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Brazilian Archives of Biology and Technology

Vol.46, n. 3 : pp. 361-372, June 2003ISSN 1516-8913 Printed in Brazil %5$=,/,$1�$5&+,9(6�2)

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.H\�ZRUGV� Gill, fish, methyl parathion, toxicity, morphology

∗ Author for correspondence

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The effects of pollutants on aquatic animals can beassessed by population studies, in particularthrough the evaluation of the survival rates andreproductive success. However, as in aquaticanimals gills are directly exposed to theenvironment, they may also be used as indicatorsof water quality (Rankin et al�, 1982).Fish gills play vital roles, since they are the mainsite of gaseous exchanges (Hughes, 1966; Hughes,1982). Furthermore, they are involved inosmorregulation (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 andDavenport, 1987), and taste (Hughes, 1982; Riosand 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 andRankin, 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

Machado, M. R. and Fanta, E.


362

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 vanOverbeeke, 1981; Oronsaye and Brafield, 1984;Oliveira Ribeiro et al, 1994), can change thebranchial epithelium and alter the activity ofATPase-Na-K, in that way, altering the normalflow of ions.Therefore, fish gills can be used as model forstudies on environmental impact (McKim andErickson, 1991). Laurent and Perry (1991)consider the morphologic changes in gills, as aconsequence of environmental changes, asadaptive attempts in conserving somephysiological functions.Rao and Rao (1981) investigated the effects of theorganophosphorous (OP) compound methylparathion on the synthesis of� lipid derivatives indifferent tissues of fish (muscle, gill, liver, andbrain). Quantitative analyses showed both, thedecrease of all lipids and phospholipids, and theincrease of free fatty acids in fish exposed to 0.09ppm for 48 hours (sublethal concentrations). Basedon these results, the authors suggested that therewas a larger demand of energy for the fish in thiscondition.Methyl parathion is one of several OP pesticidesdeveloped to substitute organochlorides. OPs areless persistent in the atmosphere, being easilylinked to organic matter, being adsorbed� tosediments and particled material in suspension(EPA, 1986).The toxicity of methyl parathion is the result of ametabolic conversion processed in theendoplasmic reticulum of the hepatocytes, wherethe group P=S is transformed into P=O. Thisprocess affects directly the morphology of thehepatocytes (Rodrigues and Fanta, 1998). Theresultant compound, paraoxon, is responsible forthe inhibition of several enzymatic systems(colinesterase, carboxilase, acetylcolinesterase andmitochondrial oxidative phosphorilase). Theinhibition of AChE is the most critical toxic effectbecause it results in the accumulation of theneurotransmitter acetylcholine in the synapsis,interrupting the neural transmission. Althoughsubstantial reductions in the activity of AChE ofthe brain of fish have not been fatal, the effect ofthis condition on activities as feeding,reproduction and relationships prey-predator is notknown (EPA, 1986; Silva et al�, 1993).

OP compounds are more biodegradable than theorganochloride, and therefore, often used to combatinsects. According to the Bayer technical informationmanual, methyl parathion is the single productrecommended officially for the control of the insect“broca” ((XWLQRERWKUXV� EUDVLOLHQVLV) in the State ofParaná, therefore, being extensively used inagriculture. It is active against the main plagues thatattack cotton, garlic, rice, potato, coffee, onion,citrus, bean, tobacco, corn, soybean, tomato andwheat cultures. Due to of rains or soil drainage, itoften contaminates water bodies, affecting non targetorganisms such as fish. This is the reason why thisproduct was chosen for this study.Acute toxicity tests in fish are required by theBrazilian legislation that regulates and classifiesthe pesticides used in Brazil. Despite %UDFK\GDQLRUHULR being a species thoroughly used in thesebioassays, due to its easy maintenance inlaboratory conditions, it is not a native species ofthe Brazilian ichthyofauna. However, 0HW\QQLVURRVHYHOWL is a native species from Brazilian rivers,where OPs are often used, and therefore a possiblevictim of their effects in the natural environment.Thus, our purpose was to use the morphology ofthe gills of 0�� URRVHYHOWL as indicators of theeffects of lethal and sublethal effects of theorganophosphorous compound methyl parathion.

