Fig. 2. Environmental exposure and heart rate response to stress in M. trossulus. (A) Heat stress; as temperature increases (broken line; beginning at 01:00 h, experimental time), heart rate increases until the animal reaches a temperature at which heart rate falls rapidly (H_crit). With continued heat stress, heart rate remains depressed, but as soon as temperature drops, heart rate returns to a normal level. (B) Cold stress; as temperature decreases from acclimation level, heart rate decreases to zero. (C) Low salinity stress; as salinity decreases from acclimation level, heart rate decreases very gradually until the animal reaches a salinity at which there is a rapid fall in heart rate (S_crit). Heart rate remains depressed during continued low salinity treatment but recovers immediately when salinity begins to increase.