Evolution and comparative haemoglobin oxygen binding in new zealand mudfishes

Abstract

New Zealand's five endemic mudfish (Neochanna spp.) species have distributions that differ both geographically and by habitat type. Differences in habitat preferences between species have led to the proposal of an evolutionary series within the group. A morphological cline can be observed from the galaxiform Chatham Island and Canterbury species inhabiting lakes and streams, respectively, to the anguilliform Northland and brown mudfishes of ephemeral wetlands. Morphological specializations proposed for wetland dwelling include loss of pelvic fins, reduced eyes, enlarged nostrils, development of caudal flanges, and elongation of dorsal and anal fin bases to become almost confluent with the caudal fin. Another expectation of adaptation to wetland dwelling is specializations in respiratory physiology to obtain oxygen from highly hypoxic or acidic waters, and the ability to cope with seasonal exposure to air during the drought season. Expected respiratory specializations to wetland dwelling include high oxygen affinity haemoglobins, high levels of cooperative oxygen binding, the presence of multiple haemoglobins and the ability to aestivate and survive long periods of emersion.

The four mainland Neochanna species were examined to determine if differences in haemoglobin expression as well as differences in haemoglobin oxygen binding correlated with differing habitats and treatments. Whole blood oxygen affinity was determined at several pH levels (6.5, 7.0, 7.5 and 8.0) and temperatures (10'C, 15'C and 20'C), as well as different treatments (aestivating, fasting and control) using a Hemox analyzer. The presence of multiple haemoglobins was determined by isoelectric focusing. All four species displayed high oxygen affinities (p50 = 6.5 to 9.5 mm Hg at pH 7.5 15'C), moderate levels of cooperativity (Hill coefficients = 1.75 to 2.00 at pH 7.5 15'C), pH sensitivity (Bohr coefficients = -0.62 to -0.94 between pH 7.5 and 7.0 at 15'C), temperature sensitivity (ΔH = -2.20 to -15.78 k cal mol-1 between 10'C and 15'C) and the presence of multiple haemoglobins. Black, brown and Northland mudfish were able to survive aestivation for six weeks but there were no changes between air-breathing and water-breathing individuals with respect to oxygen binding characteristics. Although there is evidence of habitat specialization in haemoglobin physiology between mudfish species, differences between species did not correlate with the evolutionary series proposed for specialization to dwelling in ephemeral wetlands and latitudinal distributions of mudfish species appear to strongly dictate oxygen binding properties of mudfish whole blood.