Journal of Limnology

vol. 63 (Suppl. 1), 2004

DIAPAUSE IN AQUATIC INVERTEBRATES

Guest Editors: V. Alekseev, G. Giussani, O. Ravera and N. Riccardi

CONTENTS

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1
Effects of diel vertical migration on ephippia production in Daphnia

Victor ALEKSEEV
Zoological Institute of the Russian Academy of Sciences, University emb., 1, 199034, St. Petersburg, Russia
e-mail address: alekseev@zin.ru

Fish presence in experimentally simulated temperature conditions (limno-towers) led to diel vertical migration and resulted in a decrease of ephippia production in Daphnia pulicaria. Diel fluctuation of food, temperature and day length similar to those experi­enced by migrating Daphnia were tested in laboratory experiments with flow-through-systems. Daphnids were kept under these con­ditions for 15 days and the proportions of females producing an ephippium were determined. In addition, maturation time, survival to maturation, size of the first clutch and female dry weight (without eggs) on day 15 were traced. The most important factor affect­ing ephippia production in migrating Daphnia was photoperiod change, and when mother and embryo are exposed to alternating light conditions, these are thought to have the effect on the mother of acting as a signal to stop ephippia production. Such effects might be explained by the different sensitiveness to light intensity in females carrying an embryo and an embryo itself in broods. Fish presence forced Daphnia to stay in low-light conditions during daytime hours, to avoid attacks by fish. The Daphnia were able to check light intensity constantly by short vertical jumps above a light-threshold that was confirmed experimentally in limno-towers. The dim conditions were possibly light enough for adults to check day length, but were too dark for embryos shaded by the mother's body. Food conditions played a relatively small role in the process, and no effects of temperature on ephippia production were found. As expected, food affected the size of the first clutch, and temperature controlled the time to maturation. Photoperiod had a marginally significant influence on the time to maturation in Daphnia. A hypothesis on the role of photoperiod as the key factor for Daphnia life cyclic recurrence and other seasonal adaptations is proposed.

2

Seasonal phenotypes of Daphnia: post-diapause and directly developing offspring

Kęstutis ARBAČIAUSKAS
Institute of Ecology of Vilnius University, Akademijos St. 2, LT-08412 Vilnius-21, Lithuania
e-mail address: arbas@ekoi.lt

Life-history and physiology of post-diapause and directly developing offspring in Daphnia were compared to determine differ­ences in life-history pattern between offspring origins, identify the basic traits effecting pattern distinction, assess the consequencies of differing life-history for relative fitness, and qualify this seasonal phenomenon. Although the size of ex-ephippial hatchlings was positively correlated with maternal size, ephippia produced by differently sized females of D. magna yielded neonates which body size and it’s variation were similar to those for the first-clutch parthenogenetic neonates produced under high food conditions. Otherwise, post-diapause neonates of D. pulex were larger than first-clutch parthenogenetic neonates and varied in size close to that for neonates from the later clutches. Ex-ephippial offspring showed elevated metabolic activity. Under rich food, they grew faster during juvenile development and were larger at maturation than parthenogenetic females. Post-diapause offspring of D. magna showed higher juvenile growth rates also at limited food availability. Female origins during early maturity differed in resource allocation strategy. A greater allocation to progeny in offspring exiting diapause, however, resulted in increased progeny numbers and higher relative fitness only under rich food environments. Whereas, parthenogenetic females of D. magna exhibited a trend for higher fitness across limited food concentrations. For D. pulex, variability of fitness across differing nutritional environments was lower in directly developing females than that for ex-ephippial offspring. Higher metabolic activity in combination with small initial size resulted in lower resistance to starvation of post-diapause offspring of D. magna. Descendants of female origins in this species showed similar fitness. We conclude that although some traits of ex-ephippial offspring in comparison to parthenogenetic ones may differ across Daphnia species, the life-history strategy of post-diapause females in Daphnia is the same. Ex-ephippial females are adapted to predictable optimal conditions, whereas parthenogenetic females are adapted to unpredictably varying environment. Daphnia have evolved two alternative seasonal phenotypes matching environmental conditions in which they occur, and perform in accordance with predictions of seasonal polyphenism. Due to higher metabolic activity, which must lead to increased resource acquisition and probably is associated to larger allocation to progeny, post-diapause females are superior to directly developing offspring under high food conditions, which are expected during early season, but inferior under limiting food environments, which frequently occur later in the season. In seasonal climate, the adjustment of resource acquisition with respect to resource availability may be a general evolutionary trend for multivoltine organisms resulting in a seasonal polyphenism.

