2-ethylhexyl 3-mercaptopropionate

Short-term toxicity to aquatic invertebrates

Link to relevant study record(s)

Reference

Endpoint:

short-term toxicity to aquatic invertebrates

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Justification for type of information:

This selected scenario covers the category approach for which the read-across hypothesis is based on structural similarity. For the REACH information requirement under consideration, the property investigated in studies conducted with different source substances is used to predict the property that would be observed in a study with the target substance if it were to be conducted. Similar properties are observed for the different source substances; this may include absence of effects for every member of the category.
There are expected to be differences in strength of the effects forming a regular pattern. The prediction will be based on a worst-case approach or trend analysis as appropriate for the respective endpoint. The read-across is a category approach based on the hypothesis that the substances in this category share structural similarities with common functional groups. This approach serves to use existing data on aquatic toxicity for substances in this category.
The hypothesis corresponds to Scenario 4 of the RAAF. The substances MMP, BuMP, EHMP, iOMP, iC13MP, ODMP, GDMP, TMPMP, PETMP, and Di-PETMP are esters of a common acid, 3-mercaptopropionic acid (3-MPA). The key functionality and main driver for ecotoxicity of the substances within this category is the presence of free SH-groups. It can be predicted with high confidence that the substances within this category will lead to the same type of effects.
For details, please refer to the category document attached to Iuclid section 13.

Reason / purpose for cross-reference:

read-across: supporting information

Reason / purpose for cross-reference:

read-across source

Test organisms (species):

Daphnia magna

Duration:

48 h

Dose descriptor:

EC50

Effect conc.:

0.44 mg/L

Nominal / measured:

nominal

Conc. based on:

test mat.

Basis for effect:

mobility

Conclusions:

Based on read-across, the 48 h EC50 of EHMP to Daphnia is 0.44 mg/L (nominal).

Description of key information

The toxicity of EHMP to Daphnia magna was estimated by read-across from other mercaptoacetates. The estimated EC50 is 0.44 mg/L.

Key value for chemical safety assessment

Fresh water invertebrates

Fresh water invertebrates

Dose descriptor:

EC50

Effect concentration:

0.44 mg/L

Additional information

No experimental data are available for the target substance EHMP. A short-term toxicity study in Daphnia magna was conducted with the source substance iOMP. A justification for read-across is attached to Iuclid section 13.

A study was performed to assess the acute toxicity of the test material  iOMP to Daphnia magna in accordance with OECD TG 202.

Pre-study solubility work conducted indicated that it was not possible to obtain a testable solution of the test material using traditional methods of preparation e.g. ultrasonication. The highest dissolved test material concentration that could be prepared (by visual examination) was
0.80 mg/l using a preliminary solution in tetrahydrofuran.
A pre-study media preparation trial indicated that a dissolved test material concentration of approximately 6.8 mg/l was obtained from a saturated solution method of preparation indicating this to be the limit of water solubility of this material under test conditions.
Following preliminary range-finding tests, twenty daphnids (2 replicates of 10 animals) were exposed to an aqueous solution of the test material at concentrations of 0.0068, 0.012, 0.022, 0.038, 0.068, 0.12, 0.22, 0.38 and 0.68 mg/l for 48 hours at a temperature of approximately 20°C under semi-static test conditions. The number of immobilised Daphnia were recorded after 24 and 48 hours.
A positive control conducted approximately every six months used potassium dichromate as the reference material. Daphnia magna was exposed to an aqueous solution of the reference material at concentrations of 0.32, 0.56, 1.0, 1.8 and 3.2 mg/l for 48 hours at a temperature of approximately 20°C under static test conditions. Immobilisation and any adverse reactions to exposure were recorded after 3, 24 and 48 hours.

Analysis of the saturated solution at 0 and 24 hours showed measured concentrations of 5.83 and 8.24 mg/l respectively.
Analysis of the freshly prepared media at 0 and 24 hours showed measured concentrations to range from less than the limit of quantitation (LOQ) to 0.811 mg/l. Analysis of the old media at 24 and 48 hours showed measured test concentrations to range from less than the limit of quantitation (LOQ) to 0.437 mg/l. This decline in measured concentration was in-line with the stability analysis which showed the test material to be unstable in the test medium.

The time-weighted mean measured test concentrations were calculated to be: 0.0068, 0.0032, 0.011, 0.017, 0.038, 0.076, 0.14, 0.26 and 0.49 mg/i. The 48-Hour EC50 for the test material to Daphnia magna based on time-weighted mean measured test concentrations was 0.31 mg/l with 95% confidence limits of 0.13 - 13 mg/l.

These values don't represent the real environmental conditions because the test substance is rapidly oxidized by the oxygen content in the aqueous phase. As the transformation products are less toxic than the parent substances (no free –SH groups) regulatory endpoints calculated on the basis of nominal concentrations represent a realistic worst case approach. 

The 48 hEC50 was determined to be 0.44 mg/L based on nominal concentrations.