The interactive effects of temperature,food level and maternal phenotype on offspring size in Daphnia magna

Invertebrate offspring are usually larger in colder environments. To test for possible effects of covariates (e.g. maternal phenotype and feeding conditions) on this pattern, we performed a laboratory experiment to look at the effect of temperature on newborn weight in the planktonic crustacean Daphnia magna. Three tempèratures (12°C, 16°C and 22°C) and two food levels (10,000 cells ml^–1 and 150,000 cells ml^–1) were used, and offspring were examined from the first five clutches of mothers that had been maintained under the constant experimental conditions for three generations. Preliminary analysis suggested that newborn weight was significantly affected by temperature although patterns in the data were not clear cut. In addition, the covariates mother weight and clutch size were positively and negatively correlated with newborn weight, respectively; and later clutches tended to contain heavier offspring. Therefore, in an effort to control for the effects of the covariates, repeated-measures analysis of covariance was performed using ratio values of newborn weight/mother weight (relative newborn weight) as the dependent variable, clutch size as the covariate and clutch number as the repeated measures term. Now, temperature as a main effect in an ANCOVA model did not significantly influence relative newborn weight. The repeatedmeasure term clutch number also became nonsignificant, indicating that when differences in mother weight due to age were accounted for there were no overall differences in relative newborn weight between clutches from a particular mother. Temperature effects on relative newborn weight were only significant as part of interaction terms with food concentration and with clutch number. Thus there were different weight responses to temperature within food levels, and between clutch numbers within food levels. Under the low-food conditions newborn were heaviest at 16°C, lightest at 12°C and intermediate at 22°C. Conversely, under the high-food condition newborn were lightest at 16°C, heaviest at 12°C and again intermediate at 22°C. However, newborn tended to be heavier under the low food condition, and food concentration was highly significant as a main effect. Mother growth rate showed no significant relationship with newborn weight. It is concluded that direct temperature effects on relative newborn weight are marginal and nonsignificant. Temperature effects through interactions with food concentration and clutch number are important determinants of newborn weight, but relatively speaking account for only a small proportion of observed variance in newborn weight (25%), compared with the direct effect of food concentration (67%).     

Size-scaling of zooplankton foraging in Arctic charr

Prey capture rate (number of prey s⁻¹) and the mode of feeding of Arctic charr Salvelinus alpinus were studied by performing foraging experiments with two sizes (1·1 and 1·8 mm) of Daphnia longispina prey. Arctic charr were particulate feeders at all densities tested. Adjusted for the effect of prey density, the capture rate showed a hump-shaped relationship with Arctic charr size for both sizes of D. longispina . Estimated attack rates ( a ) also tended to show a hump-shaped relationship with fish size. The estimated size-scaling exponent of the attack rate function, however, was relatively small, implying small changes in attack rate over fish sizes. Simultaneous estimations of a and handling time were used in combination with published data on fish metabolism and dry mass rations of prey to estimate maintenance resource density of prey as a function of Arctic charr mass. Maintenance resource densities increased monotonically with Arctic charr size, and rapidly as optimum fish size relative to attack rate on prey was passed.     

Alga consumption of four dominant planktonic crustaceans in Lake Balaton (Hungary)

Between 1981–83 the gut contents ofDaphnia galeata, D. cucullata, Eudiaptomus gracilis, andCyclops vicinus were examined with light and scanning electron microscope to obtain information on the feeding of these species in Lake Balaton. The twoDaphnia species feed mainly on abioseston, and it is assumed that their primary nutrient source was organic matter adsorbed onto the surfaces of the abioseston granules plus bacteria and detritus.E. gracilis feeds on algae, showing a preference for green algae and diatoms.C. vicinus is also a prodigious consumer of algae in Lake Balaton, utilizing the whole size spectrum of phytoplankton. Concerning the trophic relationships between phytoplankton and zooplankton in Lake Balaton, that between diatoms and bothE. gracilis andC. vicinus is the most conspicouos. Convincing evidence for an extensive utilization of blue-green algae was not found. Though there is no firm evidence yet, it is likely that theDaphnia are dependent on organic matter adsorbed on the abioseston.     

The biogeography of AustralianDaphnia: clues of an ancient (>70 m.y.) origin for the genus

The AustralianDaphnia (D. occidentalis, D. jollyi, D. lumholtzi, D. carinata, D. nivalis, andD. cephalata) include representatives from three distinct groups within the genus. The worldwide distribution of species within these three groups is consistent with a Gondwanan or pre-Gondwanan origin for each group. These data suggest an even more ancient origin for the genusDaphnia. The finding thatD. cephalata must have been separated fromD. carinata since Gondwanan times is consistent with recent biochemical data on the phylogenetic relations of AustralianDaphnia, but contrasts with electrophoretic evidence of continuing hybridization between these two taxa. It is suggested that continued gene exchange between otherwise discrete species, or clonal complexes, ofDaphnia may be maintained for millions of years, partly as a result of the varying degrees of cyclical parthenogenesis, and partly as a result of the highly fragmented population structures, found in these taxa.     

