Heart rate of the land crab, Cardisoma guanhumi (Latreille), during aquatic and aerial respiration

1. 1. EKG's were obtained on the land crab Cardisoma guanhumi. The heart rate after electrode implantation was initially high. (Rates up to 100/min were measured.) Within 1.5 hr after implantation, rates reached 80/min and 15 hr later, averaged 70/min. 2. 2. A diurnal rhythm in heart rate appeared, with rates highest in the late afternoon and evening. 3. 3. Periods of brief cardiac arrest were common and without known cause. Cardisoma were not particularly sensitive to tactile and visual stimuli; heart rate did not change with mild disturbances. 4. 4. Heart rate was not a function of body weight. 5. 5. Submergence of Cardisoma in normoxic water typically caused a brief tachycardia, followed by a bradycardia. Bradycardia may be a response to the relatively lower levels of oxygen in water compared to air. When crabs re-emerged from water, heart rates rapidly rose to resting levels or above, suggesting a possible response to payment of an O₂ debt. 6. 6. Heart rates of crabs submerged in 50% seawater were lower than those submerged in freshwater, 25% and 100% seawater.