3
Experiences with dormancy in tardigrades

Roberto BERTOLANI*, Roberto GUIDETTI, K. Ingemar JÖNSSON¹⁾, Tiziana ALTIERO, Deborah BOSCHINI and Lorena REBECCHI
Department of Animal Biology, University of Modena and Reggio Emilia, via Campi, 213/D - 41100 Modena, Italy
¹⁾Department of Theoretical Ecology, Lund University, Ecology Building, S-223 62 Lund, Sweden
*e-mail corresponding author: bertolani.roberto@unimore.it

Tardigrades often colonise extreme habitats, in which they survive using both types of dormancy: quiescence and diapause. To­gether with nematodes and bdelloid rotifers, tardigrades are known to enter quiescence (with several forms of cryptobiosis: anhy­drobiosis, cryobiosis, anoxybiosis, osmobiosis) at any stage of their life cycle, from egg to adult. Entering anhydrobiosis, tardigrades contract their body into a so-called tun, loosing most of their free and bound water (>95%), synthesizing cell protectants (e.g., tre­halose, glycerol, heat shock proteins) and strongly reducing or suspending their metabolism. Our research on cryptobiosis focused on some ecological and evolutionary aspects. We evaluated: i) the long-term anhydrobiotic survival by comparing quantitative data on recovery from naturally induced desiccation in several species of tardigrades; ii) differences in survival patterns between species and populations by experimentally inducing anhydrobiosis and cryobiosis; iii) phenotypic factors affecting anhydrobiotic survival. As regards diapause, we considered encystment and eggs. Encystment involves at least the synthesis of new cuticular structures. Morphological changes during cyst formation are more complex than those involved in tun formation. We analyzed more in detail encystment processes, comparing a semiterrestrial with a limnic species. Several inter-specific differences have been identified, other than the production of two types of cysts in the semiterrestrial species. Our analysis of life history traits of a laboratory reared strain of a soil tardigrade revealed a particular hatching phenology that involved the production of both subitaneous and resting eggs. The latter need a cue to hatch (dehydration followed by re-hydration). In addition, the evolutionary meaning of dormancy in tardigrades is discussed. 

4

Diapause in Calanoid Copepods: within-clutch hatching patterns

Bart T. DE STASIO
Department of Biology, Lawrence University, Appleton, Wisconsin USA 54912
e-mail address: Bart.T.Destasio@lawrence.edu

Diapause is a major life history feature of many invertebrate organisms. Determining the phenology of diapause is critical for understanding survival and reproductive success of individuals as well as the long-term viability of many populations. The time spent in dormancy by individuals and variability among offspring in the duration of dormancy are two important aspects of invertebrate life histories. Some data are available, especially on duration of diapause, for plants and insects, but little information is available concerning variability among offspring in diapause traits. This is especially true for crustacean zooplankton, where essentially no information has been published on duration of diapause or variability among offspring in diapause timing or dynamics. Here I pre­sent data on the duration of diapause, and variability among offspring for diapause characteristics. The freshwater calanoid cope­pod Onychodiaptomus sanguineus, an obligately sexual species, was collected from Bullhead Pond, Rhode Island, U.S.A., and raised under conditions in the laboratory to induce production of diapausing eggs. One hundred clutches of these diapausing eggs (920 to­tal eggs) were incubated for over two years in a full-factorial experiment testing the effects of temperature and photoperiod cycles on the hatching dynamics and duration of diapause. Overall hatching success was highest (approximately 86%) for eggs exposed to si­multaneous temperature and photoperiod cycles mimicking natural changes, and was lowest (approximately 20%) when eggs were incubated at constant temperature (4 °C) and in constant dark conditions. The highest fraction of eggs hatched at approximately 550 days of age, but the age of eggs at hatching was highly variable among clutches. There was also large variability within clutches for hatching patterns, with some clutches containing eggs that all hatched synchronously and others in which eggs hatched more con­tinuously throughout the experiment. Treatment conditions significantly affected within-clutch synchrony of hatching, as well as syn­chrony of the onset of hatching. These results of high within-clutch variability and differences among clutches in diapause dynamics have important implications for our understanding of reproductive success of individuals producing diapausing eggs, parent-off­spring conflict, and the evolution of bet-hedging strategies in invertebrates.

