Two species of amphipods, Rhepoxynius abronius Barnard (Phoxacephalidae) and Eohaustorius estuarius Bosworth (Haustoriidae) commonly used in sediment bioassays were used :to assess the effects of laboratory holding on their sensitivity to the toxicants tributyltin (TBT) and cadmium (Cd). In all tests, the LC,, decreased 2 to 3-fold for animals herd for several weeks in the laboratory versus those recently collected from the field. Additionally, the LD(50) for both toxicants was also 2 to 3 times lower for both species of laboratory-maintained animals. For both species, whole-body lipids declined by a factor of 2 to 3 during the holding period and when the LD(50) for each toxicant was lipid normalized, the critical body burdens became nearly the same. Because TBT is a lipophilic compound, it is expected to partition according to organismal body lipid, however, Cd is known to not partition preferentially to lipid. It is possible that declining lipid concentrations may be the mechanistic process by which body burdens of TBT are Controlled, but for Cd, another mechanism may be more important. It was concluded from these results that decreasing whole-body lipid content may be an indicator of declining animal health (and increased sensitivity to toxicants) but the actual mechanistic control over the toxicity response remains to be elucidated. It is assumed that the standard 10-day test, conducted with recently caught animals, probably has no impact on organism health, but that caution should be used when interpreting bioassay results from long-term studies that use animals which are not maintained in optimum condition. This study also suggests that variability in response to toxicants may occur over the natural, seasonal cycle of physiological variation that occurs in populations.
