Biogeography and systematics of the Tasmanian mountain shrimps of the family Anaspididae (Crustacea: Syncarida)

The family Anaspididae is a relictual Gondwanan group of malacostracan crustaceans now surviving only in cool waters and caves of Tasmania. Its most widespread species, Anaspides tasmaniae, is also the most morphologically plesiomorphic of the order Anaspidacea, and shows very little development since Triassic fossil anaspid forms. A number of issues relating to the systematics and taxonomy of the family Anaspididae have been raised since the first discovery of the extant species. In particular, the separate species status of Anaspides spinulae has been questioned. The level of genetic differentiation among populations of Anaspides tasmaniae has never been ascertained, but has been presumed to be high due to the long periods of isolation between populations. The relationships between the three anaspid genera are also of interest as they elucidate the early evolution of the family in Tasmania. In order to clarify the systematics of the genus Anaspides, an allozyme and mitochondrial DNA study of populations of Anaspides tasmaniae, A. spinulae, Paranaspides lacustris, Allanaspides helonomus and Allanaspides hickmani was undertaken. Anaspides populations were sampled over the entire geographic range of the genus and a wide variety of habitats, including caves. Genetic affinities ascertained from the allozyme and 16S mtDNA analyses were largely congruent. The allozyme study also provided information on within-population processes and the mtDNA analysis provided confidence levels on phylogenies and molecular clock estimates for divergence times. The results showed that: ‚Äö Anaspides spinulae was not supported as a species separate from A. tasmaniae. ‚Äö Cave populations do not form a distinct genetic group despite the common loss of pigmentation. All cave populations sampled shared genotypes with the nearest epigean populations. ‚Äö Populations of Anaspides tasmaniae (including A. spinulae) were characterised by generally low levels of within-population genetic variation, with many instances of fixed differences and private alleles, and often high levels of among-population genetic differentiation. ‚Äö The genus Anaspides contains three distinct geographical groups all of which were supported genetically as separate species. These are found in the south (Huon area), the southwest and the Central Plateau / Derwent areas. It is proposed that these groups be considered as separate species requiring detailed description. ‚Äö Differentiation among Anaspides populations within the Central Plateau was significantly less than that within the southern and southwestern group. Speciation appears to be continuing in isolated populations, particularly in the southwest. ‚Äö The Central Plateau group has a core group of 8 populations, and 5 populations on the fringe of the Plateau that are more distant. An isolation by distance test found a significant correlation between inter-population structure and geographical distance. ‚Äö The southern group of Anaspides showed the same high level of differentiation from the other two Anaspides groups as from Paranaspides. This level of differentiation is similar to genuslevel divergence in other crustacean groups. As a result, it is recommended that the southern Anaspides group be described as a new genus. A thorough examination of morphological characters should be undertaken in order to establish the southern Anaspides as a new genus. ‚Äö The emergence of Allanaspides was found to be the earliest divergence from the anaspid lineage, estimated at about 43 million years ago by molecular clock calculations. The separation of Paranaspides lacustris and the southern group of Anaspides, and the divergence of the two Allanaspides species is likely to have occurred between about 20 and 25 million years ago. The Central Plateau and southwest groups appear to have diverged approximately 10 million years ago. The estimated time at which the last common ancestor occurred within geographical groups is approximately 6 my for the southwestern group, 3.5 my for the southern group and 2 my for the Central Plateau. ‚Äö Biogeographical explanations for divergences in the family Anaspididae are suggested. These include climatic warming in the Eocene, increasing aridity in the late Oligocene and Miocene, various geological faulting and tectonic events in the Tertiary and the invasion of Tasmanian freshwaters by fish predators. Vicariant speciation in the Tertiary appears to have been the major influence on evolution in the Anaspididae, with Pleistocene glaciation and/or recent warming possibly forcing populations into cave refugia. There is evidence that glacial meltwater may have allowed some populations to mix, having a homogenising effect on genotypes, and possibly overlaying more ancient relationships between populations.

Topic: Fish Biology and Ecology Studies
Subfield: Aquatic Science
Field: Agricultural and Biological Sciences
Domain: Life Sciences