The Evolution of Queensland Spiny Mountain Crayfish of the Genus Euastacus. I. Testing Vicariance and Dispersal With Interspecific Mitochondrial Dna

Abstract

The upland mesic rainforests of eastern Australia have been described as a "mesothermal archipelago" where a chain of cool mountain "islands" arise from a warm "sea" of tropical and subtropical lowlands. An endemic freshwater crayfish belonging to the genus Euastacus is found on each of these mountain "islands." The Euastacus are particularly suitable for the study of evolution because each mountain harbors a unique species, there are many taxa present providing replication within the group and, most importantly, their distribution is linear, extending along a south-north axis. This group could have evolved by "simultaneous vicariance" where there was one vicariant separation event of a widespread ancestor, or by "south to north stepping stone dispersal" where there were long distance dispersal events from neighboring mountain islands, starting in the south and proceeding north in a dispersal-colonization wave. We used pairwise genetic distances between nearest geographic neighbors as a novel way to test the two hypotheses. If diversification was due to "south to north stepping stone dispersal," then pairwise genetic distances between nearest geographic neighbors should decrease progressively the farther north the taxon pairs are found, reflecting the decreasing periods of isolation. In this case there should be a negative correlation between the south to north rank order of nearest neighbors and pairwise genetic distances. A Spearman's correlation on 16S mtDNA pairwise genetic distances and geographic rank order was not significant, indicating there was no support for the south to north stepping stone dispersal hypothesis. If simultaneous vicariance was responsible for diversification then all nearest geographic neighbor taxon pairs should have similar genetic distances and, therefore, the variance in nearest neighbor distances should be zero, or close to it. To test if the observed variance was tending towards zero we developed a randomization test where nearest neighbor taxon pairs were assigned random genetic distances and the variances calculated. The observed variance lay in the < 0.05 range of the simulated variances, providing support for the simultaneous vicariance hypothesis. The data also suggest there was simultaneous vicariance of at least two ancestral Queensland lineages. The timing of this vicariant event was probably in the Pliocene, which is consistent with the divergence times reported for other Australian mesic rainforest restricted taxa.

Australian spiny mountain crayfish and their temnocephalan ectosymbionts: an ancient association on the edge of coextinction?

“…Both cophylogeny (linked phylogenetic histories) and host specificity (clade specificity and number of hosts) indicate consistent latitudinal patterns, which we attribute to the differential effects of long-term climate drying and habitat fragmentation (see also [32]). In particular, our results suggest that opportunities for host switching have been (and likely remain) far greater in central and southern Australia, in line with larger host species ranges, higher range overlap, and the greater host diversity [54].…”

Global diversity of crayfish (Astacidae, Cambaridae, and Parastacidae––Decapoda) in freshwater

“…These genera appear to form well-defined monophyletic groups (e.g., Schull et al 2005) as a result. The species richness of many of these genera appears to be the result of vicariance and dispersal with isolation events (e.g., Ponniah and Hughes 2004;Schull et al 2005). …”

Shared phylogeographic patterns between the ectocommensal flatwormTemnosewellia albataand its host, the endangered freshwater crayfishEuastacus robertsi

“…The area which is <100 km 2 , is believed to contain the entire range of both species (Morgan, 1997; Ponniah & Hughes, 2006; Coughran & Furse, 2010). Samples were collected, as per Ponniah & Hughes (2004), in stream reaches at elevation >750 m. On each mountaintop one site was sampled except for Mount Finnigan where two stream reaches were sampled.…”

“…Previous studies have suggested the diverse array of Euastacus species in eastern Australia evolved through vicariance of formerly widespread ancestral taxa that became isolated in upland refuges of the eastern highlands during the Pliocene drying of the Australian continent (Ponniah & Hughes, 2004; Shull et al, 2005; Ponniah & Hughes, 2006). In these studies two species, E. robertsi and E. fleckeri were found to comprise a highly divergent monophyletic group within the genus.…”