Molecular and morphological investigations into two species-complexes within the Australian freshwater crab genus Austrothelphusa Bott, 1969 (Crustacea: Decapoda: Gecarcinucidae)

Abstract

This thesis investigates the morphological and molecular diversity of the Australian freshwater crabs of the genus Austrothelphusa. Austrothelphusa currently has eight recognised species: A. transversa (von Martens, 1869); A. angustifrons (A. Milne-Edwards, 1869); A. agassizi (Rathbun, 1905); A. valentula (Riek, 1951); A. raceki (Bishop, 1963); A. wasselli (Bishop, 1963); A. tigrina (Short, 1994) and Austrothelphusa gilbertensis (Naser, Davie and Waltham, 2018) . However, recent molecular studies indicate that these may represent species-complexes containing multiple pseudocryptic or previously unrecognized species. Molecular clock estimates indicate a late Miocene origin around 8–9 mya for Austrothelphusa followed by radiation into eight deeper lineages around the Miocene–Pliocene boundary to about the mid-Pliocene (about 4–6 mya), with these lineages each subsequently showing evidence of rapid species diversification through the Pleistocene to form numerous species-complexes that date from around 1–3 mya. Explanations for this are proposed based on palaeogeographic historical changes affecting northern Australia. The present study focuses on two such species-complexes, Austrothelphusa wasselli and A. angustifrons. An integrative taxonomic approach was followed combining the analysis of both molecular (mitochondrial 16s and COI sequences) and morphological data. The Austrothelphusa wasselli species-complex consists of two major clades that appear to represent separate vicariant speciation events. A large A. wasselli species-group clade includes seven putative species from nine separate river catchments: the Stewart, Archer, Embley, Jeannie, Endeavour, Barron, Normanby, Mitchell, Gilbert and Norman Catchments. Most lineages are catchment specific, but in a few cases there is overlap. The second clade contains two putative new species, one from the eastern upper Mitchell Catchment, and the other from the adjacent Burdekin River Catchment. Within each clade, the genetic species differences are reflected in consistent morphological characters including differences in the degree of carapace region differentiation, epibranchial tooth prominence, the degree of carapace granulation and striation, walking leg proportions, male pleon shape and proportions, and G1 shape. While these lineages are likely to represent a number of new species, the present work mainly serves to confirm the identity of true A. wasselli (sensu stricto) based on a re-examination of the holotype, and through the examination of more recent topotypic material. One new species was recently described, Austrothelphusa gilbertensis Naser, Davie and Waltham, 2018, , from the Gilbert River Catchment, north-western Queensland. Samples identified as Austrothelphusa angustifrons from northern Cape York and the islands of the Torres Strait were also examined to understand the impact of insular and river catchment separation on genetic population divergence within this putative species. The analyses based on the 16s, and the barcoding portion of COI were congruent. Two major clades were found that also appear to represent separate vicariant speciation events. A large A. angustifrons species-group includes seven species found from Cape York through the Torres Strait islands and into southern New Guinea. The type of A. angustifrons is redescribed, and A. podenzanae (Colosi, 1919) is resurrected from its synonymy. The other five species are described as new. The second, species-group, contains two new species, one co-occurring on Prince of Wales and Horn Island, Torres Strait, and one from the Lockhart River Catchment on north-eastern Cape York. Morphological characters reinforce the genetic separation of the two major species-groups/clades, and in particular the shape of the apical margin of the male second gonopod is informative. Within each clade the genetic species differences are reflected in several consistent morphological characters including differences in the degree of carapace region differentiation, epibranchial tooth prominence, the degree of carapace granulation and striation, colour and colour pattern variation (where recorded), walking leg proportions, abdominal shape and proportions, and G1 shape. The large number of new species identified, just within these two species-complexes, indicate a much larger biodiversity of freshwater crabs in northern Australia than previously suspected, and further study is required. In particular, many river-catchments remain uncollected or have very limited sampling. These need to be targeted to obtain fresh specimens for DNA testing and morphological assessment and description. The recognition of so many species with limited geographical ranges and habitats, has serious conservation and management implications. At the minimum, life-history and physiological studies of representatives of each of the major species-complexes is urgently needed. Anthropogenic threats from mining, farming and land clearing are potentially serious, but the longer-term implications of climate change and consequent aridification, must also be considered. Increasing uncertainty of reliable monsoonal rains could have severe impacts on coastal-flowing river catchment zones, and potentially lead to species extinctions. It is hoped that this study will provide a platform on which to base further studies to fully understand the phylogeography, biodiversity, and conservation of Australian freshwater crabs.