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0HW\QQLV� URRVHYHOWL Eigenmann, 1915(Serrasalmidae) was obtained from fish farms ofthe State of São Paulo. Fishes of total length 4.0 -5.0 cm, mean weight of 3.0 g, were maintained in30 litres aquaria, protected by green shields toavoid external stimuli of the fish (Fanta, 1995) andto improve their adaptation. The fishes wereacclimated for two weeks to laboratory conditions(temperature 25°C; pH 7.0; photoperiod 8 hourslight/16 hours dark). They were fed once a day,one hour after the start of the light period withcommercial pellets.

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Mentox 600 CE ® is an insecticide that contains600 g/l of the active principle theorganophosphorous methyl parathion. It is acolorless liquid, and has a strong scent. The

Effects of the Organophosphorous Methyl Parathion on the Branchial Epithelium


363

product used in the experiments was supplied bythe Mentox Industry.

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Experiments were carried out in 30 liters aquaria,kept in the same conditions as during acclimation.Three tests were carried out: (i) control – 10 fisheswere maintained in water without pollutant; (ii) and(iii) experimental – 10 fishes were kept in each of theaquaria contaminated with OP. Environmentalconditions were the same in all three aquaria, withthe exception of the contaminant.0HW\QQLV�URRVHYHOWL�was contaminated with 7µl ofOP/litre and 1µl of OP/litre. These concentrationswere lower than the LC50 for %UDFK\GDQLR� UHULR(8.5µl OP/litre) in acute tests (96 hours). Sevenppm concentration caused the smallest number ofdead individuals (1 fish in 96 hours of exposition).On the other hand, the concentration of 1 ppm waschosen because it was the smallest concentrationused in toxicity tests with %��UHULR. The results ofthese tests obtained by laboratories validated byIBAMA (Brazilian Environmental Institute), areused for the establishment the toxicity degree ofthe product.

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A qualitative observation of some behaviouralitems was done with all fishes through on theexperiment, mainly respiration, activity, balance,grouping, prefered region in the experimentalaquaria. No numerical evaluation was done.

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Gills of 0��URRVHYHOWL were sampled for histologyand for scanning electron microscopy. The 2nd

right branchial arch was dissected out at 1, 4 and8h after exposure to 7 ppm OP. From fishescontaminated with 1 ppm, the second rightbranchial arch was collected at 1, 4, 8, 24, 48, 72,and 96h after T0. At the same times, gills fromcontrol fish of both experiments were alsoextracted.For histology, the gills were fixed in Bouin for 8h,routinely prepared, and the sections stained withhematoxylin�and eosin (HE) (Culling et al�, 1985).For scanning electron microscopy, 2nd leftbranchial arch was fixed in glutaraldehyde 2.5%,diluted in 0.2 M Cacodilate buffer pH 7.4 at 4 oC,

overnight. The samples were washed in 0.5 MCacodylate, pH 7.4, contrasted in 4% osmiumtetroxide diluted in 0.2 M Cacodilate, pH 7.4 for1h at 4 oC. The critical point was obtained in aBalzers CPD 030, and the metalization was donewith gold in a Balzers SCD 050. Preparations wereobserved in a Jeol JSM 5300 scanning electronmicroscope.

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Gills of 0HW\QQLV� URRVHYHOWL had 4 pairs ofcartilagenous branchial arches. Gill filaments orprimary lamellae, were lined by a stratifiedsquamous epithelium. At the epithelial surface, thepolygonal cells showed concentric microridges. Theapical portion of chloride cells was rarely observedamong the epithelial cells of the filament (Fig. 1).Respiratory or secondary lamellae were leaf likestructures in an oblique display to the primarylamellae. They were lined by a smooth squamousepithelium, without microridges at the cell surfaces,and with a prominent nucleus (Figures 1 and 2).There was a clear transition from the branchialfilament to the respiratory lamella (Fig. 1).Respiratory lamellae had a thin epithelium, sustainedby pillar cells that surround blood spaces (Fig. 2).

