Haemoglobin components and oxygen transport in relation to habitat distribution in triplefin fishes (Tripterygiidae)

Haemoglobin components were analysed for nine species of New Zealand triplefins and their isoelectric points (pI) ranged from 5.1 to 7.0. The number of well-expressed isohaemoglobins was larger in shallow-water and tidal pool species, ranging from four in Grahamina signata to eight in Grahamina capito, 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 malcolmi. The red blood cell oxygen-binding properties were determined at 15 °C and 25 °C in the pH range 6.7–7.9 for the shallow-water species G. capito, the shallow to mid-depth species Forsterygion varium, and the deep-water species F. malcolmi. Oxygen affinity was highest for G. capito and the magnitude of the Bohr effect lower (Δlog P ₅₀/ΔpH = −0.37 at 25 °C, where P ₅₀ is the half-saturation coefficient) compared to the two Forsterygion species (Δlog P ₅₀/ΔpH = −0.52 to −0.59). Further, the cooperativity factor, n ₅₀, was lower in G. capito thus maintaining oxygen transport over a wide range of environmental oxygen pressures. Oxygen binding was similarly influenced by temperature in both G. capito and F. malcolmi (maximum heat of oxygenation ΔH_max = −27 kJ mol⁻¹ and −37 kJ mol⁻¹, respectively). Thus, triplefin fishes living in shallow, thermally unstable habitats possess a greater number of cathodally migrating isohaemoglobins, and their red blood cells have a higher oxygen affinity and reduced cooperativity which is less sensitive to changes in pH than do species occurring in more stable, deeper water habitats. Our analysis of an assemblage of closely related species circumvents some of the difficulties inherent in studies where interpretation of experimental results is confounded by phylogeny. Accepted: 18 March 1999