Competition and the life table of cladocerans from a tropical shallow lake

Laboratory experiments were carried out to investigate the influence of competition for food, with different combinations of species, using life table data. In Experiment 1, the cultivated chlorophycean Desmodesmus spinosus (Chodat) Hegewald, at a low concentration (0.25 mg C L⁻¹), was used to feed the cladocerans Daphnia gessneri Herbst and Diaphanosoma birgei Kořínek; in Experiment 2, the same alga was used in an even lower concentration (0.15 mg C L⁻¹) to feed three species, D. gessneri, D. birgei, and Ceriodaphnia richardi Sars, in pairs. Three treatments were set up in Experiment 1 (four replicates each): a. control with the species alone; and b. in shared cultures. For Experiment 2, six treatments were set up (three replicates each): a. control with the species alone; and b. pairs of species in shared cultures. Competition for the chlorophycean occurred in Experiment 1, when D. birgei significantly reduced the average life span of D. gessneri, and D. gessneri significantly reduced the gross and net reproductive rates of D. birgei, despite the greater efficiency in all parameters, of survival and reproduction, of the second species. D. birgei was the only species that reproduced under extreme food limitation; in Experiment 2, despite gross and net reproductive rates, generation time, population growth rate and reproductive effort negatively influenced by C. richardi and D. gessneri, in addition to the average life span, also affected by D. gessneri. D. birgei was competitively superior to the other two species being able to reproduce and grow at very low food concentrations.     

Predator-specific inducible defenses in the rotifer Keratella tropica

1. A Patagonian strain of Keratella tropica has very different induced morphological responses to two predators – the carnivorous rotifer Asplanchna brightwelli and the interference competitor Daphnia pulex. Asplanchna induces the most exuberant morph. Compared to the basic morph, it has a fourfold longer right posterolateral spine (up to c. 115 μm), greatly elongated anterolateral and anterosubmedian spines, and no left posterolateral spine. Transitional morphs have an incompletely developed right posterolateral spine and a reduced left posterolateral spine. Daphnia induces moderate development of both posterior spines but no elongation of any anterior spines. Induction of these morphs by Asplanchna and Daphnia is mediated by kairomones. 2. The Asplanchna‐induced morph is much better defended against large (0.9 mm) Asplanchna than either the basic or Daphnia‐induced morph. The long right posterolateral spine usually prevents capture or ingestion. The Asplanchna‐ and Daphnia‐induced morphs are similarly susceptible to interference from large (3 mm) D. pulex. 3. Life‐table experiments with cohorts of the basic and Asplanchna‐induced morphs at 5 × 10³ and 2 × 10⁴ cells of Cryptomoas erosa per millilitre indicate little or no cost of the induced defense. Lifetime fecundity (13–15 offspring per female) did not differ significantly between morphs. The mean intrinsic rate of natural increase (r_m day⁻¹) of the induced morph was very slightly but significantly lower than that of the basic morph at the lower food concentration (0.46 versus 0.48) but no different from it at the higher food concentration (0.53 versus 0.54). However, spine development may involve undetermined allocation costs and environmental costs relating to interactions with other organisms. 4. It is not clear why K. tropica has separate induced responses to Asplanchna and Daphnia. Moderate spine development probably reduces damage or ingestion by small (<1.5 mm) daphniids, as in other species of Keratella, but further development may confer no protection against larger ones. Thus, the ratio of benefit to cost with daphniids (and other cladocerans) may be highest for intermediate spine development. In contrast, much greater spine development seems necessary for effective defense against Asplanchna. The more moderate response to Daphnia also may reflect less likely spatial and temporal overlap.     

Roles of parasitic fungi in aquatic food webs: a theoretical approach

1. Parasites are ubiquitous in ecosystems, but their roles in material transfer are poorly understood. Fungal parasites in freshwater ecosystems are of major importance to small heterotrophic eukaryotes and consume large phytoplankton that are resistant to zooplankton grazing. 2. To evaluate their ecosystem‐level effects, we developed a simple food web model that incorporates competition between small and large phytoplankton for nutrients, zooplankton grazing on small phytoplankton, fungal parasitism on large phytoplankton and includes a newly discovered trophic link from fungal zoospores to zooplankton (F‐Z link). 3. Our model demonstrates the likely occurrence of an indirect mutualism between fungi and zooplankton, in which fungal parasitism increases zooplankton production by reducing the biomass of inedible large phytoplankton. Contradicting the expectation from a previous short‐term experiment that the F‐Z link may benefit zooplankton, the presence of the F‐Z link can reduce material transfer from phytoplankton to zooplankton because of the negative effect of the indirect mutualism. The model indicates that high growth efficiency of fungi on host tissue and their high nutrient status for zooplankton are crucial for the F‐Z link to increase zooplankton production. 4. The model also indicates that the contribution of material transfer via F‐Z link to zooplankton increases with nutrient availability. Our results suggest that parasitic fungi may be a key player in material transfer, especially in eutrophic ecosystems.     

