The effects of temperature and photoperiod on life history parameters of Daphnia magna (Crustacea: Cladocera)

SUMMARY. 1. The life history responses of two common and three rare Daphnia magna clones to constant conditions of temperature and photoperiod were investigated, and intraspecific variation in the genes controlling growth, survivorship and reproduction was observed.
2. Rearing temperature had a significant effect on growth, survivorship and on all reproductive parameters measured (generation time, intrinsic rate of increase and male production), while photoperiod influenced significantly only survivorship and male production.
3. The maximum proportion of male offspring was produced at a low temperature (14°C) and at a mid-range photoperiod (L:D16:8).
4. The likely mechanism whereby clones with different life histories are maintained in the population is discussed.     

The effect of food availability, female culture-density and photoperiod on ephippia production in Daphnia magna Straus (Crustacea: Cladocera)

SUMMARY. A method is described whereby ephippia (sexual eggs plus protective membranes) are experimentally induced at low food levels (≤ 0.05 mg spinach D. magna ^-1), high female culture-densities (≤ 0.4 D. magna ml^-1) and in short-day photoperiods (L:D < 12:12, 12°C). The density-dependent ephippial response was related to an increased encounter rate between females at the higher densities. External metabolites had no significant effect on ephippia production. Ephippia were formed in the second generation after exposure to short day-lengths. Five clones from the same population exhibited genetic variation in their critical photoperiods for induction of sexual reproduction (≥= 10% ephippia). There was evidence of a genetic difference in photoresponse between two populations of D. magna. The evolution of timing mechanisms for ephippia production are discussed in relation to the predictability of environmental change.     

A test for the production of sexual pheromones by Daphnia magna (Crustacea: Cladocera)

SUMMARY. (1) Swarming in the Cladocera has often been associated with periods of sexual reproduction and this has led to the suggestion that members of opposite sex are attracted to one another. The most plausible mechanism by which this could occur is through the production of pheromones. The present experiments aimed to determine if Daphnia magna Straus produce such chemicals.
(2) No evidence was found for the presence of sexual pheromones. The behaviour of both males and females was unaffected by the presence of sexually receptive animals of the opposite sex.
(3) It is suggested that the cited relationship between sexual reproduction and swarming may be due to the fact that sexual reproduction occurs during periods of high population density. It is during such periods that swarming behaviour would be most apparent.     

The role of density in sex determination in Daphnia magna (Crustacea, Cladocera)

SUMMARY. 1. Cladocerans are generally believed to exhibit environmental sex determination; however, the evidence for this assertion is equivocal. Particularly, no studies have been performed which isolate potential effects of the environment on the sex of offspring (and hence sex ratio) from effects on the induction of sex (with the necessary appearance of males). We report the findings of a study of Daphnia magna Straus which is able to make this distinction.
2. We examined the sexes of broods produced by asexually reproducing females collected from high and low densities of D. magna in naturally occurring populations. We looked for an effect of density, a factor frequently implicated in male induction, on offspring sex.
3. We could find no effect of density on brood sex. Our results may indicate a true absence of interaction or may arise from our sampling procedure obscuring an interaction. More detailed field observations are suggested as a means of further examining the role of density in sex determination in Cladocera.     

Haemoglobin synthesis in Daphnia magna Straus (Crustacea: Cladocera): ecological differentiation between neighbouring populations

SUMMARY. 1. Variations in the haemoglobin index of two neighbouring populations of Daphnia magna were recorded over a range of dissolved oxygen concentrations (0.5–4.0ml O₂ 1⁻¹, 20°C). Reciprocal transfer experiments between habitats compared haemoglobin synthesis in situ.
2. An inverse relationship was found between the oxygen content of pond water and the haemoglobin indices of laboratory and natural populations.
3. Significant genetic differences in the synthesis of haemoglobin were found between the two populations. Animals from the poorly oxygenated habitat (0.8±0.18ml O₂ 1⁻¹) had consistently higher haemoglobin contents (maximum HI, 87.7±4.5) at all experimental and in situ oxygen levels. D. magna from the well oxygenated pond (4.3±0.59 ml O₂1⁻¹) had a lowered physiological ability to synthesize haemoglobin (maximum HI, 48.3±4.2). The process of ecological differentiation in Daphnia populations is discussed.     

