Possible toxic effects on Daphnia resulting from the green alga Scenedesmus obliquus

We cultured individuals of two Daphnia species and their hybrid on two different algae, Scenedesmus obliquus and Chlamydomonas globosa, in different concentrations. Our results suggest that culture conditions of S. obliquus can be such that the algal cells become toxic to Daphnia     

Zooplankton-induced unicell-colony transformation in Scenedesmus acutus and its effect on growth of herbivore Daphnia

The effects of colonial ecomorphs of the green alga Scenedesmus acutus on growth of Daphnia cucullata and D. pulex were examined. In ecologically relevant densities (up to 200 animals l^-1) the relatively small D. cucullata did not induce colony formation in Scenedesmus acutus, whereas the larger congener D. pulex significantly promoted colony formation. Both clearance rate and population growth rate (r) were significantly lower in D. cucullata when fed colonial Scenedesmus then when fed unicellular food. However, for D. pulex no effects of food type were observed. These results show that large Daphnia may influence its food in such a way that smaller congeners and competitors are negatively affected.     

Green microalgae can use naphthalenesulfonic acids as sources of sulfur

1-Naphthalenesulfonate (1-NS) was utilized by axenic cultures of Scenedesmus obliquus and by 5 other green microalgae as the sole source of sulfur. For all algae under study, 2-naphthalenesulfonate was definitely inferior to 1-NS as a source of sulfur. The rate of disapperance of 1-NS from the medium was measured by HPLC and, indirectly, by relating growth to sulfur supply. The physiological availability of 1-NS sulfur for Scenedesmus obliquus amounted to about 14% of sulfate sulfur. 1-Naphthol appeared as the major metabolite of 1-NS. Hence, it was concluded that 1-NS underwent a desulfonation which also took place in the presence of moderate concentrations of sulfate.Abbreviations HPLC high performance liquid chromatography - 1-NS 1-naphthalenesulfonic acid - 2-NS 2-naphthalenesulfonic acid - OD optical density - OD₀ optical density at time 0 of the light-and-dark cycleDedicated to Prof. Dr. W. Habsguth on the occasion of his 75th birthday     

A hypothesis on the role of Aphanizomenon in translocating phosphorus

A mechanism for the upward translocation of phosphorus by flake-forming Aphanizomenon is hypothesized. Aphanizomenon germinates on oxic sediments and rises into the water column, then the flakes diurnally migrate. Observations from Spring Lake, Minnesota, U.S.A., indicate that Aphanizomenon translocates phosphorus to the summertime epilimnion. However, the observations cannot discriminate between which of two mechanisms related to Aphanizomenon flakes, upon germination or with subsequent diurnal migrations, is more important.     

Carotenoids in the outer cell-wall layer of Scenedesmus (Chlorophyceae)

Scenedesmus obliquus, strain 633, which synthesizes ketocarotenoids and sporopollenin, also forms pink-red-colored cell walls. Both the cell walls left over after autospore liberation and those from homogenates of disrupted green cells have similar carotenoid pigmentation. Canthaxanthin, astaxanthin, an unidentified ketocarotenoid, and lutein were found as integral cell wall components. They are bound to the outer (trilaminar) layer of the complete cell wall which also contains sporopollenin.Abbreviations CWH complete cell walls isolated from the homogenates - CWM maternal cell walls accumulated in the medium - KC ketocarotenoid - SC secondary carotenoids - SP sporopollenin     

Contrasting patterns of body size for Daphnia species that segregate by habitat

Summary We investigate how body size of two coexisting Daphnia species varies among 7 lakes that represent a gradient of predation risk. The two species segregate vertically in stratified lakes; D. galeata mendotae is typically smaller and more eplimnetic than D. pulicaria. The extent of vertical habitat partitioning, however, varies seasonally within and among lakes in apparent response to predation intensity by epilimnetic planktivorous fishes. Daphnia pulicaria uses the epilimnion at low levels of fish predation but is restricted to the hypolimnion under high fish predation, whereas D. galaeta mendotae always utilizes the epilimnion. The species display contrasting patterns of genetic variation in neonate size and size at maturity. D. pulicaria is larger in lakes with higher fish and Chaoborus densities whereas D. galeata mendotae is smaller. This contrast in body size in lakes with high predation is associated with greater habitat segregation in those lakes. In lakes with low predation risk, the two species are similar in body size at birth and maturity.Authorship order alphabetical     

The costs of crest induction for Daphnia carinata

The effects of notonectid-induced crests on growth and reproduction, and resource allocation to crest construction, moult losses and eggs of Daphnia carinata were measured. An attempt was made to elucidate the mechanisms of physiological costs of crest induction for this species. The crested morph of d. carinata reached a significantly larger size than the uncrested form. Reproductive output was similar in early instars, but the crested morph produced more eggs in latter broods. Instar duration was longer for the crested morph and age at first reproduction was delayed. Survival was also lower in this form. Crest construction required significant resources (equivalent to 60 eggs over a life time) but evidence is presented that these resources were obtained primarily by re-allocation of available material rather than collection of extra resources. The crested morph allocated significantly more resources to moulting than its uncrested counter-parts. The uncrested morph produced large eggs in early instars and progressively smaller ones in later instars. The crested morph produced only small eggs. The hypothesis is presented that the crest-induction strategy of D. carinata involves at least two separate sets of responses, each with its own costs and trade-offs. The first response is production of the crest. The cost of crest production is an increased cost of moulting. D. carinata off sets this cost by increasing instar duration and thus age at reproduction. The second response is increased size. D. carinata achieves this by reducing the fraction of available resources allocated to reproduction. The cost is lower reproductive output.     

