Resting Egg Formation Research Articles

Influence of Seawater Salinity on the Survival, Growth, Development and Neonate Production of Scapholeberis mucronata (O. F. Müller) (Crustacea: Cladocera)

The effect of salinity on the survival, growth, development and neonate production of the cladoceran Scapholeberis mucronata (O. F. Müller) was studied by using Chlorella pyrenoidosa as feed at 1, 2, 3, 4 salinity gradients and control group according to the pre-experiment and median lethal concentration experiment. The acute effect of salinity on this species indicated that 4 and 4.5 was its limit superior of reproduction and survival. The survival rate and growth rate of individuals reared at 1 and 2 salinity gradients were higher than that of those reared at the other gradients. The mean size of the female adult decreased from 820 to 743 μm when the salinity increased from 1 to 4. Among individuals reared at 1 and 2, the intrinsic rate of natural increase (rm) of population was 1.021 and 0.903, respectively; the rate of egg production was 1.281 and 1.390, respectively; the cumulative egg production was 83.2 and 106.0 and the mean life span was 16.05 and 17.30, respectively. These values of life history parameters were higher than those of individuals reared at 3. No eggs were produced by females reared at 4 during the whole experiment. Furthermore, individuals reared at 1 and 2 had faster embryonic development. The above results implied that S. mucronata is relatively well adapted to low-salinity conditions (1–2). Resting egg formation and sexual reproduction did not occur at all the tested salinity gradients.

Chronic effects of PFOA and PFOS on sexual reproduction of freshwater rotifer Brachionus calyciflorus

Rotifers play an important role in the dynamics of freshwater and coastal marine ecosystems, and are also important tools for assessing toxicity in aquatic environments. In this study, the effects of perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA) on the population growth rate and resting egg production of rotifer Brachionus calyciflorus were investigated. Reproductive bioassays indicated that PFOS increased the rotifer population growth rate at the concentration ⩽2.0mgL−1, and inhibited it at higher concentrations. For PFOA, the inhibition of population growth rate was observed when the concentration was greater than 4.0mgL−1. Exposure to PFOS (0.25mgL−1) or PFOA (2.0mgL−1) increased the mictic ratios of unexposed rotifer offspring. Population variation and increased mictic ratios were likely the two major factors leading to decline of resting egg production. The resting eggs formed under exposure to PFOA/PFOS in the range of 0.125–2.0mgL−1 showed higher hatching percentages in the control medium than that without PFOA/PFOS exposure. When the resting eggs were formed in the control medium and incubated in media with different levels of PFOA/PFOS, higher hatching percentages were induced by PFOS but lower hatching percentages induced by PFOA. The effects on the hatching rate of resting eggs with PFOA/PFOS exposure during the hatching period were greater than those with exposure during resting egg formation period, and the effect of PFOS was greater than that of PFOA. Both PFOA and PFOS exhibited slight effect on the hatching pattern.

Combined effects of four Microcystis aeruginosa strains and Scenedesmus obliquus concentrations on population dynamics and resting egg formation of two Daphnia species

We examined the hypothesis that toxic effects of Microcystis aeruginosa to population dynamics, ephippial production, and resting egg formation of Daphnia were restricted by food quality levels. Four unicellular, toxic M. aeruginosa strains with contrasting concentration of microcystin and two Daphnia species were used in this study. The number of offspring at first reproduction, population densities, and maximum population growth rate of two Daphnia species were lower at the highest toxic M. aeruginosa 7820 treatment compared with other treatments under two concentrations of Scenedesmus obliquus. The maximum number of offspring at first reproduction of two Daphnia species appeared in the higher toxic M. aeruginosa 526 strain at the high concentration of S. obliquus. The effects of M. aeruginosa strains, S. obliquus, and their combination on the number of offspring at first reproduction and the maximum population growth rate of two Daphnia species were significant (P < 0.001). Two Daphnia species could not reproduce in the highest toxic M7820 treatment under two concentrations of S. obliquus. They had lower population size and maximum population growth rate in the higher toxic M526 treatment at the low concentration of S. obliquus, but they were higher at the high concentration of S. obliquus. This result suggests that high S. obliquus concentration could relieve the toxicity of M. aeruginosa to Daphnia, and Daphnia could utilize the lower toxic Microcystis as food. The cumulative ephippia numbers of two Daphnia species were more at the high concentration of S. obliquus than those at the low concentration. The percentage of ephippia containing no resting eggs of two Daphnia species was evidently higher at the low concentration of S. obliquus but was lower at the high concentration of S. obliquus. Our results indicated that the cumulative ephippia numbers of Daphnia were population density dependent at the high-level food, and the productions of ephippia in Daphnia were significantly controlled by microcystin concentration at the low-level food.