)LJXUH� � - SEM of the region of insertion of tworespiratory lamellae (L) in the branchial filament (F).Notice the absence of microridges at the respiratoryepithelium and their presence at the primary lamella(arrow head). The arrow indicates a prominent nucleus(3500x). In the detail, the surface of epithelial cells in thefilament, and a chloride cell apex (white arrow) (7000x).



364

)LJXUH� � - Respiratory lamellae with squamous cells(arrow), pillar cells (arrow head), and erythrocites (H).The double arrow indicates a prominent nucleus.(1000x).

)LJXUH� �� - Altered respiratory lamellae. Notice thecorrugated epithelium and the disorganization of thelamellar structure (7ppm–1h) (1000x).

)LJXUH� � - Epithelial detachment (arrow head) in fishexposed to OP (7ppm–4h) (1000x).

)LJXUH��- Hyperplasia (arrow head). Notice the coveringof the respiratory lamellae surface (7ppm–8h) (1000x).

)LJXUH� �� - Alterations in the cell morphology of fishexposed to 1ppm of OP, for 1h (1000x).

)LJXUH��- Epithelium wrinkling of respiratory lamellaewas observed after exposure to 1ppm OP, for 24h(1000x).



365

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The effect of methyl parathion on the branchialepithelium was quite drastic: structural changes ofthe gill lamellae organization, epithelialdetachment, necrosis, hyperplasia, loss of themicroridges, altered cellular morphology. Thesefeatures were observed in both concentrations.

Bioassay 7ppm

7LPH� RI� H[SRVXUH� ±� �K� Respiratory lamellae andinterlamellar epithelium disorganization wasobserved. In addition, epithelium wrinkling of therespiratory lamellae was noticed (Fig. 3).

7LPH� RI� H[SRVXUH� ±� �K� Epithelial detachment inthe respiratory lamellae was observed (Fig. 4).

7LPH� RI� H[SRVXUH� ±� �K� Hyperplasia was quitefrequent, and cellular proliferation ocurred mainlyon the surface of the respiratory lamellae (Fig. 5),that tended to fuse.

Bioassay 1ppm

Even at the concentration of 1 ppm, methylparathion caused changes in the branchialepithelium of 0��URRVHYHOWL.

7LPH�RI�H[SRVXUH�±��K. All branchial tissues werealtered. The interlamellar epithelium becameirregular, the surface corregated, and the cells thin,with flat nucleus. The shape of pillar cells wasaltered, and consequently the size and shape ofblood spaces. The shape of erytrocytes becameirregular, crenated with disform nucleus (Fig. 6).

7LPH� RI� H[SRVXUH� ±� �K. Degeneration of lamellarand interlamellar cells was observed. The bloodspaces collapsed and the erytrocytes were weaklystained by eosine.

7LPH� RI� H[SRVXUH� ±� �K. The respiratory lamellaewere quite narrowed with a significant flatteningof the squamous cells nucleus. The cytoplasm oferytrocytes was weakly stained by eosine.7LPH�RI� H[SRVXUH� ±� ��K� Epithelium wrinkling ofrespiratory lamellae, cellular degeneration andcollapse of the blood spaces were common.Disorganization and decrease in the amount of

microridges at the epithelial cells surface of theprimary lamellae, as well as generalized wrinklingwas observed (Figures 7 and 8).

7LPH� RI� H[SRVXUH� ±� ��K� Intense tissuedegeneration and disorganization occurred, withconsequent loss of blood spaces. Wrinkled,irregular lamellae, with discontinuous aspect,contrasted with their aspect in the controlbranchial epithelium (Fig. 9).

