The study of phenotypic variation at the level of populations and species is central to understanding the processes that lead to evolutionary diversification. Triplefin fishes (Tripterygiidae) are a diverse and ecologically important family on New Zealand rocky reefs. This assemblage provides an opportunity to explore phenotype-environment relationships and the mechanisms responsible for the ecological speciation that is thought to have driven diversification in this clade. To test these relationships, we used two main approaches. Firstly, we used a geometric morphometric approach, and secondly, we used controlled rearing experiments in aquaria to examine the mechanism behind this phenotypic variation. The results showed that wild populations of the habitat generalist Forsterygion lapillum differed in body and fin morphology across a gradient in wave exposure in a region with extensive gene flow. Results of the tank experiments demonstrated that new recruits of F. lapillum from the same location that were raised in distinct water movement environments became morphologically distinct, whereas new recruits of F. lapillum from distinct water movement environments that were raised in identical conditions did not. The reciprocal rearing experiments, in conjunction with the morphological variation shown in wild individuals across the exposure gradient, indicate that plasticity is predominantly responsible for the phenotypic variation seen in F. lapillum populations in the Hauraki Gulf, New Zealand.