The burrowing crab Austrohelice crassa mediates the impact of temperature extremes in soft sediments.

Climate change is driving increases in mean air and water temperatures as well as increasing temperature variation and extremes. Intertidal organisms are routinely exposed to rapid and wide fluctuations in both water and air temperatures, with the most extreme heating occurring during mid-day low tides when heat spikes are most intense. In summer heat spikes can be major determinants of behaviour, physiology and survival, leading to differences in ecosystem function. Sediment temperatures can be altered by biota (e.g. sediment burrows) that modify sediment topography, resulting in differences in functioning and responses compared to non-bioturbated areas. This study investigated the impact of the burrowing crab Austrohelice crassa (Decapoda: Varunidae) on estuarine sediment temperatures and CO 2 fluxes (as proxy of benthic metabolism) under ambient conditions and when exposed to in situ experimental heating via open top chambers. Temperature intensification was detected to 5 cm depth in one site and 10 cm in the other. Burrow density was associated with differences in warming at all measured heights, although the magnitude and direction of these differences varied between sites. CO₂ flux responses varied with burrow density and heating context, although effects were site- and metric-dependent. This study provides evidence of the mediating effect of burrows on temperatures, and the potential for these sediment features to drive differences in local environmental conditions and functioning from plain areas under current and warming scenarios.