Journal of Fish Biology (2014) 84, 827–843 doi:10.1111/jfb.12186, available online at wileyonlinelibrary.com The effect of sustained hypoxia on the cardio-respiratory response of bowfin Amia calva : implications for changes in the oxygen transport system C. S. Porteus*†, P. A. Wright‡ and W. K. Milsom* *Department of Zoology, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada and ‡Department of Integrative Biology, University of Guelph, Guelph, ON, N1G 2W1, Canada This study examined mechanisms underlying cardio-respiratory acclimation to moderate sustained hypoxia (6·0 kPa for 7 days at 22 ◦ C) in the bowfin Amia calva , a faculta- tive air-breathing fish. This level of hypoxia is slightly below the critical oxygen tension (p crit ) of A. calva denied access to air (mean ± s.e. = 9·3 ± 1·0 kPa). Before exposure to sustained hypoxia, A. calva with access to air increased air-breathing frequency on exposure to acute progressive hypoxia while A. calva without access to air increased gill- breathing frequency. Exposure to sustained hypoxia increased the gill ventilation response to acute progressive hypoxia in A. calva without access to air, regardless of whether they had access to air or not during the sustained hypoxia. Additionally, there was a decrease in Hb–O 2 binding affinity in these fish. This suggests that, in A. calva , acclimation to hypoxia elicits changes that increase oxygen delivery to the gas exchange surface for oxygen uptake and reduce haemoglobin affinity to enhance oxygen delivery to the tissues. © 2013 The Fisheries Society of the British Isles Key words: critical oxygen tension; facultative air breather; Hb–O 2 binding affinity; metabolic rate. INTRODUCTION In some obligate water breathers, fishes that use only gills to obtain oxygen from their environment, it has been shown that oxygen uptake and transport to the tissues are enhanced following sustained exposure to moderate hypoxia. This was due to an increase in Hb–O 2 binding affinity in rainbow trout Oncorhynchus mykiss (Wal- baum 1792) (Soivio et al., 1980; Tetens & Lykkeboe, 1981; Bushnell et al., 1984; Rutjes et al., 2007) and an increase in oxygen carrying capacity of the blood in O . mykiss and cod Gadus morhua L. 1758 (Tetens & Lykkeboe, 1981; Rutjes et al., 2007; Petersen & Gamperl, 2011). The responses of some facultative air breathers to sustained hypoxia have also been examined, but usually while the fishes had access to air and the results have been variable. For example, in one study of the facultative air breather Synbranchus marmoratus Bloch 1795, exposure to sustained hypoxia †Author to whom correspondence should be addressed at present address: College of Life & Environmen- tal Sciences, Geoffrey Pope Building, Room 109, University of Exeter, Exeter EX4 4QD, U.K. Tel.: +44 1392725853; email: c.s.porteus@exeter.ac.uk 827 © 2013 The Fisheries Society of the British Isles