1. We test the hypothesis that the physiological tolerance of corixids for saline conditions determines, at least partially, their occurrence along a salinity gradient that encompasses a variety of anionic compositions.
2. Three species of Sigara, S. nigrolineata (fresh waters), S. scripta (hyposaline waters) and S. selecta (mesosaline and hypersaline waters) were selected, and it was predicted that species occurring in the most saline waters would be ion regulation specialists that are more tolerant of higher salinity, while species living in fresh or hyposaline waters would be generalist regulators in relation to different anions, and therefore less tolerant of higher salinity.
3. Niche breadth was quantified using field data for both water conductivity and anionic composition (realised niche). In addition, physiological tolerance in relation to conductivity and anionic composition (fundamental niche) was estimated in laboratory tests using eggs, nymphs and adults of all species.
4. Sigara nigrolineata adults showed the most limited conductivity tolerance and were generalists in relation to different anionic water compositions. Sigara scripta demonstrated tolerance of hyposaline and mesosaline waters and higher survival rates in sulphated waters. Sigara selecta was the only species occupying hypersaline waters that showed a preference for anion chloride.
5. Sigara scripta was the only species coexisting with the other two species. This can be explained by the overlap found in conductivity and anionic composition niche dimensions at the lower (S. nigrolineata) and upper (S. selecta) tolerance limits.
6. Young life-stages of the species showed similar responses but less tolerance of higher salinity than adults; this was the case for eggs of S. nigrolineata and nymphs of S. scripta and S. selecta.
7. Physiological limitations were not found in relation to fresh and less saline waters in S. selecta, so other factors, probably involving biotic interactions, may play an important role in this species.