Acute thermal effects in an inland freshwater crab Austrothelphusa transversa (von Martens, 1868) occupying seasonal, tropical rivers

Crustaceans occupying drying waterholes in seasonal tropical rivers can access terrestrial areas to regulate hyperthermia. An experiment tank filled with water and containing a mesh ramp and dry platform was designed to determine the terrestrial locomotion response of the tropical inland freshwater crab Australothelphusa transversa (von Martens, 1868) when exposed to increasing water temperatures. A thermoline controller increased water temperature (acclimation 25 °C, ± 0.5) at two rates, 18 °C h–1 (commonly employed in other studies) and 3 °C h–1 (reflecting measured regional river conditions) to determine the acute effects temperature (AET50) threshold for the crab. Random movement was minimal in the 18 °C h–1 experiment, with crabs accessing and remaining on the platform until the experiment ended (AET50 = 40.2; maximum = 41.69 °C). Crabs exhibited more random movement in the 3 °C h–1 experiment, visiting the platform for 3–21 min before returning for 3–131 min (AET50 = 34.2, maximum = 37.25 °C). Crabs collected from a second, more arid, geographical location in northern Australia had a similar AET using 3 °C h–1 increase (AET50 = 35.3; maximum 37.60 °C) and a similar AET using 18 °C h–1 increase (AET50 = 39.3; maximum 41.7 °C) to the coastal crab results (ANCOVA interaction location and ramp rate F = 1.88, P > 0.15). A temperature rate altering AET estimates by several degrees (regardless of where crab population was collected: ANCOVA location, F = 0.668, P > 0.4; rate increase factor, F = 4.31, P < 0.001) highlights the need for laboratory experiments to be underpinned by relevant field data. These data outline that crab avoidance responses could include accessing river banks, as a response necessary to regulate temperature during critical periods.