The effect of body size on spatial resource competition and reproductive isolation was examined in a sister species pair of subtidal triplefin fishes (F. Tripterygiidae) in New Zealand. Ruanoho decemdigitatus and Ruanoho whero have overlapping sympatric distributions and differ in body size, attaining a total length of 12 cm and 9 cm, respectively. R. decemdigitatus was most commonly found in sheltered areas shallower than 5 m, while R. whero was frequently found in sheltered to moderately exposed areas down to 20 m. In sites where the species co-occurred, R. whero was less associated with rock substratum. The effect of body size on substratum use was investigated using laboratory trials based on the field data to test habitat preference and competitive ability in a common setting. Reproductive behaviour was assessed in courtship, mate choice and hybridisation trials. Both species exhibited similar habitat preferences, but large R. decemdigitatus were dominant in inter- and intraspecific contests for the preferred rock habitat, while small R. whero were displaced into less preferred habitats. Courtship behaviour in R. whero was a subset of that displayed by R. decemdigitatus, while no mating behaviour was observed in heterospecific trials. Female R. whero showed a strong preference for smaller males, while female R. decemdigitatus had no preference for male size. Results suggest that body size differences in the Ruanoho pair are consistent with female choice for smaller males in R. whero and competition for habitat in both species. Body size in the Ruanoho species appears to be influenced by conflicting selection pressures that may differ between the species.
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