Implications of phytoplankton chemical composition for zooplankton production: experimental evidence

Summary Growth and reproduction of twoDaphnia species (Cladocera) from Lake Constance were measured experimentally in a through-flow laboratory system under different food conditions, in order to investigate the effect of food quality on zooplankton populations. The cryptomonadRhodomonas sp. and the diatomStephanodiscus hantzschii were used as food. BothDaphnia species grew well at concentrations of 0.2–2.0 mgC/l ofRhodomonas sp., and 0.4–2.0 mgC/l ofS. hantzschii, whileStephanodiscus concentrations of 0.2 mgC/l did not supportDaphnia growth. Nitrogen content, which was very different in the two algae (C/N ratio = 4.9 forRhodomonas and 10.9 forS. hantzschii), was presumed to be the limiting factor. To test this hypothesis, nitrogen-limitedRhodomonas was offered to the cladocerans and their growth observed: the reduction in algal nitrogen content hindered animals' growth and reproduction. Results suggested that zooplankton production may also be affected by the chemical composition of the food algae, and that nitrogen potentially limits production, even in freshwater environments that are generally not N-limited.     

Behavioural and morphological changes in polymorphic Daphnia related to different predation regimes

Morphology and behaviour of Daphnia galeata in Lake Lille Lungegårdsvann, Bergen, Norway, were studied in two consecutive years. A portion of the population showed helmet cyclomorphosis in 1992, but no helmets developed in 1993. The non-helmeted individuals in 1992 were found in the littoral habitat during summer, whereas the helmeted morph stayed mainly in the pelagial. These differences in morphology and behaviour within and between years are likely to be caused by changes in predation regimes. Larvae of the predacious phantom midge, Chaoborus flavicans, were observed only in 1992, whereas the three-spined stickleback, Gasterosteus aculeatus, was present both years.     

Constraints on growth and allocation patterns of Silphium integrifolium (Asteraceae) caused by a cynipid gall wasp

Summary Life-history traits of 101 clones from two populations of Daphnia magna were measured under controlled environmental conditions in the laboratory. Some individuals had four juvenile instars, others had five. This depended on their length at birth and on the population they came from. Females in the group with five juvenile instars were smaller at birth but larger and older at maturity than those with four juvenile instars. Within groups of females with equal numbers of preadult instars (instar groups) age and size at maturity increased with size at birth. This relationship differed significantly among instar groups for both age and size at maturity. Significant differences in age and size at maturity between two populations became non-significant when size at birth was used as a covariable in AN-COVA. Within populations, size at birth depended on the clone and on the parity of the clutch. First-clutch offspring were considerably smaller than those from later clutches. The results suggest that variability in life-history traits is common within and between clones, but that most of this variation can be accounted for by size at birth and the number of pre-adult instars.     

Ephippia of Daphnia magna straus — A technique for their mass production and quick revival

A new rearing technique ideally suitable for promoting either the parthenogenetic or the gametogenetic (ephippia) type of reproduction ofDaphnia magna in the laboratory is described. Airdrying and rehydration induces rapid hatching. Using this technique it is possible to measure the toxicity of chemicals to ephippia in an 8 day test procedure.     

A new standardized sediment bioassay protocol using the amphipod Hyalella azteca (Saussure)

A new standardized bioassay procedure for testing the chronic toxicity of sediments to Hyalella azteca was developed. Tests were initiated with 0–1 wk old amphipods exposed to sediments from Hamilton Harbour, Toronto Harbour, and Lake Ontario for 4 to 8 weeks. Both survival and growth were significantly reduced in the Hamilton Harbour sediments relative to those from the lake after 4 weeks. Exposures of 8 weeks resulted in greater variability; survival of amphipods in sediments from one of the harbour stations, and growth in sediments from both harbour stations with surviving young were not statistically different from survival and growth in lake sediments. Growth and survival in lake sediments were comparable to cultures grown with cotton gauze and no sediment after 4 weeks, but survival was poorer by week 8. Replication was good in 12 out of 13 tests done in duplicate; the difference in survival between replicates averaged 2.2 animals (20 amphipods/replicate, 4 week exposure). We propose that 4 week exposures of young (0–1 wk old) Hyalella would provide a suitable standardized chronic toxicity test for sediments. A detailed protocol on the methodology is presented.     

温度对蚜科专化菌暗孢耳霉休眠孢子形成的影响

为探究虫霉休眠孢子形成的关键影响因素,通过孢子浴接种和多浓度生物测定实验,观察蚜科专化菌暗孢耳霉(Conidiobolus obscurus)在5个温度处理(10,15,20,24,28℃)和3个接种浓度梯度下感染寄主桃蚜(Myzus persicae)的情况,并观测不同处理下蚜尸内产休眠孢子的比例。暗孢耳霉对桃蚜的毒力随温度和接种浓度升高而提高。同时,感病致死的蚜尸镜检结果表明:温度显著影响休眠孢子形成,温度越高,形成几率越大;接种浓度的影响次之,在15—24℃间,形成几率随接种浓度提高而增大。这一现象可能的解释:高温环境将使寄主种群增长停滞或消退,暗孢耳霉通过感知环境温度情况,倾向于在较高温时于寄主体内形成休眠孢子来规避接下来可能出现的寄主匮乏期。