Energy resource reallocation in Daphnia schodleri (Anomopoda: Daphniidae) reproduction induced by exposure to hexavalent chromium

Ecotoxicological Effects of Potassium Dichromate on the Tadpole Shrimp Triops longicaudatus

The tadpole shrimp Triops longicaudatus is a freshwater crustacean with fast embryonic and larval development, short life cycle, and high fecundity. They are very active swimmers of a reasonable size, easy to spot and record. Such characteristics make it a promising candidate as an experimental model in ecotoxicology to evaluate the effects of aquatic pollutants, particularly using its locomotor behavior as an endpoint. To evaluate the sensitivity of T. longicaudatus and develop endpoints of interest, we conducted exposure experiments with lethal and sub-lethal concentrations of potassium dichromate, a compound known for its ecotoxicological importance and as a hexavalent chromium source. The endpoints evaluated were mortality, growth, sexual maturation, reproductive output, cholinesterase activity and locomotor/swimming behavior. The 96 h median lethal concentration was found to be 65 µg/L. Furthermore, exposure to potassium dichromate at higher concentrations had a significant negative impact on the growth rate of T. longicaudatus in terms of both body mass and length. The time for maturation was also delayed at higher concentrations. In addition, locomotor behavior allowed for the discrimination of all tested chromium concentrations and the control group and from each other, proving to be the most sensitive endpoint. Overall, the data support the potential of T. longicaudatus as a model for ecotoxicity testing, using apical endpoints with impact at the population level; in particular, results suggest that behavior assessments in this species might be useful for detecting hazardous compounds in environmental monitoring of freshwater ecosystems.

Evaluating the Impact of Individual and Combined Toxicity of Imidacloprid, Cycloxaprid, and Tebuconazole on Daphnia magna

The risks posed by chemicals in the environment are typically assessed on a substance-by-substance basis, often neglecting the effects of mixtures. This may lead to an underestimation of the actual risk. In our study, we investigated the effects of three commonly used pesticides-imidacloprid (IMI), cycloxaprid (CYC), and tebuconazole (TBZ)-both individually and in combination, using various biomarkers to assess their impact on daphnia. Our findings indicated that the order of toxicity, from highest to lowest, was TBZ, IMI, and CYC, as determined by acute toxicity as well as reproduction. The effects of the ITmix (IMI and TBZ) and CTmix (CYC and TBZ) combinations on immobilization and reproduction were evaluated by MIXTOX, revealing a higher risk of immobilization at low concentrations for ITmix. The effect on reproduction differed depending on the ration of pesticides in the mixture, with synergism observed, which may be caused mainly by IMI. However, CTmix showed antagonism for acute toxicity, with the effect on reproduction depending upon the composition of the mixture. The response surface also exhibited a switch between antagonism and synergism. Additionally, the pesticides extended the body length and inhibited the development period. The activities of superoxide dismutase (SOD) and catalase (CAT) content was also significantly induced at different dosage points in both the single and combination groups, indicating changes in the metabolic capabilities of detoxifying enzymes and target site sensitivity. These findings highlight the need for more attention to be focused on the effects of pesticide mixtures.

Predation risk affects the ecotoxicity evaluation of antibiotics: Population growth and antioxidase activity in the ciliate Paramecium jenningsi

Although predation risk exists under natural conditions, its role is usually ignored when evaluating the ecotoxicity of environmental contaminants, and the interaction between predation risk and antibiotic ecotoxicity is not yet clear. To investigate the nonconsumptive effects (NCEs) of predation on the ecotoxicity evaluation of antibiotics, the median lethal concentration (LC₅₀), relative population growth rate (RGR), and activities of three antioxidases were measured in the ciliate Paramecium jenningsi exposed to graded concentrations of the antibiotics nitrofurazone (NFZ) or erythromycin (ERY) in the presence or absence of a predator, i.e., the ciliate Didinium nasutum. The results showed that (1) NCEs significantly reduced the LC₅₀ of NFZ but had no effect on that of ERY; (2) predation pressure alone had no significant effect on the inhibitory rate of the P. jenningsi population, but the interaction with NFZ was synergistic, while that with CRY was additive; (3) the concentrationresponse (i.e., mortality) model for each antibiotic exposure with and without predation pressure differed significantly in the parameter slope; (4) RGRs were significantly reduced by antibiotic exposure or NCEs; only in NFZ-exposed groups did the RGRs decrease linearly with increasing exposure concentration; and (5) the activities of all three antioxidases significantly increased due to NCEs or following exposure to antibiotics. In brief, NCEs were detected in P. jenningsi, and these had additive or synergistic effects on antibiotic ecotoxicity, but their magnitude depended on the properties and exposure concentrations of the antibiotics. Our findings suggest that it is necessary to consider the roles of NCEs in the ecotoxicity evaluation of environmental contaminants.

