Acute and chronic effects of exposure to the juvenile hormone analog fenoxycarb during sexual reproduction in Daphnia magna

Recent studies have demonstrated that insect growth regulating insecticides are able to affect reproductive endpoints in zooplankton species at very low levels. For the cyclic parthenogenetic water flea Daphnia, most of this research has focused on the asexual part of the life cycle and induction of male offspring. Even though Daphnia and many other aquatic invertebrates rely on sexual reproduction and subsequent production of dormant eggs to recover from environmentally harsh conditions, much less is known about the effects of toxicants on the sexual reproductive phase. Using fenoxycarb as a model pesticide, we exposed male and female neonate Daphnia magna, under conditions inducing a switch to sexual reproduction, and tested for effects on dormant egg (ephippia) production and sex ratio of parthenogenetic offspring. Subsequently, we assessed whether fenoxycarb exposure affected the quality of the produced dormant eggs and viability of the hatchlings. Our results showed that exposure to sub-lethal concentrations of fenoxycarb caused a sharp decrease in parthenogenetic reproduction, while inducing male offspring. Dormant egg production was marginally negatively affected, but survival and fitness of the hatched individuals were not significantly affected. This indicates that under pesticide stress, surviving adult females invested in sexual reproduction at the expense of parthenogenetic reproduction. Exposure to toxicants during the sexual reproductive phase, could affect the active aquatic phase as well as the dormant phase in natural zooplankton populations. This indicates the need for further ecotoxicological research and development of test protocols taking into account the full life cycle of zooplankton species.

Timing matters: sensitivity of Daphnia magna dormant eggs to fenoxycarb exposure depends on embryonic developmental stage

Although Daphnia magna is a key species in many lentic freshwater ecosystems and is commonly used as model organism in ecology and ecotoxicology, very little is known about the effects of chemicals on their dormant life stages. Dormant eggs (ephippia) are produced when environmental conditions deteriorate, and Daphnia switch from clonal to sexual reproduction. Ephippia produced over different growing seasons can accumulate in the sediment of ponds and lakes, where they can be exposed to pesticides and other (anthropogenic) stressors. In the present study, we have investigated the effects of pesticide exposure on dormant eggs at different embryonic developmental stages and evaluated the degree of protection against pollution provided by the ephippial case. We therefore conducted a hatching experiment in which decapsulated and encapsulated dormant eggs were exposed to an insect growth regulator (fenoxycarb) at different stages during their development, both before and after activation of the eggs. In addition, we developed an analytical method to measure fenoxycarb concentrations in the dormant eggs. Fenoxycarb negatively affected development and hatching success and changed the timing of hatching in activated and in dormant eggs. Hatching characteristics as well as fenoxycarb concentrations inside the eggs differed significantly between exposure treatments. Final stages of embryonic development were most sensitive to pesticide exposure and had the highest tissue concentrations of fenoxycarb. Tissue concentrations did not differ significantly between decapsulated and encapsulated eggs, suggesting that the ephippial case offers limited or no direct protection against pesticide exposure. With this study we provide new evidence showing that pesticides can bioconcentrate in and affect D. magna dormant eggs. The severity of the effects on developing embryos depends on the timing of pesticide exposure. Our results stress the importance of considering the full life-cycle of model organisms used in ecotoxicological studies, since these are ultimately aimed at assessing risks of chemical exposure on natural aquatic ecosystems.

Pesticide exposure impacts not only hatching of dormant eggs, but also hatchling survival and performance in the water flea Daphnia magna

Laboratory ecotoxicity tests and biomonitoring in aquatic systems are currently based on the active component of invertebrate communities. Even though dormant egg banks are crucial for the long term survival and community dynamics of many aquatic organisms, the effects of anthropogenic activities on dormant egg bank dynamics have rarely been studied. In this study we investigated the effects of two pesticides with a different mode of action (carbaryl and fenoxycarb) on hatching of Daphnia magna dormant eggs (ephippia) as well as on survival, growth and reproduction of the hatched neonates. Dormant eggs were exposed to the pesticides simultaneously to incubation under conditions that induce hatching (long daylight and 20 °C). Carbaryl had no negative effects on embryonic development or hatching rate up to concentrations almost 1,000 times the median effect concentration (EC50) of neonate survival in acute tests. Fenoxycarb, however, had a significant dose-related effect by delaying or completely stopping the hatching process and caused severe abnormalities in developing individuals. Both pesticides had significant negative effects on survival and reproduction of the hatchlings. These results indicate that, in addition to inducing mortality of active individuals, pesticides can affect zooplankton communities by altering hatching dynamics and life history traits of hatched individuals. We briefly discuss how such pollution induced changes in the benthic-pelagic coupling could translate into trans-generational effects impacting ecological and evolutionary dynamics.

Long distance dispersal of zooplankton endemic to isolated mountaintops--an example of an ecological process operating on an evolutionary time scale

Recent findings suggest a convergence of time scales between ecological and evolutionary processes which is usually explained in terms of rapid micro evolution resulting in evolution on ecological time scales. A similar convergence, however, can also emerge when slow ecological processes take place on evolutionary time scales. A good example of such a slow ecological process is the colonization of remote aquatic habitats by passively dispersed zooplankton. Using variation at the protein coding mitochondrial COI gene, we investigated the balance between mutation and migration as drivers of genetic diversity in two Branchipodopsis fairy shrimp species (Crustacea, Anostraca) endemic to remote temporary rock pool clusters at the summit of isolated mountaintops in central South Africa. We showed that both species colonized the region almost simultaneously c. 0.8 My ago, but exhibit contrasting patterns of regional genetic diversity and demographic history. The haplotype network of the common B. cf. wolfi showed clear evidence of 11 long distance dispersal events (up to 140 km) with five haplotypes that are shared among distant inselbergs, as well as some more spatially isolated derivates. Similar patterns were not observed for B. drakensbergensis presumably since this rarer species experienced a genetic bottleneck. We conclude that the observed genetic patterns reflect rare historic colonization events rather than frequent ongoing gene flow. Moreover, the high regional haplotype diversity combined with a high degree of haplotype endemicity indicates that evolutionary- (mutation) and ecological (migration) processes in this system operate on similar time scales.

Impact of grazing on the species richness of plant communities in Mediterranean temporary pools (western Morocco)

The impact of grazing on the vegetation of Moroccan temporary pools has been studied at 2 scales: regional (inter-pools) and local (intra-pools). Half of the 16 forest pools studied is located in a reserve and ungrazed. The other half, located within public forest, is grazed. Vegetation relevés coupled to water-depths measurements were carried out in each pool. The results showed a significant effect of grazing on both scales of analysis. This effect was found in the species composition of the vegetation, which differed between the 2 types of pools, and in the lower species richness and abundance of plant species in the grazed pools. These differences are interpreted as resulting from the selection by herbivores and the differential tolerance of species to disturbance. These impacts are likely to expose certain species to local extinction by reducing their populations.