Entomology Department

Halobates - Oceanic
Insects 

Austrobates - the Missing Link?

Nils Møller Andersen
Zoological Museum, University of Copenhagen

Ó Copyright, N. M. Andersen & Zoological Museum, University of Copenhagen

Contents

General

Biogeography

Evolution

Literature cited



Austrobates rivularis, wingless male, length 3.2 mm, from Lydia Creek, Cape York, Queensland, Australia. (Photo: Zoological Museum, University of Copenhagen).  

General

In January 1993, Paul Zborowski, CSIRO Entomology, took a small sample of a water strider in Lydia Creek, a freshwater stream situated in the middle of the Cape York Peninsula, northern Queensland, Australia. It was preliminary identified by Tom A. Weir, the Australian National Insect Collection, Canberra,  as a hitherto unknown member of the Gerridae-Halobatinae and subsequently examined and described by Andersen & Weir (1994) as a new genus and species, Austrobates rivularis.

The discovery of Austrobates rivularis adds an exciting new piece to the jig-saw puzzle picturing the evolution and diversification of water striders in general, and se skaters in particular. Austrobates shares many characters with the genus Asclepios, of which the overall pale color and the incrassate and ventrally modified male fore femora are the most striking. However, the male terminalia has many of the modifications used to characterize Halobates, such as the spiracular processes and long styliform processes of segment 8. Only the shape of the male proctiger is definitely Asclepios-like. As these characters indicate, Austrobates seems to be somewhat of a "missing link" between Asclepios and Halobates. However, a phylogenetic (cladistic) analysis unequivocally places Austrobates as the sister-group of Halobates (Damgaard et al., 2000).

Biogeography

In a phylogenetic framework, the origin of a monophyletic group (clade) can be determined as the event where the group was separated from its sister group by the splitting of a common ancestor. The sister-group of Halobates is Austrobates rivularis which, as far as we know, is endemic to northern Queenland, Australia. Within Halobates the sister group of all other species is  H. (Hilliella) with three species distributed along the coasts of tropical northern Australia. A likely hypothesis is that Halobates diverged from its sister group Austrobates somewhere in the area which now constitutes the northernmost part of the Australian continent. Available fossil evidence suggests that Halobates had evolved before the Middle Eocene (45 Myr), when Australia was part of eastern Gondwana.

The sister group of Austrobates+Halobates is the genus Asclepios which is distributed along the coasts of South and East Asia (see Distribution of Sea Skaters). Assuming that the actual distribution of Asclepios also represents the ancestral area for the vicariance event separating Austrobates+Halobates from Asclepios may have taken place somewhere between Asia and Australia. Contrary to conventional views, there may have been continental fragments of Gondwanan origin lying between Australia and mainland Asia during the late Cretaceous and early Tertiary. Members of the tribe Metrocorini, the closest relatives of the Halobatini, are widely distributed in the Afrotropical region and in south and east Asia, but absent from the Australian region. Only one species, has crossed Wallace's line. Taking the present distribution of the freshwater relatives of sea skaters into account, it also seems most likely that the first members of the tribe Halobatini originated in eastern Gondwana, perhaps in areas which now comprise the Indo-Australian region. 

Evolution

The marine sea skaters undoubtedly evolved from related freshwater gerrids. All species belonging to the tribe Metrocorini live in lotic freshwater habitats, in mountains as well as in coastal lowlands. It has therefore been hypothesized that ancestral sea skaters invaded coastal marine habitats from lotic freshwater habitats, probably via estuarine habitats. The fact that Austrobates is limnic and not marine may seem puzzling because it suggests either that the marine environment has been invaded twice, or that Austrobates has recolonized freshwater habitats from marine habitats. These two hypotheses are equally parsimonious requiring two ecological transitions each. However, since organisms may have a wider spectrum of habitat tolerances than those actually preferred, a third hypothesis may apply where ancestral Halobatini lived in both limnic and marine habitats, Asclepios adopted a preference for marine habitats, whereas Austrobates and Halobates evolved from their euryhaline ancestors into species preferring only limnic and marine habitats, respectively.

Literature cited

Andersen NM, Weir TA. 1994. Austrobates rivularis, gen. et sp. nov., a freshwater relative of Halobates (Hemiptera: Gerridae), with a new perspective on the evolution of sea skaters. Invertebrate Taxonomy 8: 1-15.

Damgaard J, Andersen NM, Cheng L, Sperling FAH. 2000. Phylogeny of sea skaters, Halobates Eschscholtz (Hemiptera, Gerridae), based on mtDNA sequence and morphology. Zool. J. Linn. Soc. 130: 511-526.

 


Back to: Top of page Main page

 Last update: 04 september 2002