Coexisting species are interesting from evolutionary and ecological perspectives. Sympatry can provide insights into mechanisms that allow stable long-term coexistence, such as niche partitioning, character displacement, interspecific competition and species boundaries. In New Zealand streams, a flock of closely related non-migratory galaxiid fishes exist throughout eastern South Island (Galaxias vulgaris sensu lato). These fishes are largely parapatric, but recent studies have found streams that contain coexisting populations of two newly discovered galaxiids (Galaxias �southern� and G. gollumoides). In such streams there is a cline from allotopic (sections of streams containing predominantly one species) G. �southern� (lower altitudes) to allotopic G. gollumoides (higher altitudes), with a syntopic (both species) transition section. Such coexistence makes G. �southern� (yet to be formally described) and G. gollumoides ideal for examining the ecology and evolution of sympatric taxa. Very little is known about these two taxa. This thesis aims to detail the morphology, ecology and genetic differentiation between G. �southern� and G. gollumoides. These factors will be used to address separate species status and the possibility of interspecific competition. Data will also be used to examine resource and morphological shifts in either species between syntopy and allotopy, which maybe consistent with character displacement theory. All fish collected for analysis were first classified as G. �southern� or G. gollumoides by eye and by mtDNA. Morphological and ecological (habitat use and diet/isotope enrichment) differentiation between the two species were examined from three sympatric streams (streams containing allotopic and syntopic sites; representing three separate catchments) and four allopatric streams (streams containing only allotopic sites; representing two separate catchments). For sampling purposes, sympatric streams were subdivided into lower (allotopic G. �southern�), mid (syntopic) and upper (allotopic G. gollumoides) sections. We used this sampling regime to test for character displacement (i.e. ecological and morphological shifts between syntopy and allotopy). Diet, stable isotope, genetic structuring and hybridisation (amplified fragment length polymorphisms) were analysed from the three sympatric streams. Artificial stream channels were also used to observe the influence of interspecific competition on resource use over a complete diel cycle. G. �southern� and G. gollumoides were found to be morphologically distinct, differing significantly in 21 of 25 morphological measurements. The two species were also segregated ecologically with G. �southern� preferring shallower water, coarser substrate, and higher water velocity, feeding predominantly on mayflies (Ephemeroptera). In contrast, G. gollumoides preferred deeper water, finer substrate and lower water velocity, feeding predominantly on terrestrial invertebrates. Dietary segregation was also reflected by differences in [delta]�� Carbon and [delta]�⁵ Nitrogen enrichment. Morphological shifts consistent with character displacement were seen in overbite size and mouth size, but neither trait showed consistent statistical differences within and across streams. Ecological shifts were seen in distance to the nearest cover, with G. gollumoides being found further away from cover in syntopy and G. �southern� being found the same distance away from cover in syntopy and allotopy. In artificial stream channels, resource use of both species was not affected by the presence of a congener, but both species used faster water velocities during the night while remaining ecologically distinct. AFLP analysis revealed distinct genotypic clustering between these species with no F1 hybrids. Fine-scale genetic structuring was also observed within both species among streams, and among sites within streams. The minimal effect of a congener on resource use suggests that interspecific competition is not a strong force shaping sympatric populations of Southland galaxiids. This is also reflected by the lack of consistent morphological shifts between allopatry and sympatry. Similarly, character displacement does not seem to be a strong influencing G. �southern� and G. gollumoides. Diel cycles influenced niche expression in both fishes suggesting that abiotic factors may be important in structuring freshwater fish communities in Southland. The fine-scale genetic structuring within both species suggests limited conspecific genetic mixing and movement within streams, which possibly reflects drainage evolution. The absence of F1 hybrids suggests reproductive isolation between G. �southern� and G. gollumoides which may be associated with assortative mating. The present study provides evidence for the separate status of these sympatric fishes under several species concepts. There is conclusive evidence for morphological, ecological and genetic differentiation between sympatric G. �southern� and G. gollumoides, and such segregation maybe associated with long term coexistence in nature. Key words: coexistence, ecology, morphology, genetics, G. �southern�, G. gollumoides.

