Fig. 1. Crowding reversibly alters total breathing time (TBT). A naïve cohort of snails (N=23) received a 30-min hypoxic challenge (10–12 snails/500 ml; Pre-obs). The mean TBT (± s.e.m.) is plotted on the y-axis. These snails were then placed into crowded conditions (20 snails/100 ml) for 24h and were then given a second hypoxic challenge (Post-obs 1). Snails were then placed into an uncrowded aquarium (two snails/100 ml) for 24h. A Kruskal–Wallis (non-parametric ANOVA) was performed on these data (KW=19.326; P<0.01) followed by a Dunn's Multiple Comparison test. TBT was significantly less in Post-obs 1 compared with Pre-obs (^*P<0.01). TBT in Post-obs 2 was significantly greater than in Post-obs 1 (P<0.01). There was no statistical difference between Pre-obs and Post-obs 2 (P>0.05).