Chemosphere 55 (2004) 257–265

www.elsevier.com/locate/chemosphere

Studies on lethal concentrations and toxicity stressof some xenobiotics on aquatic organisms

M. Abul Farah a, Bushra Ateeq a, M. Niamat Ali b, Rubina Sabir a,Waseem Ahmad a,*

a Gene-Tox Laboratory, Division of Genetics, Department of Zoology, Aligarh Muslim University, Aligarh 202002, UP, Indiab P.G. Department of Zoology, University of Kashmir, Srinagar-190006, J & K, India

Received 22 January 2003; received in revised form 3 October 2003; accepted 25 October 2003

Abstract

Three widely used xenobiotics pentachlorophenol (PCP), 2,4-dichlorophenoxyacetic acid (2,4-D) and 2-chloro-2,6-

diethyl-N-(butoxymethyl) acetanilide (Butachlor) are evaluated for acute toxicity and stress behavior on freshwater fish

(Heteropneustes fossilis, Clarias batrachus, Channa punctatus) and mosquito larvae (Culex pipiens fatigans). The

experiment was carried out by medium treatment using intermittent flow-through system. Median lethal concentrations

(LC50) were calculated by probit analysis. The LC50 values and 95% confidence intervals showed variable range for

tested chemicals. Mosquito larvae generally appeared resistant than fish, while H. fossilis was found to be most sen-

sitive. Stress signs in the form of behavioral changes are also observed. Both types of organisms are recommended as

good bioindicator for the risk assessment of aquatic environment due to chemicals tested.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Acute toxicity; LC50; Behavioral changes; PCP; 2,4-D; Butachlor

1. Introduction

Chemicals play an important role in modern day

farm practices. New formulations are introduced on

regular basis. Human destructive influence on the

aquatic environment in the form of sub-lethal pollution

also contributes to chronic stress conditions. Impor-

tant compounds such as (PCP), 2,4-D and Butachlor

belonging to chlorophenoxy and acetanilide groups merit

immediate attention in this context. Together these con-

stitute the highest production figure worldwide (Seiler,

1991; Hill et al., 1997; 2,4-D Fact Sheet PAN, 1997).

The pronounced biocidal activity of PCP, known for

many years, has facilitated its use as fungicide, mollus-

cide, insecticide, herbicide, slimicide and as preservative

*Corresponding author. Tel.: +91-571-2720920; fax: +91-

571-2708336.

E-mail address: faridi2001@rediffmail.com (W. Ahmad).

0045-6535/$ - see front matter � 2003 Elsevier Ltd. All rights reserv

doi:10.1016/j.chemosphere.2003.10.063

(Seiler, 1991; Extoxnet, 1996). The other related com-

pound, 2,4-D, is a common herbicide employed in post-

emergence foliar spray and has additional use in weed

control of wheat, rice, maize and aquatic weeds (2,4-D

Fact Sheet PAN, 1997), while, Butachlor is known for

its pre-emergence selective herbicidal activity and is used

in rice fields of many Asian countries (Sinha et al., 1995).

The massive use of these chemicals has therefore, re-

sulted in their ubiquitous presence in the environment

and more seriously, exposure of people has culminated

in serious toxic symptoms (Klemmer et al., 1980). De-

spite this, no satisfactory information is available on the

toxic stress of these chemicals with special reference to

India.

For such purpose, initial toxicity tests are usually

performed on water and sediments using fish and

invertebrates. Thus standard lethal tests such as the

median lethal concentration (LC50, 96 h) are recom-

mended for preliminary exploration to establish initial

estimates of the toxicity of a substance (Sprague, 1989).

ed.

258 M.A. Farah et al. / Chemosphere 55 (2004) 257–265

The effects of pollutants on fish can be evaluated by

acute and chronic toxicity test (Sprague, 1973; Rand and

Petrocelli, 1985; Van der Merwe et al., 1993; Nussey

et al., 1996). In addition, abnormal behavior if any is

also recommended as the sign of toxicity stress (EPA

Guideline, 1996).

