Biochemical adaptation for dormancy in subitaneous and dormant eggs of <Emphasis Type="Italic">Daphnia magna</Emphasis>

Daphnia can reproduce through subitaneous and dormant eggs. The production of dormant eggs is induced by stimuli associated with deteriorating growth conditions, and enable Daphnia populations to survive temporarily harsh environmental conditions. Dormant eggs are expected to have developed special biochemical adaptations to bridge this long unfavourable period, but little comparative biochemical data are available for dormant and subitaneous eggs. We compared levels of the following molecules between subitaneous and dormant eggs: (a) triglycerides, which are the most abundant energy storage molecules in Daphnia, (b) glycerol, a cryoprotectant also involved in energy storage, and (c) the heat shock protein Hsp60, a molecular chaperone that may assist in maintaining protein structural integrity and inhibiting cell metabolism during diapause. Unexpectedly, no difference in triglycerides content between egg types was found. As expected, dormant eggs contained more glycerol and relatively more Hsp60 than subitaneous eggs. The biochemical composition of dormant eggs can therefore be seen as an adaptation to the harsh environmental conditions these eggs encounter. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Status of Daphnia Resting Egg Banks in Bohemian Forest Lakes Affected by Acidification

We studied the egg banks of Daphnia gr. longispina in four acidified lakes in the Bohemian Forest (Czech Republic, Europe). Daphnia had become extinct in three of the studied lakes due to anthropogenic acidification and we assessed the possibility of autochthonous recovery of the population after recent amelioration of the lake environment. We determined the ephippia distribution in eleven cores 10–30 cm long, and the state of Daphnia resting eggs in over 13 000 ephippia from eight cores. Apparently well-preserved eggs were used for hatching experiments and for DNA amplification. Vertical profiles of ephippia densities in several cores did not agree with historical data on the Daphnia presence in the lakes, as the sediment had been repeatedly disturbed by human activities in the past. Ephippia are present near the surface of the sediment in all lakes, and they might therefore receive hatching cues. We were not able, however, to prove that viable eggs are present in the ephippia banks of the lakes. The percentage of the well-preserved eggs in all but one core was below 1%. In addition, no egg hatched in the hatching experiments and we were not able to amplify DNA from the preserved eggs of lakes where Daphnia is extinct, although amplification from relatively young eggs from the fourth lake was successful in 60% of the cases. We conclude that the recovery of Daphnia populations in studied lakes from autochthonous sources is unlikely.     

Egg predation by copepods in Daphnia brood cavities

Both field observations and enclosure experiments show that juvenile copepods enter Daphnia brood cavities to feed on Daphnia eggs and embryos. The ability to perform such in vivo exploitation is reported for both cyclopoid and calanoid copepods. Copepodites of Acanthocyclops robustus (G.O. Sars) were found to eat eggs in brood cavities of D. magna, D. pulex and D. pulicaria in experimental enclosures rich in algae. Copepodites of Eudiaptomus gracilus (G.O. Sars) were found in brood cavities of D. hyalina in a mesotrophic lake. The copepods' intrusions into brood cavities caused dramatic declines in the clutch size of infested Daphnia, and this predation effect could easily be confused with the effect of severe food limitation.     

Inbreeding and outbreeding depression in Daphnia

Egg-to-adult viability of sexual offspring in Daphnia magna is lower for selfed (average: 43.0%) than for outcrossed families (average: 74.7%). This suggests that intraclonal mating is not the rule in Daphnia populations. For a given family, hatching rate of eggs resulting from interpopulation crosses is lower than for intrapopulation crosses. This breakdown in hatching responses may result in the effective gene flow between Daphnia populations being severely reduced, offering an explanation for the apparent paradox of genetic differentiation of Daphnia populations in spite of efficient dispersal.     

