetic differentiation of rotifers at the intercontinental scale is still limited. Polyarthra dolichoptera and P. vulgaris are cosmopolitan rotifers that are tolerant to environmental changes, offering an excellent model to address the research gap. Here, we investigated the populations in Southeastern China and eastern North America and evaluated the phylogeographical patterns from their geographical range sizes, geographic–genetic distance relationships and their responses to spatial‐environmental factors. Using the mitochondrial cytochrome c oxidase subunit I gene as the DNA marker, we analyzed a total of 170 individuals. Our results showed that some putative cryptic species, also known as entities were widely distributed, but most of them were limited to single areas. The divergence of P. dolichoptera and P. vulgaris indicated that gene flow between continents was limited while that within each continent was stronger. Oceanographic barriers do affect the phylogeographic pattern of rotifers in continental waters and serve to maintain genetic diversity in nature. The genetic distance of P. dolichoptera and P. vulgaris populations showed significant positive correlation with geographic distance. This might be due to the combined effects of habitat heterogeneity, long‐distance colonization, and oceanographic barriers. Furthermore, at the intercontinental scale, spatial distance had a stronger influence than environmental variables on the genetic differentiations of both populations. Wind‐ and animal‐mediated transport and even historical events of continental plate tectonics are potential factors for phylogeography of cosmopolitan rotifers.     

A regression analysis of the summer population dynamics of Polyarthra vulgaris in a northern Michigan bog lake

The population dynamics of a planktonic rotifer (Polyarthra vulgaris) were examined in a brown water, acid lake in northern Michigan, U.S.A. Predation by Chaoborus punctipennis and low food (Navicula spp. and Cyclotella spp.) concentrations were the main factors limiting P. vulgaris populations of all factors examined. The data presented here support a hypothesis for zooplankton limitation by an invertebrate predator.     

Predator-prey behavior and its effect on rotifer survival in associations of Mesocyclops edax,Asplanchna girodi,Polyarthra vulgaris,and Keratella cochlearis

Summary The predatory copepod Mesocyclops edax preys effectively on the rotifers Asplanchna girodi and Polyarthra vulgaris but not on the rotifer Keratella cochlearis. It readily captures individuals of this latter species but usually releases them unharmed, being unable to remove the soft parts within their loricae. The predatory A. girodi regularly eats K. cochlearis but cannot catch P. vulgaris. When P. vulgaris is contacted by the corona of A. girodi, it immediately escapes by elevating its paddles and jumping away a distance up to about ten times its own body length. In experimental communities of these predator and prey species the survival of Polyarthra and Keratella is significantly affected by the species of predator present and by predator-prey interaction between the two predators when both are present.Supported by National Science Foundation research grant DEB 76-09768 and Public Health Service career development award KO4-GM 70557 to J.J.G.     

Weak swimming response of a bdelloid rotifer to chemical cues of a native copepod predator

Journal of Ethology - Whether bdelloid rotifers can detect and respond to the chemical cues of predators has not been studied, and research on their behaviour is scarce in general. To test for such...     

Comparison of taxon-based and trophi-based response patterns of rotifer community to water quality: applicability of the rotifer functional group as an indicator of water quality

Rotifer community is often used as a taxon-based bioindicator for water quality. However, studies of the planktonic community from the viewpoint of functional groups in freshwater ecosystems have been limited, particularly for rotifers. Because rotifers have various trophi types determining their feeding strategies, thereby representing an ecological niche, their functional feeding groups can act as biological and ecological indicators in lakes and reservoirs where planktonic communities are dominant. We analyzed the patterns of spatial distribution of the rotifer community in various reservoirs and then its relationship with water quality through redundancy and regression analyses. Compared with taxon-based composition, the response of trophi-based composition appears simplistic and showed clearer tendency in relation with water-quality variables. Each trophi responded differently by the degree of eutrophication indicating that each trophi group is possibly affected by environments such as the combinations of water-quality variables in different ways.     

