Ephippia

Abstract: Emergence rates of two cladoceran species (Ceriodaphnia reticulata and Eubosmina longispina) and two Calanoid copepod species (Epischura nordenskioldi and Diaptomus sanguineus) from sediments were monitored weekly in Bullhead Pond (Rhode Island) over a 3.5-yr period with permanently deployed plastic, inverted-funnel traps. Emergence rates of C. reticulata decreased over the study period, dropping to zero in the third year, and determined timing of first appearance as well as fluctuations in population size. Emergence of E. Zongispina was sporadic and correlated poorly with population abundance variations, but continued throughout the study. Significantly fewer ephippia of C. reticzdata were recovered from under traps compared to adjacent sediments, suggesting that reserves of ephippia had been depleted during the study. Reserves of E. longispina ephippia were not significantly depleted under traps during the study. Patterns of emergence for E. nordenskioldi and D. sanguineus were very similar to each other in timing and importance in determining planktonic standing stocks. For each, nauplii emerged synchronously and initiated the population each year. Both species hatched during every year of the study, suggesting that eggs can remain viable in the sediments for at least 3 yr. Dormancy enables many animals and plants to survive harsh environmental periods (Harper 1977; Wiggins et al. 1980; Fryxell 1983; Henis 1987). Many planktonic species living in variable habitats produce dormant eggs that can persist for extended periods (see Hutchinson 1967). It is known that rotifers produce resting eggs that can remain viable for at least 35 yr (Nipkow 196 1). Some cladocerans have been reported to produce resting eggs (ephippia) that can persist for decades (e.g. Moritz 1987; Moghraby 1977), and recent work has shown that Calanoid copepods can make diapausI Current address: Center for Limnology, 680 N. Park Street, University of Wisconsin, Madison 53706. Acknowledgments This research was supported in part by NSF grants BSR 83-07350 and BSR 85-16724 to N. G. Hairston, Jr., and by a Sigma Xi Grant-in-Aid of Research. Support and facilities were provided by the Zoology Department, University of Rhode Island. I thank N. G. Hairston, Jr., for his support, discussions, and practical assistance during the study. E. Olds and A. Rothchild were a help in the field, and D. Miller helped with the computer work. This paper has benefited from the reading of earlier drafts by E. De Stasio, N. G. Hairston, Jr., K. D. Hambright, P. Jumars, M. L. Keefe, S. Twombly, T. Whitesel, and two anonymous reviewers. This manuscript was part of a disscrtation submitted in partial fulfillment of the requirements for a Ph.D. degree at Cornell University. ing eggs that last for at least 3 yr and probably > 15 (Hairston and De Stasio 1988; De Stasio 1989). The traditional emphasis on dormancy as an adaptation to avoid harsh conditions perhaps has resulted in little appreciation for its role during periods of “normal” environmental conditions. Recent studies have suggested that pools of dormant eggs in sediments have potentially important effects on the ecology and evolution of zooplankton species in both freshwater and marine environments (Kankaala 1983; Marcus 1984; Hairston and De Stasio 1988; De Stasio 1989). These studies have shown for individual species that dormancy and emergence patterns can play a major role in determining the timing of occurrence of a species (see also Tauber et al. 1986). Many studies have analyzed the dynamics of zooplankton communities that include species that undergo dormancy (e.g. Wiggins et al. 1980; Frey 1982), but there is a general lack of information on the species-specific patterns of dormancy and their direct effects on the temporal dynamics of zooplankton communities. There is a clear need for both basic field studies and theoretical investigations of dormancy during normal environmental variations to supplement the extensive laboratory data

Abstract: A chemical signal, released by a fish predator under summer-like high water temperature and long-day photoperiod, caused the formation of resting eggs in a clone of Daphnia magna. No ephippial females were recorded and no ephippia were released in the control treatment during 45 days of the experiment. When exposed to fish water, the fraction of ephippial females reached a maximum of 3.7%, a value comparable to that registered in summer in the GroBer Binnensee (North- ern Germany), a hypertrophic lake inhabited by fish, which was the source lake for our experimental clone. The number of ephippia released within 45 days was on average 34 ± 22. Ephippia formation could not result from the between-treatment differences in population density, and related patterns of food depletion, since no substantial difference between control and fish treatment was observed. Instead, specific information on the presence of a predator provided a cue which induced the formation of resting eggs in Daphnia. Under heavy predation and very low survival probability of parthenogenetic females, ephippia formation in summer can be adaptive, i.e. higher fitness can be achieved through sur- vival in the diapausing state than through the immediate reproductive gain via parthenogenesis.

Abstract: To assess the similarity, not only in community structure, but also in the factors that shape cladoceran assemblages, we analysed the contemporary zooplankton populations and their sub-fossil remains in 39 shallow UK and Danish lakes. Contemporary zooplankton populations sampled from both the lake edge and the open water in August were compared with surficial sediment assemblages. The sedimentary assemblage data combined counts of both ephippial and chitinous remains in order to provide some representation of Daphnia and Ceriodaphnia. A relatively large volume of sediment (>5 cm3) was analysed for ephippial remains so as to include those species best represented by the larger ephippia. Ephippia were identified to species level in the case of Daphnia magna, and to species aggregates for other groups such as Daphnia hyalina agg., Daphnia pulex agg. and Ceriodaphnia spp. In accordance with previous work several species found in contemporary samples (copepods and the cladocerans Polyphemus pediculus, Scapholeberis mucronata and Diaphanosoma brachyurum) were absent from the surface sediments. There were extensive supporting environmental data sets for the 39 sites. It was therefore possible to determine the factors which influenced assemblage composition for the two datasets by a combination of constrained ordination, in this case redundancy analysis (RDA), partial RDA and Procrustes rotation. The same two factors, zooplanktivorous fish density and submerged macrophyte abundance, were not only the main structuring forces for both data sets, but also explained very similar amounts of the variation in the different assemblages. Thus, we conclude that the living communities and their sedimentary remains reflect the environment they are shaped by in broadly similar ways. Calibration of sub-fossil cladoceran assemblages against modern environmental data to reconstruct environmental change can, therefore, validly employ principles elucidated by contemporary studies to determine the most appropriate modelling technique.

Abstract: SUMMARY 1 To investigate the contribution of hatchlings from lake-Daphnia resting eggs (ephippia) to population and community structure, hatching was monitored in situ from April to November 1986 in two north German lakes Hatching traps were placed on the sediments and the incidence, genetic and species composition of ephippial hatchlings determined twice-weekly 2 Hatching began in April after the thawing of ice and continued for 3–4 weeks until the lakes stratified There was no obvious relationship between the onset and duration of hatching and the environmental variables recorded, namely Secchi depth, surface temperature, temperature and oxygen concentration at the sediment-water interface 3 Among the hatchlings were D galeata, D hyalina and their interspecific hybrid 4 In the lake which contained overwintering animals the number of ephippial hatchlings was approximately one third of the total number of juveniles present in the lake 5 In the second lake, no adults were recorded during winter and the population was probably founded by ephippial hatchlings alone There was good agreement, in the short term, between the proportion of each species represented among hatchlings and the subsequent species composition in the lake 6 The hatchlings were genetically diverse, and alleles were representative of those alleles present in contemporary populations