The influence of phosphorus concentration on the population dynamics and resting egg formation of two cladocerans

Laboratory experiments were conducted to evaluate the effects of food phosphorus (P) concentrations on the population dynamics, ephippium production, and resting egg formation of two cladocerans, Moina irrasa and Daphnia similoides, fed on Chlorella vulgaris that were cultured in five different P concentrations (0.05, 0.1, 1, 5, and 10 mg/L). The body length at the first pregnancy of M. irrasa and D. similoides significantly increased with higher P concentration, whereas the maturation time significantly decreased. The maximum population density of M. irrasa (mean ± SD, 6220 ± 1129 ind/L) and D. similoides (1816.5 ± 63.5 ind/L) appeared in the 5 and 10 mg P/L groups, respectively. The maximum male density of M. irrasa (150 ± 113.0 ind/L) appeared in the low P concentration of 0.05 mg/L, whereas it was in the higher P concentrations for D. similoides. The maximum cumulative ephippia number of M. irrasa (51/female) and D. similoides (6.2/female) appeared in the 0.05 and 1 mg P/L groups, respectively. The maximum percentage of ephippia containing resting eggs of M. irrasa (69.3%) appeared in the lowest P concentration. The percentage of ephippia containing resting eggs of D. similoides increased gradually with higher P concentration, and the ratio was above 70% in higher P concentrations (5–10 mg P/L). Our results suggest that low P can advance and induce the production of ephippium in two cladocerans and that higher male density promotes the resting egg formation of cladocerans.

Formation and structure of the ephippium (resting egg case) in relation to molting and egg laying in the water flea Daphnia pulex De Geer (Cladocera: Daphniidae)

Resting eggs produced by daphnid species in response to environmental deterioration play an important role in colonizing new habitats or in re-establishing extinct populations. Females lay resting eggs into the space within the dorsal part of their carapace and form an egg case called the ephippium to protect them. Previous studies mainly reported the morphology of the completely formed ephippium and/or the forming ephippium of an uncertain stage. To understand ephippium formation and to clarify key transitions in the formation of resting eggs, we examined the structure and formation of the ephippium in the water flea Daphnia pulex De Geer (Cladocera: Daphniidae) by stereomicroscopy, histology, and scanning electron microscopy. The females used in this study produced resting eggs by obligate parthenogenesis. We divided ephippium formation into four stages based on two molts and a single ovulation, as follows. Stage I begins 13 min after molting in adult females that do not ovulate. In Stage II, immediately after the first molt, a protuberance appears beneath the neck region and the carapace begins to thicken. In Stage III, the resting eggs ovulate and the carapace in the area of the forming ephippium becomes much thicker than the normal carapace and accumulates dark pigmentation. In Stage IV, following the second molt, the female sheds the ephippium with the enclosed resting eggs and forms a new carapace. These stages will provide a useful reference for future studies on resting egg formation.

Effects of colonial Microcystis aeruginosa and interspecific competition on the population dynamics and resting egg formation of two cladocerans

We investigated the effects of colonial Microcystis aeruginosa and interspecific competition on the population dynamics and resting egg formation of two cladocerans. In the single-species culture experiments, high M. aeruginosa concentrations favored the population growth of Moina irrasa but strongly inhibited that of Daphnia carinata. Small-bodied M. irrasa was predominant over large-bodied D. carinata regardless of the presence of the colonial M. aeruginosa in the mixed cultures. The per capita cumulative number of ephippia and the percentage of ephippia containing D. carinata resting eggs both increased from under 0 to 10 mg/L M. aeruginosa in the single-species culture but decreased in the mixed culture. However, the per capita cumulative ephippia number of M. irrasa in the mixed culture was higher than those in the single-species culture at the high M. aeruginosa levels. The effects of M. aeruginosa, interspecific competition, and their combination on the per capita cumulative ephippia numbers of the two cladocerans were significant. The occurrence of ephippia was related to the higher population density of the two species. Our results suggest that interspecific competition inhibits the ephippium production and resting egg formation of the large-bodied cladocerans (D. carinata) in the presence of colonial M. aeruginosa.