Chaoborus predation and the function of phenotypic variation in Daphnia

To investigate the role of helmet formation in defense against predation, laboratory experiments were used to analyze the effects of morphological changes in Daphnia on susceptibility to Chaoborus predation. Behavioral observations of Chaoborus preying on helmeted and non-helmeted Daphnia suggest pre-contact advantages for helmeted prey but post-contact advantages for non-helmeted prey. Helmeted Daphnia are better at evading capture by Chaoborus but may also be more easily handled by the predator. Swimming behavior of the prey, which is influenced by the presence of a tailspine, may affect Chaoborus strike distance. These results re-emphasize the potential hydromechanical importance of body shape changes in defense against predation.     

Reproduction strategies of <Emphasis Type="Italic">Daphnia pulicaria</Emphasis> population in a high mountain lake of Southern Spain

For three consecutive years, a population study of Daphnia pulicaria was undertaken in Río Seco Lake, a small high-mountain lake, in order to elucidate the reproductive strategies adopted by Daphnia in this system. Daphnia appears to colonize this lake every spring by hatching from ephippia and reproduce by means of subitaneous (non-diapausing) and ephippial (diapausing) eggs. D. pulicaria in this lake is an obligate parthenogenetic population. There is a short time period for subitaneous egg production and a much longer period for ephippial egg production. The contribution of subitaneous eggs to Daphnia population density and structure appears to be low. Diapause onset showed a high temporal synchronization in the three studied years in Río Seco Lake, and day-length emerged as the main cue triggering diapause onset and the main explanatory factor for the proportion of ephippial females observed. The development and reproduction of D. pulicaria in Río Seco Lake involves taking a gamble on resting forms to guarantee inter-annual Daphnia persistence in the lake, giving priority to investment in future generations. Guest editor: Piet Spaak Cladocera: Proceedings of the 7th International Symposium on Cladocera     

Changes in epibiotic burden during the intermolt of Daphnia determined by hypodermal retraction stages

We calibrated four stages of hypodermal retraction and cuticle regeneration with five stages of parthenogenetic egg development in Daphnia. Using the hypodermal retraction stages, we found that epibiotic burden increased with elapsed intermolt time for juvenile, male, and female Daphnia bearing parthenogenetic or ephippial eggs. The rate of increase of burden was similar for adult females of two Daphnia species and for males and females of D. pulex. Rate of increase of burden was similar between juvenile and adult females of D. galeata mendotae and D. pulex.     

Food quality triggers the reproductive mode in the cyclical parthenogen <Emphasis Type="Italic">Daphnia</Emphasis> (Cladocera)

Cyclic parthenogenesis (heterogony) is a widespread reproductive mode found in diverse taxa such as digenean trematodes, gall wasps, gall midges, aphids, cladocerans and rotifers. It is of particular interest as it combines the advantages of asexual reproduction (rapid population growth) and sexual reproduction (recombination). Usually sexual reproduction is initiated when, or slightly before, environmental conditions deteriorate, and often results in the production of resting stages. The optimal timing of diapause induction must thus be under strong natural selection. Using the cladoceran Daphnia as a model system, we show here for the first time that the switch from parthenogenetic to sexual reproduction in a cyclical parthenogenetic organism can be influenced by the chemical composition of food. Under crowding conditions Daphnia reproduced parthenogenetically with subitaneous eggs when fed the algal species Cryptomonas sp., but started the production of resting eggs when fed with the green algal species Scenedesmus obliquus. Supplementation experiments with lipids and especially proteins showed that the induction of resting egg production in two clones of different Daphnia species was due to a dietary deficiency in the green alga. Hence, the low food quality induced a switch in the reproductive mode that may contribute to optimal timing of the sexual reproduction of Daphnia in nature. Furthermore, our results have two other major implications: first, they suggest that protein compounds should be added to the list of diet constituents potentially limiting or influencing Daphnia reproduction. Second, we show that the role of food quality goes far beyond the up to now documented effects of food quality on somatic growth and trophic transfer efficiency of herbivores: due to its effects on sexual reproduction and the production of resting eggs, food quality might influence genetic diversity and long-term persistence of Daphnia in lakes. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Distribution and ecology of Demospongiae from the circalittoral of the islands of the Aegean Sea (Eastern Mediterranean)

