Hypoxia tolerance in the epaulette shark (Hemiscyllium ocellatum)

The epaulette shark, Hemiscyllium ocellatum, is a tropical reef shark that can live in an environment with cyclic periods of low oxygen concentration, suggesting that it has a well-developed capacity for anaerobic metabolism. Most investigations of hypoxia-tolerant teleosts and reptiles have focused on species that inhabit cold environments. This study was carried out on a tropical reef shark in order to determine whether similar strategies for hypoxia survival are used at higher environmental temperatures. We studied the effects of a single exposure to mild hypoxia and cyclic exposure to extreme hypoxia on blood-lactate concentration and key indicators of neurological function. The basal blood-lactate concentration for the epaulette shark was determined as 0.37 mM and showed a graded increase during hypoxia. After a single exposure to mild hypoxia (20% of normoxia for 4 h), the mean blood-lactate level rose to 3.07 mM (P < 0.01). After cyclic exposure to extreme hypoxia (eight repetitions of a 120-min exposure at 5% of normoxia), there was a rise in mean blood-lactate concentration to 5.43 mM (P < 0.0001). During both hypoxic regimens, there were no observed changes in key indicators of neurological function. We conclude that the epaulette shark is tolerant to both mild hypoxia and to cyclic exposure to extreme hypoxia.