Coexisting species are interesting from evolutionary and ecological perspectives. Sympatry can provide insights into mechanisms that allow stable long-term coexistence, such as niche partitioning, character displacement, interspecific competition and species boundaries. In New Zealand streams, a flock of closely related non-migratory galaxiid fishes exist throughout eastern South Island (Galaxias vulgaris sensu lato). These fishes are largely parapatric, but recent studies have found streams that contain coexisting populations of two newly discovered galaxiids (Galaxias �southern� and G. gollumoides). In such streams there is a cline from allotopic (sections of streams containing predominantly one species) G. �southern� (lower altitudes) to allotopic G. gollumoides (higher altitudes), with a syntopic (both species) transition section. Such coexistence makes G. �southern� (yet to be formally described) and G. gollumoides ideal for examining the ecology and evolution of sympatric taxa. Very little is known about these two taxa. This thesis aims to detail the morphology, ecology and genetic differentiation between G. �southern� and G. gollumoides. These factors will be used to address separate species status and the possibility of interspecific competition. Data will also be used to examine resource and morphological shifts in either species between syntopy and allotopy, which maybe consistent with character displacement theory. All fish collected for analysis were first classified as G. �southern� or G. gollumoides by eye and by mtDNA. Morphological and ecological (habitat use and diet/isotope enrichment) differentiation between the two species were examined from three sympatric streams (streams containing allotopic and syntopic sites; representing three separate catchments) and four allopatric streams (streams containing only allotopic sites; representing two separate catchments). For sampling purposes, sympatric streams were subdivided into lower (allotopic G. �southern�), mid (syntopic) and upper (allotopic G. gollumoides) sections. We used this sampling regime to test for character displacement (i.e. ecological and morphological shifts between syntopy and allotopy). Diet, stable isotope, genetic structuring and hybridisation (amplified fragment length polymorphisms) were analysed from the three sympatric streams. Artificial stream channels were also used to observe the influence of interspecific competition on resource use over a complete diel cycle. G. �southern� and G. gollumoides were found to be morphologically distinct, differing significantly in 21 of 25 morphological measurements. The two species were also segregated ecologically with G. �southern� preferring shallower water, coarser substrate, and higher water velocity, feeding predominantly on mayflies (Ephemeroptera). In contrast, G. gollumoides preferred deeper water, finer substrate and lower water velocity, feeding predominantly on terrestrial invertebrates. Dietary segregation was also reflected by differences in [delta]�� Carbon and [delta]�⁵ Nitrogen enrichment. Morphological shifts consistent with character displacement were seen in overbite size and mouth size, but neither trait showed consistent statistical differences within and across streams. Ecological shifts were seen in distance to the nearest cover, with G. gollumoides being found further away from cover in syntopy and G. �southern� being found the same distance away from cover in syntopy and allotopy. In artificial stream channels, resource use of both species was not affected by the presence of a congener, but both species used faster water velocities during the night while remaining ecologically distinct. AFLP analysis revealed distinct genotypic clustering between these species with no F1 hybrids. Fine-scale genetic structuring was also observed within both species among streams, and among sites within streams. The minimal effect of a congener on resource use suggests that interspecific competition is not a strong force shaping sympatric populations of Southland galaxiids. This is also reflected by the lack of consistent morphological shifts between allopatry and sympatry. Similarly, character displacement does not seem to be a strong influencing G. �southern� and G. gollumoides. Diel cycles influenced niche expression in both fishes suggesting that abiotic factors may be important in structuring freshwater fish communities in Southland. The fine-scale genetic structuring within both species suggests limited conspecific genetic mixing and movement within streams, which possibly reflects drainage evolution. The absence of F1 hybrids suggests reproductive isolation between G. �southern� and G. gollumoides which may be associated with assortative mating. The present study provides evidence for the separate status of these sympatric fishes under several species concepts. There is conclusive evidence for morphological, ecological and genetic differentiation between sympatric G. �southern� and G. gollumoides, and such segregation maybe associated with long term coexistence in nature. Key words: coexistence, ecology, morphology, genetics, G. �southern�, G. gollumoides.