A review of typhloplanid flatworm ecology, with emphasis on pelagic species

The study of microturbellaria is not popular, and hence, the taxonomy and ecology of these animals are poorly known. In temperate zone ponds and lakes, the number of species may be up to 50+ per water body. The Typhloplanidae, the subject of this review, live in the littoral–benthic zone, but pelagic occurrences have been accumulating since the first case was reported in 1952. Four species are currently known to be occasionally pelagic; all live in warm–temperate or tropical lakes. Typically, one species per lake is found, but in one lake in Brazil, 2 species co-occur. One of these feeds on the other and on zooplankton, with a preference for Cladocera. Flatworms seem to trade the littoral zone for the pelagic if predation pressure in the littoral exceeds that in the open water, and if the pelagic offers better food. Most Typhloplanidae feed on arthropods, often with a preference for cladocerans, and show parallels with the cladocerans and rotifers in that 2 types of eggs (subitaneous and resting) are produced. In some flatworms, however, resting egg formation does not require amphimixis. There is also great variation between species in both the number of clutches and their composition, and within species, latitudinal differences may be important. At the cold extreme, species produce only one clutch of resting eggs; at the tropical extreme, species produce many subitaneous clutches, although resting eggs continue to be produced as well. The factors driving the life cycles of the pelagic species are discussed and include familiar variables such as temperature (circulation patterns), food, and predation. Flatworms also produce mucus and toxins that add complexity to their ecology; toxins are used for catching prey, and possibly for defense against predators (but not fish). If they become too abundant for their food supply, flatworms also seem to switch to resting egg production or block reproduction.

Resting Egg Production and Fatty Acid Enhancement in the Waterflea, Ceriodaphnia quadrangula, by Using Combination of Light Regime and Food

AbstractCombined effects of different light regimes, 6L:6D and 12L:12D (hours light : hours dark, L:D) and algal foods of Scenedesmus quadricauda (S) and Chlorella vulgaris (C) on ephippia production and fatty acid (FA) profile of Ceriodaphnia quadrangula were investigated. The highest resting egg production, density, and specific growth rate were obtained with 6L:6D+S; followed by 12L:12D+S treatment. The C16:0 varied from 16.2 to 22.16 mg/g dry weight (d.w.); C18:1n−9 from 11.03 to 14.94 mg/g d.w; C18:2n−6 from 9.79 to 20.24 mg/g d.w; and C18:3n−3 from 6.98 to 16.02 mg/g d.w. The amounts of C20:4n−6 (4.36 mg/g d.w) and C20:5n−3 (3.35 mg/g d.w) at 6L:6D+S were highest compared to other treatments. The maximum saturated fatty acids and monounsaturated fatty acids were obtained at 6L:6D+C while maximum polyunsaturated fatty acids observed at 6L:6D+S and 12L:12D+C. The resting egg production and FAs profile could be improved when the frequency of changes from light to dark were increased. Our findings also indicate a significant increase in the quantity of C18:3n−3 in C. quadrangula populations during resting egg formation, hence, implying that manipulation of light regime and algal food can improve performance of C. quadrangula as a live food in aquaculture industries, especially for larval feeding.

Correction: Dietary Essential Amino Acids Affect the Reproduction of the Keystone Herbivore Daphnia pulex