The ultrastructure of the so-called olfactory setae on the antennula ofDaphnia magna Straus (Crustacea,Cladocera)

A group of nine sensory setae is found on the tip of the antennula ofDaphnia magna in both sexes. Inside a seta four dendrites are situated, each with one receptor cilium. The receptor cilia extend through a liquor space into the exterior part of the seta. The exterior part of the liquor space is divided from the interior part by a knob-like thickening of the innermost layer of the epicuticle, the basal bead. The basal bead narrows the liquor space and the receptor cilia. The interior part of the liquor space is surrounded by five sheath cells, the exterior part by a thin cuticle. In the exterior part the receptor cilia branch partly and reach a terminal pellet on the tip of the seta. The terminal pellet is a thickened part of the epicuticle. It is permeable to several dissolved substances. It is the exterior part of the receptor that projects over the tip of the antennula and seems to be the entire seta. During the premoult the fifth sheath cell builds up the articulation of the seta, the fourth the basal bead, and the third the shaft of the seta. The first sheath cell forms the cuticular sheath. The organ seems to be a chemoreceptor, but the adequate stimulus is as yet unknown.     

Trophic interactions and habitat segregation between competing Daphnia species

Summary Two commonly coexisting species of Daphnia segregate by habitat in many stratified lakes. Daphnia pulicaria is mostly found in the hypolimnion whereas D. galeata mendotae undergoes diel vertical migration between the hypolimnion and the epilimnion. I examined how habitat segregation between these two potentially competing species might be affected by trophic interactions with their resources and predators by performing a field experiment in deep enclosures in which I manipulated fish predation, nutrient levels, and the density of epilimnetic Daphnia. The results of the experiment indicate that habitat use by D. pulicaria can be jointly regulated by competition for food from epilimnetic Daphnia and predation by fishes. Patterns of habitat segregation between the two Daphnia species were determined by predation by fish but not by nutrient levels: The removal of epilimnetic fish predators resulted in higher zooplankton and lower epilimnetic phytoplankton densities and allowed D. pulicaria to expand its habitat distribution into the epilimnion. In contrast, increased resource productivity resulted in higher densities of both Daphnia species but did not affect phytoplankton levels or habitat use by Daphnia. The two species exhibit a trade-off in their ability to exploit resources and their susceptibility to predation by fish. D. g. mendotae (the less susceptible species) may thus restrict D. pulicaria (the better resource exploiter) from the epilimnion when fish are common due to lower minimum resource requirements than those needed by D. pulicaria to offset the higher mortality rate imposed by selective epilimnetic fish predators. D. g. mendotae does not appear to have this effect in the absence of fish.     

Comparison of the roles of Ostracods and Cladocerans in regulating community structure and metabolism in freshwater microcosms

Laboratory microcosms were used to compare the effects of the littoral ostracod Cypridopsis vidua and the planktonic cladoceran Daphnia magna on community structure and metabolism. Filter-feeding by cladocerans, both in the presence and absence of ostracods, greatly reduced the abundance of planktonic algae when D. magna reached peak density around day 50; rotifers and euglenids were then limited to flocculent matter on the container bottom. Both net production and community respiration rates decreased as community composition changed. Microcosms containing ostracods as the only microcrustacean showed little reduction in total algal numbers but the otherwise dominant alga, Scenedesmus spp., was replaced by Ankistrodesmus spp. when peak ostracod density was reached around day 100. Rotifers were completely eliminated but euglenids were able to coexist with ostracods. Ostracods impacted community metabolism less than cladocerans, but depressed respiration slightly more than net production.     

Daphnia curvirostris Eylmann in Japanese high mountain waters (Cladocera: Daphniidae)

A species of Daphnia, Daphnia curvirostris Eylmann, found in high mountain lakes and ponds in central Japan is described. Although there were some differences in the shape of the male rostrum and the chromosome number between European populations as described by Johnson (1952) and Trentini (1980), and Japanese ones collected from high mountain waters, Japanese specimens had many characteristics similar to the taxon D. curvirostris of Europe.     

A test of a model for planktivorous filter feeding by bream Abramis brama

Synopsis The planktivorous feeding of bream, Abramis brama on Daphnia hyalina and Bosmina coregoni was analyzed in a stepwise regression analysis with the average size (and standard deviation) of consumed organisms as dependent variable and the size of the fish, the average size (and standard deviation) of the organisms and their density in the environment as independent variables. Three basic predictions on filter feeding were formulated and tested. It was predicted that the (average) prey size should increase with fish size, but that the standard deviation should decline. Secondly the prey size should be strongly correlated with the prey size available and thirdly the prey density should have little effect on the size selection. These hypotheses could not be rejected for bream>10 cm feeding on B. coregoni and for bream>20 cm feeding on D. hyalina. The hypotheses were not valid for smaller bream as these acted as particulate or combined filter- and particulate feeders.     

The influence of mineral salts on fecundity of the water flea (Daphnia magna) and the implications on toxicity testing of industrial wastewater

The effects of mineral salts constituting water hardness on fecundity ofDaphnia magna were assessed. Of the salts tested, increased concentrations of NaHCO₃ and MgSO₄ had no effect on fecundity, CaSO₄ significantly increased fecundity, and KCl significantly reduced fecundity. The number of offspring produced per daphnid was correlative to the CaSO₄ concentration at CaSO₄ concentrations between 91 and 2100 mg/ℓ. The effects of CaSO₄ on daphnid fecundity could influence the interpretive outcome of industrial wastewater toxicity tests using this species when the waste and dilution waters contain different concentrations of CaSO₄. It is recommended that when performing these tests, dilution water be sampled at the intake site of the industry's water source, thus assuring initial comparability of the waste and dilution waters. The CaSO₄ content of the water prior to and after industrial use should be determined to identify any alterations of CaSO₄ concentration during use. Identification of CaSO₄ concentration differences can aid in the interpretation of effects associated with the wastewater.