Feeding behavior of early life stages of the zebrafish Danio rerio is altered by exposure to glyphosate

Glyphosate levels and the transfer of glyphosate across trophic levels have rarely been studied in zooplankton. The food preferences of zebrafish during the first-feeding stage (which is critical for the survival of organisms), were analyzed because of the requirement for live food. Larval survival begins to be affected when glyphosate intake exceeds 0.3666 µg/larvae/day, in the case that only the food is contaminated; if the medium is also contaminated, the effects on survival start from 0.2456 µg/larvae/day. It was shown that glyphosate was more likely to be incorporated through the medium than through the food (zooplankton), which supports the results of previous studies that have ruled out the potential for biomagnification. The bioconcentration factor (BCF) of glyphosate was determined using an ELISA tests specific to measure glyphosate in the fish D. rerio, the rotifers Brachionus calyciflorus and Lecane papuana, and the cladoceran Ceriodaphnia dubia. The experimental design consisted in exposing seven zebrafish adults per replica (four replicates) in three treatments 1, 5, and 10 mg/L of glyphosate for 96 h to obtain bioconcentration factors in the gills, liver, and muscle. These concentrations were selected as potential glyphosate concentrations right after application as double highest reported concentration. Glyphosate levels in zooplankton can represent up to 6.26% of the total weight of rotifers (BFC = 60.35) and in zebrafish adult organs were less than 8 µg/mg of tissue (BCF values < 6). Although glyphosate does not biomagnify, our results suggest that glyphosate affected the dynamics between zooplankton and zebrafish larvae, diminishing survival and feeding rates, given that zooplankton species bioconcentrate glyphosate in large quantities. The BCF values found in this contribution are higher than expected. Glyphosate exposure affected energy metabolism and feeding behavior of zebrafish larvae, which presented high mortality rates at environmentally relevant concentrations.

Combined effects of perchlorate and hexavalent chromium on the survival, growth and reproduction of Daphnia carinata

Perchlorate and hexavalent chromium (Cr(VI)) are common cocontaminants in aquatic environments due to their high water solubility, stability, mobility, and some coapplications. However, few studies have investigated their combined toxicity to organisms. In this work, we studied the acute and chronic toxicities of perchlorate and Cr(VI), alone and in combination, with survival, growth, and reproduction as endpoints using Daphnia carinata as a model organism. For a single contaminant, Cr(VI) was found to be more toxic than perchlorate to D. carinata not only in terms of survival but also in terms of growth and reproduction. In regard to the combined pattern, the interactive effects on survival, growth, and reproduction were mainly additivity, antagonism, and synergism, respectively, suggesting that the interactive response of perchlorate and Cr(VI) is endpoint-specific. Due to significant synergism, over 21 days of observation, the inhibition of 0.1 mg/L perchlorate and 0.2 mg/L Cr(VI) on cumulative offspring per female in the first seven broods reached 63.9 ± 3.6%, suggesting that long-term exposure to perchlorate and Cr(VI) at environmentally relevant concentrations may affect D. carinata reproduction in the natural environment. Our results will be significant for understanding the complicated combined toxicity of perchlorate and Cr to aquatic organisms.

