Neochanna

McDowall, R. M., 1997, Affinities, Generic Classification and Biogeography of the Australian and New Zealand Mudfishes (Salmoniformes: Galaxiidae), Records of the Australian Museum 49 (2), pp. 121-137 : 130-135

Biogeography of the genus Neochanna

Trans-Tasman range of Neochanna: The close phylogenetic relationships between the Australian and New Zealand mudfishes, hypothesised above, raises biogeographical implications only hinted at in previous studies ( McDowall, 1970; McDowall & Frankenberg, 1981). Any hypothesis employing land-based mechanisms of vicariance/dispersal for Neochanna implies an ancient, probably late Mesozoic/early Tertiary, land connection between Australia and New Zealand involving the former existence of Gondwana and its subsequent fragmentation and dispersal around the southern hemisphere. A scenario involving the occurrence of the common ancestors of the Australian and New Zealand mudfishes on Gondwana, and their separation by a vicariance event occurring perhaps as long as 85 million years ago ( Cooper, 1989), is conceivable, and perhaps consistent with scenarios presented for the biogeography of other taxa common to Australia and New Zealand.

However, given the fact that the Australian mudfish is diadromous and thus has a marine juvenile life-stage probably lasting several months, and given the fact that ocean currents sweep across the Tasman Sea from Australia to New Zealand, an origin of the mudfishes in Australia, with dispersal to New Zealand in these ocean currents is an equally likely scenario. This is consistent with the hypothesised phylogeny of the genus, in which the Australian species is the most primitive and the sister group of the three more derived New Zealand species.

Choosing between these scenarios is not simple. Some would argue that congruence of distribution patterns between disparate and unrelated taxonomic groups points conclusively to a common vicariance-related, explanation for the distributions ( Croizat et al., 1974; Rosen, 1974; Craw, 1989). I am not, however, of the view that such common patterns necessarily have the same explanations or that dispersal explanations should be invoked only when phylogenies are inconsistent with possible patterns of vicariance suggested by other taxa ( Croizat et al., 1974; Rosen, 1974; see McDowall, 1978; Andersson, 1996). Berra et al. (1996) have recently reported on results of electrophoretic comparisons of populations of G. maculatus from Australia, New Zealand and South America, and they concluded that in this diadromous species differences between populations over its vast range are so slight as to suggest recent, and even continuing, gene flow-though this is not a new idea (McDowall & Whitaker, 1975; McDowall, 1978). The only way this is possible is by transoceanic dispersal in the southern ocean. If this is possible for G. maculatus between AustralialNew Zealand and South America, dispersal of the diadromous Australian mudfish from eastern Australia to New Zealand is also highly conceivable.

Distribution of the Australian species: The Australian mudfish occurs primarily in Tasmania, though there are records from Flinders Island (Bass Strait-Green, 1984), and in coastal, southern Victoria ( Jackson & Davies, 1982; Koehn & O'Connor, 1990; Koehn & Raadik, 1991 - Fig. 10 ). Freshwater fish distributions that span Bass Strait occur also in other Australian species. Some are diadromous, like G. maculatus , G. brevipinnis and G. truttaceus (Galaxiidae-McDowall & Frankenberg, 1981), Prototroctes maraena Giinther (Prototroctidae­ McDowall, 1996a), Anguilla australis Richardson and A. reinhardtii Richardson (Anguillidae-Beumer, 1996), and Pseudaphritis urvillii Cuvier and Valenciennes (Bovichtidae-Andrews, 1996). Others, however are not diadromous, such as Galaxiella pusilla (Mack) (Galaxiidae-McDowall & Frankenberg, 1981) and Nannoperca australis Giinther (Nannopercidae-Kuiter et al., 1996). The distributions of the diadromous species may be explained by either dispersal of marine life stages through the sea across Bass Strait, between the Australian mainland and Tasmania, or by the known presence of a land connection across the strait as recently as the Pleistocene «14,000 years before present-Davies, 1974). The same alternative dispersal explanations are unlikely to apply to the non-diadromous species and the land bridge scenario is a much more likely explanation for these.