5

Population density, sexual reproduction and diapause in monogonont rotifers: new data for Brachionus and a review

John J. GILBERT
Department of Biological Sciences, Dartmouth College, Hanover, New Hampshire, 03755 U.S.A.
e-mail address: John.J.Gilbert@Dartmouth.edu

Diapausing, fertilized eggs in monogonont rotifers typically are formed after an environmental signal induces amictic females to produce mictic daughters. Mictic females lay haploid eggs that may develop parthenogenetically into males, or that may be fertilized and develop into diapausing, female embryos called resting eggs. Laboratory experiments demonstrate that crowding is the signal for production of mictic females in strains of Brachionus calyciflorus from Texas, Spain and Germany, and in a strain of Brachionus variabilis from Spain. In these experiments, newborn amictic females were cultured singly in large and small volumes to give low- and high-density treatments. Therefore, the induction of mictic females is due exclusively to population density and cannot be attrib­uted to a grouping effect involving some interaction among individuals. B. variabilis is very sensitive to population density; females had to be cultured in 30 ml to prevent appreciable production of mictic daughters. Crowding is now known to be the signal for ini­tiation of sexuality and diapause in four species of Brachionus, two species of Epiphanes, and Rhinoglena frontalis. The chemical that mediates this response somehow influences oocytes in the maternal body cavity to differentiate into mictic females. Aggregation of individuals in natural systems may facilitate the production of mictic females. In some of these rotifers, the response to crowding is suppressed in early generations from the resting egg, assuring that clonal populations will attain higher population densities before committing to sexual reproduction. Fitness benefits of the mictic-female response to crowding, and to other environmental signals in other rotifers (dietary tocopherol, long photoperiod), are discussed.

6

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

Dmitry L. LAJUS* and Victor R. ALEKSEEV¹⁾
Department of Ichthyology and Hydrobiology Faculty of Biology and Soil Sciences, St. Petersburg State University, 16 line V.O. 29, 199178, St. Petersburg, Russia
¹⁾Zoological Institute of the Russian Academy of Sciences, University emb., 1, 199034, St. Petersburg, Russia
*e-mail corresponding author: dlajus@yahoo.com

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 & 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.

7

Survival strategies of freshwater insects in cold environments

Valeria LENCIONI
Section of Invertebrate Zoology and Hydrobiology, Museo Tridentino di Scienze Naturali, Via Calepina 14, I-38100 Trento, Italy
e-mail address: lencioni@mtsn.tn.it

At high latitudes and altitudes, ice formation is a major variable affecting survival of freshwater fauna and hence the abundance and composition of invertebrate communities. Freezing, but also desiccation and anoxia, are lethal threats to all life stages of aquatic insects, from the eggs to the adults. During cold periods, the aquatic stages commonly remain in or move to a portion of the water body that will not freeze or dry (e.g., deep waters of lakes, springs and hyporheic zone) where they can remain active. Less frequently they migrate to habitats that will freeze at the onset of winter. Insects have developed a complex of strategies to survive at their physiological temperature minimum, comprising (a) morphological (melanism, reduction in size, hairiness/pubescence, brachyptery and aptery), (b) behavioural (basking in the sun, changes in feeding and mating habit, parthenogenesis, polyploidy, ovoviviparity, habitat selection and cocoon building), (c) ecological (extension of development to several years by quiescence or diapause and reduction of the number of generations per year), (d) physiological and biochemical (freezing tolerance and freezing avoidance) adaptations. Most species develop a combination of these survival strategies that can be different in the aquatic and terrestrial phase. Freezing avoidance and freezing tolerance may be accompanied by diapause. Both cold hardiness and diapause manifest during the unfavourable season and: (i) involve storage of food resources (commonly glycogen and lipids); (ii) are under hormonal control (ecdysone and juvenile hormone); (iii) involve a depression or suppression of the oxidative metabolism with mitochondrial degradation. However, where the growing season is reduced to a few weeks, insects may develop cold hardiness without entering diapause, maintaining in the haemolymph a high concentration of Thermal Hysteris Proteins (THPs) for the entire year and a slow but continuous growth. A synthesis of literature regarding adaptation strategies in aquatic insects is presented, highlighting the scarcity of information on freshwater insects from Alpine regions. Most references are on Diptera Chironomidae from North America and North Europe. Some recent findings on aquatic insects from Italian Alpine streams are also presented.

