WPrinted in Great Britain BLOOD GAS TRANSPORT IN THE CEPHALOPOD, SEPIA OFFICINALIS

Blood gas transport was studied in unrestrained free-swimming cuttle-fish, Sepia officinalis, following cannulations of an efferent branchial (arterial) vessel and the vena cava cephalica with indwelling catheters. In well-aerated water the arterial pOi averaged about ioo-oo mmHg and was fully saturated with O2. Mixed venous pOi varied between 17 and 40 mmHg, typically corresponding to blood O2 utilizations of 80 % or higher. Some blood samples showed venous pH to exceed arterial, a tendency becoming more distinct during exposure to hypoxic water. The resulting higher O2 affinity of venous compared to arterial blood discourages O2 unloading in the tissues, while promoting efficient O2 loading in the gills. A high n-value of Sepia blood ( « = 4-7) is important for maintaining a large arteriovenous Oa content difference and a high utilization of circulating O2