Selenium deficiency in Crustacea

Summary The effect of selenium deprivation onDaphnia magna was examined under controlled rearing conditions in a synthetic culture medium. After three generations, fertility was significantly reduced in deprived (Se^–) animals. Growth and mortality of parent daphnids and development of parthenogenetic eggs were not affected during this period. In the fourth generation Se^– daphnids rejected parts of their second antennae. At the ultrastructural level antennal muscle tissue was severely affected. Animals deprived of selenium had mitochondria and sarcoplasmic reticulum with myelin-like alterations. Giant lysosomes were present and complete lysis of muscle fibrils was observed in antennal muscle cells. These alterations are characteristic features of peroxidic damage in tissues. This interpretation is consistent with the function of selenium as a constituent of the enzyme glutathione peroxidase which protects cells from peroxidation. Selenium should be included in synthetic culture media for daphnids.Abbreviations GSH-Px selenium dependent glutathione peroxidase - Se⁺/Se^– selenium supplemented (control)/selenium deprived animals - SOD Superoxide dismutase - SR sarcoplasmic reticulum     

A behavioral bioassay employing Daphnia for detection of sublethal effects: Response to polarized light

The swimming response of Daphnia pulicaria to polarized white light was examined. This response was employed as a behavioral bioassay to investigate the effects of sodium bromide (NaBr) on Daphnids physiology. The control response to linearly polarized white light involved an orientation at 90° to the e‐vector. With the addition of NaBr the response became more random. An EC₅₀ (concentration which eliminates the response in 50% of the test individuals) of 1.31 x 10^‐1 M NaBr was calculated using the statistic von Mises K. A possible mechanism to account for the toxicity of this mild sedative is the blocking by the bromide ion of chloride channels involved in transmitting nerve impulses.     

Evidence for epistasis: reply to Trouve et al.

Salathé and Ebert (2003, J. Evol. Biol. 16: 976–985) have shown that the mean logarithmic fitness of Daphnia magna clones declined faster than linearly with increasing inbreeding coefficient F. They interpreted this result as evidence for synergistic epistasis. Trouve et al. (2004, J. Evol. Biol., doi: 10.1111/j.1420‐9101.2004.00755.x) suggested that hybrid vigour could be an alternative explanation for this finding. We use a population genetic model to support the original claim that the marked deviation from linearity cannot be explained without epistasis. We further argue that the relevant reference population is the metapopulation and not the subpopulation. Taken together, we believe that synergistic epistasis between recessive deleterious alleles segregating in the D. magna metapopulation is the most likely explanation for the finding of Salathé and Ebert.     

Fish kairomone regulation of internal swarm structure in Daphnia pulex (Cladocera: Crustacea)

In this laboratory experiment, swarms of D. pulex were artificially created by placing thirty individuals in small chambers containing 130 ml of water. The swimming behaviour of the animals was measured using a video camera and a programme to digitize observations. The D. pulex swimming in water with fish kairomones had a more uniform swimming speed compared with animals in control water. If aggregated prey individuals benefit from a reduced predation risk, and this risk is further reduced by uniformity of swarm members, the uniformity of swimming speed can be interpreted as an behavioural adjustment to minimize the vulnerability to predation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Development,Reproduction and Growth Pattern of two Coexisting,Pond Dwelling Cladocerans

Laboratory investigations on life history parameters of 2 coexisting cladocerans (Daphnia obtusa. Moina brachiata) from a nearly temporary pond in South Germany revealed that the species have different temperature tolerances and temperature optima. D. obtusa experienced the highest reproductive success at 15 and 20 °C and was able to survive and to reproduce at 2 °C but died at 30 °C. The reproductive success of M. brachiata was highest at 25 and 30 °C and the species could not withstand temperatures <15 °C and ≥35 °C. At temperatures between approximately 20 and 25 °C, where both cladocerans coexisted in nature, M. brachiata showed a faster population growth due to its approximately twofold higher egg production rates (10–12 eggs female⁻¹ day⁻¹ compared to approximately 5 eggs female⁻¹ day⁻¹ in D. obtusa) and its shorter juvenile development (3.3 and 2.4 days compared to 6.3 and 5.3 days in D. obtusa); M. brachiata needs generally only 3 molts to reach maturity while D. obtusa requires 5–6 molts.     

Five hundred and twenty‐eight microsatellite markers for ecological genomic investigations using Daphnia

Until recently, an enormous effort was needed to apply genomic tools to ecological investigations, especially when striving to uncover the functional mechanisms of phenotypic plasticity and the genetic basis of evolutionary adaptation within natural populations. This present study aimed to develop a genomic resource for an organism ideally suited for functional ecology and evolutionary research. Over 760 unique DNA fragments containing microsatellite loci were isolated and characterized from Daphnia to provide more than 500 molecular markers for constructing a genetic map and for localizing chromosomal regions containing genes of ecological importance via quantitative trait locus analyses. Although primarily developed to genotype members of the Daphnia pulex species complex, a significant fraction of these markers is potentially valuable for population genetics and recombination mapping of distantly related species. Over 60% of markers tested in cross‐specific amplifications are possibly conserved within the subgenus Daphnia, whereas 48 and 18% of tested primers are found to amplify subgenus Hyalodaphnia and subgenus Ctenodaphnia DNA, which represents ～140 and 200 million years of evolutionary preservation.