The systematics of Australian Daphnia (Cladocera: Daphniidae). Electrophoretic analyses of the Daphnia carinata complex

Genetic variation at eleven allozyme loci was scored in approximately 1600 individual Daphnia. The samples included representatives of all nine taxa described from the D. carinata complex in Australia. Principal coordinates and cluster analyses revealed only three groups of genotypes within the complex, corresponding to the taxa D. cephalata, D. nivalis, and a conglomerate of all other taxa described from the complex, D. carinata (sensu lato). These results are consistent with recent multivariate morphological analyses of the complex.     

The components of life-history traits variation in Daphnia magna population

The variation of six growth and reproduction traits in the parthenogenetic clones of Daphnia magna and its relationships with the genotypes at 5 isozyme loci were studied under controlled conditions at three food concentrations. A significant genetic component of age at maturity, growth rate and fecundity variation was detected. Nested ANOVA revealed the influences of the genotypes of the isozyme loci, that are more pronounced at the high food concentration. In some cases the genotype-environment interactions were found.The variation of body length at 6 days and total number of offspring includes a significant heterozygosity — environment interaction. The most heterozygous clones show the least differences in life history traits between food conditions.Life-history traits heritability, genotype-environment interactions and different reproductive strategies of the clones are discussed.     

Genetic variation in a subtropical population of Daphnia

We assayed a subtropical population of Daphnia ambigua for genetic variation using protein electrophoresis (9 loci) and quantitative genetics approaches (life history characters). Our goal was to obtain information about relative levels of variation in a subtropical population, and compare them with extensive previous studies in the temperate and arctic zones. The observed level of allozymic variation (H = 0.11) was consistent with those previously observed in other temperate zone Daphnia populations. However, variation for quantitative traits (heritability) was lower than typically observed in previously-studied temperate populations: estimates were not statistically different from zero. Because allozyme heterozygosity was consistent with previous temperate zone estimates, and the polymorphic allozyme loci did not depart from the expectations of Hardy-Weinberg equilibrium, we concluded that a period of clonal selection was the most likely explanation for the low heritabilities observed. We do not conclude that this study provides evidence to suggest that subtropical populations harbor lower levels of genetic variation because of their location.     

Colony formation in Scenedesmus: a literature overview and further steps towards the chemical characterisation of the Daphnia kairomone

Semiochemicals play an important role in interactions between living organisms in aquatic environments. Although the presence of chemical cues is confirmed in more and more systems, the chemical structures remain predominantly elusive. To create more accurate prey–predator interaction models and to advance the research on chemical communication, it is essential to identify these compounds. A literature overview of cues involving Daphnia (either as producer or receiver) is given and the progress towards their isolation and structure elucidation is described. Most of the research so far has concentrated on the elucidation of kairomones produced by predators of Daphnia (especially Chaoborus and several species of fish). Although some progress has been made, these cues have not been isolated and identified yet. Additionally new results on the isolation and identification of the kairomone responsible for the colony formation in Scenedesmus using differential diagnosis and bioassay-directed fractionation of Daphnia exudates are presented. The importance of suitable and well performing bioassays herein cannot be underestimated. Some preliminary results with solid-phase extraction with C₁₈ proved to be reproducible for extracting the active compound from Daphnia water, although it was not possible to get the biological activity into a single fraction. The cue was not extractable with an anion exchanger (SAX). Subjecting the extract to HPLC led to one active fraction.     

Experimental interspecific hybridization in Daphnia

Hybridization is a common phenomenon in Daphnia (Cladocera; Anomopoda); interspecific hybrids have been found between several species and hybrids are found in many European lakes. Although much information on the morphology, ecology and genetics of hybrids is available, little is known about the level of reproductive isolation among species or about the relative fitness of hybrids and parental species. In order to facilitate studies on differentiation and speciation processes and comparative experimental studies on hybrids and recombinant genotypes, we present the first successful laboratory crossing experiments of two different Daphnia species, D. galeata and D. cucullata. Males and sexual females from two D. galeata and two D. cucullata clones were reciprocally crossed, juveniles hatched from resting eggs and reared until maturity. Hatching and juvenile survival rates of hybrids were relatively low (12.1% and 24%, respectively). D. galeata and D. cucullata clones vary in their level of successful interspecific matings and in the number of subsequent offspring. In general, hybrid crosses between D. cucullata females and D. galeata males were more successful than reciprocal crosses.     

