Haemoglobin components and oxygen transport in relation to habitat distribution in triplefin fishes [Tripterygiidae]

Haemoglobin components were analysed for nine species of New Zealand triple fins and their isoelectric points (pI) ranged from 5.1 to 7.0. The number o f well-expressed isohaemoglobins was larger in shallow-water and tidal pool species, ranging from four in Grahamina signata to eight in Grahamina capi to, and were relatively cathodal. Two strongly anodal isohaemoglobins were expressed in the mid-depth species Ruanoho decemdigitatus and Ruanoho whero , and one in the deeper water species Karalepis stewarti and Forsterygion m alcolmi. The red blood cell oxygen-binding properties were determined at 15 degrees C and 25 degrees C in the pH range 6.7-7.9 for the shallow-water s pecies G. capito, the shallow to mid-depth species Forsterygion varium, and the deep-water species F. malcolmi. Oxygen affinity was highest for G. cap ito and the magnitude of the Bohr effect lower (Delta log P-50/Delta pH = - 0.37 at 25 degrees C, where P-50 is the half-saturation coeffcient) compare d to the two Forsterygion species (Delta log P-50/Delta pH = -0.52 to -0.59 ). Further, the cooperativity factor, n(50), was lower in G. capito thus ma intaining oxygen transport over a wide range of environmental oxygen pressu res. Oxygen binding was similarly influenced by temperature in both G. capi to and F. malcolmi (maximum heal of oxygenation Delta H-max = -27 kJ mol(-1 ) and -37 kJ mol(-1), respectively). Thus, triplefin fishes living in shall ow, thermally unstable habitats possess a greater number of cathodally migr ating isohaemoglobins, and their red blood cells have a higher oxygen affin ity and reduced cooperativity which is less sensitive to changes in pH than do species occurring in more stable, deeper water habitats. Our analysis o f an assemblage of closely related species circumvents some of the difficul ties inherent in studies where interpretation of experimental results is co nfounded by phylogeny.