Size-selective predation by fish is often considered to be a primary driver of seasonal declines in large-bodied Daphnia populations. However, large Daphnia commonly exhibit midsummer extinctions in ponds lacking planktivorous fish. A number of empirical and theoretical studies suggest that resource competition and its interaction with nutrient enrichment may determine variable dominance by large Daphnia. Low resource levels may favor competitive dominance by small-bodied taxa while large Daphnia may be favored under high resource conditions or following a nutrient/productivity pulse. Nutrient enrichment may also influence the strength of invertebrate predation on Daphnia by affecting how long vulnerable juveniles are exposed to predation. We investigated these hypotheses using an in situ mesocosm experiment in a permanent fishless pond that exhibited seasonal losses of Daphnia pulex. To explore the effects of nutrient enrichment, Daphnia plus a diverse assemblage of small-bodied zooplankton were exposed to three levels of enrichment (low, medium, and high). To explore the interaction between nutrient enrichment and invertebrate predation, we crossed the presence/absence of Notonecta undulata with low and high nutrient manipulations. We found no evidence of competitive reversals or shifts in dominance among nutrient levels, Daphnia performed poorly regardless of enrichment. This may have been due to shifts in algal composition to dominance by large filamentous green algae. Notonecta had significant negative effects on Daphnia alone, but no interaction with nutrient enrichment was detected. These results suggest that Daphnia are not invariably superior resource competitors compared to small taxa. Though predators can have negative effects, their presence is not necessary to explain poor Daphnia performance. Rather, abiotic conditions and/or resource-based effects are probably of greater importance.     

Density-dependent predation ofChaoborus flavicans onDaphnia longispina in a small lake: the effect of prey size

Functional response curves of fourth instar larvae ofChaoborus flavicans preying on two size classes ofDaphnia longispina were examined throughout three summer seasons in a small forest lake. Data for each size class were fitted to Holling's disc equation. The parametersa (attack rate) andTh (handling time) were calculated for each prey size from these curves. Attack rate was greater and handling time was shorter for small (0.77 mm) than for large (1.82 mm)Daphnia. In 1:1 mixture of these prey size classes the predation rates ofChaoborus on smallDaphnia at prey densities above 20 l^–1 were greater than predicted from the single size-class experiments. The observed predation rates on largeDaphnia were lower than predicted at all prey densities. Since both single size-class and two size-class experiments were run during the same period of time the difference in observed and predicted predation rates could not be attributed to seasonal changes in prey preference ofChaoborus larvae. In experiments with a concentrated mixture of lake zooplankton (dominated byD. longispina)Chaoborus preference forDaphnia decreased as prey body size increased. There was no obvious correlation between selectivity coefficients and size-frequency distributions ofDaphnia. When medium-sizedDaphnia were omitted from calculations the preference of small over large prey did not differ significantly from the predictions of the single size-class model.     

Life history response of <Emphasis Type="Italic">Daphnia galeata</Emphasis> to heterogeneous conditions within a reservoir as determined in a cross-designed laboratory experiment

