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Sediment toxicity testing with very hydrophobic organic chemicals (VHOCs) is challenging because of the chemicals' low aqueous solubilities and slow kinetics. The present study presents the results of experiments investigating whether the standard exposure duration of 28 days with benthic invertebrates is sufficient for VHOCs; above which concentrations in sediment VHOCs are present as "free phase," that is, crystals or non‐aqueous‐phase liquids (NAPLs); and whether it is possible to discriminate between actual VHOC toxicity and physical effects caused by NAPLs through fouling of the test organisms. The results suggest that the standard sediment toxicity test duration is sufficient for obtaining steady‐state VHOC concentrations in Hyalella azteca and Lumbriculus variegatus, provided that spiking and equilibration are performed properly (i.e., no free phase present). Under these conditions, transient (days 3–20) peak‐shaped toxicokinetics were observed, with steady‐state concentrations reached at approximately 28 days. The concentration above which NAPLs are present, the so‐called critical separate phase concentration (CSPC), was determined for several VHOCs by modeling and two experimental methods. Modeling resulted in unrealistic and variable data and therefore should be applied with caution. Experimentally determining CSPCs was successful and yielded values of approximately 1000 (400–2000) mg/kg dry weight, depending on the chemical. Finally, it was demonstrated that distinguishing actual toxicity from physical effects is possible by applying a well‐considered test setup, combining toxicity tests with multiple invertebrates (including Lumbriculus, which serves as a negative control for fouling); a broad test concentration range, preferably up to at least 30 000 mg/kg; and passive sampling to localize the CSPC. Applying this setup, false‐positive effects due to fouling, as well as false‐negative results due to testing at too low concentrations (trying to stay below the CSPC), can be avoided. Environ Toxicol Chem 2024;43:1728–1739. © 2023 The Authors. Environmental Toxicology and Chemistry published by Wiley Periodicals LLC on behalf of SETAC. [ABSTRACT FROM AUTHOR]

The freshwater amphipod genus Hyalella Smith, 1874 is widely distributed in the Neotropics, with several biogeographically restricted species and a high cryptic diversity throughout South America. Tens of species of Hyalella have been documented from nearby Brazil and Argentina, but no systematic record of the genus exists for Paraguay. Here we describe two new species of Hyalella: H. mboitui sp. nov. and H. julia sp. nov. from the Ñeembucú wetlands of southwestern Paraguay. Hyalella mboitui sp. nov. and H. julia sp. nov. are characterised by a dorsally smooth body, pigmented eyes, uropod 1 endopod with a curved seta, the dorsal margin of uropod 3 ramus without setae, and uropod 3 peduncle longer than wide and with six setae apically. The two species are distinguished by their diagnostic mouthparts, with a median serrated edge on the lacinia mobilis in H. mboitui sp. nov. and two elongated lateral denticles with a serrated edge in H. julia sp. nov., and by the presence of a pronounced cup for the dactylus on gnathopod 2 in H. mboitui sp. nov. In addition, they show differences in the number of articles on antennae 1 and 2, in the relative length of the pereiopods, and in the numbers and types of setae on their gnathopods and uropods 1-3. Hyalella mboitui sp. nov. and H. julia sp. nov. represent the first taxonomically documented occurrence of Paraguayan freshwater amphipods. These new taxa attest to the largely unmapped species richness of freshwater invertebrates in the Humid Chaco of Paraguay. This potential biodiversity hotspot is currently under threat from land conversion, highlighting the need for more systematic studies and effective conservation of the local invertebrate biodiversity. [ABSTRACT FROM AUTHOR]

The amphipods of the genus Hyalella have many endemic species throughout South America, and these may have poorly studied associated species as epibionts. The aim of the present study was to review the literature on the presence of the protozoon Lagenophrys lenticulata (Kellicott, 1885) on species of Hyalella and contrast the results with the geographical distribution of the reported hosts. The literature records the presence of L. lenticulata on populations of H. azteca from Canada, Mexico and the United States of America, and on populations of H. curvispina from Uruguay. The literature also reports that H. curvispina is widespread throughout southern South America, including Argentina and Chile, and that it would be possible to find the presence of epibionts on other populations of H. curvispina. The corresponding biogeographical and evolutionary topics are discussed. [ABSTRACT FROM AUTHOR]

The freshwater amphipod genus Hyalella Smith, 1874 is widely distributed in the Neotropics, with several biogeographically restricted species and a high cryptic diversity throughout South America. Tens of species of Hyalella have been documented from nearby Brazil and Argentina, but no systematic record of the genus exists for Paraguay. Here we describe two new species of Hyalella : H.mboitui sp. nov. and H.julia sp. nov. from the Ñeembucú wetlands of southwestern Paraguay. Hyalellamboitui sp. nov. and H.julia sp. nov. are characterised by a dorsally smooth body, pigmented eyes, uropod 1 endopod with a curved seta, the dorsal margin of uropod 3 ramus without setae, and uropod 3 peduncle longer than wide and with six setae apically. The two species are distinguished by their diagnostic mouthparts, with a median serrated edge on the lacinia mobilis in H.mboitui sp. nov. and two elongated lateral denticles with a serrated edge in H.julia sp. nov. , and by the presence of a pronounced cup for the dactylus on gnathopod 2 in H.mboitui sp. nov . In addition, they show differences in the number of articles on antennae 1 and 2, in the relative length of the pereiopods, and in the numbers and types of setae on their gnathopods and uropods 1-3. Hyalellamboitui sp. nov. and H.julia sp. nov. represent the first taxonomically documented occurrence of Paraguayan freshwater amphipods. These new taxa attest to the largely unmapped species richness of freshwater invertebrates in the Humid Chaco of Paraguay. This potential biodiversity hotspot is currently under threat from land conversion, highlighting the need for more systematic studies and effective conservation of the local invertebrate biodiversity., Competing Interests: The authors have declared that no competing interests exist., (Giovanni Mussini, Nicole D. Stepan, Gersey Vargas.)