Cladogenesis and loss of the marine life-history phase in freshwater galaxiid fishes (Osmeriformes : Galaxiidae)

Switches from migratory (diadromous) to nonmigratory (freshwater) life hist ories are known to have occurred repeatedly in some aquatic taxa. However, the significance of the loss of diadromy as an initiator for speciation rem ains poorly understood. The rivers of New Zealand's South Island house a sp ecies Rock of recently derived nonmigratory galaxiid fishes known as the Ga laxias vulgaris complex. Members of this complex are morphologically and ge netically similar to the diadromous G. brevipinnis found in New Zealand and southeastern Australia. We hypothesised that South Island's G. vulgaris co mplex (at least 10 nonmigratory lineages) represents a number of independen t radiations from a migratory G. brevipinnis stock, with repeated loss of d iadromy. Sequence data were obtained for 31 ingroup samples (G. vulgaris co mplex and G. brevipinnis) plus four outgroup taxa. A well-resolved phylogen y based on 5039 base pairs of the mitochondrial genome suggests that diadro my has been lost on three separate occasions. Thus, speciation in these gal axiid fishes is partly an incidental phenomenon caused by switches from dia dromous to nonmigratory strategies. However, much of the subsequent nonmigr atory diversity is monophyletic, suggesting that drainage evolution (vicari ance) has also played a major role in cladogenesis. Levels of sequence dive rgence among major ingroup lineages (1.6-12.7%) suggest that the radiation is considerably older relative to Northern Hemisphere (postglacial) complex es of salmonid, osmerid, and gasterosteid fishes. Sympatric taxa are not mo nophyletic, suggesting that their coexistence reflects secondary contact ra ther than sympatric speciation. The monophyly of New Zealand G. brevipinnis is well supported, but both mitochondrial DNA and nuclear sequences indica te that G. brevipinnis is paraphyletic on an intercontinental scale. The di vergence (maximum 11.5%) between Tasmanian and New Zealand G. brevipinnis, although large, supports marine dispersal rather than vicariance as the pri nciple biogeographic mechanism on an intercontinental scale.