Phylogenetic Systematics of the Australian Fairy Shrimp Genus Branchinella Based on Mitochondrial Dna Sequences

Phylogenetic reconstructions derived from analyses of DNA sequence variation in a segment of the mitochondrial ribosomal RNA (16S) gene revealed an extensive radiation of species within the genus Branchinella from Australia. Genetic differentiation was evident among all of the species analyzed, including two recently discovered species. There was limited agreement, however, concerning taxon affinities deduced from the 16S data and a previous morphological study. Members of the genus appear to have diversified rapidly, but analyses of other genes and broader taxon sampling are crucial to confirm this hypothesis. Disparities between levels of morphological and molecular change were noted between certain taxa. Whether the radiation of Australian Branchinella happened in situ or was the product of independent colonization events will require the analysis of branchinellids that occur outside Australia. Species comprising the fairy shrimp genus Branchinella occur in freshwater habitats in Europe, Asia, North and South America, southern Africa, and Australia. They are exceptionally diverse, however, in Australia where 27 species are currently recognized, all of which are endemic. Eighteen of these species have been described in detail by Geddes (1981) based on his critical review of earlier morphological works (e.g., Sayce, 1903; Linder, 1941) along with his own analysis of additional collections. Recent surveys and morphological work have led to the discovery of nine more species of Branchinella from this continent (Timms, 2001, 2002; Timms and Geddes, in press). Two of these new species, e.g., B. campbelli and B. budjiti (Timms, 2001), have been analyzed in the present study; the remaining seven species are formally described elsewhere (Timms, 2002; Timms and Geddes, in press). It should be noted that in Belk and Brtek’s (1995) checklist, two of the designated subspecies of B. nichollsi, i.e., B. n. hattahensis and B. n. buchananensis (Geddes, 1981), were elevated to species rank, albeit without justification. Because the present results (see below) support Geddes’ (1981) recognition of these two subspecies, we have adopted his taxonomic scheme. Characters that have been commonly used to differentiate species of Australian Branchinella are the presence or absence of frontal appendages (organs) on the second antennae of males, structural details of the phyllopods, and penes. Geddes (1981) proposed a hypothesis of species relationships based primarily on these characters, partitioning branchinellids into three species groups (Table 1). It has been recognized, however, that these characters not only vary among closely related species but also within species, and they may overlap between species (Geddes, 1981; Timms, 2001). Affinities among species based on one set of morphological features and those based on a different set are not always mutually inclusive, suggesting that these characters may have been influenced by convergent evolution. For these reasons, a purely morphological approach is unlikely to produce a robust indication of species relationships. The high level of species diversity and endemism in Australian Branchinella, along with their ability to occupy inland waters with a broad range of salinities (i.e., 0 up to 43 g/L) (Geddes, 1981; Geddes, 1983; Timms and Sanders, 2002), makes this genus an ideal group for evolutionary and ecological studies. Here we used DNA sequences coding for a region of the mitochondrial (mt) ribosomal RNA gene (16S) as an independent source of phylogenetic information to provide an estimate of the evolutionary relationships within Australian Branchinella, test the validity of the species groups proposed by Geddes (1981), and distinguish between phylogenetically informative and homoplasious morphological traits within the genus. 436 JOURNAL OF CRUSTACEAN BIOLOGY, 23(2): 436–442, 2003