A few studies on median lethal dose of forestated

chemicals have appeared in terrestrial vertebrates: for

example, 50–328 mg/kg for PCP (RTECS, 1980; Renner

et al., 1986), 100–2000 mg/kg for 2,4-D (Gehring and

Betso, 1977; Young, 2000) and 2000 mg/kg for Buta-

chlor (Tomlin, 1994). The situation however, in aquatic

organisms is complicated since completely or partially

dissolved chemicals maintain a constant pressure. PCP,

2,4-D and Butachlor used terrestrially end up in the

water bodies and create such pressure, affecting non-

target aquatic organisms.

This study is aimed at examining the toxicity of these

compounds on some freshwater fish (Heteropneustes

fossilis, Clarias batrachus and Channa punctatus) and the

larval stages of the mosquito (Culex pipiens fatigans) by

determining LC50 values and analysing behavioral

changes due to toxic stress. The study further focussed

on more reliable evaluation of the risk to freshwater

organisms and to strengthen the base line data that

could be used to estimate a comparative sensitivity to

these xenobiotics.

2. Materials and methods

2.1. Chemicals

The herbicides, pentachlorophenol (CAS No: 87-86-

5, 99% purity), 2,4-dichlorophenoxyacetic acid (CAS

No: 94-75-7, 98% purity) and 2-chloro-2,6-diethyl-N-

(butoxymethyl) acetanilide, 50% EC (Butachlor: CAS

No: 23184-66-9, technical grade) were purchased from

Fluka Chemika (Switzerland), Loba Chemie and Shree

Rasayan Udyog (India) respectively.

2.2. Experimental organisms

Fish: Various freshwater fish: H. fossilis (30± 5 g and

18± 2 cm), C. batrachus (35± 5 g, 16± 2 cm) and C.

punctatus (30± 5 g, 15± 2 cm) were procured from local

out let and acclimatized for two weeks to the laboratory

conditions. All the stocks were maintained in polypro-

pylene troughs containing water at 22± 2 �C and the

dissolved oxygen at the constant (5 ppm) level. Specimen

were subjected to a prophylactic treatment by bathing

twice in 0.01% KMnO4 for 30 min at an interval of 24 h

before commencing the routine experiments during

which feeding was stopped. Only healthy fish showing

little or no mortality for two weeks were used in the

experiments.

Mosquito larvae: Fourth instar larval stages of Culex

pipiens fatigans with uniform size were used. All the

stocks were maintained at constant conditions of tem-

perature (25± 2 �C) and humidity (80± 5%).

2.3. Treatment

Since 2,4-D and PCP are not soluble or have low

solubility, a stock solution of the two chemicals was

prepared by dissolving desired amount in ethanol. The

multiple exposures comprised of 10 concentrations in

each chemical (Tables 1 and 2). Replicates on negative

control (water) and solvent control (0.2% ethanol) were

set up simultaneously for comparison. The acute toxicity

test was worked out in medium treatment by intermit-

tent flow-through test by replacing toxicant and the

dilution of water at regular intervals (Adams, 1993). A

group of 10 fish and 100 larvae were exposed in each

concentration. Treatments were given in polypropylene

troughs by dissolving desired quantity of stock solution

in 50 and 10 l of water for fish and mosquito larvae

respectively. Remaining test conditions like temperature

(22± 2 �C), pH of water (6.5–8.5) dissolved oxygen

>60% of saturation and hardness (140–160 mg/l as

CaCO3), etc. were kept constant. All variables and test

concentrations were measured at the beginning and the

termination point of the study.

2.4. Determination of LC50 and toxicity stress

Mortality of test organisms was recorded when

opercular movements stopped. Dead individuals were

removed instantly. Accurate records of mortality counts

were maintained at a regular interval of 6 h till 96 h (fish)

and 48 h (C. p. fatigans). LC50 values were based on the

cumulative mortality observed at the end of a desired

exposure. LC50 values and 95% confidence intervals (CI)

were calculated by probit analysis transformation

method and by plotting graph of percent mortality

(probit value) against log concentrations (Musch, 1996).