Plankton community of a polyhumic lake with and without Daphnia longispina (Cladocera)

The impact of Daphnia longispina (Cladocera) on the plankton food web was studied in a polyhumic lake where this species comprised almost all zooplankton biomass. Plastic enclosures (volume 7 m³) were inserted into the lake retaining the initial water stratification except that in one enclosure zooplankton was removed. After the removal of Daphniaa rotifer, Keratella cochlearis, ciliates and heterotrophic nanoflagellates increased markedly and the density and biomass of bacteria decreased. Edible algal species, Cryptomonas rostratiformisand three small chrysophytes,Ochromonas, Pedinella and Spinifermonas, took advantage of the removal of Daphnia, while more grazing-resistant species declined. In spite of the changes in the species composition of phytoplankton, the removal of Daphnia did not affect the biomass, primary production or respiration of plankton. The results implied that the density of heterotrophic flagellates and ciliates was controlled by Daphnia, but in its absence the former took its role as the bacterial grazers.     

The costs of crest induction for Daphnia carinata

The effects of notonectid-induced crests on growth and reproduction, and resource allocation to crest construction, moult losses and eggs of Daphnia carinata were measured. An attempt was made to elucidate the mechanisms of physiological costs of crest induction for this species. The crested morph of d. carinata reached a significantly larger size than the uncrested form. Reproductive output was similar in early instars, but the crested morph produced more eggs in latter broods. Instar duration was longer for the crested morph and age at first reproduction was delayed. Survival was also lower in this form. Crest construction required significant resources (equivalent to 60 eggs over a life time) but evidence is presented that these resources were obtained primarily by re-allocation of available material rather than collection of extra resources. The crested morph allocated significantly more resources to moulting than its uncrested counter-parts. The uncrested morph produced large eggs in early instars and progressively smaller ones in later instars. The crested morph produced only small eggs. The hypothesis is presented that the crest-induction strategy of D. carinata involves at least two separate sets of responses, each with its own costs and trade-offs. The first response is production of the crest. The cost of crest production is an increased cost of moulting. D. carinata off sets this cost by increasing instar duration and thus age at reproduction. The second response is increased size. D. carinata achieves this by reducing the fraction of available resources allocated to reproduction. The cost is lower reproductive output.     

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.     

Possible toxic effects on Daphnia resulting from the green alga Scenedesmus obliquus

We cultured individuals of two Daphnia species and their hybrid on two different algae, Scenedesmus obliquus and Chlamydomonas globosa, in different concentrations. Our results suggest that culture conditions of S. obliquus can be such that the algal cells become toxic to Daphnia     

The systematics of Australian Daphnia (Cladocera: Daphniidae). Multivariate morphometrics

Forty nine characters were measured in each of 76 male and 200 female Daphnia including specimens from all eleven taxa described from the genus in Australia, except D. jollyi. Separate multivariate analyses (cluster, principal components and discriminant) of males and females each revealed only five morphological groups in the genus corresponding to the species D. occidentalis, D. lumholtzi, D. cephalata, D. nivalis, and a conglomerate of all other taxa described from the D. carinata complex, D. carinata (sensu lato). Interpretation of the male results was straightforward, but that for females was not because of the occurrence of an environmentally induced seasonal change in morphology (cyclomorphosis) in females. Log transformations of the female data successfully isolated the effects of cyclomorphosis. The seasonal morphs formed two groups which were further differentiated into taxonomic groups.     

An in situ test of the effects of food quality on Daphnia population growth

The temporal variation of the surface deposit feeders in a mudflat of Chuwei Mangrove Forest showed a significant input of populations from the recruitments in the spring. The chemical analysis of the sediment in the top 8 cm showed that the sedimentary organic content had a subsurface maximum value in the deeper layer in most cases. Most of the temporal variations in the organic carbon and nitrogen contents occurred in the top few centimeters. The high C:N ratio and low organic nitrogen content in the sediment suggests the supply of carbon (or energy) to the benthos was low. The increase of labile protein content in the deep layer might be due to the downcore transport of sediment nutrients by the bioturbation. Laboratory growth experiment suggests that some the chemical characters examined in this study may represent the sediment nutrients to the animals.     

A note about Daphnia in Venezuelan waterbodies

After examining numerous water samples of 19 lakes and reservoirs in Venezuela, only two species of Daphnia were found: D. laevis (in colder, high altitude lakes) and D. gessneri. This observation supports the theory of the scarcity of Daphnia species in tropical inland waters.     