Studies on the intestine of Daphnidae (Crustacea,Cladocera) ultrastructure of the midgut of Daphnia magna and Daphnia obtusa

The fine structure of midgut cells in two species of Cladocera (Crustacea: Branchiopoda) has unique features when it is compared to that in the midguts of other arthropods. Features which suggest that food is absorbed in digested form are: the presence of the peritrophic membrane, the glycocalyx, the almost complete absence of pinocytotic vesicles, and the large surface area of a great number of microvilli. Digested products presumably pass into the hemolymph through the network of extracellular canals and cisterns surrounding the basal ends of intestinal epithelial cells. Cells of the midgut in Cladocera differ from those of other arthropods in the simplicity of their basal plasma membranes, which are not highly folded. The small number of membrane invaginations suggests that water reabsorption is very slight, as is the usual condition in aquatic animals. The origin and evolution of peculiar structures we call “multivesicular-like bodies” have been investigated. These display a variety of different morphological features. Some contain acid phosphatase activity and are considered as specialized lysosomes.     

Genetic variation in the cellular response of Daphnia magna (Crustacea: Cladocera) to its bacterial parasite

Linking measures of immune function with infection, and ultimately, host and parasite fitness is a major goal in the field of ecological immunology. In this study, we tested for the presence and timing of a cellular immune response in the crustacean Daphnia magna following exposure to its sterilizing endoparasite Pasteuria ramosa. We found that D. magna possesses two cell types circulating in the haemolymph: a spherical one, which we call a granulocyte and an irregular-shaped amoeboid cell first described by Metchnikoff over 125 years ago. Daphnia magna mounts a strong cellular response (of the amoeboid cells) just a few hours after parasite exposure. We further tested for, and found, considerable genetic variation for the magnitude of this cellular response. These data fostered a heuristic model of resistance in this naturally coevolving host–parasite interaction. Specifically, the strongest cellular responses were found in the most susceptible hosts, indicating resistance is not always borne from a response that destroys invading parasites, but rather stems from mechanisms that prevent their initial entry. Thus, D. magna may have a two-stage defence—a genetically determined barrier to parasite establishment and a cellular response once establishment has begun.     

Underestimation of the Effect of Biologiocal Invasions in Phylogeographic Reconstructions as Seen in Daphnia magna (Crustacea,Cladocera)

Biology Bulletin - A phylogeographic analysis of Daphnia magna Straus 1820 (Crustacea, Cladocera) was used to study the consequences of underestimating the effects of biological invasions, namely,...     

Levels of protein, RNA, DNA, glycogen and lipid during growth and development of Daphnia magna Straus (Crustacea: Cladocera)

1. Protein, RNA, DNA, glycogen and lipid content were determined in Daphnia magna on days 0, 2, 4, 6, 8 and 21 of growth and development. The composition of D. magna as percentage of reconstituted dry weight was similar to other zooplankton with the exception of DNA content, which was lower than values previously reported.
2. The relative content of protein, RNA, DNA and reconstituted dry weight changed during the 21-day growth period, and these changes were related to growth rate and total growth of D. magna . RNA:protein, RNA: reconstituted dry weight, and protein:RNA : DNA ratios were highly correlated to relative growth rate and total growth as measured by protein content or reconstituted dry weight.
3. Addition of progeny biomass to adult biomass increased correlations between biochemical ratios and absolute growth rate, but had little effect on relationships involving relative growth rate or total growth.
4. The relationship between biomolecule ratios and growth established for D. magna grown under optimal conditions was not successful in predicting growth of D. magna reared under crowded conditions.
5. These data indicate that variation in biochemical ratios among life, stages of D. magna may be used to predict growth of organisms grown under similar conditions, but may not be extended to other situations. It is suggested, however, that variation in biochemical ratios in a particular life stage of a zooplankton species may be related to the productivity for that species.     

Final thermal preference in parthenogenetic females of Daphnia magna straus (Crustacea: Cladocera) acclimated to various temperatures

The final thermal preference FTP) range in parthenogenetic females of cladoceran Daphnia magna was assessed by “acute” and “chronic” methods. The first method included 4-month acclimation to different temperatures in the range of 14.2 ± 0.7 to 27.1 ± 0.3°C; the “chronic” method was characterized by long-term acclimation to +20°C. Two ranges of FTP were found for D. magna, 13.3–15.4°C and 20.2–26.2°C. The thermal preference of daphnids and the temperature of acclimation were correspondingly linearly. The range of FTP was independent of the season. The searching activity of D. magna rose in April, when the FTP range increased, and the FTP was less pronounced.     