Change in behavior related to stress and symptoms were

closely monitored during the course of experiment.

3. Results

3.1. LC50 determination

The relation between various concentrations of PCP,

2,4-D and Butachlor and the percent mortality with

their respective probit values for the test organisms are

shown in Tables 1 and 2. The estimated LC50 values and

confidence intervals are listed in Table 3. Graphic rep-

resentation of the probit transformation for the chemi-

cals under study are presented in Figs. 1–3. No mortality

Table 1

Showing the percent mortality and their respective probit values obtained by acute toxicity testing by various concentrations of PCP, 2,4-D and Butachlor in H. fossilis and

C. batrachus

Test

organism

PCP 2,4-D Butachlor

Conc.

ppm

Log

conc.

% Mortality Probit

value

Conc.

ppm

Log

conc.

% Mortality Probit

value

Conc.

ppm

Log

conc.

% Mortality Probit

value

H. fossilis 0.35 )0.52 0 – 30 1.47 0 – 0.5 )0.30 0 –

0.40 )0.45 3.34 3.12 40 1.60 6.67 3.5 1.0 0 16.67 4.05

0.45 )0.39 10 3.72 50 1.69 23.34 4.26 1.5 0.17 20 4.16

0.50 )0.34 16.67 4.05 60 1.77 23.34 4.26 2.0 0.30 40 4.75

0.55 )0.30 33.34 4.56 70 1.84 36.67 4.67 2.5 0.39 53.34 5.08

0.60 )0.25 43.34 4.82 80 1.90 46.67 4.92 3.0 0.47 63.34 5.33

0.65 )0.22 53.34 5.08 90 1.95 60 5.25 3.5 0.54 73.34 5.61

0.70 )0.18 63.34 5.33 100 2 73.34 5.61 4.0 0.60 80 5.84

0.75 )0.15 80 5.84 110 2.04 83.34 5.95 4.5 0.65 90 6.28

0.80 )0.12 100 – 120 2.07 100 – 5.0 0.69 100 –

C. batrachus 0.35 )0.45 0 – 70 1.84 0 – 1.0 0 0 –

0.40 )0.39 3.34 3.12 80 1.90 6.67 3.52 1.5 0.17 13.34 3.87

0.45 )0.34 10 3.72 90 1.95 10 3.72 2.0 0.30 26.67 4.39

0.50 )0.30 23.34 4.26 100 2 13.34 3.87 2.5 0.39 33.34 4.56

0.55 )0.25 36.67 4.67 110 2.04 26.67 4.39 3.0 0.47 43.34 4.82

0.60 )0.22 46.67 4.92 120 2.07 43.34 4.82 3.5 0.54 53.34 5.08

0.65 )0.18 56.67 5.18 130 2.11 63.34 5.33 4.0 0.60 60 5.25

0.70 )0.15 60 5.25 140 2.14 70 5.70 4.5 0.65 80 5.84

0.75 )0.12 76.67 5.74 150 2.17 83.34 5.90 5.0 0.69 83.34 5.95

0.80 )0.09 100 – 160 2.20 100 – 5.5 0.74 100 –

M.A.Farahet

al./Chem

osphere

55(2004)257–265

259

Table 3

Calculated LC50 values (ppm) and 95% confidence intervals for PCP, 2,4-D and Butachlor in fish and mosquito larvae

Test organism Durations (h) LC50 (95% confidence intervals)

PCP 2,4-D Butachlor

H. fossilis 96 0.58 (0.40–0.70) 81 (50–100) 2.34 (1.5–4.0)

C. batrachus 96 0.64 (0.45–0.75) 122 (100–130) 3.25 (2.5–4.5)

C. punctatus 96 0.77 (0.65–0.90) 107 (90–130) 2.82 (2.0–4.5)

C. p. fatigans 48 98 (60–140) 302 (150–400) 35 (20–70)

Table 2

Showing the percent mortality and their respective probit values obtained by acute toxicity testing by various concentrations of PCP,

2,4-D and Butachlor in C. punctatus and C. p. fatigans

Test

organism

PCP 2,4-D Butachlor

Conc.

ppm

Log

conc.

