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