8

Role of diapause in dispersal and invasion success by aquatic invertebrates

Vadim E. PANOV*, Piotr I. KRYLOV and Nicoletta RICCARDI¹⁾
Zoological Institute of the Russian Academy of Sciences, University emb., 1, 199034, St. Petersburg (Russia)
¹⁾CNR Institute of Ecosystem Study, L.go V. Tonolli 50, 28922 Verbania Pallanza, Italy
*e-mail corresponding author: rbic@zin.ru

Review of existing literature shows an important role of diapause in dispersal of aquatic invertebrates. There is evidence that among aquatic invertebrates, the role of diapause in dispersal can be important in crustaceans, specifically for species that produce resting eggs. Analysis of dispersal vectors of diapausing species revealed the increasing role of human-mediated vectors of species dispersal during last century (intentional and unintentional introductions, specifically associated with shipping) in comparison with natural vectors (currents, wind, birds). Generally, the role of human-mediated vectors is most important for species dispersal across geographical barriers and into large aquatic ecosystems affected by shipping. Current human-mediated transport vectors increase rates of aquatic species introductions in many orders of magnitude in comparison with historical nature-driven species dispersal rates. Ability to develop diapausing resting stages facilitates species survival during movement across geographical barriers under extreme conditions, such as in ballast tanks of ships. Case studies for invasive species of Cladocera show that some invaders may possess adaptive life cycles, switching to the early prolonged gamogenetic reproduction, which facilitates their invasion success into novel ecosystems and further dispersal by both natural and human-mediated vectors.

9

Environmental plasticity of fish avoidance diapause response in Daphnia magna

Mirosław ŚLUSARCZYK
Department of Hydrobiology, University of Warsaw, Banacha 2, 02-097 Warszawa, Poland
e-mail address: m.slusarczyk@hydro.biol.uw.edu.pl

Organisms cope with harsh environmental conditions in various ways: either by tolerating environmental stress (through physiological adaptations), or by avoiding it in space (through migration) or time (diapause). Some species rely on a single strategy while others may choose from an array of options when facing different environmental stressors. Planktonic crustaceans may utilise different active (morphological, behavioural, life-history) or passive (diapause) defences to survive periods of high risk of fish pre­dation. Recent evidence has indicated that resting egg production could be induced in Daphnia magna by chemical cues associated with fish predation. This suggests that contrary to most known cases of diapause, which are triggered well in advance of catastrophic events (here termed "predictive diapause"), fish avoidance diapause in D. magna may exhibit a "responsive nature" and be initiated only after intensive predation appears. Experimental evidence discussed here indicates that the reaction of D. magna to chemical signals of fish predation could be conditional and determined by key environmental conditions, which in nature affect relative gains of activity vs dormancy. At high risk of fish predation, the decision of Daphnia to produce resting eggs was disfavoured by high food concentration. This reaction was claimed adaptive since high food allows for higher reproductive rates and better survival of off­spring. All this may assure higher benefits due to activity despite some risk of predation (once predation pressure is not fatal to all active descendants) and disfavour resting eggs production. Moreover, at moderate food conditions the decision of Daphnia to pro­duce resting eggs was disfavoured by the availability of a dark refuge from fish visual predators and thus likely lowering the risk of being preyed upon. Furthermore, when food was at a moderate level and a dark refuge was not present the decision of Daphnia to produce resting eggs was favoured by low water temperature. This could be explained as an adaptive reaction again, once low water temperature (due to its effect on a rate of metabolism) should have affected the gains derived from active life and reproduction more seriously than ones of inactive stages. The evidence presented here indicates that a responsive diapause allows D. magna to maxi­mise reproductive output by taking advantage of opportunities presented by an unpredictable environment.

10

Use of ephippial morphology to assess richness of anomopods: potentials and pitfalls

Jochen VANDEKERKHOVE*, Steven DECLERCK, Maarten VANHOVE, Luc BRENDONCK, Erik JEPPESEN^1,2), José Maria CONDE PORCUNA³⁾ and Luc DE MEESTER
Laboratory of Aquatic Ecology (Katholieke Universiteit Leuven), Ch. de Bériotstraat 32, 3000 Leuven, Belgium
¹⁾Department of Freshwater Ecology (The National Environmental Research Institute), Vejlsøvej 25, P.O. Box 314, DK-8600 Silkeborg, Denmark
²⁾Department of Plant Ecology, University of Aarhus, Nordlandsvej 68, DK-8240 Risskov, Denmark
³⁾Instituto del Agua (Universidad de Granada), Ramón y Cajal 4 (edif. Fray Luis de Granada) E-18071 Granada, Spain
*e-mail corresponding author: Jochen.Vandekerkhove@bio.kuleuven.ac.be

