The effects of habitat on phenotype, growth and fitness in New Zealand triplefin fishes (Family: Tripterygiidae)

Abstract

New Zealand triplefins have previously been shown to have diverged in habitat use. This thesis aimed to determine whether they differed in habitat performance, with a view to determine whether processes that are hypothesized to have taken place over evolutionary timescales (i.e. ecological speciation), are evident over ecological timescales. Of particular interest is how variation in the environment can lead to fitness differences that have evolutionary consequences. This was achieved by testing several proximate indicators of fitness in different habitats, and included: (i) describing the patterns and variation in settlement and abundance of the four most common species of triplefins (Forsterygion lapillum, F. varium, Ruanoho whero, and Notoclinops segmentatus) among habitats, (ii) testing the effect of habitat on the relationship between male size and clutch size, (iii) utilising the age and growth rate information of two species of triplefin, F. lapillum and F. varium, as a proxy for post-settlement performance, (iv) testing the relationship between habitat and morphology in F. lapillum populations, and (v) testing the hypothesis that the mechanism for the environmentally-induced morphological variation seen in the wild populations (chapter 4) is phenotypic plasticity. Intraspecific variation was found in all of the ecological traits examined, with phenotypeenvironment correlations observed in density and reproductive output, growth rates and longevity, and fin and body morphology. The main factors associated with driving selection pressures were wave exposure and microhabitat type, and the former was consistently evident across all traits measured. Additionally, plasticity was shown to be the mechanism responsible for the observed morphological variation in F. lapillum, and theoretically has the potential to generate disruptive selection by creating the raw material for natural selection to act upon. By identifying characters under selection, these results suggest a basis for divergent selection in habitat use. This informs our understanding of niche diversification in triplefins and supports the hypothesis that differentiation in habitat use is an important mechanism in the ecological speciation seen in the New Zealand triplefin fish assemblage. As few field studies have examined this process in marine fishes, empirical studies of this nature on marine fishes are important in working towards an understanding of the drivers that promote and maintain diversity in these systems.