Maternal Responses and Adaptive Changes to Environmental Stress via Chronic Nanomaterial Exposure: Differences in Inter and Transgenerational Interclonal Broods of Daphnia magna

There is increasing recognition that environmental nano-biological interactions in model species, and the resulting effects on progeny, are of paramount importance for nanomaterial (NM) risk assessment. In this work, Daphnia magna F0 mothers were exposed to a range of silver and titanium dioxide NMs. The key biological life history traits (survival, growth and reproduction) of the F1 intergenerations, at the first (F1B1), third (F1B3) and fifth (F1B5) broods, were investigated. Furthermore, the F1 germlines of each of the three broods were investigated over 3 more generations (up to 25 days each) in continuous or removed-from NM exposure, to identify how the length of maternal exposure affects the resulting clonal broods. Our results show how daphnids respond to NM-induced stress, and how the maternal effects show trade-offs between growth, reproduction and survivorship. The F1B1 (and following germline) had the shortest F0 maternal exposure times to the NMs, and thus were the most sensitive showing reduced size and reproductive output. The F1B3 generation had a sub-chronic maternal exposure, whereas the F1B5 generation suffered chronic maternal exposure where (in most cases) the most compensatory adaptive effects were displayed in response to the prolonged NM exposure, including enhanced neonate output and reduced gene expression. Transgenerational responses of multiple germlines showed a direct link with maternal exposure time to 'sub-lethal' effect concentrations of NMs (identified from standard OECDs acute toxicity tests which chronically presented as lethal) including increased survival and production of males in the F1B3 and G1B5 germlines. This information may help to fine-tune environmental risk assessments of NMs and prediction of their impacts on environmental ecology.

Exposure to the herbicide 2,4-D produces different toxic effects in two different phytoplankters: A green microalga (Ankistrodesmus falcatus) and a toxigenic cyanobacterium (Microcystis aeruginosa)

The extensive use of 2,4-dichlorophenoxiacetic acid (2,4-D) in agriculture is an important source of pollution to water and soil. Toxicity of commonly used herbicides to non-target, planktonic photosynthetic organisms has not been described completely yet. Therefore, we determined the effect of subinhibitory 2,4-D concentrations on the Chlorophycean alga Ankistrodesmus falcatus and on a toxigenic strain of the cyanobacterium Microcystis aeruginosa. Population growth, photosynthetic pigments, macromolecular biomarkers (carbohydrates, lipids, and protein), and antioxidant enzymes (catalase [CAT], glutathione peroxidase [GPx], and superoxide dismutase [SOD]) were quantified, and the integrated biomarker response (IBR) was calculated. Scanning electron microscope (SEM) and transmission electron microscope (TEM) observations were also performed. The 96-h median inhibitory concentration (IC₅₀) for 2,4-D was 1353.80 and 71.20mgL^-1 for the alga and the cyanobacterium, respectively. Under 2,4-D stress, both organisms increased pigments and macromolecules concentration, modified the activity of all the evaluated enzymes, and exhibited ultrastructural alterations. M. aeruginosa also increased microcystins production, and A. falcatus showed external morphological alterations. The green alga was tolerant to high concentrations of the herbicide, whereas the cyanobacterium exhibited sensitivity comparable to other phytoplankters. Both organisms were tolerant to comparatively high concentrations of the herbicide; however, negative effects on the assessed biomarkers and cell morphology were significant. Moreover, stimulation of the production of cyanotoxins under chemical stress could increase the risk for the biota in aquatic environments, related to herbicides pollution in eutrophic freshwater ecosystems.

Accumulation, depuration dynamics and effects of dissolved hexavalent chromium in juvenile Japanese medaka (Oryzias latipes)

We previously demonstrated that chronic exposure to hexavalent chromium (Cr(VI)) causes a variety of adverse effects in the Japanese medaka (Oryzias latipes). The present study investigated the transition of acute to chronic effects by assessing the influences of Cr(VI) exposure concentration and exposure duration on Cr accumulation and their effects on fish growth and antioxidant physiology. Juvenile fish were exposed to 0.05, 0.5, 4 or 8mg/L Cr(VI) for 28 days. Endpoints were evaluated on days 3, 7, 14, 21 and 28. In addition, Cr depuration was examined for fish from two exposure groups (0.5 and 8mg/L). Chromium accumulation was rapid initially, then continued at a slower rate till the end of the exposure period without showing signs of reaching a steady state. Depuration patterns differed between the two exposure groups, but both reached 50% in 14 days. The rapid initial accumulation was accompanied by increased lipid peroxidation and elevated activities of antioxidants (e.g., GST, SOD and CAT). Activities of these enzymes had mostly returned to baseline levels by day 7, but there was no evidence of further cellular damage from ROS. Effects on fish length and weight continued to be evident over the 28-day exposure period. Our study suggest that the initial effects of Cr(VI) exposure may not be a good predictor of more-chronic effects in fish as a consequence of an efficient acclimation response by the antioxidant system that limits ROS-mediated toxicity.

