Physiological adaptation in the radiation of New Zealand triplefin fishes (Family Tripterygiidae)

Abstract

Physiological adaptation to divergent environments is a poorly understood factor in adaptive radiation. New Zealand (NZ) triplefin fishes (Tripterygiidae) have undergone a radiation associated with habitat diversification within NZ's coastal waters, where 26 closely-related endemic species occur in overlapping but divergent habitats, partitioned by depth and exposure. By investigating the relationship between respiratory physiological traits and habitat in these fishes, this thesis examines whether there is evidence in this group to support two proposed criteria for adaptive radiation; phenotype-environment correlation and trait utility. Significant interspecific differences were observed in rates of oxygen consumption (VO2) and critical oxygen concentration (O2 crit) in 12 species of triplefin examined. O2 crit correlated with species' habitat depth, with intertidal species displaying greater hypoxia tolerance than subtidal species, thus demonstrating phenotype-environment correlation and trait utility in relation to hypoxia exposure. Interspecific differences in VO2 were significantly influenced by phylogeny, indicating a lack of strong environmental selection on VO2. However, there was some indication of lower VO2 in species occupying more exposed habitats. Mitochondrial respiration was also examined in three species; the intertidal species displayed higher cytochrome c oxidase activity and was able to maintain efficient oxidative phosphorylation at higher temperatures than the two subtidal species, further indicating phenotype-environment correlation and trait utility. Haemoglobin (Hb) isoform expression was examined in 23 species. Isoform multiplicity declined with habitat depth, supporting the hypothesis that higher multiplicity may be associated with greater environmental variability. A lack of phylogenetic signal in Hb expression, and latitudinal variation in the relative isoform abundance in some species, indicated potential selection on this trait. However, there was no pattern in expression of cathodal Hbs, and the trait utility of this multiplicity is unknown. Overall, there is strong evidence that differences between intertidal and subtidal environments in exposure to high temperatures and hypoxia may have lead to divergence in O2 crit and mitochondrial function between intertidal and subtidal species. Therefore physiological adaptation may have enabled the expansion of species into the more demanding habitats such as the intertidal zone. Hb isoform multiplicity and VO2 were correlated with habitat in both intertidal and subtidal species, however the trait utility associated with these correlations is unknown and thus there remains a lack of evidence to support a direct role of physiological adaptation in habitat divergence of subtidal species - and therefore for adaptive radiation of the group as a whole.