The effects of walking on heart rate, ventilation rate and acid-base status in the lobster homarus americanus

American lobsters Homarus americanus were exercised on an underwater treadmill at speeds from 1.7 to 8 m min-1 to determine the effects of exercise on heart rate, ventilation rate and acid-base status. Heart and ventilation rates showed almost instantaneous increases at the start of exercise, but the magnitude of the increase was not related to speed. Maximum heart rate was approximately 80-90 beats min-1 and maximum ventilation rate was 175-180 beats min-1 at all speeds tested. Exercise at all speeds caused a decrease in haemolymph pH, with the acidosis after exercise at 8 m min-1 being significantly greater than at the other three speeds. Concomitant with this acidosis was a large increase in partial pressure of carbon dioxide, with the largest increase occurring after exercise at 8 m min-1. The concentration of lactate in the haemolymph increased to similar levels at all speeds of walking. Davenport analysis indicates that the acidosis was predominantly respiratory in nature. Although it was anticipated that heart and ventilation rates would show increases proportional to the speed of exercise, this was not the case. Rather, the responses were fixed regardless of walking speed. The reason for this phenomenon remains unexplained.