The Effects of Hypoxia on Three Sympatric Shark Species: Physiological and Behavioral Responses

Abstract

Behavioral and physiological responses to hypoxia were examined in three sympatric 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 about 234.6 mg O2 kg −1 h −1 for S. tiburo, 437.2 mg O2 kg −1 h −1 for C. acronotus, and 161.4 mg O2 kg −1 h −1 for M. norrisi. For ram-ventilating sharks, mouth gape averaged ∼1.0 cm whereas M. norrisi gillbeats averaged 56.0 beats min −1 . Swimming speeds, mouth gape, and oxygen consumption 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 O2 kg −1 h −1 under hypoxic conditions (2.5–3.4 mg l −1 ), respectively. M. norrisi decreased swimming speeds to 16 cm s −1 and oxygen consumption rate remained similar. Results support the hypothesis that obligate ram-ventilating sharks respond to hypoxia by increasing swimming speed and mouth gape while buccal-ventilating smoothhound sharks reduce activity.