The effects of hypoxia on three sympatric shark species: physiological andbehavioral responses

Behavioral and physiological responses to hypoxia were examined in three sy mpatric species of sharks: bonnethead shark Sphyrna tiburo, blacknose shark , Carcharhinus acronotus, and Florida smoothhound shark, Mustelus norrisi, using closed system respirometry. Sharks were exposed to normoxic and three levels of hypoxic conditions. Under normoxic conditions (5.5-6.4 mg l(-)1) , shark routine swimming speed averaged 25.5 and 31.0 cm s(-1) for obligate ram-ventilating S. tiburo and C. acronotus respectively, and 25.0 cm s(-1) for buccal-ventilating M. norrisi. Routine oxygen consumption averaged abo ut 234.6 mg O-2 kg(-1) h(-1) for S. tiburo, 437.2 mg O-2 kg(-1) h(-1) for C . acronotus, and 161.4 mg O-2 kg(-1) h(-1) for M. norrisi. For ram-ventilat ing sharks, mouth gape averaged similar to1.0 cm whereas M. norrisi gillbea ts averaged 56.0 beats min(-1). Swimming speeds, mouth gape, and oxygen con sumption rate of S. tiburo and C. acronotus increased to a maximum of 37-39 cm s(-1), 2.5-3.0 cm and 496 and 599 mg O-2 kg(-)1 h(-1) under hypoxic con ditions (2.5-3.4 mg l(-1)), respectively. M. norrisi decreased swimming spe eds to 16 cm s(-1) and oxygen consumption rate remained similar. Results su pport the hypothesis that obligate ram-ventilating sharks respond to hypoxi a by increasing swimming speed and mouth gape while buccal-ventilating smoo thhound sharks reduce activity.