Growth and reproduction were studied in the laboratory in a cross-designed experimental set-up in four Daphnia galeata subpopulations collected from different locations (with respect to water characteristics) in a reservoir (epilimnion, metalimnion and hypolimnion in the deepest part of the reservoir near the dam and epilimnion of the upstream part of the reservoir) and in a laboratory clone of the same species. The results of two-way ANOVA revealed significant effects of the two parameters manipulated – source of water used for cultures and Daphnia subpopulation – on the life history characteristics of growth and reproduction. The water from the upstream part of the reservoir was the most favourable culture medium for all characteristics of the Daphnia groups studied (the largest primiparae, clutches and eggs, the shortest postembryonic development time and filtering setae). The poorest performances were recorded in the downstream, epilimnetic and metalimnetic waters. The primiparae in the hypolimnetic water were smaller but had relatively larger clutches of smaller eggs and slightly longer postembryonic development times. The Daphnia subpopulation originating from the hypolimnion had the smallest primiparae, the largest clutches, the smallest eggs and the shortest postembryonic development, whereas the opposite was found in animals from the epilmnion. These differences in ecologically relevant traits were supported by analysis of the quasi-neutral genetic markers that indicated significant site-dependent differences in clonal structure between the subpopulations. There was no consistent trend to higher within-group variance in the life history traits in the genetically heterogeneous subpopulations from the reservoir compared to the laboratory clone.     

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.     

Embryology of Chaoborus-induced spines in Daphnia pulex

Daphnia pulex (Crustacea: Cladocera) embryos were found to be sensitive to a chemical cue (kairomone) in an extract of the predator Chaoborus americanus (Insecta:Diptera). Sensitivity of embryos to the kairomone remains throughout embryonic development. Apparently declining sensitivity as development proceeds may be due to the amount of time the embryos are exposed to the kairomone. Male embryos were also found to be sensitive to the kairomone. The smallest eggs within a brood produced small offspring, which showed the antipredator morphology to a significantly lower degree than largest eggs. The production of the neckteeth is described, at the developmental stage in the maturation of the Daphnia coinciding approximately with the escape of the embryos from the brood chamber.     

<Emphasis Type="Italic">Daphnia</Emphasis>species diversity in Kenya,and a key to the identification of their ephippia

The distribution of Daphniaspecies in tropical Africa is poorly known and understood. Daphniaare assumed rare in tropical regions, but systematic studies covering large areas are sparse. We sampled the active community (live zooplankton) and/or the dormant community (diapausing egg banks in the sediment) of 41 standing water bodies in Kenya in search for Daphnia.Overall the dormant communities yielded 11 species of Daphnia, a species richness more than twice the species richness found in the active communities. Dormant community species diversity better reflects the spatial, and particularly the temporal (multi-annual) variation in environmental conditions available to Daphniain these tropical standing waters. Hence, we suggest that the dormant community be taken into account when assessing local zooplankton diversity, especially in fluctuating tropical lake ecosystems, where the presence of each local Daphniaspecies in the active community may be strongly seasonal or erratic. Geographic distribution data from this study are supplemented with previous records of Daphniain East Africa to provide an overview of the known distribution of Daphniain Kenya and neighbouring countries. We also present a detailed key for morphological identification of the ephippia of the 11 Daphniaspecies encountered, complemented with photographs and drawings of diagnostic characters.     

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.     

Effects of Microcystis aeruginosa on interference competition between Daphnia pulex and Keratella cochlearis

Laboratory experiments tested the hypothesis that a toxic strain of Microcystis aeruginosa decreases the ability of Daphnia pulex to interfere with Keratella cochlearis. To test a variety of conditions, juvenile and adult Daphnia were exposed to the cyanobacterium for different time periods prior to, and during the experiments. Adult Daphnia not only suppressed rotifers over successive two-day intervals, but also had a significant impact within a 24-hour period. However, the presence of Microcystis (5 × 105 cells ml^–1) decreased the Daphnia effect in both experiments. Although juvenile Daphnia also significantly suppressed Keratella population growth, the presence of Microcystis (10⁵ and 5 × 10⁵ cells ml^–1) caused a significant reduction in daphniid body size and decreased the ability of both nonacclimated and acclimated daphniids to suppress rotifers. Keratella inhalation and mortality are positively correlated with filtering rates and body size of Daphnia. Therefore, the feeding rates and size structure of a Daphnia population will determine its potential to interfere with vulnerable rotifers. In all experiments the presence of Microcystis significantly decreased the ability of Daphnia to interfere with this rotifer despite the fact that Keratella was also inhibited. In the field this effect might be augmented if Microcystis colonies are more easily ingested by cladocerans than by the rotifers.     