7LPH�RI� H[SRVXUH� ±� ��K. At this time, epitheliumdetachment of respiratory lamellae was observed.However, the most evident alteration was observedat the surface of the filament epithelium, wheremicroridges were substituted by cell membranefolds (Figures 10 and 11).

7LPH� RI� H[SRVXUH� ±� �� K� The effects of the OPwere similar to those observed after 72h. Thedetachment of the lamellar epithelium wasintensified. At the surface of the filament cellsshowed changes in the shape of the microridgesthat either became punctiform or disappeared insome areas, giving place to a fold that covered thewhole extension of the cells (Figures 12 and 13).



366

)LJXUH� � - Corrugation of respiratory lamellaeepithelium. Notice the alterations in the microridges inthe filament epithelium (arrow head). (1ppm–24h)(SEM 2000x).

)LJXUH��- Corrugation persists after 48 h in 1ppm OP.Notice the reappearance of microridges in the centralpart of epithelial cells of the filament (arrow head)(SEM 2000x).

)LJXUH�� - Detachment of the epithelium in 1ppm OP,after 72 h (1000x).

)LJXUH�� - Corrugation in the branchial filament (SEM2000x). In the detail, folds of the cell membrane(1ppm–72h).

)LJXUH�� - Intensive detachment (arrow head) and celldegeneration (1ppm–96h) (1000x).

)LJXUH� �� - Punctiform or absent microridges (arrowhead) (1ppm–96h) (SEM 2000x).



367

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Bioassay�7ppm

The concentration of 7 ppm of methyl parathionwas lethal for 0�� URRVHYHOWL��All fish died after 8hours of exposure. After 15 minutes in water withOP, fish lost balance and intensified respiratoryfrequency. After 8 hours, the opercular and jawsmovements connected to respiration stopped.Fishes kept the mouth constantly opened, andswam without rest, subsequently becominglethargic and dying.

Bioassay 1ppm

The concentration of 1ppm of methyl parathionwas sublethal to 0�� URRVHYHOWL, and all fishessurvived for more than 96 hours of exposure.However, some behavioural changes wereobserved in the first 8 hours. All fishes remainedclose to the water surface. They did not formgroups and the decrease of activity was evident.The fishes were most of the time in rest, whiletheir respiratory movements were accelerated.There was recovery and noticeable return to thenormal in the subsequent period until the end ofthe experiment.

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Fish gills were chosen for this investigationbecause they were in direct contact with theaquatic environment and, therefore, could be goodindicators of water quality.Epithelial wrinkling and detachment, hyperplasiaand alterations in the shape of microridges arecommon when the water quality changes. Mallatt(1985) analyzed 130 publications in whichmorphological changes of the respiratoryepithelium due to the action of chemicals wereregistered. Lesions observed less than 10 timeswere not considered in his statistical analysis, andthe wrinkling of the respiratory epithelium isamong these lesions. However, in 0HW\QQLVURRVHYHOWL, this change appeared in the first hourafter contamination, at both concentrations. Thisindicate probably an initial loss of the osmoticbalance in the cells. The regression of thewrinkling afterwards could mean that the