This correction is issued to acknowledge the previous work by Dr Ulrike Koch and her colleagues which had stimulated our research and influenced our experimental approach but was unfortunately omitted from the list of cited references. In her PhD thesis, Dr. Koch investigated food quality effects on and clonal differences in resting egg production of Daphnia sp. (Koch 2009). In a part of her thesis, she addressed the potential role of single amino acids for the reproductive mode of Daphnia. This work has stimulated our subsequent research presented in our paper (Fink et al. 2011). In her studies, Dr. Koch could demonstrate that the addition of specific dissolved amino acids (namely arginine and histidine) to a diet of the green alga Scenedesmus sp. (which together with crowding conditions induced resting egg formation in Daphnia, Koch 2009) triggered a change in Daphnia’s reproductive strategy from the production of resting eggs to the production of subitaneously developing eggs; this change also increased the population growth rate of daphnids (Koch et al. 2011). In our study using a similar experimental setup, we corroborated several of Dr. Koch’s findings, but also have extended her findings as well as reported some striking differences. Using a different green algal genus (Chlamydomonas), we could confirm that addition of the amino acids arginine and histidine to a green algal diet induces a switch from resting egg production to the development of subitaneous eggs under crowding conditions. Interestingly, Koch (2009) has reported quite different effects of the two green algae Scenedesmus and Chlamydomonas on the reproduction of daphnids in her thesis: While she found daphnids to produce mostly subitaneous eggs on a diet of Chlamydomonas, but mostly resting eggs on a diet of Scenedesmus (under crowding conditions), our results suggest that also Chlamydomonas as a food organism leads to an increased resting egg production of daphnids compared to a high quality cryptophyte diet. Because of the effects of amino acid addition on the reproduction mode of Daphnia, Koch et al. (2011) investigated the effect of amino acid additions on the daphnids’ population growth rate. In our study, we followed another, independent hypothesis that the amino acid addition could also affect somatic growth of Daphnia. In an extension to the investigations of Koch et al. (2011), we further have demonstrated that the addition of arginine and histidine also influenced either the the timing of the release of eggs from the ovary into the brood chamber or the actual embryonic development of cladoceran eggs. The most striking differences in the results of Dr. Koch and our study lie in the reported relative difference of the amino acid contents in the hydrolysates of the food algae. Koch (2009) and Koch et al. (2011) related the effects of algal diets on Daphnia reproduction to these algae’s amino acid contents. In contrast to their findings, we could not detect significant differences in the arginine and histidine contents of the algal diets in our experiment, despite their strongly different effects on the daphnids’ reproductive strategy (Fink et al. 2011). This corroborates earlier work by Ahlgren et al. (1992) who found no taxon-specific amino acid profiles in numerous phytoplankton taxa. Hence, our data suggests that the algal amino acid composition cannot explain the observed effects of algal diet on the reproductive strategy of Daphnia (Fink et al. 2011). These partially contradicting results on the content of essential amino acids of freshwater phytoplankton species and their potential consequences for herbivorous zooplankton remain to be resolved by future studies. Taken together, the results of Koch and colleagues (2009, 2011) and our findings (Fink et al. 2011) indicate a high, but previously overlooked importance of specific essential amino acids for the reproductive strategy of the aquatic keystone herbivore Daphnia. Unfortunately, the citation of the work of Dr. Koch was accidentally deleted from a previous version of our manuscript during the assembly of our paper and was therefore unintentionally not included in the paper. These references, for the omitted acknowledgements, were not listed in the published References list: 52.Koch U (2009) Resting egg production in Daphnia: food quality effects and clonal differences. PhD thesis. Limnological Institute, University of Konstanz, Germany 53.Koch U, Martin-Creuzburg D, Grossart HP, Straile D. (2011) Single dietary amino acids control resting egg production and affect population growth of a key freshwater herbivore. Oecologia 167 (4): 981-989. DOI: 10.1007/s00442-011-2047-4

Survival Strategies of the Rotifer Brachionus rotundiformis for Coexisting with the Copepod Apocyclops borneoensis in Laboratory Culture

Interspecific relationship between a euryhaline rotifer Brachionus rotundiformis and a cyclopoid copepod Apocyclops borneoensis was investigated in the laboratory culture. In a mixed culture of B. rotundiformis and A. borneoensis, population growth of B. rotundiformis was suppressed from day 10, while growth in a monoculture population continuously increased throughout the experimental period. However, the population growth of A. borneoensis in the mixed culture did not markedly differ from that in a monoculture population. Suppression of B. rotundiformis growth coincided with a decrease in the numbers of both non-eggbearing and egg-bearing females, and increasing resting egg formation. Growth of A. borneoensis was not affected by the presence of the rotifer. However, relative growth index of ovisac bearing females in the mixed culture was 1.62 times higher than that in the monoculture. Presence of the copepod did not greatly reduce the food available to the rotifer population. The rotifer B. rotundiformis responded in a unique way, to stresses such as physical damage (filtering by A. borneoensis) with the production of many resting eggs to increase its chances of survival.