Comparison of pulmonary endothelial cell and fibroblast proliferation using time-lapse cinematographic analysis

Time-lapse cinematography was used to study and compare the proliferation and migration activity of pulmonary endothelial cells and fibroblasts, two cell types with very different structural and functional properties. Endothelial cells were found to have a mere rapid growth rate than fibroblasts. Contributing to the shorter population doubling time of the endothelial cells were lower interdivision times and a tendency for these cells to remain in division cycle with successive generations of growth. Striking differences between endothelial cells and fibroblasts were seen in migration behaviour. Endothelial cells had lower migration rates and tended to remain within a restricted growth area, whereas fibroblasts migrated freely throughout the growth area.     

The analysis of planktonic rotifer populations: A plea for long-term investigations

Short-term species succession, and long-term community development, of planktonic rotifer populations of the temperate zone and factors influencing species' abundance (ie., physical and chemical limitations, food and exploitative competition, mechanical interference competition, predation, parasitism) are described and discussed. The long-term development of plankton communities in three European lakes is described and the major events are discussed in relation to ecological interactions. Frequently, the shortcomings of our knowledge about population ecology and ecophysiology of rotifers prevent explanations of short-term and, especially, of long-term developments of these plankton organisms. The need for qualitative and quantitative observations in the field and in the laboratory over long periods becomes obvious.     

Experimental evolution of bet hedging in rotifer diapause traits as a response to environmental unpredictability

The adaptive response of organisms to unpredictable environments is increasingly recognized as a central topic in fundamental and applied evolutionary ecology. Selection due to environmental unpredictability can act on multiple traits of an organism's life cycle to reduce the impact of high environmental variance. The aim of this research was to study how unpredictability selects for diapause traits: 1) the timing of sex (a proxy of the timing of diapausing egg production), and 2) the diapausing egg hatching fraction (a proxy of diapause duration). We used an experimental evolution approach with the facultative sexual rotifer Brachionus plicatilis. Laboratory populations experiencing two contrasting regimes of environmental fluctuation (predictable versus unpredictable) evolved divergently over a short time span (< 77 days). The populations under the unpredictable regime showed an earlier initiation of sexual reproduction and a lower hatching fraction of diapausing eggs than populations under the predictable regime. These findings demonstrate empirically the existence of bet‐hedging strategies in B. plicatilis regarding both traits, consistent with theoretical predictions of bet‐hedging evolution under conditions of unpredictable environmental variance. Given that scenarios of increased environmental variability are expected to occur in the near future, a comprehensive understanding of the role of bet‐hedging strategies is necessary for predicting population responses to environmental change.     

Effects of activin A and follistatin on developmental kinetics of bovine embryos: cinematographic analysis in a chemically defined medium

The effects of recombinant human activin A and follistatin on the developmental kinetics of bovine presumptive zygotes matured and fertilized in vitro using time-lapse cinematography were investigated. The presumptive zygotes were cultured for 9 days in a chemically defined medium (modified synthetic oviduct fluid, control) and modified synthetic oviduct fluid supplemented with activin A or follistatin. Development under cine-recording conditions was similar to that in an incubator. Addition of activin A to modified synthetic oviduct fluid increased, while addition of follistatin decreased, the percentage of zygotes that developed to morulae and blastocysts. Follistatin significantly prolonged the timing of development to the 9-16-cell stage compared with the control and activin A media. Activin A significantly shortened the duration of the third cell cycle compared with the control, but follistatin significantly prolonged the fourth cell cycle compared with the control and activin A. Developmental arrest ('lag-phase') during the 4-8-cell stage was observed in 95% of embryos developed to more than the 9-16-cell stage in all treatments. The greater the number of cells at the onset of the lag-phase, the earlier the onset of the phase and the shorter the duration of the phase, the further embryos were able to develop by day 9 in all treatments. The number of cells at the onset of the lag-phase in the medium containing activin A was significantly higher than it was in control or follistatin-containing media. Moreover, activin A significantly shortened the duration of the lag-phase compared with follistatin. The present results indicate that activin A may enhance in vitro development of bovine embryos by improving developmental kinetics, especially by increasing the number of cells at the onset of the lag-phase and shortening the duration of this phase.