Oxygen levels, acid-base status and heart rate during emersion of the shore crab Carcinus maenas (L.) into air

1. When the ambient oxygen tension of the water was lowered, unrestrained shore crabs acclimated to 15, 20 and 25°C emerged into air to ventilate the branchial chambers when the inspired oxygen tension ( {Mathematical expression}) was reduced to mean values of 37±4 mmHg, 51±6 mmHg and 119±3 mmHg respectively. There is a positive correlation between {Mathematical expression} at emersion and acclimation temperature (Fig. 1). 2. The extent to which the {Mathematical expression} of the water in the branchial chambers was increased by emersion was related to the length of time emerged (Fig. 2), and inversely proportional to the {Mathematical expression} at emersion (Fig. 3). 3. Following 60 s emersion at 15°C there was a marked increase in oxygen content of the postbranchial haemolymph ( {Mathematical expression}), pHa was reduced and {Mathematical expression} increased (Table 1). The same trend in acid-base balance was observed at 25°C but {Mathematical expression} was unchanged as emersion was from virtually normoxic water (Table 2). 4. Emersion from water at 25°C into air at 25°C and 60% R.H. caused no significant change in heart rate or core temperature. Emersion into air at 17°C was accompanied by a decreased heart rate and core temperature. 5. At relatively low temperatures emersion from hypoxic water serves to aerate the branchial chambers and lead to an increase in {Mathematical expression}, which together with a recovery tachycardia restores the oxygen supply to the tissues. CO₂ accumulates during emersion but to a lesser extent than during air-breathing, so that CO₂ exchange with water is retained during emersion. At 25°C emersion into cool air may serveto reduce the crab's temperature thus reducing its oxygen demand.