%

Mortality

Probit

value

Conc.

ppm

Log

conc.

%

Mortality

Probit

value

Conc.

ppm

Log

conc.

%

Mortality

Probit

value

C. punctatus 0.50 )0.30 0 – 60 1.77 0 – 0.5 )0.30 0 –

0.55 )0.25 6.67 3.52 70 1.84 10 3.72 1.0 0 13.34 3.87

0.60 )0.22 16.67 4.05 80 1.90 16.67 4.05 1.5 0.17 26.67 4.39

0.65 )0.18 23.34 4.26 90 1.95 33.34 4.50 2.0 0.30 33.34 4.56

0.70 )0.15 36.67 4.39 100 2 40 4.75 2.5 0.39 43.34 4.82

0.75 )0.12 46.67 4.92 110 2.04 56.67 5.18 3.0 0.47 52 5.05

0.80 )0.09 53.34 5.08 120 2.07 73.34 5.61 3.5 0.54 56.67 5.18

0.85 )0.07 66.67 5.44 130 2.11 80 5.84 4.0 0.60 73.34 5.61

0.90 )0.04 80 5.84 140 2.14 86.67 6.13 4.5 0.65 80 5.84

0.95 )0.02 100 – 150 2.17 100 – 5.0 0.69 100 –

C. p. fatigans 20 1.30 0 – 50 1.69 0 – 5 0.69 0 –

40 1.60 21 4.19 100 2 7 3.52 10 1 20 4.16

60 1.77 25 4.33 150 2.17 15 3.96 20 1.30 37 4.67

80 1.90 25 4.33 200 2.30 25 4.33 30 1.47 43 4.82

100 2 51 5.03 250 2.39 39 4.72 40 1.60 52 5.05

120 2.07 64 5.36 300 2.47 48 4.95 50 1.69 63 5.33

140 2.14 75 5.67 350 2.54 57 5.18 60 1.77 75 5.67

160 2.20 80 5.84 400 2.60 71 5.55 70 1.84 83 5.95

180 2.25 87 6.13 450 2.65 84 5.99 80 1.90 94 6.55

200 2.30 100 – 500 2.69 100 – 90 1.95 100 –

260 M.A. Farah et al. / Chemosphere 55 (2004) 257–265

was observed in any fish or in mosquito larvae in control

group.

The LC50 values estimated inH. fossilis, C. batrachus,

C. punctatus and C. p. fatigans were 0.58, 0.64, 0.77 and

98 ppm for PCP, 81, 122, 107 and 302 ppm for 2,4-D

and 2.34, 3.25, 2.82 and 35 ppm for Butachlor respec-

tively. Concentrations below 0.35 ppm (PCP), 30 ppm

(2,4-D) and 0.5 ppm (Butachlor) showed no mortality in

any fish, whereas, no mortality appeared in mosquito

larvae at or below 20, 50 and 5 ppm of the respective

chemicals. In terms of relative sensitivity, H. fossilis was

found to be more sensitive among the fish and mosquito

larvae comparatively resistant than the fish, as the esti-

mated LC50 values were higher for each chemical.

3.2. Toxic stress and poisoning symptoms in fish

Fish subjected to higher concentrations of chemicals

under observation displayed behavioral changes such

as restlessness, swimming at the surface or abnormal

swimming behavior, vigorous jerks of body, loss of

balance, myotonia and anorexia. The concentrations

between 0.65–0.90 ppm, 110–150 ppm and 4.0–5.0 ppm

of PCP, 2,4-D and Butachlor induced lethality accom-

panied by tremors and coma. Severe poisoning was

found to be terminated into death along with the

observable signs of congestion, breathing difficulties,

edema and hemorrhages of the viscera.

