Toxicological data for rotifers and cladocerans.
The increase of worldwide population and the need to control pests are some of the factors that have led to the application of agrochemicals on agricultural areas to protect and increase crop production. Nevertheless, these substances are of environmental concern since they can reach water reservoirs and act on non-target organisms. Therefore, different aquatic species have been tested to evaluate their sensitivity to different toxicants, including pesticides, so as to elucidate the secondary effects of these chemicals to estimate “safe levels” in aquatic media. A wide variety of toxicity tests can be found in literature to evaluate the toxicity of xenobiotics in the environment at organismal and sub-organismal levels under different regimes. This chapter focuses on those tests performed with some freshwater invertebrates (cladocerans and rotifers) to study the toxicity of four important classes of pesticides.
The need to provide enough food to the growing worldwide population and control pests are some factors that have led to the application of agrochemicals (pesticides) on agricultural areas to protect and increase the crop production . Despite the advantages offered by pesticides, these substances can turn into an environmental concern since they can leave their action point mainly by surface water runoff and reach water reservoirs, which could alter the aquatic environment and pose a threat to human health [2-4]. The majority of these chemicals have a synthetic basis, and different categories have been established to classify them depending on their chemical structure. Some of the most representative agrochemicals with great ecological impact are organochlorine hydrocarbons (DDT), organophosphates (parathion and diazinon), carbamates (carbaryl and methiocarb) and pyrethroids (deltamethrin), where the first group is characterized by its stability in the environment after being released. Pollution of freshwater ecosystems with these chemicals is well known and has been reported for several regions worldwide and it represents a problem of consideration for the preservation of the aquatic environment [5-8]. All these pesticides act by altering the organism’s nervous system .
In this context, scientists have worked to develop and standardize protocols to evaluate the toxic effects of a wide variety of pollutants on certain living organisms known as “sentinel organisms” or “bioindicators” . Bioassays are toxicity studies that can be performed with organisms that represent an important component of ecosystems and are able to respond to xenobiotics, and therefore, bioassays may be used to predict “safe levels” of toxicants in the environment. Among bioindicators, freshwater invertebrates are used frequently due to their importance as primary consumers of algae and herbivores representing a key link in trophic webs [10-12]. Moreover, some aspects like a) abundance, b) wide distribution, c) maintenance and easy culture in the laboratory, d) genetic stability and e) sensitivity are considered to select test organisms [13, 14].
Standard toxicity tests are usually performed with a single species to assess the toxicity in water samples and different endpoints can be evaluated, such as motility, reproduction and enzymatic inhibition. The endpoint “motility”, usually corresponds to a short-term (acute) toxicity assay and represents the concentration of chemicals that reduces the motility to 50% of the animals after 24 or 48 h exposure and the result is expressed as EC50. This assay also can be interpreted as the lethal concentration for 50 percent of individuals (LC50). For the long-term (chronic) tests, behavioral changes (grazing and filtration rates, phototaxis and survival) and reproduction assays can be conducted. The reproduction assay evaluates the effects on reproduction typically after 21 days of exposure and is represented by EC50. This parameter estimates the concentration that inhibits 50% of reproductive effort [15-19].
Within the chronic test category, ecotoxicologists have used another approach to evaluate toxicity known as “sub-lethal effects tests” by estimating variations in biochemical or physiological components (biomarkers). Their importance is based on their capability to indicate damage to the organism following exposure to concentrations of contaminants that are not acutely toxic. Some examples are the enzymatic inhibition and genomic responses (genotoxicity) that indicate disturbances occurring at the sub-organismal level [20-22].
Measurement of different endpoints can provide valuable toxicological information to derive water quality criteria for the safe release of compounds into aquatic bodies [13, 23]. This chapter focuses mainly on those studies performed with freshwater invertebrates that are representative for comparison purposes according to their availability in literature.
2. Freshwater invertebrates as sentinel organisms
In general terms,
2.1.1. Daphnids as bioindicators
188.8.131.52. Daphnia magna
Another modality of toxicity test corresponds to the behavioral response under toxicant exposure. Martins
Toxic effects of pesticides also have been evaluated considering food availability. Pereira and Golcalves (2007)  evaluated acute and chronic toxicity of methomyl (carbamate) to different daphnid species, including
Sensitivity between same species has been tested by Toumi
184.108.40.206. Ceriodaphnia dubia
Metabolic activation of pesticides via cytochrome P450 (a protein superfamily involved in the metabolism of xenobiotics and endogenous compounds) has been tested by El-Merhibi
220.127.116.11. Daphnia carinata
The Australian native species
Acute and chronic toxicity of chlorpyrifos was evaluated by Zalizniak and Nugegoda (2006)  using three successive daphnid generations. For the lethal toxicity, a 48h LC50 was estimated for parent generation (0.5 µg/L). In long-term toxicity assays (21-day survival), fecundity, time to the first brood and female size were monitored. The number of offspring per female in parent individuals was significantly reduced. The main endpoints altered in the first generation were survival and fecundity, whereas the time to the first brood and an indication of hormesis (response stimulation and inhibition at low and high concentrations, respectively) were evident in the second generation. Moreover, the lowest concentration tested (0.005 µg/L) yielded the lowest number of offspring per female. For the third generation, daphnids showed a remarkable sensitivity at low concentrations of chlorpyrifos (0.025 µg/L).