Zooplankton species richness is typically assessed through analysis of active community samples. These samples ought to be collected at many different locations in the lake and at multiple occasions throughout the year so as to cover the spatial and temporal heterogeneity in active community structure. A number of studies have shown that high numbers of species can be retrieved with a limited effort through hatching of dormant eggs isolated from lake sediments. However, dormant eggs of different species differ in their propensities to hatch, resulting in biased assessments of species composition, abundance and richness. In this paper, we explore the potentials and pitfalls of a third method to assess cladoceran species richness. For twenty European lakes, we identified the num­ber of ephippium morphotypes in sediment samples taken on a single occasion. The morphotype richness was well correlated with species richness as assessed through hatching of dormant forms and through analysis of active community samples covering a six month period. However, not all species had a species-specific ephippial morphotype, consequently resulting in an underestimation of true species richness.

11

Intraspecific differences in hatching phenology of the fairy shrimp Chirocephalus diaphanus Prévost, 1803 (Crustacea, Anostraca) in relation to habitat duration

Paola ZARATTINI
Dipartimento di Biologia Animale e dell’Uomo, Università La Sapienza, V.le dell’Università 32, I-00185 Roma, Italy
e-mail address: paola.zarattini@uniroma1.it

The influence of environmental characteristics on hatching phenology in the fairy shrimp Chirocephalus diaphanus Prévost, 1803 was investigated by comparing the hatching behaviour of two populations living in mountain pasture pools differing in size and in duration of filling. Significant differences observed at population level confirmed that an influence of the environment in determining hatching pattern does actually exist. C. diaphanus seems to be able to adjust the cyst reactiveness to the stochastic nature of its habitat. The cysts laid by females adapted to a predictable and relatively constant environment are characterized by higher hatching percentages with respect to those produced in a temporary pool having erratic hydrocycles.

Workshop on:

Diapause in Aquatic Invertebrates

SUMMARIES

12

Study of different steps of a diapause of Diacyclops thomasi (Cyclopoida, Copepoda): comparison of the ultrastructure of the midgut epithelium in active specimens, in the beginning of encysting and during profound diapause

Alexandra BAUD¹⁾, C. CUOC and Victor ALEKSEEV^2)
1)Biological Institute, University of Provence,  Marseille, France
e-mail address: alexandrabaud@hotmail.com
²⁾Zoological Institute, The Russian Academy of Sciences, St. Petersburg 199034, Russia
e-mail address: Alekseev@zin.ru

Diacyclops thomasi is an interesting Canadian species, whose copepodids 4 form a protective cyst while in diapause. Based (bottomed) on its thickness and form, one can easily evaluate the degree of the entrance into the diapause of this crustacean. We used this criterion to compare ultrastructural features of the midgut epithelium in the beginning of the diapause, at its advanced stage and in active adult female specimens. In this cyclopoid species, the gut is composed of three distinct parts: the foregut, the midgut and the hindgut; posterior part of the foregut is transformed into a voluminous chitin-lined caecum, separated from the midgut by a valve. The epithelial midgut represents the greatest part of the digestive tract, and ultrastructural changes relative to digestion mostly take place here.
Thus, the ultrastructure of the midgut epithelial cells completely opposes active adult females and diapausing copepodids 4 of D. thomasi. Two cell types were found in the former, numerous vacuolar cells, presenting figures of active endocytosis, with great number of vesicular pits and vacuoles containing phagocytized material (?). These cells are found in great number, essentially in the middle part of the midgut. Cells of another type (ph. 19) have more densely packed microvilli, and contains no vacuoles, but principally RER and mitochondria. These are present on the entire length of the midgut.
While in active individuals the epithelium is differentiated, in diapausing  at profound stage D. thomasi we could hardly determine the three parts of the gut, because of a deep transformation of the midgut epithelial layer. The better conserved was the foregut with its caecum and valve having intact cuticle layer; however, the underlying cells appeared degenerated. As for midgut epithelium, only nuclei remained from cells in middle and posterior zones; these were encircled by a membrane with a rim of hyaloplasm rich in ribosomes. Both post abdominal and especially thoracic compartments of the body were filled with large lipid lacunas, adjacent  dorsally to muscle fibers, ventrally to nervous chain or muscles, and to membrane and nuclei from the midgut epithelium.
The midgut epithelium at the beginning of the encysting shows an intermediate transformation: all zones are well discernable, but vacuolar cells are infrequent and seem to discharge their content into the gut lumen, the microvilli of the totality of epithelial cells are much fewer then in active adults. Moreover, we could observe epithelial cells likely "peeling off": their apical membrane was detaching, letting cellular content rich in ribosomes and sometimes in little lipid droplets (150 µm) out into the lumen, so this mass of cytoplasm was membrane-free. The apical  nucleus peeled off with, was voluminous (1.15 µm) with chromatin concretions. An intermediate ultrastructural figure at the beginning of encysting confirms progressive physiological modifications of the organism during the diapause.