Toxicity evaluation of cypermethrin, glyphosate, and malathion, on two indigenous zooplanktonic species

In Aguascalientes, Mexico, there is a special concern about pesticides because of their intensive use on guava production areas, which are located in the vicinity of water reservoirs; thus, non-target organisms could be exposed. Thereafter, the aim of this work was to assess the effect of cypermethrin, Faena® (glyphosate), and malathion, which are the most used pesticides in Aguascalientes' guava production, on the indigenous freshwater species Alona guttata (cladoceran) and Lecane papuana (rotifer). Acute 48-h toxicity tests were carried out, and LC50 values were calculated. Then, five sublethal concentrations (1/80, 1/40, 1/20, 1/10, and 1/5 of the respective LC50) were selected for the chronic assays: (a) intrinsic growth rate analysis in the rotifer and (b) partial life table analysis in the cladoceran. The results of the acute toxicity tests showed that A. guttata was more sensitive to malathion (LC50 = 5.26 × 10-3 mg/L) at concentrations found in natural environments with continuous application on guava fields, whereas L. papuana was more sensitive to Faena® (LC50 = 19.89 mg/L). The somatic growth of A. guttata was inhibited for the chronic exposure to cypermethrin. In addition, cypermethrin and Faena® seemed to exert endocrine disruptive effects on A. guttata. Moreover, malathion chronic exposure significantly decreased the survival of A. guttata. Moreover, L. papuana was affected chronically for the three pesticides.

Fitness Trade-Off Associated With Spinosad Resistance in Frankliniella occidentalis (Thysanoptera: Thripidae)

Frankliniella occidentalis (Pergande) is an economically important pest of agricultural crops. High resistance has been detected in field populations of F. occidentalis against the insecticide spinosad. In this study, we compared life history traits, body sizes, and feeding behaviors (recorded via an electrical penetration graph) of spinosad-susceptible (Ivf03) and spinosad-resistant (NIL-R) near-isogenic lines of F. occidentalis. Life table analysis showed that NIL-R had reduced female longevity and reduced fecundity. The relative fitness of NIL-R (0.43) was less than half that of Ivf03. NIL-R individuals were smaller than Ivf03 individuals, both in body length and body width at every stage. The number and duration of feeding activities were significantly reduced in NIL-R, with the exception of total duration of long-ingestion probes. These results suggest that there is a fitness trade-off associated with spinosad resistance in F. occidentalis, and that the development of resistance in this pest species may be reduced by rotating spinosad with other pesticides lacking cross-resistance.

Maternal-embryonic metabolic and antioxidant response of Chapalichthys pardalis (Teleostei: Goodeidae) induced by exposure to 3,4-dichloroaniline

Chapalichthys pardalis is a viviparous fish, microendemic to the Tocumbo Region in the state of Michoacán, Mexico. Despite the peculiar type of reproduction of goodeid fish and their mother-embryo interaction, the effects on embryos induced by maternal exposure to aquatic xenobiotics are still unknown. The objective of the present work was to determine the maternal-embryonic metabolic and antioxidant response of C. pardalis exposed to 3,4-dichloroaniline (3,4-DCA), a compound considered highly noxious to the environment because of its high toxicity and persistence, which has been used as reference toxicant in toxicological bioassays. We determined the median lethal concentration (LC₅₀, 96 h) and then exposed pregnant females to 3.3, 2.5, and 0.5 mg L^-1 of 3,4-DCA (equivalent to LC₁, LC_0.01, and LC_50/10, respectively) during 21 days. We assessed the activity of antioxidant enzymes like superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), macromolecules content (proteins, lipids, carbohydrates), glucose, and lactate concentration, as well as the oxidative damage, by measuring thiobarbituric acid reactive substances, and protein oxidation. To interpret results, we used the integrated biomarker response (IBRv2). The average LC₅₀ was of 5.18 mg L^-1 (4.8-5.5 mg L^-1; p = 0.05). All females exposed to concentrations of 3.3 and 2.5 mg L^-1 lost 100% of the embryos during the bioassay, whereas those exposed to 0.5 mg L^-1 showed alterations in the antioxidant activity and oxidative damage, being the embryos and the maternal liver the most affected, with IBRv2 values of 10.09 and 9.21, respectively. Damage to macromolecules was greater in embryos and the maternal liver, with IBRv2 of 16.14 and 8.40, respectively. We conclude that exposure to xenobiotics, like 3,4-DCA, in species with a marked maternal-embryonic interaction represents a potential risk for the development and survival of the descendants, thereby, potentially affecting the future of the population.

