The influence of hydrological environments on the morphology and reproductive biology of non-migratory Galaxias fishes in New Zealand

Abstract

Across eastern South and Stewart islands, non migratory Galaxias fishes occupy wetland and stream habitats with very different hydrological characteristics. This study sought to examine the influence of the hydrological environment in contrasting habitat types on members of the Galaxias vulgaris species complex, particularly Galaxias gollumoides. This group was once considered a single species, but has long been recognised as having high morphological variation, a situation which has challenged formal taxonomic description of lineages.

Morphological variation between habitat types was investigated by comparison of populations of Galaxias brevipinnis, G. gollumoides and G. vulgaris. Inter-specific convergence and intra-specific divergence of morphological characters were demonstrated, representing general shifts in morphology towards a common functional form in particular habitat hydrologies. 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, whereas lotic Galaxias had shorter bodies; more slender caudal peduncles; broader pectoral fins; and shorter more flatter heads, with smaller mouths. The hypothesis that observed morphological differences are induced by the hydrological environments of habitats during the larval and juvenile phases was tested in a reciprocal transplant experiment. Experimental results indicated that when reared in a flow treatment opposite to that of their source, shifts in overall head morphologies of wetland and stream G. gollumoides occurred. Experimental results, in combination with examination of field collected specimens, thus confirm that hydrology is an important factor both moulding an individual’s morphology to a particular habitat type, early in their ontogeny; and maintaining such intra-specific morphological divergence in adult G. gollumoides.

The influence of contrasting hydrological environments on growth and reproduction was examined in G. gollumoides populations in paired wetland and stream habitats, sampled seasonally over a two year period, and in sixty captive G. gollumoides from both habitat types. Captive wetland and stream G. gollumoides persistently utilised different spawning substrates across multiple tanks and years, even when held in common conditions, suggesting substrate choice may be a collective behavioural response within populations occurring in different habitat types. Field observations and experimental investigations revealed that the timing of spawning in G. gollumoides is associated with water temperature and barometric pressure. An ability to respond to the earlier rise in water temperature in wetlands may allow wetland G. gollumoides to spawn and hatch earlier compared to stream conspecifics. Wetland G. gollumoides females also had higher fecundity. Moreover, the presence of more nursery habitat, readily accessible micro-crustacean prey, and a longer feeding and growth period, may have led to wetland 0+ G. gollumoides attaining longer lengths in each season, and a greater number reaching sexual maturity and recruiting to the adult populations in autumn.

Overall, G. gollumoides displays morphological traits and reproductive and early growth strategies that are highly influenced by, and responsive to, the type of habitat occupied. It is suggested that these, in part, represent phenotypic plastic responses to the contrasting hydrological environments of ephemeral wetlands and tributary streams that were once likely widespread in Southland.