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Long-term toxicity to aquatic invertebrates

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Reference
Endpoint:
long-term toxicity to aquatic invertebrates
Type of information:
read-across based on grouping of substances (category approach)
Adequacy of study:
key study
Justification for type of information:
REPORTING FORMAT FOR THE CATEGORY APPROACH
Please refer also to the read-across statement attached in section 13

1. HYPOTHESIS FOR THE CATEGORY APPROACH (ENDPOINT LEVEL)
The target and the source substances are structurally similar substances that share the common organometallic core structure consisting of a central zinc metal bonded to four alkyldithiophosphate esters (ligands) by coordinate covalent bonds -Zn[(S2P(OR)2]2. Structural variations between the target and the source substances are related only to the alkyl (R) groups of the alkyldithiophosphate ligands. The substances in this category give thus rise to an (identical) common compound Phosphorodithioic acid moiety that can be released by the breakage of ester bonds and dissociation from the Zinc complex to which the organism would be exposed if the target substance was tested in the toxicity studies. Exposure to the parent compounds (non-transformed constituents) and to the counter alkyl alcohols, possibly released by hydrolysis of P-O bonds – non-common compounds – would not influence the prediction of the (eco)toxicological properties because they are considered to have the same biological targets and to cause the same type of effects through a common underlying mechanism due to the same functional groups (zinc cation, phosphorodithioic cation and aliphatic alcohol anionic moieties). The impurities of the target and the source substances are not expected to impact the prediction because they are identical or, if slightly structural different, belong to the same class of compounds with the same functional groups and their percentages are very low.

2. CATEGORY APPROACH JUSTIFICATION (ENDPOINT LEVEL )
The aquatic toxicity of the ZDDP category members is mainly influenced by the molecular weight (depends on the length of alkyl rests), and the water solubility. Numerous studies demonstrate decreasing toxicity with the increased chain length. The water solubility of the source substance (CAS 84605-29-8) is with 2764 mg/L above the one of the target substance. Therefore, this source substance is expected to be more bioavailable than the target substance. Thus, data on CAS 84605-29-8 can serve as worst-case data for the target substance. In general, intrinsic toxicities of substances may be the same, because they have the same functional groups and thus the same mode of action. Therefore, taking into account the similar predicted environmental fate and behaviour, the target substance is considered not to possess significantly higher toxicity potential to aquatic environment then the source substance. Thus, the results of the toxicity studies in aquatic invertebrates available for the source substance are considered to be similar if these studies were conducted with the target substance. However, the target substance should be classified as aquatic toxic with long-lasting effects because its alkyl chains lengths are shorter (< C8) and thus aquatic toxicity potential cannot be ruled out.
Duration:
21 d
Dose descriptor:
IC50
Effect conc.:
> 0.8 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
reproduction
Duration:
21 d
Dose descriptor:
IC50
Effect conc.:
> 0.8 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
growth
Duration:
21 d
Dose descriptor:
IC50
Effect conc.:
> 0.53 mg/L
Nominal / measured:
meas. (not specified)
Conc. based on:
test mat.
Basis for effect:
reproduction
Duration:
21 d
Dose descriptor:
IC50
Effect conc.:
> 0.53 mg/L
Nominal / measured:
meas. (not specified)
Conc. based on:
test mat.
Basis for effect:
growth
Duration:
21 d
Dose descriptor:
NOEC
Effect conc.:
0.4 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
reproduction
Duration:
21 d
Dose descriptor:
NOEC
Effect conc.:
0.8 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
growth
Duration:
21 d
Dose descriptor:
LOEC
Effect conc.:
0.8 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
reproduction
Duration:
21 d
Dose descriptor:
LOEC
Effect conc.:
> 0.8 mg/L
Nominal / measured:
nominal
Conc. based on:
test mat.
Basis for effect:
growth
Details on results:
- Mortality of parent animals: At test completion, there was < 10% mortality observed in all treatments.
- No. of offspring produced per day per female: The mean number of neonates produced were as follows:
Lab Control: 83.2
WAF Control: 87
0.05 (mg/L): 64.9
0.10 (mg/L): 70.2
0.20 (mg/L): 73.7
0.40 (mg/L): 73.2
0.80 (mg/L): 68.9

