Architecture of Burrows and Ecological Aspects of Thronistes rouxi (Coleoptera: Scarabaeidae): A Potential Bioindicator for Coastal Dune Conservation

Thronistes rouxi is a subterranean beetle species strictly associated with coastal dune ecosystems, which face increasing threats from urbanization and coastal erosion. The objectives of this study were as follows: (1) to characterize the morphology and dimensions of burrows in situ and analyse the relationship between their dimensions and adult body size on two beaches of the Río de la Plata, (2) to determine the influence of sand grain size on the dimensions of T. rouxi burrows on both beaches, (3) to evaluate whether both sexes construct burrows and (4) to analyse the temporal variation in burrow density and determine the spatial distribution patterns within a population. Field surveys for burrow characterization were conducted during the summer seasons of 2020–2021 at Lagomar and Araminda beaches (Canelones, Uruguay) using beeswax casts. To determine which sex digs the tunnels, males and females were placed in individual laboratory terrariums. Temporal variation of burrow density was monitored weekly, from December 2021 to February 2022, at Araminda Beach. The spatial distribution pattern was analysed during the population peak using the dispersion index (DI) and Morisita's index (MI). The burrows exhibit a sinuous tubular structure with a single entrance, reaching up to 2 m in length and 0.9 m in depth. Grain size differences between beaches did not influence gallery dimensions. Although length and depth were similar between sites, burrow width showed significant differences between beaches and was positively correlated with adult body size and pronotum width. The highest mean density and abundance were observed in early January (0.6 ind/m 2 ± 0.3; 61.2 ± 23.1), and the spatial distribution was significantly aggregated (DI = 1.30; MI = 1.49). In both laboratory and field observations, only males exhibited construction behaviour, often remaining motionless at the burrow entrance in a vertical position with extended forelegs. This behaviour is consistent with a potential role in mate attraction and/or guarding behaviour. These findings provide valuable information on the burrowing behaviour and ecology of T. rouxi and its potential use as a bioindicator for the conservation of coastal ecosystems.