  13

Diapause and ecological strategies in Chironomidae (Diptera)

Boudewijn GODDEERIS
Royal Belgian Institute of Natural Sciences, Vautierstraat 29, B-1040 Brussels, Belgium
e-mail address: Boudewijn.Goddeeris@naturalsciences.be

The larvae of non-biting midges (Chironomidae) constitute a major part of the benthic macro-fauna in freshwater ecosystems. About one fifth of all aquatic insect species belong to this family, this means about 1,500 species for Europe only. Diapause, i.e. a developmental arrest or retardation in particular stages of the life cycle, appears to determine a large part of the ecological strategies in Chironomidae. In permanent aquatic habitats, it is mostly linked to overwintering, but the season(s) and period covered by a diapause may differ significantly between species: sometimes the overwintering diapause has already started in summer. Aestivation of dried larvae is known from temporary waters, but strict summer diapauses in permanent waters have also been observed. The developmental stage(s) in which diapause occurs is also species-specific, and even in the same genus diapause may occur as well in the egg stage, as in instar II, III or IV, depending on the species. Diapause in Chironomidae seems to be mainly governed by photoperiod and temperature.
Overwintering diapause is linked to unfavourable environmental conditions for the larvae; it also prevents adult emergence during ice-covering. It is the key factor in the yearly synchronisation of larval development and, consequently, of the spring emergence. Synchronisation of emergence may lower the predation on adults and may increase mating chances. However, a major consequence of species-specific diapauses appears to be the temporal niche-segregation of the larvae of cohabiting species, as demonstrated by an ecological guild in Tanytarsus. Also, diapausing larvae are more resistant to extreme conditions such as freezing and desiccation.
Furthermore, a lot of species display an active diapause during which the larvae still feed and grow to a certain extent. By doing so, they are very opportunistic: they avoid maturation during unfavourable seasons, but any food resource is still used when possible, increasing especially the fitness of the females, as has been demonstrated in Chironomus.

  14

Implications of dormant propagule banks on the structure of genetic variation within and among populations of cyclic parthenogenetic zooplankton

Luc DE MEESTER, Koen DE GELAS and Joost VANOVERBEKE
Laboratory of Aquatic Ecology, Katholieke Universiteit Leuven, Ch. De Beriotstraat 32, 3000 Leuven, Belgium
e-mail address: Luc.DeMeester@bio.kuleuven.ac.be

Many aquatic organisms produce dormant eggs that build up resting egg banks. Although the primary role of dormant eggs is the survival of unfavourable conditions, the ecological and evolutionary implications of their production reaches far beyond the mere persistence of the population in variable environments. We here explore the population genetic consequences of the production of dormant egg banks. The production of dormant egg banks has profound impacts on both the genetic diversity maintained within natural populations as well as on the level of genetic differentiation among populations. With respect to the development of genetic differentiation among populations, we focus on the role of dormant egg banks in reinforcing founder effects upon colonization of new habitats, as outlined in the recently formulated "Monopolization Hypothesis" (De Meester et al. 2002, Acta Oecologica). With respect to the development of genetic structure and the maintenance of genetic variation within populations, we focus on the role of dormant egg banks in determining the genetic diversity at the beginning of the growing season. Using this as a starting point, we develop a general conceptual model to explain the difference in genetic structure and genetic diversity between temporary and small and large permanent habitats in cyclic parthenogenetic Daphnia. This general model involves variables such as the size of the dormant egg bank, the length of the growing season and the strength of clonal selection. Although the impact of dormant egg banks is profound in all aquatic organisms producing dormant stages, we point to their special role in cyclical parthenogens. This is because in cyclic parthenogens, the production of dormant stages is associated with sexual reproduction. Hatching of dormant propagules is therefore an important driver increasing genetic diversity, which can subsequently be eroded by clonal selection

Acknowledgments
The conceptual model on interpopulational genetic differentiation (Monopolization hypothesis) was developed together with Klaus Schwenk, Beth Okamura and Africa Gomez, whereas Piet Spaak and Raquell Ortells contributed to the conceptual model on genetic diversity within populations. Financial support was obtained mainly by project OT/00/14 of the KULeuven Research Fund.

