2-ethylhexanoic acid, molybdenum salt

Long-term toxicity to aquatic invertebrates

Description of key information

Relevant and reliable data that could be used for the determination of a PNEC for the freshwater environment - using the statistical extrapolation method - were retrieved for 4 different species: the rotifer Brachionus calyciflorus, the cladocerans Daphnia magna and Ceriodaphnia dubia, and the midge Chironomus riparius. Test results were obtained in experiments that followed international accepted guidance, and were based on measured levels of dissolved molybdenum. Values were 193.6 mg Mo/L, 89.5 mg Mo/L, 63.0 mg Mo/L and 121.4 mg Mo/L, respectively.

Relevant and reliable data that could be used for the determination of a PNEC for the marine environment - using the statistical extrapolation method - were retrieved for 2 different species: the copepod Acartia tonsa and the mysid Americamysis bahia. Test results were obtained in experiments that followed international accepted guidance, and were based on measured levels of dissolved molybdenum. Values were 7.96 mg Mo/L and 116 mg Mo/L, respectively. Other reliable studies only reported unbounded NOECs for three other marine species (no effect at the highest concentration testes); these data are only used as supportive information.

 

Additional information

Freshwater compartment

For the rotifer Brachionus calyciflorus the 48h-EC10 value of 193.6 mg Mo/L is retained for the derivation of a predicted no-effect concentration for molybdenum (De Schamphelaere et al, 2008). EC10 values are preferred over NOEC values as the latter are test design-dependent values. In this specific case the 48h-EC10 is even more sensitive than the 48h-NOEC of 244 mg Mo/L.

For Ceriodaphnia dubia two reliable studies were identified: GEI (2009) and De Schamphelaere et al (2008). K1-values were available for two endpoints, with reproduction being more sensitive than survival. The geometric mean of both studies, 63.0 mg Mo/L, was selected for the derivation of a predicted no effect concentration of molybdenum.

For the midge Chironomus riparius the 14d-EC10 value of 121.4 mg Mo/L is retained for the derivation of a PNEC for molybdenum (De Schamphelaere et al, 2008). EC10 values are preferred over NOEC values as the latter are test design-dependent values. In this specific case the 14d-EC10 is more sensitive than the 14d-NOEC of 393 mg Mo/L.

For the cladoceran Daphnia magna there were three studies that provided K1 effects data: GEI (2009), Rodriguez (2007) and De Schamphelaere et al (2008a). Each of these studies reported 21d-EC10 values based on reproduction. The geometric mean of 89.5 mg/L that is derived with these three values (62.8, 105.6, 108 mg Mo/L) is put forward for D. magna for the derivation of a PNEC for molybdenum. 

For the freshwater amphipod Hyalella azteca there was one water-only exposure study that provided K1 effects data (Ziese et al, 2016). Both 42d-EC10 and 42d-NOEC values were reported on various endpoints (mortality, growth, reproduction). EC10 values are preferred over NOEC values as the latter are test design-dependent values, and the lowest 42 d-EC10 was noted for reproduction, and this value of 44.6 mg Mo/L is put forward for H. azteca for the derivation of a PNEC for molybdenum.

 

Marine compartment 

A reliable 20d-EC10 of 7.96 mg Mo/L has been generated for the marine copepod Acartia tonsa (Aquasense, 2009). EC10 values are preferred over NOEC values as the latter are test design-dependent values. In this specific case the 20d-EC10 is markedly lower than the 20d-NOEC of 26 mg Mo/L. The reported EC10 for A. tonsa is used in a weight of evidence approach for the derivation of a marine PNEC for molybdenum. A chronic 28d-growth and reproduction test has been conducted with the marine mysid shrimp Americamysis bahia (Lehman, 2010). No significant effects were noted at the highest exposure concentration level of 116 mg Mo/L. Evaluated endpoints were survival (first and second generation), reproduction (time of first brood, interbrood time, number of young produced per female), and growth (length and dry weight of male and female adult first generation organisms). The unbounded value of 116 mg Mo/L is considered as a reliable and conservative NOEC-value for A. bahia.

Researchers at the North Australian Marine Research Alliance developed several long-term testing methods various marine species, using different inorganic substances as testing material (incl. Mo as molybdate) (Trenfield et al, 2016, van Dam et al, 2016, 2018a).The three species tested were the marine snail Nassarius dorsatus, the barnacle Amphibalanus amphitrite and the hermit crab Coenobita variabilis. No adverse effects were observed up to the highest concentration tested (7.0 – 9.5 mg Mo/L). Data could therefore not be used in an SSD-approach but are taken into account when assessing whether the derived PNECmarine is sufficiently protective for species that are not included in the SSD (reliable supportive information).