The ecological role of chemical stimuli for the zooplankton: predator-induced morphology in Daphnia

Summary Numerous adaptive predator-induced responses occurred when eight clones representing seven Daphnia (Crustacea: Cladocera) species were tested against three common predators: fourth instar larval phantom midge Chaoborus americanus, adult backswimmer Notonecta undulata, and small sunfish Lepomis macrochirus. The predators were confined within small mesh bags, suggesting that the signal for induction is chemical. The induced responses included longer tail spines, longer heads, smaller bodies, increased egg clutches, and decreased lipid reserves. Each Daphnia species responded to each of the three predators in a unique manner. Induced responses in the above characters showed no significant association. The induced morphological changes are generally consistent with current theories of what is an adaptive response for the various sizes of Daphnia exposed to tactile and visual predators. The abundance of induced responses in these experiments suggests that predator-induced responses are a widespread and ecologically important phenomenon of the freshwater zooplankton.     

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.     

Aliesterase- (Ali-E) activity in Daphnia magna straus as a parameter for exposure to parathion

In vivo exposure of Daphnia magna Straus to parathion in concentrations from 0.05 to 5 µg/l resulted in a concentration-dependent decrease in aliesterase activity of the homogenate of the animals. Decrease in this activity after 48 hours of exposure was maximal for concentrations > 1.5 µg/l and amounted to about 90% of the activity of the blank.     

Mechanisms of the filtering area adaptation in Daphnia

The changes of selected parameters of the filtering comb of the third thoracic limb were studied in a natural population of Daphnia pulicaria Forbes, as well as in experimental enclosures and in lab cultures, including individual life history. Two hypotheses were tested: 1. either these changes are related to the succession of clones coexisting within one population, or 2. the size of the filtering area changes gradually as an individual adaptation during the moulting. No evidence supporting the clonal hypothesis was found. On the contrary, the adaptability of the filtering comb is the same in a natural population as it is in a clone and in individuals.     

Synergistic effects of different food species on life-history traits of Daphnia galeata

In this paper we describe the life history consequences of feeding Daphnia galeata with different food types in different concentrations. We fed the animals with four concentrations of two green algae Scenedesmus obliquus and Chlamydomonas globosa, given separately as well as in a 1:1 mixture. Growth, reproduction and condition of the animals and consequently the intrinsic rate of population increase, r, increased with the concentration of the food for all three experimental series. The life history parameters measured were lowest in the animals raised on C. globosa. Animals fed on S. obliquus showed higher values for r. We observed a distinct synergistic effect of the two algal species: growth, reproduction, condition and r were significantly higher for the animals raised on the mixed media as compared to the animals raised on the mono-algal media. We conclude that the two algal species represented complementary food resources.     

Sexual reproduction in Daphnia: interspecific differences in a hybrid species complex

To investigate whether reproductive variation in the Daphnia galeata-cucullata-hyalina hybrid species complex can explain the occurrence of hybrids and backcrosses, a total of 43 clones from three north German lakes were tested for allocation to sexual reproduction under equal stress conditions. Six replicates per clone were followed until the seventh adult instar. I used the following cues to promote sexual reproduction: short photoperiod, water from a crowded Daphnia culture, fish-conditioned water and low food concentration. For each animal, clutch size and clutch sex were recorded. Ephippia, which were empty since the animals were cultured individually, were only produced by D. cucullata and D. cucullata x hyalina (26% and 6% of the broods, respectively), whereas almost all taxa produced males (the range was 2–15%). Intraspecific variation for male and ephippial production was also found. The fact that the taxa tested show different responses to the stimuli of-fered may indicate that there is reproductive isolation between them. This is supported by field data from the Tjeukemeer (The Netherlands) which shows that D. galeata mostly produces ephippia in spring and D. cucullata in autumn. The presence of hybrids with ephippia in both seasons, however, shows that backcrossing is also possible.     

Intersex and male development in Daphnia magna

Sex induction is environmentally stimulated in Daphnia and involves a cue-dependent response for sex determination. Somatic growth was shown to be similar in males and females during juvenile instars, but divergent due to a reduction in male somatic growth, at about the time that females produce ovaries. At this time, males appeared morphologically adult with respect to secondary sex characteristics. Intersex was rare and is unlikely to be important in natural populations. Intersex could be induced in both sexes, and observed more frequently after longer exposure to high temperature, or in the second generation following a temperature change. This indicates an impact on the ability of mothers to determine the sex of the offspring. It may be possible to use intersex characteristics for manipulative investigation of sex-determination mechanisms in Daphnia. Although sex-determination is initiated before birth, intersex occurrence suggests that development of male characters requires an additional process, probably involving hormone activity, during juvenile development and maturation.