How do toxic metals affect harmful cyanobacteria? An integrative study with a toxigenic strain of Microcystis aeruginosa exposed to nickel stress

Nickel (Ni) is an essential metal for some organisms, but also a common toxic pollutant released into the water. Toxicity of Ni has not been completely established for cyanobacteria; for this reason, we evaluated the effect of sub-inhibitory Ni concentrations on a toxigenic strain of Microcystis aeruginosa and on microcystins production. Population growth, photosynthetic pigments concentration, biomarkers, including antioxidant enzymes (catalase [CAT], glutathione peroxidase [GPx], and superoxide dismutase [SOD]), as well as macromolecules (proteins, carbohydrates and lipids) were quantified; SEM and TEM observations were also performed. Population growth was affected starting at 3µgL(-1), and at 24µgL(-1) growth was completely inhibited; the 96-h Ni(2+) IC50 was 3.7µgL(-1). Ni exposure increased pigments concentration, augmented all the macromolecules, and increased activities of CAT and GPx; alterations on the internal cell structure were also observed. The integrated biomarker response revealed that Ni(2+) augmented the antioxidant response and the macromolecules content. Ni stress also increased microcystins production. M. aeruginosa was affected by Ni at very low concentrations, even lower than those established as safe limit to protect aquatic biota. Aside from the toxic effects produced in this cyanobacterium, stimulation to produce toxins could potentiate the environmental risks associated with water pollution and eutrophication.

Simulating ocean acidification and CO2 leakages from carbon capture and storage to assess the effects of pH reduction on cladoceran Moina mongolica Daday and its progeny

In order to evaluate the effects of pH reduction in seawater as a result of increasing levels of atmospheric CO2, laboratory-scale experiments simulating the scenarios of ocean acidification (OA) and CO2 leakages of carbon capture and storage (CCS) were performed using the model organism Moina mongolica Daday. The LpH50s calculated in cladoceran toxicity tests showed that M. mongolica exhibited intermediate sensitivity to OA, which varied among species and with ontogeny, when compared with different phyla or classes of marine biota. Survival, reproduction and fecundity of parthenogenetic females were evaluated after 21-day exposures. Results showed that increased acidity significantly reduced the rate of reproduction of M. mongolica resulting in a decreased intrinsic rate of natural increase (rm) across the gradients of pH reduction. The analysis of macromolecule contents in neonates suggested that nutritional status in progeny from all broods were significantly reduced as seawater pH decreased, with increasing magnitude in latter broods, except the contents of protein from two former broods and lipids from the first brood. Our findings clearly showed that for this ecologically and economically important fish species, the negative effects of pH reduction on both "quantity" and "quality" of progeny may have far-reaching implications, providing direct evidence that OA could influence the energetic transfer of marine food web and ecosystem functions in acidified oceans in the future.

Nickel has biochemical, physiological, and structural effects on the green microalga Ankistrodesmus falcatus: An integrative study

In recent years, the release of chemical pollutants to water bodies has increased due to anthropogenic activities. Ni(2+) is an essential metal that causes damage to aquatic biota at high concentrations. Phytoplankton are photosynthesizing microscopic organisms that constitute a fundamental community in aquatic environments because they are primary producers that sustain the aquatic food web. Nickel toxicity has not been characterized in all of the affected levels of biological organization. For this reason, the present study evaluated the toxic effects of nickel on the growth of a primary producer, the green microalga Ankistrodesmus falcatus, and on its biochemical, enzymatic, and structural levels. The IC50 (96h) was determined for Ni(2+). Based on this result, five concentrations were determined for additional tests, in which cell density was evaluated daily. At the end of the assay, pigments and six biomarkers, including antioxidant enzymes (catalase [CAT], glutathione peroxidase [GPx], superoxide dismutase [SOD]), and macromolecules (proteins, carbohydrates and lipids), were quantified; the integrated biomarker response (IBR) was determined also. The microalgae were observed by SEM and TEM. Population growth was affected starting at 7.5 μg L(-1) (0.028 μM), and at 120 μg L(-1) (0.450 μM), growth was inhibited completely; the determined IC50 was 17 μg L(-1). Exposure to nickel reduced the concentration of pigments, decreased the content of all of the macromolecules, inhibited of SOD activity, and increased CAT and GPx activities. The IBR revealed that Ni(2+) increased the antioxidant response and diminished the macromolecules concentration. A. falcatus was affected by nickel at very low concentrations; negative effects were observed at the macromolecular, enzymatic, cytoplasmic, and morphological levels, as well as in population growth. Ni(2+) toxicity could result in environmental impacts with consequences on the entire aquatic community. Current regulations should be revised to protect primary producers.