epithelium was still able to make someadjustments after an initial aggression.Mueller et al� (1991), studying the effects of thealuminium on the branchial epithelium, proposedtwo phases to explain the morphological changesin the respiratory system of fishes. An initial phasethat could last from some hours to days, whichwas characterized by the efflux of ions, theinhibition of the active reception of ions andinterference in the gaseous exchanges. In asubsequent phase, there would be a return to thenormal morpho-physiological characteristics.Goss et al� (1992) propose that the squamous cellsof the respiratory epithelium of freshwater fishtook part in ionic regulation mechanisms. In fact,the ionic change Na+/H+, and the enzymesinvolved in this process were in the membrane ofthese cells. It was possible that a sublethal dose ofmethyl parathion changed the acid-base balanceand the absorption of Na+ in this freshwater fish,0��URRVHYHOWL, since the wrinkling reached the cellmembrane.It should also be taken into consideration that inthe case of methyl parathion, one could observetwo subsequent effects. The first effect was aresult of the direct contact of the OP with theepithelial cells, including those of the respiratoryepithelium, through which it entered the organism.Later, the organophosphorous compound, afterhaving been metabolized in the liver, wasdistributed to the tissues through the blood flow inthe form of methyl paraoxon.Epithelial detachment in 0�� URRVHYHOWL was evenobserved in the sublethal concentration of OP. InMallatt’s review (1985), this was the mostcommon branchial change that occurred infreshwater fishes rather than in seawater fish. Thiscould be due to the fact that the first ones werehyperosmotic in relation to the environment,facilitating the influx of water through theepithelium lesion, increasing the volume in theoedema, and consequently the detachment.Abel (1976) repeated this process as being adecrease of the superficial area of the gills whatwas necessary to maintain the internal osmoticsurrouding regarding the functional loss of theepithelial cells. Nowak (1992) found that therespiratory epithelium detachment resultedin the increase of the diffusion distance, affectingthe gaseous exchanges. This phenomenonhas also been described in another type ofenvironmental contamination such as inacid waters (Kawall, 1993), heavy metals



368

(Oliveira Ribeiro et al�, 1994) and salinity(Luvizotto, 1994; Fanta et al�, 1995).In 0��URRVHYHOWL, the hyperplasia was subsequent tothe process of epithelial detachment, whichsuggested the former was a consequence of thelatter. The hyperplasia was characterized bycellular proliferation in the interlamellar region ofthe repiratory lamellae, decreasing the surface areaand making gaseous exchanges more difficult.Magor (1988) repeated that these cells stemedfrom the epithelium of the filament in theinterlamellar space and could act as a barrierimpeding the diffusion of harmful substances tothe blood of the fish. Khan and Kiceniuk (1984),studying the histopathological effects of raw oil onfishes, found that hyperplasia, together with themucus secretion, protected the gills against futuredamages caused by intoxicants. This alteration wasextremely intense after contamination with heavymetals (Oliveira Ribeiro et al�, 1994).The branchial responses could be good to hinderthe entrance of intoxicants in the blood flow, butthey have the undesirable effect of decreasing theoxigenation of tissues, generating a barrier togaseous exchanges (Mallatt, 1985). This breathingdifficulty was confirmed by the behaviouralresponse of 0�� URRVHYHOWL, because when exposedto the 7ppm lethal dose, the animals had strongbreathing difficulties. It is possible, therefore, totrace a parallel among the hyperplasia and thebehavioural effects of the intoxicated animals.Nowak (1992) did not detect any significantdifference in the diameter of the blood spacessurrounded by the pillar cells in fish exposed to theendosulfan organochloride. However, somestudies report damages in the pillar cells caused bypollutants (Schimid and Mann, 1961; Bettex-Galland and Hughes, 1973; Abel, 1976). Suchcellular damages are often associated to highdoses in which the animals are close to death(Mallatt, 1985).However, in 0�� URRVHYHOWL a different result wasobtained after contamination with OP. At the firsthour of exposition to the sublethal concentration,the whole structure of the respiratory lamella of 0�URRVHYHOWL��including the shape of pillar cells, wasaltered. This collapse of the pillar cells íprogressive along the 96 hours of the experiment íwas followed by a loss of shape of the erytrocytes,what can indicate osmotic and ionic alterations. The action of methyl parathion causes enzymaticinhibition, blocking the acetylcholinesterasis andother enzymes. It is not the methyl parathion that

