AbstractAccumulation of hydrophobic organic xenobiotics is thought to occur by passive diffusion across the respiratory membrane of aquatic organisms. This route has been confirmed with fish. However, aquatic invertebrates tend to remove organic xenobiotics from water much more efficiently than oxygen, based on the relative uptake clearances. Uptake clearance is the rate coefficient that describes the volume of water stripped of analyte per g of organism per h. For the amphipodDiporeiasp., formerly classified asPontoporeia hoyi, ratios of the contaminant uptake clearances to oxygen clearance were essentially constant at 3.9 ± 0.4 (X+SE) for benzoαpyrene (BaP), 3.8 ± 0.3 for hexachlorobiphenyl (HCBP), and 4.2 ± 0.6 for phenanthrene (Phe). Therefore, based on the membrane transport efficiencies of nonpolar xenobiotics (60–80) and oxygen (approximately 63) in fish, and the uptake clearance for organic xenobiotics byDiporeiasp., either xenobiotics are accumulated through routes other than across the respiratory membrane or the accumulation efficiency of oxygen from water is much lower in amphipods than it is in fish. The variability in the uptake clearance for both BaP and HCBP was best described by regressions with the surface area‐to‐volume ratio, whereas the uptake clearance for the more hydrophilic Phe was best described by a total surface area rel
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