Ephippial Production Research Articles

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.

Cladoceran community responses to biomanipulation and re‐oligotrophication in Lake Vesijärvi, Finland, as inferred from remains in annually laminated sediment

Summary1. We studied the role of zooplankton in biomanipulation and the subsequent recovery phase in the Enonselkä basin of Lake Vesijärvi, using subfossil cladocerans in annually laminated sediment. Measures to restore the Enonselkä basin included reduction in external nutrient loading and mass removal of plankti‐ and benthivorous fish. Water clarity increased and the lake changed from a eutrophic to a mesotrophic state. However, some signs of increased turbidity were observed after 5–10 years of successful recovery.2. Annual laminae in a freeze core sample were identified and sliced, based on the seasonal succession of diatoms. Cladoceran remains and rotifer eggs were counted, and Daphnia ephippia and Eubosmina and Bosmina ephippia and carapaces were measured. Annual changes in pelagic species composition were studied with principal component analysis. Individual species abundance, size measurements and various cladoceran‐based indices or ratios (commonly used to reconstruct changes in trophic state and fish predation) were tested for change between four distinct periods: I (1985–1988) dense fish stocks, poor water quality; II (1989–1992) fish removal; III (1993–1997) low fish density, improved water quality; IV (1998–2002) slightly increased fish density and poorer water quality.3. After the removal of fish, the mean size of Daphnia ephippia and Eubosmina crassicornis ephippia and carapaces increased significantly. In contrast, the percentage of Daphnia did not increase. When based on ephippia, the ratio Daphnia/(Daphnia + E. crassicornis) increased, but the interpretation was obscured by the tolerance of fish predation by small Daphnia and by the fact that bosminids were the preferred food of roach. Moreover, ephippial production by E. crassicornis decreased in recent years.4. The abundance of Diaphanosoma brachyurum and Limnosida frontosa increased significantly after the fish population was reduced, while that of Ceriodaphnia and rotifers decreased.5. The expanding littoral vegetation along with improved water clarity was clearly reflected in the concentration of littoral species in the deep sediment core. The species diversity index for the entire subfossil community also increased.6. The period of faltering recovery was characterised by greater interannual variability and an increased percentage of rotifers. Nevertheless, the mean sizes of Daphnia ephippia and E. crassicornis ephippia and carapaces indicated a low density of fish. The deteriorating water quality was apparently related to multiple stressors in the catchment after rehabilitation, such as intensified lakeshore building, as well as to exceptional weather conditions, challenging the management methods in use.

Winter storms: Sequential sediment traps record Daphnia ephippial production,resuspension, and sediment interactions

For species that do not over‐winter, an essential part of the life cycle is the production of diapausing eggs. We use sequential sediment traps in southern Lake Michigan (1997‐2000) to capture settling Daphnia ephippia and sediments, characterizing ephippial production and sediment interactions during hazardous conditions. Each year, there was an enormous pulse of D. mendotae ephippia (ca. 7.2 × 1013 ephippia, 1.2 × 1014 diapausing eggs) that coincided with autumn population decline. Most ephippia settled through the water column, although a few were captured at the water surface and blown shoreward. The duration and amplitude of the autumn ephippial fluxes were similar among years. Ephippial production was positively correlated with water column depth, a consequence of Daphnia spatial abundance. In contrast, resuspension of ephippia was inversely related to water column depth and spatially complex, influenced by waves, coastal currents, and offshore gyre circulation. Large winter storms created nearshore sediment plumes, could resuspend vast numbers of ephippia (e.g., 10 March 1998 storm; 1.9 × 1012 ephippia and 3.1 × 1012 diapausing eggs), and were important in the formation of ÂÂegg banks.ÂÂ Almost all newly produced and resuspended diapausing eggs came from D. mendotae, with very few from two other species (D. retrocurva, D. dentifera) that dominated waters 12 yr ago. These observations suggest a relatively short relaxation time for species cycling out of egg banks in Lake Michigan (&lt;10 yr), due in part to (1) differential resuspension of unconsolidated versus consolidated sediments and (2) the spatially restricted nature of ÂÂhigh‐sedimentationÂÂ zones. Our study is the first use of sequential sediment traps to document diapause egg production.

