The Study Of The Organophosphorus Pesticide Dimethoate On The Behavior Of Freshwater Rotifer

The aquatic environment is often the final depository of most chemical contaminants. Among them, pesticides pose some of the most serious ecological problems because of their toxicity to both target and non-target organisms and their wide distribution. Rotifer is one of the largest micro-invertebrate phyla in terms of biomass, ecological importance, and number of species. For one thing, they are a major component of the zooplankton in freshwater and coastal marine habitats, functioning as grazers on phytoplankton and nutrient recyclers. Furthermore, rotifers are food for many larval fish and invertebrates such as crab, shrimp and so on. Therefore, rotifers play an increasing role in assessing the impacts of environmental contaminants in aquatic ecosystem. Because of their body structures and life-history, the main physiological processes in lifespan only include feeding, swimming and reproduction. Feeding behavior lead the relationship between rotifer and the phytoplankton, swimming behavior is importment in the interspecies interaction between rotifers and their predators and competitors, while reproductive behavior means that they contribute to the species population stability and development. Thus, the use of rotifers behavior characteristics as test endpoints is consequently ecotoxicologically well justified under the contaminants effect. The present research study the effect of a widely used organophosphorus pesticide(OPs), dimethoate on the behavior of the rotifer Brachionus calyciflorus.Firstly, the effects of dimethoate on the feeding behavior of the rotifer were examined. We test the flitration rate and ingestion rate of the rotifer exposed to dimethoate in 0.2,1.0 and 1.8 mg.L-1. At the lower concertrations (0.2 and 1.0 mg.L-1), rotifer feeding activity decreased following enhanced with increase in exposure time. A three stepwise stress were observed clearly including stimulation, accumulation and toxic effect while at the highest concentration (1.8 mg.L-1), only toxic effects were observed. In addition, We also tested the recovery in feeding behavior of the rotifer when after exposure. At 0.2 mg.L-1, the filitration rate and ingestion rate increased with increase in the recovery time but they could not resume to the normal levels in 12h after exposure. While at 1.0 and 1.8 mg.L-1, the recovery in feeding behaviors were observed only in the first 6 h after exposute. Since 6 h, the significant decline in filtration rate and ingestion rate were observed, which implied the postexposure effect of dimethoate. It suggested that dimethoate has latant toxic effect on the feeding behavior of rotifer.Second study analyzed the exposure time-dependent aquatic toxicity of dimethoate on the rotifer, using swimming angular and linear speed alteration as the sub-lethal endpoints. Response surface methodology (RSM) was applied in experimental design and data analysis to consider two related factors:toxic concentration, exposure time and their interaction. In general, inhibition (negative alteration in swimming angular and linear speed) of the pesticide was observed clearly at any given toxicant concentration. The highest inhibition rates in angular and linear speed were obtained in the shortest exposure time (11.36 min) and the highest dimethoate concentration (1.85 mg.L-1). The RSM used for the analysis of treatment combinations showed that a cubic polynomial regression model was in good agreement with experimental results, with R2=0.992 and 0.9997, for swimming angular speed inhibition rate and linear speed inhibition rate (p<0.01, F-test, respectively). 3D reference surface plots and contour plots showed that the toxic effect was influenced not only by dimethoate concentration, but also by the exposure time. A time-step effect was observed clearly. Thus, rotifer displays a high concentration-time course sequence of alteration in both swimming angular and linear speed under dimethoate toxic stress.Thirdly, the alteration of swimming behavior was examined firstly when the rotifer under five dimethoate concentrations after 6 and 12 h. Our results showed that imbalance of the swimming direction under toxic stress. In addition, the inhibition on swimming was observed clearly. Secondly, alterations of swimming behavior under toxic stress were analyzed and classified into three aspects:swimming direction, angle and speed change, using principal component analysis (PCA). Finally, our results indicated that how the disruption of dimethoate on swimming behavior is high concentration dependent. Additionally, different toxic effect was observed when rotifer exposed after 6 and 12 h even if under the same toxic concentration. The mode of dimethoate on different aspects of the swimming behavior of rotifer was reported first time.In addition, we assessed the effect of dimethoate on reproduction of the rotifer. The eggs production, offspring production, eggs hatching rate, the duration time of juvenile period, reproductive period, post-reproductive period and life span were evaluated. High concentrations of dimethoate caused an inhibition in the eggs and offspring productions, shorten the life span and reduced the duration of reproductive period. In addition, of particular interest in our study was that reproduction is also accelerated by low concentrations. Our results indicated that dimethoate has effect as endocrine disrupting chemicals on the maturation and reproduction of rotifer.Finally, although as important members of the zooplankton community and sources of food for fish, rotifers are used extensively in ecotoxicological research to assess the health of the environment and safety of compounds. However, most rotifer toxicity tests are only conducted using rotifer neonates derived from unexposed mothers, thus ignoring the potential transfer of contaminants from mother to offspring. To understand better the mother to offspring exposure, a multigeneration study was conducted using three successive generations (F0, F1 and F2) of the common freshwater rotifer Brachionus calyciflorus to investigate the toxic effects of dimethoate. When the F0 generation was exposed to five pesticide concentrations, the population growth rate (r) displayed symptoms of hormesis, characterized by the conversion of low-concentration stimulation to high-concentration inhibition. Despite this observation, the exposure to any given concentration of dimethoate reduced the population growth rates of the F1 and F2 generation rotifers. Significant differences existed between the F0, F1 and F2 rotifers for the population growth rate under dimethoate stress:F2 individuals were more sensitive than F1, whereas the F1 individuals were more sensitive than F0. The results indicated that the parental exposure to a given toxic stress could result in increased sensitivity and decreased fitness in the offspring. This study illustrates the utility of multigeneration toxicity tests, which may better reflect and more accurately predict the effects of long-term pesticide exposure to aquatic organisms at the population level.