acts but the methyl paraoxon, that results fromenzymatic oxidation mainly in the hepatocytes.The cholinesterase inactivation by methylparaoxon is taken, chemically, as aphosphorilation, leaving the endogenousacetylcholine free (Barberá, 1976). Bettex-Gallandand Hughes (1973), suggest that the contraction ofpillar cells is facilitated by the acetylcholine in theblood.Morphologic alterations of the pillar cells can haveseveral secondary consequences. These cellscontrol the blood pressure of the fish, and changesin the blood pressure and flow can affect thenumber of irrigated lamellae, the distribution ofthe blood within the lamellae, the permeability ofthe branchial epithelium and, as a consequence,the osmorregulatory and gaseous exchangemechanisms (Randall, 1982), causing severalphysiological disorders.The deformation of erytrocytes was obvious, andhas possibly reduced the capacity of oxygentransport, consequently causing a certain level ofhypoxia. Consequently, the fish tries tocompensate the lower levels of oxygen in its tissueby an increase of the respiratory frequency, as wasobserved in 0�� URRVHYHOWL. This is observed notonly after intoxication with chemicals, but alwayswhen there is a change in the respiratory lamellae,caused by any environmental changes (Fanta-Feofiloff et al�, 1986; Fanta et al�, 1989; 1995).The microridges are structures commonly foundon the branchial surface of the fish (Eiras-Stofellaet al�, 2001). Their location and functions,however, generate discussions. Datta Munshi andHughes (1986) studied the breathing surface of thegills of $QDEDV�WHVWXGLQHXV and found microridgesin the epithelial cells of the respiratory lamella.Hossler et al� (1986) showed a branchial structuresimilar to the one found in 0HW\QQLV� URRVHYHOWL�through ultrastructural studies. The microridges,called microfolds, are present in the arch andbranchial filaments of 0RURQH� VD[DWLOLV, but theyare absent in the respiratory lamellae that arecovered by squamous cells with smooth surface.The authors remind that the presence ofmicroridges in the epithelium of the respiratorylamellae has been related to the mucus anchor inorder to protect this epithelium against microbialor solid agents in suspension. But they suggest thatthe absence of such structures in this epitheliumcan be good to reduce the thickness of theblood/water barrier, what facilitates the gaseousexchanges.



369

Hughes (1979) proposes different functions for themicroridges that were found on the surface of therespiratory lamellae of trout�� 6DOPR� JDLUGQHUL,such as the increase of breathing surface, thepossibility of gaseous exchanges and theproduction of microturbulence that would facilitatethe reception of O2. A possible function, however,would be the occupation of the space among themicroridges by mucus, therefore promoting a flatsurface for the flow of water on top of theepithelium of the gills.The microridges in 0�� URRVHYHOWL areconcentrically arrangement in the gill arches andbranchial filaments. However, under the action ofthe sublethal concentration of 1ppm OP, aprogressive alteration was observed in the shape ofthe microridges, that disappeared in some cellsafter 96h of exposition.Mallatt et al� (1985) showed similar results whenthey investigated the way of action of a specificpoison in the branchial system of lamprey larvae.The authors describe both rare microridges withvaried length and forms and alterations in thecontact border among underlying cells, thatbecame less evident, after treatment with sublethaldoses. The alarming fact in this result is that thesubstance that was used by the authors was apoison that is specific against the tested organism.If these alterations on the surface of the gillsreflect the way of toxic action of the poison, onecan assume that 1ppm methyl parathion, evenbeing a sublethal dose, was extremely toxic for 0�URRVHYHOWL.Sprage (1973) defines bioassay as a test in whichthe quality or intensity of a substance isdetermined by the reaction of an organism exposedto this substance. Some questions, such as "Is thatsubstance toxic?” or still "How toxic is thatsubstance?”, can be answered by toxicity tests.However, the question "How do these substancesexercise their toxic effects?” cannot be answeredby these tests because they are only based on themortality caused by these substances. Using sometools, such as the cellular biology, the fragility ofthe use of the single concentration in that 50% ofthe individuals died (CL 50) as a safe limit for thepopulation, can be noticed. Although the OPmethyl parathion has not caused death of any fishafter 96h, it generated morphologic alterations inthe respiratory system of these animals, thatcertainly have ecological consequences.In the present study, the fish were exposed tocontrolled conditions in laboratory bioassay.

