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 Galaxias vulgaris species complex was examined. Morphological variation between habitat types was investigated by comparison of populations of Galaxias brevipinnis (inferred ancestor), Galaxias gollumoides (roundhead morphotype) and Galaxias vulgaris (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 Galaxias had longer bodies; shorter, more stout caudal peduncles; longer, narrower pectoral fins; and longer, wider heads with larger mouths. In comparison, lotic Galaxias 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.