Carapace epithelia are rich in large filamentous actin bundles in Daphnia magna,Daphnia pulex,and Sida crystallina (Crustacea: Cladocera)

Cladocerans (water fleas) are planktonic crustaceans that typically have a bivalved carapace. Each valve of the carapace consists of two cuticle‐secreting epithelial layers that are separated by a hemolymphatic chamber and joined by pillar structures. Ultrastructural analyses in several species of Cladocera have shown that the carapace epithelia and pillars contain filamentous structures of unknown composition. In the present study we used a fluorescent phalloidin conjugate to show that the carapaces of three cladocerans, Daphnia magna, D. pulex, and Sida crystallina, are rich in large bundles of filamentous actin (F‐actin). In D. magna we employed confocal microscopy and orthogonal views of three‐dimensional reconstructions to show that these bundles extend radially from foci in the pillars towards the integument surfaces, and their structure is consistent with that of contractile stress fibers. Using a fluorescent lipophilic stain, DiOC₆(3), we show that the F‐actin bundles are distributed in membrane‐rich regions within the carapace epithelia, and that, in the superficial epithelium, these may be large membrane‐bound organelles. In D. magna, the F‐actin bundles are present in embryonic, juvenile instar, and adult, developmental stages, and through development the bundles become larger, contain more F‐actin, and become more widely spaced. We present an alignment of the deduced amino acid sequences of six putative D. pulex actin genes, and discuss the implications that their respective sequences have on the likelihood of their inclusion into the F‐actin bundles of the carapace. Our identification of these large F‐actin bundles within the pillars of three cladocerans provides new insight into the role these structures play in influencing carapace dynamics within this order.     

Effect of previous feeding conditions on survival in Daphnia pulicaria (Crustacea: Cladocera)

Survival of natural Daphnia populations without previous acclimatization in filtered water under laboratory conditions (starvation) was used as a parameter describing the actual competitive ability of populations in periods of food limitation. Two populations (1 and 2) of Daphnia pulicaria Forbes coming from two enclosures with a low (< 1 mgC l^–1) and a high (about 3 mgC l^–1) seston level were examined in their resistance to starvation. Juveniles of both populations survived better than adult females in the conditions without seston as well as in the control experiment, despite their much lower biomass. Animals from population 1 had lower mortality than those from population 2 in all variants of the experiment. The results confirm the hypothesis that the ability of daphnids to starve depends also on adaptation to previous feeding conditions.     

Pokey,a New DNA Transposon in Daphnia (Cladocera: Crustacea)

We report the complete sequence of two representatives of the transposable element, Pokey, isolated from the ribosomal DNA of the cladoceran, Daphnia pulicaria. We describe the general features of this element, which confirms its classification as a DNA transposon. We show that Pokey is similar to piggyBac and, as such, is a member of the TTAA-specific family of elements. Pokey is putatively autonomous, possessing an open reading frame that encodes a putative protein with similarity to piggyBac's transposase, in addition to putative proteins from Drosophila and human LOOPER elements, and several human proteins of unknown function. We show that these proteins all contain amino acid motifs that are perhaps conserved for similar functions. We demonstrate that Pokey is carrying several regions of similarity to Daphnia pulex IGS sequences, likely the result of some sort of recombination event with the host genome. Pokey inserts into a conserved region of the large subunit ribosomal RNA gene known to contain other arthropod elements, suggesting that this location is a hot spot for insertional activity. However, Pokey is unique in that it is the only DNA transposon yet known to insert into this region, as other such insertions are non-LTR retrotransposons. It is now clear that this "ecological niche" has been effectively exploited by both retrotransposons and DNA transposons.     

Flow cytometry of cell proliferation through the incorporation of bromodeoxyuridine as an index of growth rate in the water flea, Daphnia magna (Crustacea, Cladocera).

BACKGROUND: In this paper, we used a small crustacean as a model to develop a method for quantifying growth rates through the measurement of a cell proliferation marker. This was done in order to study the feasibility of this assay for estimating zooplankton production in the ocean. Flow cytometry immunodetection of bromodeoxyuridine (BrdU) was performed to detect and quantify the cycling nuclei of Daphnia magna. METHODS: A combination of mechanical dissociation and cell enucleation procedures proved to be the most convenient method for preparing nuclear suspensions from whole organisms. Up to three populations of nuclei with different ploidy were observed. The relative abundance of these nuclear populations changed with the size of the flea. RESULTS: The staining technique has been optimized. The time and concentration for the maximum detection of BrdU-labeled nuclei were 3 h at 300 microM BrdU. Whole organisms can be frozen (-20 degrees C) after incubation with no changes in the final results. The method was used in different physiological conditions under controlled food and temperature in order to test the inverse relationship between physiological rates and size of organisms at several developmental stages. The quantification of BrdU-labeled nuclei in 1-6 day-old larvae showed the highest labeling index, with a mean of 95 +/- 1% (n = 22). In contrast, young animals (0.8-1.2 mm) had 25 +/- 4% (n =16, P < 0.001) and adults (>1.4mm) had 14 +/- 3% (n = 4, P < 0.001). The results obtained show an expected tendency, suggesting that a direct relationship exists between the labeling index and the instantaneous growth rate. CONCLUSIONS: Certain features of our method, such as the short times required for labeling and the possibility of preserving the samples during field experiments and under different conditions (including natural concentrations and types of food), are advantageous to the study of processes governing energy fluxes in pelagic ecosystems.     