- Body length and weight of parent animals: The mean length of adult daphnids at test completion ranged from 3.7 to 3.8 mm in controls and from 3.6 to 3.7 mm in test solutions. The mean weight of adult daphnids at test completion ranged from 0.45 to 0.54 mg in controls and from 0.47 to 0.52 mg in test solutions.
- Type and number of morphological abnormalities: No.
- Type and number of behavioural abnormalities: No.
- Time to first brood release or time to hatch: 9 days was the minimum time to first brood for the control as well as all test concentrations.
- Egg development time: NDA
- Brood size: NDA
- Time to sexual maturity: NDA
- Type and magnitude of biochemical changes: NDA
- Other biological observations: NDA
- Effect concentrations exceeding solubility of substance in test medium: No.
Results with reference substance (positive control):
Results with reference substance valid:
The 48 hour LC50 (95% confidence limits) for the reference item test was 0.40 mg Zn2+/L (0.19 to 0.6 mg/L). This value was within the acceptable range of + 2 SD of previous tests conducted and indicates that the test organisms responded normally to the reference item.
Reported statistics and error estimates:
The total number of offspring per parent animal was calculated for each replicate. The means and associated standard deviations are reported for all endpoints, which are calculated based on both the nominal concentrations and the measured concentrations. The mean measured concentrations of the test item were calculated by a time weighted mean. The NOEC and LOEC values for reproductive output, length, and weight were determined by ANOVA. Two sample t-tests were performed comparing the lab and WAF controls for reproduction, weight, and length. There were no significant differences between the controls using analysis of variance and thus all statistical analyses were performed using the WAF control. All IC50 values are presented as “greater than” values since none of the endpoints measured showed a >50% effect. Statistical analyses were performed using the statistical program CETIS.
Table I: Test Item Results

 

Nominal Loading Rate / Measured Concentration (mg/L)

 

Surviving Adults (%)

Mean # Neonates

Mean Length (mm)

Mean Weight (mg)

Lab Control

100

83.2

3.7

0.45

WAF Control

100

87.0

3.8

0.54

0.05 / 0.04

90

64.9

3.7

0.52

0.1 / 0.06

100

70.2

3.7

0.49

0.2 / 0.13

90

73.7

3.7

0.47

0.4 / 0.26

100

73.2

3.6

0.48

0.8 / 0.53

100

68.9

3.7

0.52

Validity criteria fulfilled:
yes
Conclusions:
The 21 day IC50s based on nominal loading rates for length, weight, and neonate production were all >0.8 mg/L. Based on measured test item concentration the IC50s were all >0.53 mg/L. The NOECs were 0.4 mg/L nominal for reproduction and 0.8 mg/L nominal for both daphnid length and daphnid weight. The LOECs were 0.8 mg/L nominal for reproduction and >0.8 mg/L nominal for daphnid length and daphnid weight.
Executive summary:

Introduction:

A study was performed under GLP to assess the effect of the test material on the reproduction of the freshwater invertebrate Daphnia magna over a 21-day period. The method followed that described in the OECD Guidelines for Testing of Chemicals No 211 (1998) "Daphnia magna, Reproduction Test". 

Methods:

Based on the results of an acute range finding test, the definitive static renewal test assessed the effect of the test material on daphnids over a 21 day period by observing survival, growth, and reproduction under controlled conditions and exposure to the following nominal loading rates: 0, 0.05, 0.1, 0.2, 0.4 and 0.8 mg/L prepared from dilutions of a 100 mg/L WAF. The test dispersions were renewed 3 times per week.  Survival, reproduction, and growth were recorded. The Daphnia were fed daily with an algal and YCT suspension.

Results:

The measured concentrations of the test substance differed by greater than 20% from the start of some exposure periods to the end, and therefore time-weighted means were calculated for each of the concentrations. The 21 day IC50s for neonate production, adult daphnid length and adult daphnid weight were > 0.8 mg/L based on nominal loading rates and >0.53 mg/L based on measured concentrations. The NOECs were 0.4 mg/L nominal (0.26 mg/L measured) for reproduction and 0.8 mg/L nominal (0.53 mg/L measured) for both daphnid length and daphnid weight, the latter being the highest level tested. The LOECs were 0.8 mg/L nominal for reproduction and >0.8 mg/L nominal for daphnid length and weight. Measured concentrations were calculated using the time weighted mean method.

Description of key information

Source substance Phosphorodithioic acid, mixed O,O-bis(1,3 -dimethylbutyl and iso-Pr) esters, zinc salts (CAS 84605 -29 -8):

A study was performed under GLP to assess the effect of the test material on the reproduction of the freshwater invertebrate Daphnia magna over a 21-day period. The method followed that described in the OECD Guidelines for Testing of Chemicals No 211 (1998) "Daphnia magna, Reproduction Test".

The measured concentrations of the test substance differed by greater than 20% from the start of some exposure periods to the end, and therefore time-weighted means were calculated for each of the concentrations. The 21 day IC50s for neonate production, adult daphnid length and adult daphnid weight were > 0.8 mg/L based on nominal loading rates and >0.53 mg/L based on measured concentrations. The NOECs were 0.4 mg/L nominal (0.26 mg/L measured) for reproduction and 0.8 mg/L nominal (0.53 mg/L measured) for both daphnid length and daphnid weight, the latter being the highest level tested. The LOECs were 0.8 mg/L nominal for reproduction and >0.8 mg/L nominal for daphnid length and weight. Measured concentrations were calculated using the time weighted mean method.

For CSA the measured reproduction-NOEC was choosen as the lowest value.

Key value for chemical safety assessment

EC10, LC10 or NOEC for freshwater invertebrates:
0.26 mg/L

Additional information