However, it is possible to extrapolate the resultsfor natural environmental conditions. Firstly,because some morphologic alterations wereregistered after a short period of exposition (1h),what can be considered enough time to reachconfined fish to some restrict places, for example,small lakes. In addition, the fish did not exhibitescape reaction when exposed to the sublethalconcentration. Secondly, the used doses (1ppmand 7ppm) are insignificant if compared to thedoses recommended for crops. In order to reducethe plague in the culture of tomato, for example,110ml is recommended for each 100 liters ofwater, or 1,100ppm of the organophosphorouscompound! Of course, this volume will be quitediluted if it contaminates a waterbody. Even so,reflection on the difference among theconcentrations needs to be addressed.A third point refers to how to use the product andthe final label on the packagings. The product canbe applied by costal vehicles, bulldozers orairplanes. The manual of the product alerts that,when using airplanes, the wind should be calm orweaker than 8 km/h, in order to avoid thecontamination of close atmosphere. It also warnsthe consumers not to wash the packagings or thetools either in rivers or other waterbodies. Most ofour farmers are not preparared to understand thesewarnings, therefore ignoring the cares with theirown health (Bull and Hathaway, 1986).The generalized use of %UDFK\GDQLR� UHULR intoxicity tests all over the world, but also in Brazil,is also debatable, once this species does not belongto the Brazilian ichthyofauna. This problem seemsalready solved in developed countries. In themanual of U.S. Environmental Protection Agency,we found out that some data regarding the effectsof the methyl parathion on aquatic organisms werenot used, because such studies were carried outwith non-resident species in the USA (EPA, 1986).Sprage (1973) is more demanding when notrecommending the use of 3LPHSKDOHV�SURPHODV inlaboratories of a certain area of the USA, becausethis species is non-native of this area.The great difference of sensitivity to methylparathion, by %UDFK\GDQLR� UHULR and 0HW\QQLVURRVHYHOWL, reinforces the need of toxicity testswith native species for the establishment of “safeconcentrations”. On the other hand our resultsshow also that further studies on the action ofpesticides� should be developed, since, even in“safe doses”, they can be harmful to vital organs.This, on the other hand, will impair the state of



370

health of individuals or a whole population in acertain region, affecting potentially the wholeecosystem.

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The authors are grateful to CAPES/Brazil forfunds, supporting the Master Course inMorphology, Departament of Cellular Biology,Universidade Federal do Paraná, and a scholarshipto Marcelo Rubens Machado during the course ofhis study.

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As brânquias são estruturas vitais para peixes, poissão o principal local de trocas gasosas, assim comoparcialmente responsáveis pela osmorregulação,pelo equilíbrio ácido-básico, pela excreção decompostos nitrogenados e pela gustação.Compostos químicos na água podem alterar amorfologia das células branquiais de peixes, quese constituem, dessa forma, em um útil modelopara estudos de impacto ambiental. Visandoinvestigar os efeitos de um compostoorganofosforado, o metil paration, em brânquiasde peixes, exemplares de 0HW\QQLV� URRVHYHOWLforam expostos a doses letais (7ppm) e subletais(1ppm) de Mentox 600 CE. Observações emmicroscopia óptica e eletrônica de varreduraevidenciaram enrugamento do epitélio branquial,seguido por descolamento e hiperplasia.Externamente, os filamentos branquiaisapresentaram desaparecimento gradual dasmicrossaliências. Os resultados mostram que,mesmo em doses subletais, o organofosforadopoderá levar o animal a sofrer conseqüênciassecundárias decorrentes das alterações dasuperfície de trocas gasosas e iônicas.

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Received: August 17, 2001;Revised: January 03, 2002;

Accepted: September 27, 2002.

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