Morphological convergence and divergence in Galaxias fishes in lentic and lotic habitats

The influence of contrasting lentic and lotic hydrological environments on the morphology of members of the <i>Galaxias vulgaris</i> species complex was examined. Morphological variation between habitat types was investigated by comparison of populations of <i>Galaxias brevipinnis</i> (inferred ancestor), <i>Galaxias gollumoides</i> (roundhead morphotype) and <i>Galaxias vulgaris</i> (flathead morphotype). Interspecific convergence and intraspecific divergence of morphological characters were demonstrated, representing general shifts in morphology towards a common functional form in particular hydrological environments. In all species, more lentic <i>Galaxias</i> had longer bodies; shorter, more stout caudal peduncles; longer, narrower pectoral fins; and longer, wider heads with larger mouths. In comparison, lotic <i>Galaxias</i> had relatively shorter bodies; more slender caudal peduncles; broader pectoral fins; and shorter flatter heads, with smaller mouths. This study suggests that the hydrological environment of a habitat is an important factor moulding and maintaining an individual fish’s morphology to a particular habitat type, most likely representing a phenotypic plastic response.