Barbara Santer

Summer diapause in cyclopoid copepods: adaptive response to a food bottleneck?

Summer diapause in freshwater cyclopoid copepods is often regarded as an adaptation to avoid fish predation. We tested the alternative hypothesis that the ultimate reason for fourth instar copepodites to enter the sediment during summer is to avoid a food bottleneck for herbivorous juvenile stages caused by the grazing of competing cladocerans. Seasonal life cycles of three co-existing cyclopoid copepod species, Cyclops kolensis, C. vicinus and C. abyssorum, which spend the summer in anoxic sediments, were studied in a eutrophic lake. All three species undergo an ontogenetic shift in their feeding habits from herbivory in the naupliar stages to omnivory in the later juvenile and adult stages. Nauplii depend on flagellates as food. Copepods were cultured in the laboratory without a diapause. Nauplii were subjected to lake water containing seston that was freshly collected during the different seasons, and their growth and development was recorded. Nauplii developed quickly into copepodites in lake water that was drawn in late winter (February/March), when they were abundant in the lake. In late spring and early summer, when cladocerans dominated the plankton, lake water did not support naupliar growth. Development of C. abyssorum was retarded and nauplii from the other two species died before they reached the first copepodite stage. However, all species developed nearly optimally during spring and summer if the water was supplemented with flagellates. We conclude that all species must pass a serious bottleneck as they need high concentrations of flagellates for their naupliar development. Hence, summer diapause may be an adaptation to avoid poor food conditions for nauplii. Fish predation may be important for shaping the timing of diapause in C. abyssorum, as this is the largest species and is least affected by food limitation. This hypothesis is supported by a comparison of diapause behaviour of cyclopoids in lakes of varying trophy.

Life cycle strategies of free-living copepods in fresh waters

Freshwater copepods live in habitats characterized by a high degree of instability. To survive occasional deterioration of their environment copepods have evolved adaptive mechanisms like dormancy or migration in order to avoid lethal conditions and to synchronize growth and reproduction with favourable abiotic and biotic conditions. Typical life cycles of harpacticoid, calanoid and cyclopoid copepods are presented to show strategies that have evolved to survive threatening environmental conditions.

Cannibalism in Cyclops abyssorum

We study cannibalism in the cyclopoid copepod Cyclops abyssorum (Sars). The functional response of predatory stages of C. abyssorum on inter- and conspecific prey is determined. Methods are developed to calculate the impact of cannibalism on populations. Four whole-lake studies found in the literature provide sufficient data to apply the methods. For these populations we calculate: 1. The cannibalism induced naupliar death-rate, CID. 2. The probability that a nauplius falls victim to cannibalism before reaching the copepodite state, P cann . 3. The percentage of the total amount to food ingested by the predator which is due to cannibalism, PWIC

Differential impacts of copepods and cladocerans on lake seston, and resulting effects on zooplankton growth

In an enclosure study in Schöhsee, a small mesotrophic lake in Northern Germany, the impact of copepods and daphniids on the seston community was studied. In general, these two guilds differ in their feeding behaviour. Copepods actively select their food, with a preference for larger particles, whereas most cladocerans are unselective filter-feeders. In this study we investigate how the impact of the two different grazers affects zooplankton growth. We combine results obtained in the laboratory with results measured in situ in the enclosures. Copepods and cladocerans were cultured on seston from enclosures that were inhabited by density gradients of copepods or daphniids. We observed that Daphnia grew faster on seston that was pre-handled by copepods than on seston that was pre-handled by daphniids, and that somatic growth decreased with increasing densities of daphniids in the enclosures. In contrast, we observed no differences in development rates for copepods grown on the different media. The population growth rates of Daphnia in the Daphnia treatments were determined in the enclosures. Growth differences in both somatic- and population growth of Daphnia were correlated to food quality aspects of the seston. In the laboratory we found that Daphnia growth was correlated with several fatty acids. The strongest regression was with the concentration of 20:4ω3 (r2= 0.37). This particular fatty acid also showed the highest correlation with growth after normalisation of the fatty acids to the carbon content of the enclosures (r2= 0.33). On the other hand, in the enclosure the population growth correlated most to the particulate nitrogen content (r2= 0.78) and only to the N:C ratio, when normalised to carbon (r2= 0.51).

Food niches of cyclopoid copepods in eutrophic Plußsee determined by stable isotope analysis

Feeding studies have suggested interspecific differences in the food sources and trophic position of cyclopoid copepod species. We measured δ13C and δ15N of five sympatric cyclopoid copepod species and of bulk POM from a eutrophic lake over a one-year period, to determine if stable isotope derived-data from the field supported the hypotheses from feeding studies. Cyclops abyssorum and C. vicinus differed significantly in δ15N but not in δ13C, inferring assimilation from similar carbon sources but differentiation in their trophic position. Cyclops abyssorum δ15N typically was 1– 2‰ higher than C. vicinus, suggesting a more predatory feeding mode compared to its congener. The summer species Mesocyclops leuckarti, Thermocyclops oithonoides and Diacyclops bicuspidatus exhibited considerable differences in δ13C. The lower δ13C of the hypolimnetic D. bicuspidatus compared to the two epilimnetic species indicates a food niche differentiation by vertical separation. All copepod δ13C and δ15N signatures showed seasonal fluctuations, with low values in winter and high values in summer. The δ13C of seston was generally higher than copepod δ13C. We regard selective feeding from the bulk POM, spatial separation of feeding and assimilation of 13C depleted food sources responsible for this phenomenon.

