We are discussing here the question why the Baikalian endemic and the typical Palaearctic faunas do not intermix in light of differences between species of those faunas regarding their abilities to deal with humic substances (HS) in the water. Water of Lake Baikal contains less humic substances than adjacent water bodies. HS cause stress in aquatic organisms generally resulting in an upregulation of cellular stress response systems. However, we found that amphipods endemic to Lake Baikal respond differently to HS in the water than a non-Baikal amphipod species. Whereas this species showed upregulation of stress-response related genes upon HS exposure transcript levels of those genes were lowered in Baikalian endemic amphipods. We see it as conceivable that as a consequence of down-regulation of stress-response related genes by HS those species are more susceptible to other stressors and therefore less able to survive in waters with high HS content.