18.104.22.168. Daphnia galeata
Some researchers have studied the effects of chlorpyrifos using
22.214.171.124. Biomarkers in
In relation to agrochemical toxicity assessment using biomarkers, Guilhermino
A genotoxicity study was conducted by Pereira
An assay to elucidate toxicity mechanisms of carbamates using a biomarker (AChE) was implemented by Jeon
In another study, Toumi
Some digestive enzymes have been used as biomarkers. De Coen
Another aspect of interest in ecotoxicology is that some pesticide metabolites can exhibit more toxicity than their parental compounds as it has been reported previously by Belden and Lydy (2000) . Guilhermino
Within the phylum Rotifera, rotifers of the genus
2.2.1. Brachionus calyciflorus
This researcher group also evaluated chronic toxicity of the organophosphorus methylparathion by feeding the rotifers with
In another study, ingestion rate was proposed as a sub-lethal stress indicator by Juchelka and Snell (1994)  for this species.
2.2.2. Brachionus patulus
In , the effects of different sub-lethal concentrations of DDT under high and low food levels on
2.2.3. Lecane quadridentata
In , a study was conducted to perform three toxicity tests “lethal (48h mortality), sub-lethal (inhibition of AChE activity) and chronic (5 day inhibition of the instantaneous growth rate) assays” using carbaryl and methylparathion. The carbaryl pesticide exhibited the higher chronic toxicity (EC50 2.22 mg/L) but greater lethal and sub-lethal toxicity was registered with the organophosphorus pesticide (9.4 mg/L for both bioassays). Moreover, the growth rate was more sensitive in comparison to the esterase activity and was proposed by the authors as a biomarker to assess the toxicity of anticholinesterase pesticides.
3. Relevance of sensitivity of cladocerans and rotifers against pesticides
As mentioned earlier, freshwater invertebrates tend to be sensitive against pollutants and are key factors to maintain freshwater ecosystem quality, thus, preserving these organisms in their habitat is important to guarantee the entire water reservoir health. In the present chapter, different studies to assess the toxicity of four main classes of pesticides on freshwater invertebrates were reviewed.
According to the available data on literature,
In this context, to obtain a more accurate toxicity estimation of water samples polluted with agrochemicals, different endpoints should be evaluated. For example, Pérez-Legaspi
Additionally, when evaluating toxicity in water samples, some other considerations become important. In natural water bodies, it is likely that aquatic organisms are exposed to different agrochemicals and for longer periods, thus, phenomena such as synergism between pesticides and bioaccumulation (accumulation of substances in an organism) are possible and can aggravate the ecological impact by an increase in their toxicity as the pollutants could move through food chains and reach final consumers including the human being. When considering these two phenomena in ecotoxicological studies a more realistic and representative result can be obtained.
In relation to the water quality criteria for aquatic life protection (table 1), some values seem to be appropriate to protect aquatic organisms (lindane, PCP, DDE), however, in some cases there is no criteria available (methylparathion) and for other agrochemicals the recommended concentration could be not that protective, such as those proposed for malathion, chlorpyrifos, diazinon, cypermethrin, deltamethrin, carbaryl and DDE, as these values are close to the toxicity values registered for some endpoints. Pesticide toxicity on freshwater organisms is evident and the need for continuous generation of ecotoxicological data to protect aquatic life and human health still remains.
|24h LC50 (mortality)|
|48h LC50 (immobilization) (mg/L)||Reproduction|
48h IC50 (AChE)
|Organochloride pesticides||Lindane||1.8p||1.7w||0.34w (16 d)||-||2x10-4**|
0.019+ at pH 7.8 (short term)
0.015+ at pH 7.8
|DDE||-||5.08x10-3 d||-||-||3x10-5 **|
|-||0.012w||8.6x10-3w (7 day)||-|
|Pyrethroids||Deltamethrin||9.4x10-3 g||3.2x10-4 g,i||-||5.8x10-5 i||1x10-7**|
|8.86x10-3 g||6.3x10-4 g,i||-||1.8x10-5 i|
|8.4x10-4 h||6x10-5 h||3.47x10-5 h (8 day)||-|
|9.78x10-5 h (8 day)|
|Organophosphorus pesticides||Diazinon||8.6x10-4 a||1.1x10-3w||2x10-4w (21 day)||-||1x10-5**|
|31c||31w||11w (2 day)||-|
|4.48x10-4 o||7.12x10-3 n||-||5.2 x10-3 o|
|12c||12w||0.36w (2 day)|
|-||9.49r||6.6r (5 day)||9.4r(45 min)|
|Malathion||4.08x10-3 o||1.6x10-3w||3.6x10-4w (16 day)||5.2 x10-3 o||5x10-5**|
I would like to thank to Dr. Roberto Rico-Martínez and Dr. Gustavo E. Santos-Medrano for sharing their points of view during the elaboration of the manuscript and for their technical assistance at the laboratory to get the pictures of