Fig. 1. Breathing observation experiments. (A) Three 45 min observations were performed, first in standard hypoxia, then in hypoxic H₂S (100 µmol l^–1), and lastly in standard hypoxic conditions again with 22 naïve snails. In between observation periods the snails were returned to eumoxic pondwater for 1 h. A significant increase in the second session vs the first session was seen (ANOVA F_(21,1)=98.5381, P<0.05), and there was no significant difference between sessions 1 and 3 (P>0.05). Session 2 was also significantly different when compared to session 3 (ANOVA F_(21,1)=65.8242, P<0.05). (B) A 45 min observation in standard hypoxia followed by 1 h in eumoxia, and then 45 min submerged in hypoxia (i.e. the `more intense hypoxic challenge') and another 45 min observation period in standard hypoxia. The `more intense hypoxic challenge' significantly increases the total breathing time (ANOVA F_(28,1)=31.6277, P<0.01). A separate group t-test between the breathing observation for the H₂S group (N=22) and the `more intense hypoxic challenge' group (N=29) reveals that there is no statistical difference between the two (t=1.4211, P=0.1714 where d.f.=45.9649).