Effect of salinity during resting egg formation and hatching on descendent reproduction in the rotifer Brachionus rotundiformis Tschugunoff

This study investigated the effect of salinity experienced during resting egg formation and hatching on the descendents' reproduction and on resting egg hatching in the monogonont rotifer Brachioraus rotundiformis Tschugunoff. The study was divided into two parts. First, the resting eggs formed at 17 psu were incubated at four salinities (8, 11, 22 and 33 psu) and then the hatchlings were cultured at the same salinities as during incubation. Second, the resting eggs formed at the four salinities were incubated at the same salinities as during their formation, and the hatchlings were cultured at 33 psu. The resting eggs formed at a salinity of 17 psu and incubated at four different salinities showed a higher hatching percentage at lower salinities (8 and 11 psu). When resting eggs were incubated at the same salinities as at formation (8, 11, 22 and 33 psu), the eggs showed no significant differences in a total hatching rate among treatments (83.3-86.7%). There was no significant difference in population growth when hatchlings did not experience a change in salinity. However, the population growth of hatchlings at 8 psu was comparatively arrested and at 33 psu caused by the acute increase in salinity. With regard to sexual reproduction, rotifers showed different patterns that were associated with the salinity experienced during resting egg hatching. This study shows that the salinity experienced during resting egg formation is the optimal one for egg hatching. Moreover, the salinity experienced during resting egg hatching strongly affects the descendents' sexual reproduction more than the salinity experienced during egg formation.

Late Embryogenesis Abundant (LEA) Proteins in Nondesiccated, Encysted, and Diapausing Embryos of Rotifers1

Two genes encoding for late embryogenesis abundant proteins (LEAs) are expressed in encysted diapausing embryos (or resting eggs) of rotifers (Brachionus plicatilis O.F. Müller) and females forming them. The two genes (bpa-leaa and bpa-leab) share approximately 50% of their nucleotides sequence, and bpa-leaa is more than twofold longer than bpa-leab. The deduced amino acid sequences show high abundance of alanine, glycine, lysine, and glutamic acid; a hydropathy index of lower than one; and a relatively high (81-82%) predicted probability of forming alpha-helices in their secondary structure, all of which are characteristic features of LEAs. The predicted molecular masses of bpa-LEAA ( approximately 67 kDa) and bpa-LEAB ( approximately 27 kDa) are similar to the molecular mass determined by Western-blot analyses, suggesting a low probability of posttranslational modifications. In silico analysis reveals that the two LEAs resemble group 3 LEAs based on the repeats for 11mer motifs, although they also display several putative amino acids typical of the 20mer motif of group 1 LEAs. The rotifer LEAs do not contain a predicted target sequence and are more likely localized in the cytosol. LEAs were expressed in resting eggs and females producing them, but not in other female forms or males. LEA transcripts and proteins are degraded during hatching, suggesting that LEAs are developmentally programmed during resting egg formation and hatching. LEAs probably equip the resting eggs to withstand desiccation if that occurs during dormancy. The present study expands our knowledge about the biological pathways associated with formation of rotifer resting eggs and also demonstrates the occurrence of LEAs in dormant, nondesiccated, encysted animal embryos.

Discovering genes associated with dormancy in the monogonont rotifer Brachionus plicatilis.

BackgroundMicroscopic monogonont rotifers, including the euryhaline species Brachionus plicatilis, are typically found in water bodies where environmental factors restrict population growth to short periods lasting days or months. The survival of the population is ensured via the production of resting eggs that show a remarkable tolerance to unfavorable conditions and remain viable for decades. The aim of this study was to generate Expressed Sequence Tags (ESTs) for molecular characterisation of processes associated with the formation of resting eggs, their survival during dormancy and hatching.ResultsFour normalized and four subtractive libraries were constructed to provide a resource for rotifer transcriptomics associated with resting-egg formation, storage and hatching. A total of 47,926 sequences were assembled into 18,000 putative transcripts and analyzed using both Blast and GO annotation. About 28–55% (depending on the library) of the clones produced significant matches against the Swissprot and Trembl databases. Genes known to be associated with desiccation tolerance during dormancy in other organisms were identified in the EST libraries. These included genes associated with antioxidant activity, low molecular weight heat shock proteins and Late Embryonic Abundant (LEA) proteins. Real-time PCR confirmed that LEA transcripts, small heat-shock proteins and some antioxidant genes were upregulated in resting eggs, therefore suggesting that desiccation tolerance is a characteristic feature of resting eggs even though they do not necessarily fully desiccate during dormancy. The role of trehalose in resting-egg formation and survival remains unclear since there was no significant difference between resting-egg producing females and amictic females in the expression of the tps-1 gene. In view of the absence of vitellogenin transcripts, matches to lipoprotein lipase proteins suggest that, similar to the situation in dipterans, these proteins may serve as the yolk proteins in rotifers.ConclusionThe 47,926 ESTs expand significantly the current sequence resource of B. plicatilis. It describes, for the first time, genes putatively associated with resting eggs and will serve as a database for future global expression experiments, particularly for the further identification of dormancy related genes.

