Aspects of Biology, Distribution, and Conservation of the Smooth Crayfish Euastacus girurmulayn Coughran, 2005 (Decapod: Astacidea: Parastacidae): A Dwarf Freshwater Crayfish from the Southern Rim of the Tweed Caldera, New South Wales, Australia

Abstract This study documents new aspects of the distribution, ecology, and conservation of the smooth crayfish, Euastacus girurmulayn Coughran, 2005, a dwarf freshwater crayfish endemic to rainforest headwater streams of northeastern New South Wales. Field surveys extended its known distribution from the Wilsons River system to include the Tweed and Brunswick River basins and the Cudgera Creek drainage, increasing the extent of occurrence (EOO) to 200.7 km² and area of occupancy (AOO) to 60 km². The species is a habitat specialist confined to shaded, permanently flowing upper catchment streams between 59–718 m in elevation. New records provide the first detailed observations of fecundity, confirm a maximum size of 34.7 mm OCL, and extend its altitudinal range. Molecular analyses validated the distinctiveness of E. girurmulayn and revealed marked divergence among populations, indicating the presence of distinct evolutionary significant units (ESUs). Major threats include climate change, high-intensity bushfires, drought, habitat loss, invasive species, illegal collection, and potential disease introduction. These findings support the recent listing of E. girurmulayn as Endangered under the EPBC Act (criteria B1ab(iii, v)) and highlight the urgent need for targeted conservation actions to ensure the persistence of this highly range-restricted species.