Ultrastructural investigations on the labral glands of Daphnia obtusa (Crustacea,Cladocera)

The labral glands of Daphnia consist of three distinct functional units on each side: (1) several cells at the base of the head, (2) two large cells at the base of the labrum and one large cell (cell A) in the median part of the labrum and (3) one large cell (cell B) in the median part of the labrum. These gland cells do not form a syncytium, contrary to reports by previous investigators. With the exception of cell B, they have a well-developed rough endoplasmic reticulum and many active Golgi complexes. The Golgi activity changes during the molt cycle. The Golgi activity of the cells of the head base is different from that of the large cells of the labrum. Since clear exocytotic phenomena were not observed, the secretion can be assumed to flow into the hemolymph after accumulation in the enlarged intercellular spaces. Cell B has a distinctive cytoplasmic ultrastructure the function of which is not yet understood. The four large cells of the labrum are in contact with a duct cell (or several duct cells) characterized by a deep infolding of the plasma membrane. This delimits a narrow lumen, which contains no secretion. No passage of substance is visible from the gland cells to the duct cell(s).     

Effect of vertebrate and invertebrate kairomones on the life history of Daphnia magna Straus (Crustacea: Branchiopoda)

The history of selection of Daphnia magna populations living in North African temporary ponds may differ from populations inhabiting permanent ponds. Laboratory experiments were conducted to examine the effect of fish Gambusia holbrooki and invertebrate Notonecta glauca kairomones on the life history traits of the freshwater Cladocera Daphnia magna Straus. With fish kairomones, Daphnia reproduced early and had a significantly smaller size at first reproduction (SFR) and a smaller size of neonates compared to control. In contrast, daphnids reared in water treated with Notonecta glauca had no effect on the age at first reproduction but females were also smaller and produced smaller neonates.     

Life history variation within a parthenogenetic population of Daphnia parvula (Crustacea: Cladocera)

Summary Evidence for genetically determined life history variability within a population or a species is rare. In this three year experimental examination of a parthenogenetically reproducing population of the planktonic crustacean Daphnia parvula, we found evidence for a succession of clones or groups of clones that exhibited distinctive body size and reproductive differences that were maintained after numerous generations under standardized conditions in the laboratory. The D. parvula population reached maximum density in the fall and maintained relatively high densities through the winter and spring. Isolates from this fall-winter-spring period all had a larger body size at death and higher fecundity when compared with summer isolates under natural food and temperature conditions. These differences could not be accounted for by differences in temperature and food abundance among the seasons. An additional difference in these experiments was a shift in reproductive effort by the summer isolate which produced a higher proportion of its offspring in the first two broods. The shift in life history characteristics and a summer decline of the Daphnia parvula population was correlated with both an increase in inedible and perhaps toxic blue-green algae and an increase in a dipteran predator Chaoborus. Comparison of the survivorship curves for all of the seasonal life history experiments indicated that D. parvula survivorship was not lower during the summer discounting a toxic effect from blue-green algae. Positive population growth on natural food in the laboratory at this time indicated food was not limiting and that predation was the probable cause of the population decline.Laboratory life history experiments under standardized food and temperature conditions were run with D. parvula isolates from the spring and summer plankton. Genetically based differences as determined in these experiments were smaller body size, lower fecundity, smaller brood size, and shorter life span for the summer animals relative to spring animals. Thirty seven percent of the summer animals also reproduced at an earlier age under standardized conditions. The shift in reproductive effort to earlier broods by summer animals rnder natural conditions appeared to be a phenotypic response as the summer isolate did not produce a higher proportion of its offspring in early broods under standardized conditions.When estimates of predatory mortality were added to the life tables of the standardized experiments, the earlier reproduction of some of the summer animals allowed a population increase under a regime of intense predation. Life tables for the spring animals predicted a population decline under these circumstances. Predictable seasonal changes in biotic factors such as predation suggest a mechanism whereby diverse life history patterns with corresponding differences in r may be maintained within a population.