Cholinesterase inhibition and impacts on behavior of the Asian clam, Corbicula fluminea, after exposure to an organophosphate insecticide
Introduction
Organophosphorus pesticides (OPs) are one of the most widely used classes of pesticides in the world (Fulton and Key, 2001). The toxicity of OPs result from their inhibition of cholinesterase enzymes (ChEs), which catalyze hydrolysis of the neurotransmitter acetylcholine (ACh) after it is released at the nerve synapse (Landis and Yu, 1995). Reductions in cholinesterase (ChE) activity have been widely used to indicate exposure to organophosphorous and carbamate pesticides in vertebrate and invertebrate species (Hill and Fleming, 1982, Moulton et al., 1996, Fulton and Key, 2001, Van Erp et al., 2002, Jarrard et al., 2004), including freshwater bivalves (Moulton et al., 1996, Winchester, 1997, Doran et al., 2001).
One issue related to the use of ChE activity as a biomarker is the extent to which reductions in enzyme activity relate to effects on ecologically-relevant parameters such as survival, growth or behavior. A number of studies have found significant reductions in ChE activity were correlated with effects on behavior that may impair subsequent survival of the exposed organisms (Dell’Omo and Shore, 1996, Fryday et al., 1996, Kumar and Chapman, 1998, Beauvais et al., 2001, Brewer et al., 2001). For example, Kumar and Chapman (1998) determined that chronic exposure of the eastern rainbow fish (Melanotaenia duboulayi) to sublethal levels of profenofos resulted in a 70% reduction in acetylcholinesterase (AChE) activity with associated decreases in growth rates, food consumption rates, and food conversion efficiency. In addition, Fryday et al. (1996) assessed the impact of chlorfenvinphos on captive starlings at a near lethal concentration and found that feeding activity and ChE activity decreased significantly immediately following pesticide exposure.
While some studies with bivalves have examined the influence of chemical contaminants on behaviors or parameters such as valve closure, filtration rate or burrowing into sediment (Doherty et al., 1987, McCloskey and Newman, 1995, Byrne and O’Halloran, 2000, Byrne and O’Halloran, 2001), there is a lack of data pertaining to the effects of pesticides on these organisms or the relationship between reductions in ChE activity and behavior. With many populations of freshwater bivalves in decline (Bogan, 1993, Williams et al., 1993), it is important to understand the potential impact that chemicals like pesticides could have on these organisms, particularly since many of the most threatened forms exist across agricultural landscapes where run-off contaminated with pesticides could enter surface waters (Leard et al., 1980, Uno et al., 2001).
In this study, we evaluated the effects of short-term exposure to a pesticide formulation containing the OP chlorpyrifos on cholinesterase activity and behavior of the Asian clam, C. fluminea. Chlorpyrifos is the active ingredient in a number of commonly used household and agricultural insecticide formulations (USEPA, 2002). While measured concentrations of chlorpyrifos in large rivers and lakes often fall at concentrations below the μg/L level (USGS, 1999), studies undertaken by Moore et al. (2002) and Mazanti et al. (2003) determined potential chlorpyrifos concentrations in small streams and wetlands adjacent to agricultural fields could range from 73 to over 700 μg/L. In addition, a study undertaken by the USEPA (2002) found concentrations of 486 μg/L in a water body adjacent to a recently sprayed corn field. The acute lethality of chlorpyrifos to freshwater invertebrates ranges from 0.11 μg/L in the amphipod, Gammarus lacustris, to more than 806 μg/L in the snail, Aplexa hypnorum (USEPA, 1998). The lethal and sublethal effects of chlorpyrifos on freshwater bivalves, however, is largely unknown. In a study by Doran et al. (2001), which assessed the effect of chlorpyrifos on the threeridge mussel, Amblema plicata, an LC50 value was not determined, however, at a concentration of 0.96 mg/L, chlorpyrifos was not lethal to these organisms.
The Asian clam was chosen as the study organism because of its general abundance, the ease with which it could be collected and because it shares some basic life history characteristics with native freshwater bivalves (Unionidae) in that it is an infaunal filter feeder. For the behavioral endpoints, we chose siphoning and burrowing because of their ecological significance to the organisms. The ultimate goal was to determine what concentrations of the pesticide caused a reduction in enzyme activity of the clams and if these reductions coincided with changes in the behavioral parameters.
Section snippets
Collection, transport and storage of test species
Corbicula fluminea ranging in size from 16 to 23 mm were collected from a population in the Little Black River, Carter County, Missouri. This site was selected because it contains an abundance of clams and has fast flowing water year round. Prior to testing, clams were held in 40 L temperature controlled (24 ± 2 °C) aerated tanks containing dechlorinated laboratory water for between 6 and 10 days to allow acclimation to laboratory conditions. Clams in the holding tanks were fed 200 mL of Selenastrum
Water quality
Water quality parameters were as follows with DO, pH, temperature, conductivity, alkalinity and hardness in the ranges 5.85–7.35 mg/L, 6.87–7.92, 22.2–25.7 °C, 705–1123 μS/cm, 38–57 mg/L (as CaCO3), and 120–132 mg/L (as CaCO3), respectively.
Pesticide concentrations
The average nominal and measured concentrations of chlorpyrifos in the test solutions are presented in Table 1. All measured concentrations were within 25% of the nominal concentrations with all initial concentrations, and all but two of the final concentrations,
ChE activity
Under the test conditions used, ChE activity in C. fluminea was reduced by up to 87% of control levels during a 96-h exposure to 0.5 and 1 mg/L chlorpyrifos (as the pesticide Dursban), but was not influenced at concentrations above or below these treatments. A limited number of previous studies have evaluated the effect of OP pesticides on ChE activity in freshwater bivalves, and these allow for some comparisons with our data. Doran et al. (2001) exposed the threeridge mussel (A. plicata) to
Summary
Freshwater bivalves often occur in close association with agricultural landscapes and this, along with their near continuous contact with waterborne contaminants during feeding, makes the potential for exposure to agricultural chemicals quite high (Moulton et al., 1996). While some studies indicate these organisms are not particularly sensitive to pesticides (which facilitates their use for monitoring chemical residues in their tissue, e.g. Foster and Bates, 1978, Pereira et al., 1996), at
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