Allibone

Population structure of Galaxias divergens, a widespread non-migratory galaxias in New Zealand

Presented at Fresh Perspectives, joint conference of NZ Limnological Society,
NZ Hydrological Society and Meteorological Society of NZ,
Christchurch, November 2000.

R.M. Allibone

Galaxias divergens is a small non-migratory galaxiid found in the northern South Island and the southern and central North Island, New Zealand. Genetic and morphological investigations were carried out on 28 populations collected from throughout the species' geographic range. Isozyme studies involved 26 of the populations, at eight loci. Twenty-four morphological measurements and seven meristic counts were taken from up to ten fish at each site. Multidimensional scaling and ANOVA were used to examine these measurements and counts to determine if there were distinct morphologies associated with distinct genetic groups detected in the isozyme investigation.

Five genetic groups were distinguished within G. divergens (Fig. 1, below), by fixed allelic differences or very high frequency differences among alleles at one or more loci. All genetic differentiation was between catchments, none within catchment and Nei's genetic distance (D) was not correlated with geographic distance. Populations on the periphery of the species' distribution showed some genetic distinction, but the most distinct populations occurred in the middle of the species geographic range. Morphological analyses failed to distinguish the five genetic groups.

Figure 1. The distribution of genetically distinct groups of G. divergens.

No new species are proposed despite the genetic differentiation within G. divergens. The lack of concurrent morphological characters to distinguish the genetic groups means that descriptions and identification of any new species would rely on genetic markers and geographic location. This is inconsistent with other recently described galaxiid species for which genetic and morphological distinction is apparent. However, it is proposed that these distinct genetic groups are recognised as Evolutionary Significant Units as each group is a distinct unit and the loss of any group would reduced the genetic diversity within G. divergens.

Geological processes in central New Zealand are implicated as the central cause for the observed population structure. Four of the recognised groups occupy areas of New Zealand that were part of a southern landmass in the early Pleistocene. Gene flow appears to have occurred from some of these populations across Cook Strait in a northward direction during low sea level periods associated with Pleistocene glacials. However, populations in rivers draining the southern North Island and emptying into Cook Strait were isolated during this time. Gene flow has also been constrained during glacial periods by glacial barriers to fish passage in the upper reaches of South Island rivers. Uplift along the Alpine Fault and along North Island faults has also isolated populations. The Manawatu River appears to have provided the pathway for G. divergens to pass through the North Island ranges and gain access to rivers in the southern Hawkes Bay area. The most northerly G. divergens populations appear to have been isolated by recent volcanic events in the central North Island that may have extirpated G. divergens populations that formed "stepping stone" populations linking the most northerly populations to genetically similar southern North Island populations.