Sustained branchial apnea in the Australian short-finned eel, Anguilla australis

Branchial and cutaneous O2 uptake as well as branchial ventilatory stroke volume and frequency were measured by twin-chamber respirometry in conscious unoperated eels (Anguilla australis) at 20 degrees C. The branchial ventilatory pattern comprised alternating periods of apnea and eupnea, together constituting a "Standard Breathing Cycle" (SBC = 1 apneic period plus the ensuing period of eupnea). SBC time in 17 eels averaged 15.5 min of which only 23% was devoted to eupnea. Reduction of the PO2 of inspired water from 155 to 80 torr significantly increased the proportion of time devoted to eupnea to 49% of SBC time without significantly changing SBC time. Absolute cutaneous O2 uptake (5.8 ml O2 X h-1 X kg-1) was largely independent of the frequency or occurrence of eupnea, with the relative contribution of the skin to total O2 uptake averaging 47%. The histological appearance of the skin in A. australis is very similar to that described in A. anguilla: eel skin is only poorly vascularized. It was concluded that the skin may contribute to O2 requirements other than its own, and that this adaptation is a consequence of the unusual intermittent pattern of gill ventilation.