Choosing between these scenarios to explain the distribution of the Australian mudfish (which is diadromous-Fulton, 1986) is, again, not simple; either mechanism seems equally plausible. Koehn & Raadik (1991) suggested that the known distribution of the Australian mudfish in Victoria "closely conforms to the region formerly encompassed by a land bridge" across Bass Strait, and from that I infer that they favoured a causal relationship. They thought that if the occurrence of the fish in Victoria was due to marine dispersal, "the species would be expected to be more widely distributed", thus tending to discount the role of diadromy in the species' presence in Victoria. On the other hand, they stated that "geomorphological conditions that existed during and after the last glaciation... restricted nondiadromous freshwater species" as a result of which "only diadromous species inhabit the short coastal streams. The diadromous life cycle of G. cleaveri accounts for its distribution in the Wye River " ( Fig. 10 *). They thereby apparently attribute the range of the species in Victoria to diadromy. Moreover, they were also equivocal about the nature of its diadromy, suggesting that the "more restricted distribution of Galaxias cleaveri [compared with other diadromous Galaxias species] suggests that the larvae may be confined to estuaries" .

Thus they appear to attribute the presence of the Australian mudfish in Victoria to the presence of a land connection, but its distribution in Victoria perhaps to a diadromous life cycle and therefore perhaps dispersal through the sea. Dispersal across Bass Strait by a marine stage of the Australian species seems, to me, a viable option.

Distribution and biogeography of the New Zealand species: Any interpretation of New Zealand historical biogeography must be strnctured around the fact that its land mass has had a "rough tectonic history" ( Gibbs, 1989). In particular, there was very little emergent land during the Oligocene, about 30 million years ago, perhaps so little that virtually the entire biota dates from post-Oligocene dispersal to New Zealand, if Pole (1994) is correct. Thus, all terrestrial and freshwater taxa must, at least, have passed through a very severe evolutionary bottleneck, with intensive and extensive taxonomic "winnowing" and extinction at that time ( Cooper, 1989; Cooper & Cooper, 1995). What we observe, today, results from speciation and dispersal of the survivors of that event/period, as well as the addition of taxa that have dispersed to the New Zealand region since then and have speciated as the land area increased again. The effects of any more ancient events, and in particular the supposed influences of"terranes" of allochthonous origins on the distributions ofbiotic elements ( Gibbs, 1989) are bound to have been profoundly altered by the reduced land area of the Oligocene, as well as later marine transgression, mountain building, land connections and volcanism, to the extent that today's taxa and their distributions are unlikely still to reflect their origins in such terranes ( Cooper, 1989; Cooper & Cooper, 1995).

If we assume a single ancestry/common derivation of the New Zealand species as shown in the cladogram in Fig. 9 , then the biogeography of the genus Neochanna in New Zealand is structured around the speciation and divergence of that single ancestral stock. Distributions of the three New Zealand mudfishes are entirely allopatric, with:

Neochanna diversus present in the north of the North Island;

N apoda in the western and southern North Island and West Coast of the South Island, and N burrowsius in the eastern South Island ( Fig. 11 ).

None of the three species is diadromous (though their common ancestor may have been), so that their present distributions will probably not have been affected by

recent or continuing dispersal of marine life stages through the sea. They are likely therefore to reflect, at least in part, the geological history of the landscapeprobably over la'e Tertiary to Recent times. Likely influential events in this geological history include the following:

the fact, noted above, that most of the New Zealand region was submerged beneath ocean during Oligocene times, about 30 million years

ago with emergent land of minimal size, ( Cooper, 1989, and Cooper & Cooper, 1995 show only several relatively small emergent islands-Fig. 12); the Kaikoura orogeny during the Pliocene and early Pleistocene, that resulted in the uplifting of the present mountain ranges of New Zealand, particularly of the Southern Alps (which bisect the South Island distributions of N apoda to the

north-west of the Alps and N burrowsius to the south-east) ( Fig. 11 );

formation of the Canterbury Plains during the late Pliocene and Pleistocene, as the product of extensive erosion and outwash from the rising Southern Alps ;

