Eophreatoicus in Kakadu National Park and Arnhemland, Northern Territory

Buz Wilson & Chris Humphrey




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Australian Biological Resources StudyThis research is supported by the Australian Biological Resources Study


Scientists from the Environmental Research Institute of the Supervising Scientist ( ERISS, Federal Department of the Environment) and the Australian Museum (Sydney) have discovered a flock of species in Eophreatoicus, phreatoicidean isopods endemic to Kakadu National Park and Arnhemland in the Northern Territory. Over the last 20 years, ERISS has been monitoring fresh waters around Kakadu National Park. The overall aim of their work is environmental due diligence related to uranium mining within the national park boundaries. One of the project ecologists, Dr Chris Humphrey, and colleagues collected invertebrates from numerous streams arising in the 'stone country' on the western margin of the Arnhem Plateau. At certain times of the year, they found isopod crustaceans in the genus Eophreatoicus (order Phreatoicidea). These isopods live in springs at the base of the Arnhem Plateau and come out into the streams during the summer wet season. As the dry season commences in late summer and stream flows recede, they move back upstream to their springs and, if necessary, burrow underground until the next wet. In the 1990s, Dr Buz Wilson and Dr Winston Ponder visited the park and collected a few large samples of these animals, and provided ERISS with preliminary identifications. Buz has been studying the Phreatoicidea for over a decade, and is preparing a large monograph on all Australian genera, so these Kakadu isopods were of substantial interest. Chris has been sending Eophreatoicus specimens to Buz since that initial visit, and in 2004 they successfully applied for an ABRS grant to describe the species, which they thought amounted to around 10 species. This new project was a bit like opening "Pandora's box" because Buz's laboratory study uncovered at least 30 new species. A taxonomic database (DELTA) on the morphology of these species is being used, in conjunction with genetic studies. These morphological discoveries were subsequently confirmed by genetic research done in the Australian Museum's DNA lab, with the assistance of Dr Don Colgan, Ms Karen Gray and Dr Rebecca Johnson. One surprising discovery was a cluster of distinct morphological species in the vicinity of Jabiluka, one of the sites proposed for mining uranium: 13 species were found within less than 10 km of the Jabiluka Outlier, with 2 or 3 species at each site. Buz, Chris and colleagues are preparing a series of papers on the species in Eophreatoicus and this species flock. Names for the new species will be derived from information provided by the indigenous traditional owners of Kakadu.


Specimens collected at Leichhardt Springs and at Magela Creek in the Aligator Rivers Region show considerable differences (Figure 1), even though these sites are separated by only 24 km.

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Figure 1. Comparison of Eophreatoicus species from two sites. These two females from North Magela Creek and Leichhardt Springs belong to separate species. Their eyes are different sized, their cuticle differs in roughness, the last body segment (pleotelson) has a different shape and the appendages on the pleotelson (uropods) have different decorations.

Genetic Research

We have begun the study of the genetic differences between populations from different sites. This genetic work, being carried out in laboratories of the Evolutionary Biology Unit at the Australian Museum, used the mitochondrial genes 16S and cytochrome oxidase subunit I. Preliminary results (Figure 2) show that the DNA sequences collected from the two specimens in Figure 1 show a high degree of genetic differentiation between the populations of the two sites, thus confirming the morphological system being developed. This study will obtain sequences from many of the separate populations that were sampled by ERISS over the last 10 years including a new collection made a the type locality. .

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Figure 2. Eophreatoicus species: Sequence of DNA bases from a section of the 16s mitochondrial genome (screen image from a sequence analysis program). The top two rows (E4 & E5,6,7) are from two specimens from Leichhardt Springs and the bottom three rows (E9,10,11) are from two specimens from Magela Creek Tributary site 2, with the last two rows from the same individual as a check on the consistency of the sequencing reaction. Each DNA base (A G C T) is assigned a colour. These sequences show a high degree of divergence between the two sites but are nearly constant within each population. The level of divergence (approximately 15%) is typical of that found between different genera in other crustaceans.

Eophreatoicus kershawi Recollected

In November 2004, a 3-man team led by Chris Humphrey visited the King River type locality of E. kershawi Nicholls, 1926 in Arnhemland. The type locality proved to be a sandstone outlier west of the King River Estuary. Chris found a cave-like crevice with small number of specimens.

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Chris Humphrey squeezing into the crevice to look for specimens.

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Eophreatoicus kershawi Nicholls, 1926. Adult male collected in November 2004 by Chris Humphrey and party.