Reproductive strategies of two similar Daphnia species

Cladocerans, like many other invertebrates, continue to grow after beginning to reproduce, and their reproductive capability increases with size. What are the demographic consequences of this strategy of indeterminate growth? Here we present results of laboratory experiments in which we measured reproductive strategies of two large, similar cladoceran species, Daphnia pulex and D. pulicaria under high and moderately limiting food conditions. From the experimental data, we calculated the intrinsic rates of increase r for each reproductive strategy. We also estimated the effect on r of changing the pattern of allocation of biomass to reproduction. The results suggest that the daphnid strategy may represent an evolutionary compromise between strategies that maximize r and strategies that maximize total reproductive output

Induction of male production in clones of Daphnia pulex by the juvenoid hormone methyl farnesoate under short photoperiod

The juvenile hormone, methyl farnesoate (MF), and its analog insecticides have been used successfully to induce the production of males in cladocerans under long-day conditions in the laboratory. However, without hormone addition Daphnia do not usually produce male offspring under long photoperiods, while short photoperiods are a stimulus for the induction of males. We used 21 clones of Daphnia pulex differing in their propensity to produce males under short-day conditions to test whether the treatment with MF would result in an additive effect of shifting the sex ratio towards males. Contrary to our expectations, clones with a high tendency of male production showed a reduced sex ratio in response to MF treatment under short-day conditions, but clones that produced normally few males or did not produce males were stimulated by 700 nM MF to produce up to 40% males. We suggest that the endocrine disruptive effect of MF or juvenile hormone analogs in the field may depend on the clonal composition of the cladoceran population and on the natural photoperiod. This may affect the seasonal occurrence of sexual reproduction and eventually cause a mismatch between the presence of males and ephippial females

Periodicity-dependence of the ferroelectric properties in BiFeO3/SrTiO3 multiferroic superlattices

Artificial superlattices of (BiFeO3)m(SrTiO3)m (m= 1 to 10 unit cells) consisting of multiferroic BiFeO3 and insulating SrTiO3 layers were fabricated on (100)-oriented SrTiO3 substrates by pulsed laser ablation. The remnant polarization and leakage current behavior were studied varying the periodicity (8-80A) of the superlattice. The leakage current was reduced by few orders of magnitude on increase of periodicity compared to single layer BiFeO3 thin films. Reduced leakage and intrinsic polarization hysteresis was observed and was confirmed by PUND analysis for periodicities in the range 20-60A. The leakage current was observed to be dominated by space charge limited conductionComment: Submitted to Applied Physics Letter

Finding the optimal vertical distribution: behavioural responses of Daphnia pulicaria to gradients of environmental factors and the presence of fish

1. The vertical distribution of zooplankton results from active habitat choice aiming to optimise fitness gain in a system of trade-offs. 2. Using large, controlled indoor mesocosms (Plo¨n Plankton Towers), we monitored the behavioural response of Daphnia pulicaria to vertical gradients of temperature, food, oxygen and light, in the presence and absence of fish predation. 3. In the absence of fish, Daphnia distributed as predicted by an ideal ‘free distribution with costs’. If the food was distributed homogeneously, they stayed in the warm epilimnion, while they balanced their time dwelling in epi- and hypolimnion if the food was concentrated in a deep-water maximum. 4. However, oxygen depletion in the hypolimnion, representing an additional cost, prevented Daphnia from completely exploiting the hypolimnetic food maximum. Consequently, the proportion dwelling in the hypolimnion was larger if oxygen was not limiting. 5. Fish predation had an overwhelming effect, driving Daphnia into the hypolimnion under all experimental conditions. If permitted by oxygen availability, Daphnia used the whole hypolimnion, but oxygen depletion reduced their possible habitat to the upper hypolimnion with oxygen concentrations above c. 0.7 mg L)1. As fish were less tolerant of low oxygen, the layer below the thermocline formed a predation refuge for Daphni

Role of hydrology in development of a vernal clear water phase in an urban impoundment

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72334/1/j.1365-2427.2007.01808.x.pd

Predator-mediated genotypic shifts in a prey population: experimental evidence

We demonstrate the effect of fish predation on genotype frequencies in a laboratory population composed of two Daphnia magna clones, with historically contrasting exposures to fish predation. The two clones differed in their responsiveness to predation via differential avoidance/escape behavior. The clone which coexists with fish in nature is more responsive to the presence of a fish predator, while the clone not exposed to fish predation does not exhibit the defensive reaction. Fish caused a rapid (within 18 h) and significant shift in Daphnia clonal composition, from 1:1 to 8:1, in favor of the responsive clone. Genotype-specific defensive abilities (modus defendi) can contribute greatly to the phenomenon of genotype replacement under selective predation