3.3. Toxic stress and poisoning symptoms in mosquito

larvae

Behavioral and the swimming patterns were normal

in control group without any mortality. Concentrations

>LC50 that is 120–200 ppm (PCP), 350–500 ppm (2,4-D)

and 50–90 ppm (Butachlor) showed stress in larval

stages. Respiratory problem was observed in most cases

forcing the larvae to float on the surface in congrega-

0

10

20

30

40

50

60

70

80

90

100

-0.45 -0.39 -0.34 -0.3 -0.25 -0.22 -0.18 -0.15 -0.12 -0.09

Log Concentrations PCP (ppm)

% M

orta

lity

0

1

2

3

4

5

6

7

8

9

10

Prob

it Va

lue

0102030405060708090100

-0.3 -0.35 -0.22 -0.18 -0.15 -0.12 -0.09 -0.07 -0.04 -0.02

Log Concentrations PCP (ppm)

% M

orta

lity

012345678910

Prob

it Va

lue

0

10

20

30

40

50

60

70

80

90

100

-0.52 -0.45 -0.39 -0.34 -0.3 -0.25 -0.22 -0.18 -0.15 -0.12

Log Concentrations PCP (ppm)

% M

orta

lity

0

1

2

3

4

5

6

7

8

9

10

Prob

it Va

lue

0

10

20

30

40

50

60

70

80

90

100

1.3 1.6 1.77 1.9 2 2.07 2.14 2.2 2.25 2.3

Log Concentrations PCP (ppm)

% M

orta

lity

0

1

2

3

4

5

6

7

8

9

10

Prob

it Va

lue

H. fossilis C. batrachus

C. punctatusC. p. fatigans

Fig. 1. Graphic representation of log concentrations versus the probit values of the percent mortality determining LC50 of PCP in the

organisms tested.

M.A. Farah et al. / Chemosphere 55 (2004) 257–265 261

tional form. Affected larvae gradually become inacti-

vated avoiding vertical movements, followed by death

as a consequent.

4. Discussion

Toxicity of chemicals to aquatic organisms has

shown to be effected by age, size and health of the spe-

cies. Physiological parameters like quality, temperature,

pH, dissolved oxygen and turbidity of water, amount

and kind of aquatic vegetation, concentration and for-

mulation of chemical and its exposure also greatly

influence such studies (EPA Pesticide Fact Sheet, 2000;

Young, 2000). Johnson and Finley (1980), found PCP as

highly toxic to many species of fish as evident from the

reported 96-h LC50 in Chinook (68 lg/l), rainbow trout

(52 lg/l), fathead minnow (205 lg/l), channel catfish (68

lg/l) and bluegill sunfish (32 lg/l). Against these, the

present values recorded for the same chemical are 0.58,

0.64 and 0.77 ppm in H. fossilis, C. batrachus and C.

punctatus respectively.

As for the 2,4-D, some formulations are highly toxic

(esters), therefore, LC50 ranges between 1.0 and 100 mg/l

in cutthroat trout depending on the formulations used

(NRCC, 1978). Other LC50 values reported are 1.1 mg/l

for rainbow trout (Tomlin, 1994) and 63.24 mg/l for C.

carpio (Sarikaya and Yilmaz, 2003). The corresponding

LC50 values for the present, range from 81 to 122 ppm

in all the fish studied. LC50 (96-h) values of Butachlor

has been reported by Tomlin (1994). These are 0.52 mg/l

for rainbow trout, 0.44 mg/l for bluegill sunfish, 0.32

mg/l for carp and 0.14 mg/l for channel catfish, in

comparison present value ranges between 2.34 and 3.25

ppm for this chemical showing significant toxicity to

the fish under study and possibly to other organisms as

well.