extensive marine transgression across the now southern half of the North Island during the Pliocene and early Pleistocene, when there was sea across the present North Island almost as far north as Lake Taupo; however, parts of the southwestern present North Island are thought to have been connected to the northern tip of the South Island at that time, but separated by sea from the rest of the North Island ( Fig. 13 ); re-emergence from the sea of the above area, and the closure of all sea connections across central New Zealand during the Pleistocene, at a time of lowered sea levels ( Fig. 14 );

later reopening of the sea connection across central New Zealand at what is now known as Cook Strait;

extensive Pleistocene to Recent glaciation, with permanent snow and ice to low levels, especially along the West Coast of the South Island ;

intensive Pleistocene to recent volcanism in the central/northeastern North Island (from about 25,000 years ago).

(These events are described variously by Suggate et al., 1978; Fleming, 1979; Gage, 1980; Stevens, 1980; Wilson & Walker, 1985; Cooper, 1989; Wilson & Houghton, 1993; Lewis & Carter, 1994; Cooper & Cooper, 1995.)

In examining the biogeography of the mudfishes in New Zealand, it is important to know the extent to which present distributions are a product of these various known geo-historical events that affected the landscape and its biota, as opposed to the influences of any more ancient processes.

The three species of Neochanna could have been isolated on the various Oligocene islands ( Fig. 12 ), and have spread from these centres as the New Zealand land mass emerged from the sea during the mid-Tertiary. Such a scenario would probably have required a distinct " Canterbury " island ( Cooper, 1989, shows a possible such land area-Fig. 12), particularly if the phylogeny ofNew Zealand mudfishes, with the Canterbury mudfish a sister group of the two others, as presented in Fig. 9 , is accepted .

Alternatively, however, the total allopatry of the three species is consistent with the hypothesis that their distributions reflect relatively recent (lac Tertiary and since) geological history. Where distributions seem consistent with such more recent events, it is probably appropriate in the first instance to assume that there are causal relationships. The scenario developed here is offered on that basis, and again under the assumption that the phylogenetic relationships of the three species to the Australian species, and to each other, are as proposed above ( Fig. 9 ), i.e. that N apoda and N diversus are sister species, and together form a sister group of N burrowsius .

How far spread Neochanna was in New Zealand prior to the uplift of the Southern Alps is, of course, unknown. One possibility is that the common ancestor of N apoda and N diversus , and perhaps the common ancestor of all three species, was widespread, perhaps as far north as the Waikato and even Northland prior to the Pliocene marine transgression across the southern North Island ( Fig. 13 ). Neochanna burrowsius is now separated from N apoda in the South Island by the Southern Alps, the uplift of which can be interpreted as a major vicariant event ( Fig. 11 ).

The present distribution of N diversus (Northland, south through the Hauraki Plains, and Waikato as far as the Mokau River) equates closely with the proto-North Island ofPliocene times of extensive marine transgression ( Fig. 13 : Fleming, 1979; Lewis & Carter, 1994), which may suggest that the fish was already in that area before the marine transgression developed. Neochanna diversus does not occur in the eastern North Island (Bay of Plenty), and this is consistent with the east coast of the North Island having not adopted its present connections to the remainder of the North Island until less than lO million years ago ( Cooper, 1989). Mudfish may never have reached the region. Alternatively, if there were any populations of mudfish there, they may have been extirpated by events relating to volcanism known to have occurred in the central/eastern North Island over the past 25,000 years ( Wilson & Houghton, 1993; Wilson & Walker, 1985; McDowall, 1996b). Other non-diadromous North Island freshwater fishes that might have been expected in the north-east (Bay of Plenty/East Cape) are notably sporadic in distribution east of about the longitude of Coromandel ( McDowall, 1990, 1996b), and so absence of mudfish is not inconsistent with this.

