PHYSIOLOGICAL AND BEHAVIORAL-RESPONSES TO HYPOXIA IN THE BONNETHEAD SHARK, SPHYRNA-TIBURO - ROUTINE SWIMMING AND RESPIRATORY REGULATION

We examined the effect of hypoxia on the swimming speed, respiration r ate (oxygen uptake), gape and ventilation volume of the bonnethead sha rk, Sphyrna tiburo. We used a sonic flowmeter developed for this study to examine swimming speed changes of sharks held in artificial lagoon s during diurnal dissolved oxygen changes. Sharks were observed to swi m at about 34 cm s(-1) during the day but increased to about 40 cm s(- 1) at night when dissolved oxygen levels fell to < 3 mg l(-1). Using a closed system respirometer we examined changes in swimming speed, res piration rate and gape at four dissolved oxygen levels. Swimming speed s averaged 24 to 25 cm s(-1) under normoxic conditions but increased t o 38 to 40 cm s(-1) during hypoxia. Similarly, respiration rate increa sed with increasing speed and decreasing dissolved oxygen. Gape averag ed about 1.0 cm under normoxic conditions and increased to a maximum o f about 3.5 cm during hypoxia. Using assumed oxygen extraction efficie ncies of 25, 50 and 75% and observed respiration rates, we estimated t hat ventilation volumes of about 25 to 470 l h(-1), depending upon oxy gen concentration, would be necessary for gill ventilation. These expe riments suggest that changes in swimming speed and mouth gape are impo rtant for respiratory regulation in ram ventilating sharks.