The behaviour and respiratory physiology of the shore crab, Carcinus maenas (L.) at moderately high temperatures

1. When gradually warmed from either 15 °C or 25 °C Carcinus voluntarily migrated into air at a mean temperature of 28 °C. Once in air, each crab remained there for the duration of the experiment (3 h). This behaviour was termed "emigration". 2. Emigration into cool air (17 °C) served to lower body temperature and heart rate was proportionally decreased, whilst emigration into warm air (26 °C) caused no marked reduction in body temperature and heart rate was unaffected (Fig. 2). 3. The {Mathematical expression} of crabs in dry air (R.H. 40%) was significantly lower than that of crabs in damp air (R.H. 80%) at 30 °C, due to a significant mean difference in body temperature of 1.8 °C (Fig. 3). 4. Exposure to air (R.H. 55%) for 12 h caused a progressive reduction in mass accompanied by a proportional increase in haemolymph Na⁺ concentration (Table 1). 5. 3 h exposure to air at 25 °C caused a marked increase in CO₂ tension ( {Mathematical expression}) and content ( {Mathematical expression}) in the haemolymph accompanied by a reduced pH, which caused a Bohr shift. The oxygen content of the postbranchial haemolymph ( {Mathematical expression}) was maintained high in air (Table 2). 6. The advantage of emigration may be the potential cooling effect of aerial exposure, with a consequent reduction in oxygen demand, as well as the avoidance of the possibility of low availability of oxygen in small bodies of warmed seawater.