Effect of progesterone on sexual reproduction of Brachionus manjavacas (Rotifera)

Using hormones to regulate reproduction is a common theme among animals, but little is known about the hormones of most invertebrates. The use of oxidized sterols as hormones is widespread, as is binding to specific nuclear receptors. As part of a transcriptome sequencing project, we discovered a receptor similar to the vertebrate progesterone receptor in the rotifer Brachionus manjavacas. This motivated us to investigate the effects of progesterone on various aspects of rotifer reproduction, including the switch from asexual to sexual reproduction, male production and behavior, resting egg production and hatching, and comparative species sensitivity. Progesterone had no significant effect on asexual population growth rate (r) of B. manjavacas up to 10 mg/L, however, at 14 mg/L r turned strongly negative. At 5 mg progesterone/L, resting egg production was 3.9X greater than in the control and at 10 mg/L resting egg production was 4.4X higher. Resting egg production was completely inhibited at 14 mg progesterone/L. The higher resting egg production at 10 mg/L was accomplished by a 2.2X higher rate of sexual reproduction than the control. The number of males and resting eggs produced per mictic female were not significantly different at 10 mg progesterone/L. The progesterone antagonist mifepristone at 10 mg/L had no significant effect on resting egg production, sexual reproduction rate, and males or resting eggs produced per mictic female. Exposure of males to concentrations of progesterone up to 10 mg /L for one hour had no significant effect on their circling behavior, however, exposure to 4 mg/L increased male copulation by 2.1X over the control. When females were exposed to 4 mg/L progesterone during resting egg formation, a 1.8X greater percentage of their resting eggs hatched. The enhancement of resting egg production by progesterone exposure was greater in B. manjavacas (7.8X higher than control) than B. ibericus (3.9X higher). Progesterone exposure did not enhance resting egg production in B. plicatilis or B. calyciflorus.

Development of rotifer strains with useful traits for rearing fish larvae

The euryhaline rotifer Brachionus plicatilis is a species complex, which is commonly used for rearing marine fish larvae. Providing cultures with an appropriate size of rotifers facilitates size dependent selectivity of the feeding larvae and results in larvae with higher survival, growth and stability. It is also important to obtain rotifers with higher growth rate and tolerance against environmental stress and better nutritional quality after enrichment. This paper reviews the importance of feeding rotifers with appropriate size for fish larvae, as well as how much variation of rotifer traits can be expected from natural population of rotifers, as well as from artificial manipulation. These artificial manipulations include (1) manipulation of culture history to establish descendant rotifer clones with different reproductive traits, (2) manipulation of culture environmental conditions and chemical treatments on rotifer traits, such as size and reproduction, (3) cross-mating and a molecular approach for obtaining rotifer strains with useful traits in clones of their descendants. Recent progress on the production of rotifer strains with ideal characteristics in terms of size, population growth, tolerance against external conditions and resting egg formation is discussed.

Phenotypic variation and developmental instability of life-history traits: a theory and a case study on within-population variation of resting eggs formation in Daphnia

It is commonly accepted that within-population phenotypic variation is caused by genotypic and environmental heterogeneity. Non-genotypic variation is thought to result from diversity of environmental conditions alone. This however contradicts experimental data showing that even when both genetic and environmental sources of phenotypic variation are neglected, residual variation still exists. This residual phenotypic variation is caused by deviations of developmental trajectories from the target trajectory determined for particular genotype and environment, i.e. by developmental instability. This variation is well-known for morphological traits where it is measured most often by fluctuating asymmetry, i.e. random deviations from perfect symmetry, which can be easily separated from the other type of phenotypic variation. In morphological characters which do not possess symmetry or in non-morphological characters this variation cannot usually be separated from other type of non-genotypic variation, caused by environmental heterogeneity. Most researchers still believe that all non-genotypic variation, even under standardised experimental conditions, is caused by insufficient standardization of environment, rather than by other sources of phenotypic variation. When special efforts are undertaken to analyse variation caused by developmental instability, this variation contributes substantially to non-genotypic variation. To exemplify how variation caused by developmental instability can be separated from phenotypic plasticity we analysed phenotypic variation of resting egg formation in Daphnia pulicaria. The proportion of females carrying resting eggs has been shown to depend on the photoperiod of their mothers, revealing transgeneration effects (Alekseev &amp; Lampert 2001). Developmental instability manifests itself in differences between clonemates within an experimental vessel in a standardized environment. Some females produce resting eggs whereas others do not. Our estimations based on results obtained in experimental conditions and extrapolation to conditions in the wild showed that phenotypic plasticity, i.e. phenotypic response to changes of day duration in maternal environment (phenotypic plasticity) explains only 1-2% of phenotypic variation whereas developmental instability explains approximately 98-99% of phenotypic variance of this trait (i.e. producing or not producing resting eggs), if other factors causing phenotypic plasticity are absent. This example demonstrates a major role of developmental instability in variation of the trait under study. The contribution of developmental instability to phenotypic variation should be considered in studying evolutionary patterns of phenotypic traits.