A review of literature revealed little information on

toxicity of these herbicides in invertebrate group using

mosquito larvae. Aquatic invertebrates do not in

general seem to be very sensitive to 2,4-D and the

toxicity level show wide range )0.1 to >100 ppm (2,4-

D Fact Sheet PAN, 1997). The median tolerance limit

at 48 h in Culex pipiens by alkyl benzene sulfonate

(detergent) was estimated by Lal et al. (1983). In the

present case, LC50 48-h is estimated between 35 and

302 ppm. Toxic effects of heavy metals on Aedes ae-

gypti larvae reported LC50 endpoints as 3.1, 16.5 and

0

10

20

30

40

50

60

70

80

90

100

1.47 1.6 1.69 1.77 1.84 1.9 1.95 2 2.04 2.07

Log Concentrations 2,4-D (ppm)

% M

orta

lity

0

1

2

3

4

5

6

7

8

9

10

Prob

it Va

lue

0

10

20

30

40

50

60

70

80

90

100

1.69 2 2.17 2.3 2.39 2.47 2.54 2.6 2.65 2.69

Log Concnetrations 2,4-D (ppm)

% M

orta

lity

0

1

2

3

4

5

6

7

8

9

10

Prob

it Va

lue

0

10

20

30

40

50

60

70

80

90

100

1.84 1.9 1.95 2 2.04 2.07 2.11 2.14 2.17 2.2

Log Concentrations 2,4-D (ppm)

% M

orta

lity

0

1

2

3

4

5

6

7

8

9

10

Prob

it Va

lue

0

10

20

30

40

50

60

70

80

90

100

1.77 1.84 1.9 1.95 2 2.04 2.07 2.11 2.14 2.17

Log Concentrations 2,4-D (ppm)

% M

orta

lity

0

1

2

3

4

5

6

7

8

9

10

Prob

it Va

lue

H. fossilis C. batrachus

C. punctatus

C. p. fatigans

Fig. 2. Graphic representation of log concentrations versus the probit values of the percent mortality determining LC50 of 2,4-D in the

organisms tested.

262 M.A. Farah et al. / Chemosphere 55 (2004) 257–265

33 ppm for Hg, Cd and Cu respectively (Ryms-Keller

et al., 1998).

Other chemicals like carbaryl, malathion and

ammonium sulfate have also been tried on H. fossilis

(Singh et al., 1984; Banerjee, 1993). LC50 value of car-

baryl (15.08 mg/l, 96 h) in C. batrachus induces pertur-

bations at the level of certain biochemical components

(Sharma, 1999). Similarly few acute toxicity studies in

Channa punctatus are carried out and LC50 values cal-

culated for malathion (4.51 mg/l, 96-h), endosulfan (5.78

mg/l, 96-h), carbaryl (8.71 ppm, 48-h) and phenthoate

(0.473 ppm, 48-h) (Rao et al., 1985; Haider and Inbaraj,

1986).

From a comparative account, H. fossilis seems to be

more sensitive among the fish examined (C. batrachus

and C. punctatus) as the LC50 values are found to be

lower for all the tested chemicals. Comparatively,

mosquito larvae appear generally resistant and their

LC50 values are higher than those in the fish. This

suggests that these chemicals have greater margin of

safety to mosquito larvae in comparison to fish. In a

similar study by Lal et al. (1983), Culex pipiens was

found to be more resistant than other species of aquatic

fauna like snails, slug worms, tadpoles and fish finger-

lings.

Behavioral changes as a result of stress are further

accepted as the most sensitive indication of potential

toxic effects. Various behavioral changes observed dur-

ing the coarse of the study are restlessness, swimming at

the surface or abnormal swimming behavior, vigorous

jerks of body, loss of balance, myotonia and anorexia.

More or less similar changes were reported in common

carp (C. carpio) (Sarikaya and Yilmaz, 2003) and gup-

pies (Peocilia reticulata) by Polat et al. (2002) and Baser

et al. (2003) using various chemicals including 2,4-D.