Daphnia vertical distribution and the presence of toxic cyanobacteria

Daphnia can alter its vertical position in the water column in response to chemical cues from predators. In this study we tested the hypothesis that a Daphnia pulex clone with little evolutionary exposure to cyanobacteria would move away from patches of cyanobacteria (Anabaena affinis and A. flos-aquae) which contain potent endotoxins. Daphnia was censused at 2 h intervals for 6 h in laboratory columns in which there was a steep vertical gradient of cyanobacteria. Data were analyzed by repeated-measures ANOVA. Control (no Anabaena) and experimental columns showed no significant differences in Daphnia distributions. Daphnia in experimental columns frequently moved into areas with dense concentrations of Anabaena and stayed there for long periods of time. Our results show that this D. pulex clone does not exhibit a rapid (within 6 h) avoidance response to toxic Anabaena.     

Density and depth variations of Daphnia multilocus genotypes during a summer period in Lake Maarsseveen

The genotype composition of a Daphnia population complex during a summer period in Lake Maarsseveen (The Netherlands) was determined by allozyme analysis. The depth distribution, diel vertical migration and several parameters of the total population were measured. Young-of-the-year (0⁺) perch (Perca fluviatilis) were caught and species and allozyme types of Daphnia in the perch gut were also analysed. During May 1997, the densities of D. hyalina, D. galeata, the back-cross D. g × h — hyalina and the multilocus allozyme genotypes of the hybrid D. g × h decreased, except one multilocus genotype (MMMF). Total population size decreased and the ratio of females with eggs to those without eggs decreased as well. Food limitation during this clear-water phase in the lake is considered responsible. All genotypes, except MMMF, gradually descended in the water column. This drift is thought to be a reaction to the abundantly present 0⁺ perch or to the kairomones of this fish, although predation on the daphnids was still absent. In June, diel vertical migration started, except again part of the MMMF subpopulation. The other part migrated over a short distance compared with the other taxa and allozyme types. Within two weeks, the upper 5 m of the epilimnion was devoid of Daphnia, and guts of perch were predominantly filled with MMMF. The daphnids in the gut and the lake did not differ in allozyme type composition. By the end of July, population density had increased again. The size and composition of the Daphnia population complex continuously changed during the study period, as did the depth distribution of the components. Different genotypes within the population complex seem to have developed different strategies to cope with starvation and predation and the state at a particular moment can be understood only if past and present factors are considered.     

Growth and fecundity of Daphnia after diapause and their impact on the development of a population

Laboratory and field investigations revealed that the life history traits of exephippial and parthenogenetic generations of Daphnia differ substantially. Daphniids hatching from resting eggs grow faster and their definitive body sizes are bigger than of hatchlings from subitaneous eggs. Size at maturity for exephippial animals is significantly larger. In spite of this, they mature a few days earlier than parthenogenetic females. In this study, the difference was 3–4 days for the laboratory experiments and 1–3 days for the field. Fecundity of the exephippial generation is markedly higher. Here, the clutch size for this generation was up to 3.5–4.0 times as large as for the parthenogenetic generation. Moreover, obtained results suggest that the relationship between clutch size and body length for both generations differ significantly.Estimates of the intrinsic rate of increase for field Daphnia populations demonstrated that life history traits of exephippial animals lead to a two or threefold higher rate of increase in the conditions of invertebrate predation pressure. Under moderate fish pressure, obtained r values for the daphniids hatching from resting eggs were larger than those from subitaneous. High growth rate of exephippial females is disadvantageous only under the conditions of severe pressure by fish. Obtained results suggest that hatchlings from diapausing eggs an acceleration of population increase by several times during the beginning of the development of a population with periodical re-establishment from resting eggs.     

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.