Linking sub-individual and population level toxicity effects in Daphnia schoedleri (Cladocera: Anomopoda) exposed to sublethal concentrations of the pesticide α-cypermethrin

Synthetic pyrethroids are classified as moderately toxic to mammals and birds; nevertheless, they are highly toxic to non-target aquatic organisms such as fish and zooplankters. Chemical pollutants produce different effects in exposed organisms, ranging from biochemical to population responses. Cladocerans can modify the energy content of their offspring according to the surrounding medium as a way to improve their odds in case they have to cope with stressful conditions at birth. In this study, the effect of a synthetic pesticide on two levels of response in a Daphnia species different from those traditionally used as test organisms was evaluated. With this aim, Daphnia schoedleri neonates (<24 h) were exposed for 21 days to three sublethal concentrations of α-cypermethrin, 0.54, 5.4, and 54 ng L(-1), which correspond to 48-h EC1/100, EC1/10, and EC1, respectively. Effects were measured through a life table analysis for fecundity and survivorship. For effects on progeny, protein, carbohydrates, and lipids were determined and then transformed to caloric content. Biomarkers (BM) were expected to be the most sensitive evaluated response; nevertheless, population parameters such as survivorship and net reproductive rate (R0) were more sensitive since they presented significant differences with respect to controls at the lowest tested concentration. Neonates' caloric content varied during the reproductive period assessed and was negatively correlated to fecundity: as more neonates were born, less energy was provided by the adult females. Macromolecules concentration and caloric content values in cypermethrin-exposed adults were not different from those recorded in the control at the end of exposure time. The results herein presented suggest that stressed daphnids allocate more energy reserves to their offspring, although this strategy can vary depending on the number of reproductive events during the lifecycle, and on the toxicant's concentration. Sub-individual approaches to assess toxicant effects should be accompanied by demographic studies, which support population effect predictions inferred from BMs assessment.

Asymmetric patterns in the cranial skeleton of zebrafish (Danio rerio) exposed to sodium pentachlorophenate at different embryonic developmental stages

Bilaterally symmetric organisms display mirror copies of their structures on both sides of the body, and the development of both sides is regulated by the same set of genes. Environmental variations can directly affect phenotype, and exposure to chemical contaminants at certain stages may modify embryonic development. The pesticide sodium pentachlorophenate (NaPCP) was used at the no-observable-effect concentration (NOEC) to determine the degree of susceptibility of zebrafish (Danio rerio) embryos in different developmentally susceptible windows (zygote, blastula, gastrula, segmentation, pharyngula and larva). Shape variation in the zebrafish viscerocranium and fluctuating asymmetry (FA), which increases in direct proportion to environmental stress, induced by exposure to NaPCP were measured with geometric morphometrics. Procrustes ANOVA was performed to estimate the shape variation around a symmetric consensus that accounted for the following factors: shape variation in individuals (I), variation by sides (S), the Individuals×Sides interaction (I×S), and the stages of exposure to the toxicant (Stages). Factors I, S and IxS accounted for most of the morphological variation (p<0.0001). Extensive deformities throughout the viscerocranium occurred during the window of exposure from gastrula to larva. Embryonic mortality occurred and was dependent on the stage of exposure. The NOEC concentration of NaPCP affected embryonic development in D. rerio and also induced lethal effects in embryos. FA was determined in both unexposed and NaPCP-exposed embryos and was greater in the control than in some exposure windows; besides, no correlation was found between FA and developmental stages, so our results do not support FA as a bioindicator of chemical stress but confirm its value in the study of morphological effects of toxicants.