Genetic versus environmental influence on pigment variation in the ephippia ofDaphnia pulicaria

Summary1. The dormant eggs ofDaphnia(Crustacea: cladocera) are encased in a protective, chitonous casing known as an ephippium. Ephippia are pigmented with melanin, and the degree of pigmentation ranges from transparent to opaque. Variation in pigmentation exists within and across populations and species, raising questions about the factors that influence the natural distribution of pigmentation.2. We used image analysis to quantify pigmentation in ephippial casings ofDaphnia pulicariathat were produced both in the field and in the laboratory. The percentage of the surface area of ephippia that is darkly pigmented ranged from 0.5 to 99.5%. The range, mean and variance of ephippial pigmentation varied across our 11 study lakes.3. Laboratory experiments compared clonal variation (five clones/lake) and population variation (five lakes) at three temperatures (15 °C, 20 °C and 25 °C). The degree of pigment variation between clones was much greater than pigment variation between temperatures, indicating that there is a genetic component to pigmentation in the ephippia which is stronger than the effect of temperature.4. Comparisons of pigment levels and lake characteristics were used to identify physical and biological factors associated with ephippial pigmentation. Investment in ephippial production was the strongest predictor with darker ephippia occurring more often in lakes with the highest ephippial production.

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.

The Midsummer Dynamics of Two Daphnia Species in Wintergreen Lake, Michigan

During 1976, midsummer reductions in densities of Daphnia pulicaria and Daphnia galeata mendotae populations occurred in Wintergreen Lake, Michigan. Previous attempts to determine the causes of such declines have indicated correlations between predator populations, the presence of unpalatable algae or high midsummer water temperatures, and the reduction or disappearance of the daphnid populations. In this study, in situ life table experiments were performed to assess the influence of natural food and temperature conditions on population growth. A weekly sampling program showed that D. pulicaria was dielly concentrated, after early summer, at depths between the anaerobic hypolimnion and the warmer epilimnion, consistent with previous information indicating it to be a cold—water species. However, in situ life table data showed that D. pulicaria was capable of surviving in epilimnetic water up to 27°C, although reproduction was greatly reduced for a 6—day period in mid—July. This reduction in reproduction appeared to be the result of an interaction of high temperatures, declining standing crops of small algae and increasing amounts of Anabaena, Ceratium and Volvox. Daphnia galeata mendotae did not show any adverse response to these mid—July algae—temperature conditions. Analysis of predator gut contents suggested that both daphnids were heavily preyed upon by bluegills (Lepomis macrochirus). Shifts in the size structure of the daphnid populations were also consistent with intense size—selective predation by bluegills, as was the tendency for the age distribution of parthenogenic eggs to become skewed to younger eggs as the midsummer decline proceeded. Differences in body size, ephippial production and habitat preferences between the 2 species are also consistent with the hypothesis that planktivory is a strong selective force in their evolution.

Effects of Varying Wavelengths, Intensities and Polarized Light On Population Dynamics and Ephippial Production of Daphnia Pulex Leydig, 1860, Emend. Richard, 1896 (Cladocera)

[Des populations de D. pulex ont ete elevees sous une lumiere dont la longueur d'onde, l'intensite, et la polarisation etaient connues. Il y a une correlation positive entre le taux de survivance et la longueur d'onde pour les animaux adaptes a la lumiere normale. Les differences entre les populations elevees sous des longueurs d'onde differentes disparaissent apres une semaine. L'effet de la longueur d'onde ne semble pas etre un effet d'intensite. Il y a un correlation negative entre l'intensite de la lumiere, a l'exception des tenebres completes, et la grandeur de la population. L'effet de la lumiere polarisee sur la grandeur de la population n'est pas separable de l'effet d'intensite; cependant, la polarisation de la lumiere reduit de facon significative la population des adultes. La production des ephippia a ete augmentee par les longueurs d'onde bleues. Tandis que de la lumiere etait necessaire pour la production des ephippia, son intensite et sa polarisation n'avaient pas d'effet significatif., Des populations de D. pulex ont ete elevees sous une lumiere dont la longueur d'onde, l'intensite, et la polarisation etaient connues. Il y a une correlation positive entre le taux de survivance et la longueur d'onde pour les animaux adaptes a la lumiere normale. Les differences entre les populations elevees sous des longueurs d'onde differentes disparaissent apres une semaine. L'effet de la longueur d'onde ne semble pas etre un effet d'intensite. Il y a un correlation negative entre l'intensite de la lumiere, a l'exception des tenebres completes, et la grandeur de la population. L'effet de la lumiere polarisee sur la grandeur de la population n'est pas separable de l'effet d'intensite; cependant, la polarisation de la lumiere reduit de facon significative la population des adultes. La production des ephippia a ete augmentee par les longueurs d'onde bleues. Tandis que de la lumiere etait necessaire pour la production des ephippia, son intensite et sa polarisation n'avaient pas d'effet significatif.]