The Pliocene marine transgression of the southern North Island would have excluded all freshwater fishes (and all other terrestrial and freshwater biota) from that area, but N apoda may have occupied the continuous land from the Wellington peninsula southwards through the northern West Coast of the South Island ( Fig. 13 ), to the northwest of the Southern Alps. Following retreat of the seas, re-emergence of the southern North Island, and perhaps also with bridging of Cook Strait as recently as lO,000-20,000 years ago ( Lewis & Carter, 1994), there was ample opportunity for N apoda to disperse north into the southern and western North Island, undoubtedly assisted by confluence of river systems that drained the south-western North Island and northern South Island ( Fig. 14 ). As a result, N apoda now occurs along the west coast of the North Island almost as far north as Cape Egmont (or did until locally extirpated by human impacts), in the Wairarapa in and south of the Manawatu River system, as well as widely in the West Coast of the South Island ( Fig. 11 ). The northern part of this distribution equates closely with river systems that are believed formerly to have drained the emergent land before Cook Strait took its present conformation in quite recent times ( Fig. 14 ). Distribution of N apoda extends well south along the West Coast of the South Island. There seem to be no existing habitat-suitability reasons why the fish should not occur further south. However, as Main (1989) has shown, extensive Pleistocene glaciation probably extirpated all terrestrial and freshwater life in the southern West Coast, including any populations of N apoda . This mudfish occurs there only north of about the estimated limits of low elevation glaciation.

Neochanna burrowsius occurs only as far south as the Waitaki River, and is thus confined largely to the Canterbury Plains. Dispersion of N. burrowsius southwards, as the plains were formed by erosion and outwash of gravels from the uplifting Southern Alps to the west during the late Pliocene, Pleistocene to Present, seems a likely scenario. Its absence south of the Waitaki, i.e. in the ancient, long emergent, Otago peneplain to the south, suggests that it was never present there. Nor does it occur in Central Otago or the Southland Plains, where there are plenty of apparently suitable habitats. The absence of any mudfish from southern New Zealand is consistent with a more northern origin of N burrowsius with dispersion south as far as the Waitaki, as the Canterbury Plains were formed during late Pliocene to recent times.

Overall, the patterns of distribution and speciation of the three New Zealand mudfishes conform easily to a scenario established around the lat Tertiary and subsequent geological history of New Zealand. In the absence of evidence to the contrary, the small radiation of these species appears to be of about that age; there is nothing presently known that suggests a much more ancient, Gondwana, origin. The prospect that the common ancestor of all four mudfishes in Australia and New Zealand was a Gondwana form, which survived the Oligocene bottleneck in New Zealand ( Cooper & Cooper, 1995), and which dispersed and speciated in New Zealand in response to these same lat Tertiary and subsequent geo-historical events, discussed above, cannot be excluded. Dating of speciation events, particularly the separation of the Australian and New Zealand lineages, using DNA sequencing technology may provide an estimate of the time of divergence of the New Zealand species from N cleaveri in Australia, and thereby throw light on this question, and allow a more rigorous choice of historical scenarios for the origins, distribution and speciation of the genus Neochanna .

ACKNOWLEDGMENTS. Clive Roberts, Museum of New Zealand, and Roger Cooper, Institute of Geological and Nuclear Sciences reviewed this paper and provided helpful criticisms. I acknowledge with gratitude, the assistance that Jim Lowry, Australian Museum, provided with the cladistic analysis. Loan of specimens from the Museum of New Zealand is acknowledged. The insights and attention to detail of an anonymous referee made for significant improvements in the manuscript.

Figures

Fig. 10. Distribution of Australian mudfish, Neochanna cleaveri (* Wye River).

Fig. 9. Cladogram showing hypothesised relationships of Neochanna species (length 28, consistency index 0.92, retention index 0.75). A black box indicates an apomorphic character; a shaded box indicates a homoplastic character. Superscripts refer to characters and subscripts indicate direction.

Fig. 11. Distribution of the three New Zealand mudfishes, Neochanna apoda, N. burrowsius, and N. diversus (data from NIWA New Zealand Freshwater Fish Database).

Fig. 13. New Zealand during the Pliocene, showing extensive marine transgression across the southern North Island (redrawn from Lewis & Carter, 1994).

Fig. 14. Central New Zealand during the Pleistocene, indicating likely confluences of river systems between North and South Islands as a result of lowered sea levels (redrawn from Fleming, 1979).

Fig. 12. The New Zealand area during the Oligocene (c. 30 million years ago): dark stippling indicates areas considered to have been above sea level (from Cooper, 1989).