Haemoglobin function and respiratory status of sub-adult sharks, Heterodontus portusjacksoni was investigated for up to 1 week following transfer from 100% to either 75% or 50% seawater. Metabolic rates were unusually low and arterial-venous differences in blood O2 small. Haemodilution from osmotic inflow lowered haematocrit and reduced blood O2 content by up to 50%. There was no change in O2 consumption rate, blood O2 partial pressure, cardiac output, or the arterial-venous O2 content difference, and thus O2 delivery was maintained. Ventilation was acutely elevated but returned to normal within 24 h. The O2 delivery to the tissues was facilitated by decreased blood O2-affinity that could not be simply ascribed to changes in the osmolyte concentration. The Hb was unaffected by changes in intra-erythrocyte fluid urea or trimethylamine-N-oxide (TMAO) but was sensitive to changes in NaCl. The Bohr shifts in whole blood were low and there was little role for pH in modulating O2 transport. Venous Hb saturation remained close to 65%, at the steepest part of the in vivo O2 equilibrium curve, such that O2 unloading could be facilitated by small reductions in pressure without increasing cardiac or ventilatory work. H. portusjacksoni tolerated 50% seawater for at least 1 month, but there was little evidence of respiratory responses being adaptive which instead appeared to be consequential on changes in osmotic and ionic status.