Records of the Western Australian Museum 18: 121-127 (1996). 
Status of Anadara trapezia (Deshayes) (Bivalvia: Arcoida) from 
Oyster Harbour, Albany (Western Australia) as compared with 
east Australian populations 
M. Roseline Yardin and Barry J. Richardson 
School of Science, University of Western Sydney (Hawkesbury), Richmond, New South Wales, 2753, Australia 
Abstract - A study of allozymes of the arcoid Anadara trapezia was carried 
out to estimate the degree of divergence associated with the temporal and 
geographical isolation of the southwestern Australian population from the 
east Australian population. A survey of electrophoretic variation at 27 
putative loci coding for enzymes revealed no evidence of divergence or 
speciation of the Western Australian population. At all surveyed loci in all 
sample sets, the same most common alleles were found. Questions are raised 
regarding the geological time of separation of those populations. Could the 
time span have been more recent than paleogeological records suggest or is 
this species made up of a set of well adapted gene complexes that have been 
stable for thousands of years? 
INTRODUCTION 
In the marine environment, gene flow is 
regulated by the rate of exchange of waters 
between populations. In the absence of such water 
movement, geographic barriers to gene flow may 
result in genetic divergence between populations 
of species with pelagic larval dispersal (Burton 
1986; Hedgecock 1986). The degree of divergence 
and speciation is related to the length of time of 
separation between populations. The detection of 
genetic differences or its lack between populations 
from different regions provides evidence of the 
isolation or continuity of those populations. Ideally 
a multidisciplinary approach in the delineation of a 
species would include data on the geographic 
distribution, both past and present, morphology, 
ecology, physiology and biochemistry, 
reproduction, and genetics (White 1978). However, 
there is a paucity of cases where all this 
information has been collated (see Gosling 1994). 
Anadara trapezia (Deshayes 1839), a bivalve 
mollusc of the order Arcoida, is a common species 
found in hyposaline estuaries from Cairns, 
northern Queensland (Dixon 1975) to Port Philip 
Bay, southern Victoria (e.g., Macpherson 1966; 
Poore and Rainier 1974; Smith, Coleman and 
Watson 1975; Dixon 1975) and in southern Western 
Australia at Oyster Harbour near Albany 
(Kendrick and Wilson 1959). In South Australia, 
the vast areas of tidal flats and sea grass beds in 
Gulf St. Vincent are considered suitable habitats 
for Anadara trapezia. However, no live individual 
has been found in recorded history (e.g.. Cotton 
1957, 1961; Macpherson and Gabriel 1962; Dixon 
1975), despite an attempt to reintroduce A. trapezia 
in Gulf St Vincent in 1956. The eastern and western 
population can be considered isolated from one 
another at the present time. 
The Quaternary was a period of enormous 
change. The dramatic changes in climate were 
accompanied by large fluctuations in sea level, 
temperature and circulation. These processes 
resulted in the repeated production and removal of 
geographic barriers between the populations of 
temperate eastern and western Australia. Such 
conditions are believed to favour speciation 
(Heatwole 1987). 
The fossil record shows that the distribution of 
A. trapezia around the Australian coast was more 
widespread in the past than it is today (Kendrick 
1990). In Western Australia, the most northerly 
published fossil record is Lake McLeod at the 
mouth of the Lyndon River (Kendrick 1990; 
Kendrick et al. 1991). This is of particular 
importance as the disappearance of the species 
from this area provides a good indication of the 
drying up of rivers and change in the character of 
estuaries in that region (Kendrick et al 1991). In 
South Australia during the Late Pleistocene the 
Glanville Formation was laid down during a 
period when the water in the region was much 
warmer than present day conditions and A. trapezia 
flourished (see Ludbrook 1984). A. trapezia, 
together with other molluscs then disappeared 
from South Australia. Fossils are also found in 
Tasmania, near Launceston (e.g., Goede et al. 1993) 
and also North Island, New Zealand (e.g., Crozier 
1962; Beu and Maxwell 1990; Bryner and Grant- 