Maternal aging in the univoltine freshwater copepod Cyclops kolensis: variation in egg sizes, egg development times, and naupliar development times

The constraints imposed on reproduction by decreasing lipid reserves and a univoltine life cycle were studied in the copepod Cyclops kolensis. Egg sizes, clutch size, egg development times, interclutch times, naupliar development times and sizes of CI copepodids were measured for successive clutches of eggs of Cyclops kolensis. There was a decrease in egg size, clutch size and CI size with clutch order. Egg development times and interclutch times did not vary with clutch order. Naupliar development times were negatively related to CI size. Small nauplii took twice as long to develop as large nauplii. Thus, later clutches make a smaller contribution to fitness than early clutches. The lack of a trade-off between clutch size and egg size may promote synchronisation of the univoltine life cycle.

The population genetic consequences of diapause in Eudiaptomus copepods

Prolonged diapause (extended dormancy) is thought to greatly influence evolution in freshwater invertebrates by lengthening generation time, promoting higher levels of dispersal among populations by wind or animal vectors, and increasing effective population size. However, empirical tests of these predictions are relatively rare. Comparative studies can be informative in this regard, if the comparisons involve sympatric, closely related species that differ only in the presence or absence of a dormant life history stage. We built upon a previous study by ZELLER et al. (2006), which used this approach to study patterns of microsatellite variation in Eudiaptomus copepods from northern Europe. E. graciloides possesses diapausing eggs and adults, whereas the closely related species E. gracilis is ecologically and trophically similar but lacks diapause. To separate further historical processes from recent anthropogenic influences, we examined mitochondrial DNA sequence variation in these species from three lakes in northern Germany where they are sympatric. Biotic and abiotic variation among contemporary and historical populations was minimized by focusing on hydro-logically linked lakes separated by less than 10km. E. gracilis and E. graciloides possess very different patterns of mtDNA variation. Both species possess significant population structure on small spatial scales, but E. graciloides has far fewer alleles despite a larger average divergence. Phylogeographic analyses and Bayesian skyline plots revealed evidence for historic population expansions in both species, with the growth phase beginning thousands of generations earlier in E. gracilis than in E. graciloides. Levels of genetic diversity suggest that effective population size may be an order of magnitude larger in E. gracilis than in E. graciloides, even though E. graciloides possesses a diapausing egg bank. Although this would seem to be an ideal system for quantifying the genetic role of diapause, we found no support for the assumption that freshwater invertebrates with diapause categorically possess larger effective population sizes and higher rates of gene flow than closely related species that lack diapause.

The life cycle of Cyclops vicinus in Lake Søbygård: new aspects derived from sediment analyses

Cyclops vicinus is the only copepod species in the pelagic zone of Lake Søbygård and can be found there all year round. We studied the population dynamics of this copepod over a one-year period. In contrast to earlier studies we included the copepods in the pelagic zone as well as the copepods resting in the sediment. Cyclops vicinuswas found not only in the open water, but also in the sediment throughout the year. From the fluctuations of the abundances in both habitats we suggest that the life cycle is more complex than known so far: different diapause pattern appear within the population and the induction of diapause is different than in other populations. We assume that these features contribute to the predominance of C. vicinusin Lake Søbygård.

Diapause of Cyclops vicinus (Uljanin) in Lake Søbygård: indication of a risk-spreading strategy

Cyclops vicinus is reported to enter summer diapause triggered by day length in order to survive food scarcity and fish predation. Development ceases and the fourth or fifth copepodid stages persist in the sediment for several weeks. In Lake Søbygård, however, a small eutrophic lake in Denmark, C. vicinus is found in the sediment as well as in the open water throughout the year. We performed laboratory experiments to elucidate the life cycle strategy of this population. In order to find the factors that induce diapause we tested the influence of food composition and light regime on the proportion of copepods entering diapause. Further we examined the diapause pattern of the offspring from diapausing and non-diapausing parents to see whether the co-occurrence of diapausing and non-diapausing copepods results from genetic differentiation within the population. In all experiments some of the copepods developed directly into adults, while others remained at the fourth or fifth copepodid stage and displayed diapause features. The proportion of copepods that developed directly into adults was influenced by food supply. Fewer individuals entered diapause when they received a mixed diet of algae and ciliates or algae, ciliates and seston compared with a pure algal diet. The response to light was different than described in literature: about 30% of the copepods entered diapause under dark conditions, and the diapause frequency was not higher when copepods had been exposed to continuous light. The offspring from diapausing parents showed no higher tendency to enter diapause than the offspring of non-diapausing parents, indicating no genetic differentiation. We conclude that diapause in C. vicinus represents a risk-spreading strategy, modified by food. The simultaneous production of diapausing and non-diapausing offspring ensures survival under harsh conditions; the influence of the food supply on the proportion of individuals entering diapause may adjust the population to the actual environmental condition. This flexible life cycle strategy might contribute to the exclusive dominance of C. vicinus in Lake Søbygård.