Ferrando et al. (1991), in their study on the effect of

eight selected organochlorine pesticides on eels (Anguilla

anguilla), while determining 96-h LC50 values also re-

ported behavioral changes like anxiety, disorders in

swimming patterns, loss of balance excessive mucous

secretion and lightening in colour. Sub-lethal concen-

trations of carbaryl in catfish Mystus vittatus, acceler-

ated the swimming activity and increased the frequency

of opercular beats (Arunachalam et al., 1980). In other

aquatic species including mosquito larvae (Culex pipi-

ens) behavioral changes as an index of detergent toxicity

0

10

20

30

40

50

60

70

80

90

100

-0.3 0 0.17 0.3 0.39 0.47 0.54 0.6 0.65 0.69

Log Concentrations Butachlor (ppm)

% M

ort

alit

y

0

1

2

3

4

5

6

7

8

9

10

Pro

bit

Val

ue

0

10

20

30

40

50

60

70

80

90

100

0 0.17 0.3 0.39 0.47 0.54 0.6 0.65 0.69 0.74

Log Concentrations Butachlor (ppm)

% M

ort

alit

y

0

1

2

3

4

5

6

7

8

9

10

Pro

bit

Val

ue

0

10

20

30

40

50

60

70

80

90

100

-0.3 0 0.17 0.3 0.39 0.47 0.54 0.6 0.65 0.69

Log Concentrations Butachlor (ppm)

% M

ort

alit

y

0

1

2

3

4

5

6

7

8

9

10

Pro

bit

Val

ue

0

10

20

30

40

50

60

70

80

90

100

0.69 1 1.3 1.47 1.6 1.69 1.77 1.84 1.9 1.95

Log Concentrations Butachlor (ppm)

% M

ort

alit

y

0

1

2

3

4

5

6

7

8

9

10

Pro

bit

Val

ue

C. p. fatigans

C. batrachus

C. punctatus

H. fossilis

Fig. 3. Graphic representation of log concentrations versus the probit values of the percent mortality determining LC50 of Butachlor in

the organisms tested.

M.A. Farah et al. / Chemosphere 55 (2004) 257–265 263

are known (Lal et al., 1983). The mode of action of all

these chemicals may be markedly different, behavioral

changes are similar to ours.

It is widely accepted that the stress response as a

whole is characterised by physiological changes. These

changes tend to be similar for stressors and could be

as varied as anesthesia, flight, forced swimming, disease

treatments, handling, scale loss, or rapid temperature

change (Wedenmeyer and McLeay, 1981). A study by

Wendelaar Bonga (1997) showed that stressor increases

the permeability of the surface epithelia, including the

gills to water and ions and thus induces hydromineral

disturbances. Therefore, it is good indicator of toxicity

in fish than in mammals, as the fish are exposed to

aquatic pollutants by extensive and delicate respiratory

surface of the gills. The high bioavailability of many

chemicals in water is an additional factor in their sen-

sitivity to stress.

The study showed that PCP and Butachlor induced

the stress well below the LC50 concentrations, while in

case of 2,4-D, the stress response appeared at >LC50

concentrations. Stress response by Butachlor was gen-

erally severe with the possible neurotoxic effect through

inhibition of cholinesterase (Rajyalakshmi et al., 1996).

In contrast the respiratory problem in mosquito larvae

could be due to external factors such as water pH,

mineral composition and ionic calcium levels (Wend-

elaar Bonga, 1997).

The present study on the acute toxicity in aquatic

organisms is expected to help assessment of possible risk

to similar species in natural environment, and as an aid

in determination of water quality criteria for regula-

tory measures. Correlation with acute toxicity testing

of other species for comparative purpose is further

achieved by such studies.

Acknowledgements

The Council of Scientific and Industrial Research

(CSIR), New Delhi, India is profusely acknowledged for

financial help no. 9/112 (339) 2002 EMR-I-SRF; 9/112

(340) 2002 EMR-I-SRF and 13(7740-A)/2002-pool. The

Chairman, Department of Zoology, AMU, Aligarh is

gratefully acknowledged for providing necessary facili-

ties.

264 M.A. Farah et al. / Chemosphere 55 (2004) 257–265

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