Effect of temperature on resting egg formation of the tropical SS-type rotifer Brachionus rotundiformis Tschugunoff

ABSTRACT: Tropical minute rotifer strains (SS-type) induce mixis at 30–35°C but sexual reproduction and resting egg formation do not proceed well due to rapid environmental change. The present study examined the effect of temperature regulation on rotifer Brachionus rotundiformis (Langkawi strain, SS-type) resting egg formation in small (500 mL in culture volume)- and large-scale (500 L in culture volume) experiments. Rotifers were cultured at 30°C in 15–17 p.p.t. seawater with an initial density of 1 individual (ind.)/mL. After 4 days, when cultures were in exponential growth stage with active mixis induction, the culture temperature of the experimental rotifers was changed to 25°C. Control rotifers were cultured at 30°C throughout the experiment. Fresh or frozen Nannochloropsis oculata and condensed freshwater Chlorella vulgaris were used as the rotifer diets in the small- and large-scale experiments, respectively. Significantly higher resting egg production was observed with the experimental rotifers (30→ 25°C) versus the control rotifers. In the large-scale trial, experimental rotifers produced 2.6 × 106 resting eggs during a 9-day experiment, which was 1.6-fold more than the control rotifers. Moreover, the efficiency of resting egg formation was found to increase by a factor of 1.8. The present study indicates that decreasing culture temperature from 30 to 25°C after active mixis increased resting egg formation in B. rotundiformis (SS-type).

Parthenogenetic reproduction of Diaphanosoma celebensis (Crustacea: Cladocera): influence of salinity on feeding, survival, growth and neonate production

Effect of salinity on the feeding rate and parthenogenetic reproduction of asexual females of the cladoceran Diaphanosoma celebensis Stingelin was studied. Short-term (10 h) grazing experiments were conducted using Isochrysis galbana as feed at 5, 17, 25 and 30 psu salinity. Gut pigment concentration showed a significantly higher rate of feeding at lower salinities. Survival, growth, maturity attainment and neonate production of asexual females reared in the above four test salinities indicated preference for lower salinities (5 and 17 psu). The mean size of adult females decreased from 909 to 593 μm, mean life span from 24 to 5 d, mean neonate production from 12 to 2 and mean size of neonates from 434 to 400 μm as the salinity increased from 5 to 30 psu. Salinity variations also affected the size and age of primiparous females. Resting egg formation and sexual reproduction did not occur at the tested salinities. The results indicate that D. celebensis is adapted to low saline, estuarine environments.

Life history, resting egg formation, and hatching may explain the temporal-geographical distribution ofArtemia strains in the Mediterranean basin

Life history, resting egg formation, and hatching may explain the temporal-geographical distribution ofArtemia strains in the Mediterranean basin

Increased sexual reproduction in brachionus plicatilis (rotifera) with the addition of bacteria and rotifer extracts

Previous works revealed that sexual reproduction of brachionid rotifers increases with increasing population density. It is also known that the renewal of environmental water suppresses sexual reproduction in Brachionus plicatilis O.F. Müller populations. The cause of these actions, however, remains unknown. In this study, we investigated the effects of bacterial additions to axenic rotifer B. plicatilis cultures as well as the the effect of rotifer extracts on sexual and asexual reproduction. Out of 17 bacterial strains tested, rotifer sexual reproducion (mixis) rates were 4–10 times higher than controls with addition of three Pseudomonas strains, a Moraxella strain and a Micrococcus strain. Rotifer mixis rate also increased with the addition of a water soluble extract from 1000 rotifers to 5 ml axenic rotifer cultures. Similar results were obtained for resting egg formation.