The effects of pollutants on aquatic animals can be assessed by population studies, in particular through the evaluation of the survival rates and reproductive success. However, as in aquatic animals gills are directly exposed to the environment, they may also be used as indicators of water quality (Rankin et al., 1982).
Fish gills play vital roles, since they are the main site of gaseous exchanges (Hughes, 1966; Hughes, 1982). Furthermore, they are involved in osmorregulation (Gonzales and McDonald, 1992; Flik and Verbost, 1993; Romão et al., 2001; Verbost et al., 1994), acid-base balance (Epstein et al., 1980; Evans et al., 1982; Lin and Randall, 1991; McDonald et al., 1991; Goss et al., 1992), excretion of nitrogenous compounds (Goldstein, 1982; Evans and Cameron, 1986; Sayer and Davenport, 1987), and taste (Hughes, 1982; Rios and Fanta, 1998).
Organic Pesticides (Davis and Wedemeyer, 1971; Rao and Rao, 1981; Mallatt, 1985; Evans, 1987; Laurent and Perry, 1991; Nowak, 1992; Wendelaar Bonga and Lock, 1992), detergents (Schimid and Mann, 1961; Abel, 1976; Bolis and Rankin, 1980), acids (Daye and Garside, 1980; McDonald, 1983; Kawall, 1993), salts (Hossler, 1980; Luvizotto, 1994; Fanta et al, 1995), industrial waste (Mitz and Giesy, 1985; Stoker et al, 1985; Lindesjöö and Thulin, 1994), ammonia (Smart, 1976; Arillo et al., 1979; Soderberg et al., 1984) and heavy metals (Skidmore, 1970; Matthiessen and Brafield, 1973; Lock and van Overbeeke, 1981; Oronsaye and Brafield, 1984; Oliveira Ribeiro et al, 1994), can change the branchial epithelium and alter the activity of ATPase-Na-K, in that way, altering the normal flow of ions.
Therefore, fish gills can be used as model for studies on environmental impact (McKim and Erickson, 1991). Laurent and Perry (1991) consider the morphologic changes in gills, as a consequence of environmental changes, as adaptive attempts in conserving some physiological functions.
Rao and Rao (1981) investigated the effects of the organophosphorous (OP) compound methyl parathion on the synthesis of lipid derivatives in different tissues of fish (muscle, gill, liver, and brain). Quantitative analyses showed both, the decrease of all lipids and phospholipids, and the increase of free fatty acids in fish exposed to 0.09 ppm for 48 hours (sublethal concentrations). Based on these results, the authors suggested that there was a larger demand of energy for the fish in this condition.
Methyl parathion is one of several OP pesticides developed to substitute organochlorides. OPs are less persistent in the atmosphere, being easily linked to organic matter, being adsorbed to sediments and particled material in suspension (EPA, 1986).
The toxicity of methyl parathion is the result of a metabolic conversion processed in the endoplasmic reticulum of the hepatocytes, where the group P=S is transformed into P=O. This process affects directly the morphology of the hepatocytes (Rodrigues and Fanta, 1998). The resultant compound, paraoxon, is responsible for the inhibition of several enzymatic systems (colinesterase, carboxilase, acetylcolinesterase and mitochondrial oxidative phosphorilase). The inhibition of AChE is the most critical toxic effect because it results in the accumulation of the neurotransmitter acetylcholine in the synapsis, interrupting the neural transmission. Although substantial reductions in the activity of AChE of the brain of fish have not been fatal, the effect of this condition on activities as feeding, reproduction and relationships prey-predator is not known (EPA, 1986; Silva et al., 1993).
OP compounds are more biodegradable than the organochloride, and therefore, often used to combat insects. According to the Bayer technical information manual, methyl parathion is the single product recommended officially for the control of the insect "broca" (Eutinobothrus brasiliensis) in the State of Paraná, therefore, being extensively used in agriculture. It is active against the main plagues that attack cotton, garlic, rice, potato, coffee, onion, citrus, bean, tobacco, corn, soybean, tomato and wheat cultures. Due to of rains or soil drainage, it often contaminates water bodies, affecting non target organisms such as fish. This is the reason why this product was chosen for this study.
Acute toxicity tests in fish are required by the Brazilian legislation that regulates and classifies the pesticides used in Brazil. Despite Brachydanio rerio being a species thoroughly used in these bioassays, due to its easy maintenance in laboratory conditions, it is not a native species of the Brazilian ichthyofauna. However, Metynnis roosevelti is a native species from Brazilian rivers, where OPs are often used, and therefore a possible victim of their effects in the natural environment.
Thus, our purpose was to use the morphology of the gills of M. roosevelti as indicators of the effects of lethal and sublethal effects of the organophosphorous compound methyl parathion.