The oxygen-carrying capacity of the blood of decapod crustaceans fluctuates widely. Salinity stress results in doubling of haemocyanin concentration within 24–48 h in Carcinus maenas (Boone & Schoeffeniels, 1979) while in the lobster, Homarus gammarus respiratory pigment levels are very low prior to and following moulting (Spoek, 1974). In general, however, the most important factor regulating haemocyanin concentration is nutritional state. Following starvation low values are recorded (Wieser, 1965; Uglow, 1969; Djangmah, 1970) and there are concomitant reductions in ventilation, oxygen consumption and cardiac output (Ansell, 1973; Marsden, Newell & Ahsanullah, 1973; Wallace, 1973). The interrelationships between these events are poorly understood.