4.3. Conformers versus regulators—mechanisms of toxicity?
While Na+ gradients are powerful in explaining differences in sensitivity to Cu within a single fish species, no apparent difference in sensitivity to acute exposure is seen between osmoregulating and osmoconforming animals. Under
the assumption that Cu is an osmoregulatory toxicant this is surprising since osmoconformers exhibit no or limited Na+
gradients with respect to the environment. In contrast, osmoregulating fish maintain relatively high Na+ gradients and rely
on Na+ transport. From these considerations, an expectation of osmoconforming organisms in SW to be more tolerant
than osmoregulating fish seems reasonable. However, the available data lends no support to this expectation since organisms fall on the same regression line regardless of osmoregulatory strategy, which prompted us to question the assumption that Cu is an osmoregulatory toxicant in saline environments.