  15

Summer cyclopid diapause in Québec lakes: the role of lake morphometry and productivity

Bernadette PINEL-ALLOUL¹⁾ and Victor ALEKSEEV^2)
1)Biological Department, Montréal University, Québec, Canada
e-mail address: bernadette.pinel-alloul@umontreal.ca
²⁾Zoological Institute,The Russian Academy of Sciences, St. Petersburg 199034, Russia
e-mail address: Alekseev@zin.ru

Summer diapause is a reason for some planktonic cyclopids to produce high-density accumulations in lake sediments. This phenomenon was studied in 23 mountain lakes in vicinity of Montréal, Québec province, CANADA during two seasons in 1998 and 2000. Planktonic cyclopids undergo diapause at major copepodid stages. Highest densities of diapausing cyclopids were found in the lakes sediments for Diacyclops thomassi (400,000 ind m^-2) and copepodid 3-5 of Cyclops scutifer (334,000 ind m^-2) in thermally stratified mesotrophic lakes. In others lakes a big variety in density and species compositions in summer diapausing cyclopids (SDC) have been discovered too.
We used Spearman correlative analysis to elucidates the role of biotic and abiotic factors that could be responsible for the differences in SDC accumulations found in studied lakes. We descovered that the key factor for effectiveness of SDC was lake morphometry. The ratio between lake surface area in hectares and the maximal depth in meters negatively correlated with cyclopid density in lake sediment during summer and early fall months.
Chlorophyll concentration as an indicator of primary productivity in lakes showed positive correlation with SDC accumulation in lake sediments. A negative correlation between plankton predator Chaoborus americanus and SDC densities in strong stratified lakes was significant too.
Combination of biotic and abiotic factors in each lake resulted in local diapause inducing conditions that were unique for summer cyclopid species in each lake. This could explain the big variety of SDC accumulations found in studied lakes. A hypothesis on the role of biotic interactions for effectiveness of the summer diapause in planktonic cyclopid is discussed.

Acknowledgments
This study was fulfilled as a cooperative research  between Québec Government and the Russian Academy of Sciences and partly supported by the grant 01-04-49074 from the Russian Foundation for Basic Researches.

  16

Dormancy in Bdelloid Rotifers: cues, mechanisms and limits

Claudia RICCI¹⁾, Manuela CAPRIOLI and Nadia SANTO
University of Milan, via Celoria 26, 20133 Milan, Italy
¹⁾e-mail address: Claudia.Ricci@unimi.it

In contrast to monogonont rotifers, bdelloids are able to enter dormancy at any age in response to unfavorable conditions. Their dormancy is promptly broken by the removal of the adverse condition. Such dormancy has been called quiescence, as opposed to diapause, occurring at a defined ontogenetic stage and cued by some environmental stimulus directly or indirectly predictive of environmental deterioration. The usual cue for quiescence in the bdelloids is the absence of water. This may occur because of water evaporation (anhydrobiosis), freezing (cryobiosis), osmotic stress (osmobiosis). While virtually nothing is known about cryobiosis and osmobiosis, anhydrobiosis of bdelloids has been studied in terms of its consequences on the population, of animal morphological adaptations, of its increased resistance to external injuries.
Costs and limits of anhydrobiosis of bdelloids have been investigated, and all results agree in pointing out that dormancy produce a sort of time shift of the animal life history, while the duration of the dormancy affects its recovery ability, only. The model proposed for this time shift effect is the "Sleeping Beauty" as opposed to the "Portrait of Dorian Gray".
Among the factors that affect bdelloid recovery after dormancy are substrate nature, duration of dormancy and, unexpectedly, poor food. The duration of dormancy can be safely extended, if the storage of the dry animals occurs under controlled conditions. Recovery rates decrease at increasing duration of dryness. Nevertheless, the response to dryness differ among species, and hardly the results obtained in lab experiments can reflect the conditions that are faced by animals in the field.
The contraction of the rotifer body during the induction of anhydrobiosis has been explored at SEM and at Confocal Laser Scanning Microscope, in order to test how inactive animals keep muscle contraction. Some results will be presented and discussed.
Differently from what is known about the mechanisms of protection of the dormant forms, recent studies have demonstrated that threalose as osmoprotectant molecule is absent from the bdelloids. This leads to the investigation of alternative adaptations, like presence of LEA, that are currently explored.
Some recent results suggest that cues different from those related to availability of water could also induce a sort of dormancy to the bdelloids. Long lasting starvation of both newborns and reproductive bdelloids produced suspension of activity, that was promptly resumed on addition of food. Several aspects of the observed inactivity strongly resemble the dormancy induced by the absence of water. The dormancy ability of the bdelloids can be easily related to their capacity to live in unstable and unpredictable habitats, but while the absence of water is likely to represent a common stress, the same cannot apply to long lasting absence of food.

