Respiratory and hematological responses of the bonnethead shark, Sphyrna tiburo, to acute changes in dissolved oxygen

Behavioral and physiological responses to acute changes in dissolved oxygen were examined in the bonnethead shark, Sphyrna tiburo. In two sets of respirometry experiments, sharks were randomly exposed to seawater in oxygen contents of 6.0, 5.0, 4.0 and 3.0 mg l −1. During exposure, bonnetheads increased mouth gape from 0.8 cm at 6.0 mg l −1 to 2.2 cm at 3.0 mg l −1, while ventilation volume increased from 0.61 to 5.28 l min −1 kg −1. Standard oxygen consumption remained unchanged (163–181 mg O 2 kg −1 h −1) throughout all treatments and was not significantly different. Utilization (%) declined from 52.3% at 6.0 mg l −1 to 21.3% when oxygen levels reached 3.0 mg l −1. Changes in oxygen content of ambient water also caused no significant change in either blood oxygen content or hematocrit. Using cellulose acetate electrophoresis, a single hemoglobin profile was identified at seawater of 6.0 mg l −1 and hypoxic conditions. Results suggest bonnetheads are physiologically able to tolerate moderate levels of hypoxia.