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Egg polyphenism in Heterocypris clonal lineages (Ostracoda, Crustacea)

Valeria ROSSI¹⁾, Andrea GANDOLFI and Paolo MENOZZI
Dep. of Environmental Sciences, University of Parma, Parco Area delle Scienze 11/A, I-43100 Parma, Italy
¹⁾e-mail address: valeria.rossi@unipr.it

Studies on the factors controlling the dormancy timing and hatching phenology in the apomictic ostracod Heterocypris incongruens have emphasised the responsiveness to external cues such as temperature and photoperiod and the significant interaction between genotype and environment in seasonal predictable habitat. Here we report data on intraclonal variability in Heterocypris females from ephemeral ponds of Lampedusa (Sicily, Italy). In standardised laboratory conditions, clonal females produce at least three types of eggs: resting eggs, fast hatching eggs and slow hatching eggs. Percentage of different egg types varies according to the laboratory experimental conditions. At a near optimal initial rainy season condition (16 °C and 12:12 L:D photoperiod), clonal females produce about equal proportions of resting eggs, eggs that hatch gradually within 25 days from deposition (fast hatching eggs) and synchronised eggs that hutch about 120 days from the birth of their mothers (slow hatching eggs). We evaluate the role of maternal biomanipulation and age on deposition and hatching of eggs: clonal females do not produce increasing proportion of resting eggs as they ageing but lay a variable amount of resting and non resting eggs in any clutches. Egg polyphenism is not affected by maternal behaviour or age but, in one half of non resting eggs (delayed hatching eggs), mean developmental time decreases significantly with mother's age. These eggs (that do not seem able to survive drying) hatch synchronically about 4 months after the hatching of the maternal generation. The physiological mechanisms of the observed intraclonal variability is not clear. Its evolutionary meaning is discussed considering our observation as a good example of bet-hedging strategy by females that live in unpredictable temporary ponds.

  18

Diapause in a Daphnia hybrid species complex

Piet SPAAK¹⁾ and Barbara KELLER
Department of Limnology, EAWAG, P.O. Box 611, Dübendorf, Switzerland
¹⁾e-mail address: spaak@eawag.ch

Sexual reproduction in Daphnia results in the production of diapausing eggs, which are enclosed in a protective structure called ephippium. If a male of one species fertilizes a female of another species hybrid diapausing eggs can be formed. Several studies of/in European lakes have shown that hybrids hatched from those eggs combine life history characters of both parental species in an advantageous way which might explain their success. Hybrids can remain in the population because daphnids reproduce cyclic parthenogenetic. In permanent lakes clonal animals also survive the winter; therefore hybrids do not have to be produced each season by the parental species.
But genetic variation within hybrid populations is comparable to the parental populations, indicating regular hybridization events. Not much is known about sexual reproduction of hybridizing Daphnia species. We investigated the Daphnia galeata – cucullata – hyalina species complex. The taxon composition of sexual and parthenogenetic reproducing individuals from two lakes (Tjeukemeer, The Netherlands and Greifensee, Switzerland) was compared to investigate how frequent hybrids and backcrosses are produced. We also hatched ephippia collected during these sampling periods in the lakes and compared the taxon composition of the hatchlings with their parental lake populations. Finally, we collected ephippia from sediment cores taken from Greifensee to see if hybrids and parental taxa could be traced from the past.
In Greifensee we found significant differences between the genetic composition as well as the backcross-level of pelagic asexual females, sexual females, males, and ephippial eggs (Daphnia hatchlings). The asexual daphnids are dominated by hybrids. In contrast, sexual females and especially Daphnia hatched from ephippial eggs are dominated by D. galeata. Correspondingly we found in Tjeukemeer genetic differences between sexual and asexual females. We conclude that hybrid Daphnia have a lower sexual reproductive success compared to the parental D. galeata. This might be one reason why the parental species in the species complex remain distinct.